Abstracts
Drosophila paulistorum populations colonizing the urban area of Porto Alegre, southern Brazil, were studied with the objective of characterizing their chromosomal polymorphism in this new environment. Despite being geographically and ecologically marginal and the fact that the colonization of the urban area seems to be a recent event, the populations showed a large number of inversions on all chromosome arms. Differences regarding inversion frequencies and percentage of heterozygosis were found when we compared the samples with respect to geographical, microenvironmental and temporal aspects. Such differences, however, could be attributed to both selective and stochastic factors
Populações de Drosophila paulistorum colonizadoras da área urbana de Porto Alegre, no sul do Brasil, foram estudadas com o objetivo de caracterizar o seu polimorfismo cromossômico neste novo ambiente. A despeito de serem geograficamente periféricas e ecologicamente marginais, e do processo de colonização ser um evento recente, as populações apresentaram um grande número de inversões em todos os braços cromossômicos. Diferenças nas freqüências de algumas inversões e na porcentagem de heterozigose foram encontradas quando se compararam as amostras quanto aos aspectos geográfico, microambiental e temporal. Tais diferenças, entretanto, podem ser atribuídas tanto a fatores seletivos como estocásticos.
Chromosomal polymorphism in urban populations of Drosophila paulistorum
Victor Hugo Valiati and Vera Lucia S. Valente
Departamento de Genética, Instituto de Biociências, Universidade Federal do Rio Grande do Sul, Caixa Postal 15053, 91501-970 Porto Alegre, RS, Brasil. Send correspondence to V.L.S.V. Fax: (051) 336-2011. E-mail: valiati@if1.if.ufrgs.br and valente@if1.if.ufrgs.br.
ABSTRACT
Drosophila paulistorum populations colonizing the urban area of Porto Alegre, southern Brazil, were studied with the objective of characterizing their chromosomal polymorphism in this new environment. Despite being geographically and ecologically marginal and the fact that the colonization of the urban area seems to be a recent event, the populations showed a large number of inversions on all chromosome arms. Differences regarding inversion frequencies and percentage of heterozygosis were found when we compared the samples with respect to geographical, microenvironmental and temporal aspects. Such differences, however, could be attributed to both selective and stochastic factors.
INTRODUCTION
Studies on giant chromosomes in the larval salivary glands of Drosophila have disclosed a wide variation in gene arrangements both within and between species due to paracentric inversions. Many observations in nature and on laboratory populations of the Drosophila species have extensively documented the adaptive role of chromosomal polymorphism (review in Krimbas and Powell, 1992). According to Dobzhansky and Pavlovsky (1962) and widely confirmed for several species, the relative fitness of the carriers of inversions is extremely sensitive to environmental conditions.
Drosophila paulistorum is a neotropical species occurring from Guatemala to southern Brazil. It belongs to the sibling species group willistoni and in fact is a superspecies consisting of six races or incipient species (Dobzhansky and Spassky, 1959). The entity that occurs in southern Brazil corresponds to the Andean Brazilian race.
Until the review performed by Spassky et al. (1971), the southernmost collecting point of D. paulistorum was Osório (29o54 S; 51o16 W), placed along the northern coast of Rio Grande do Sul State. Santos and Valente (1990), however, registered D. paulistorum in a much more extreme place, the city of Porto Alegre. Beyond being geographically peripheral, such D. paulistorum populations seems to be also ecologically marginal, since they inhabit a quite unpredictable environment. Although it is known as a species that does not occur in places disturbed by man (review in Ehrman and Powell, 1982), D. paulistorum and its sibling D. willistoni have been found perfectly integrated in Drosophilidae communities in the urban area of Porto Alegre city (Santos and Valente, 1990; Valiati and Valente, 1996). Cities are a man-made ecosystem in which the life conditions are extreme, due to a varying climate and poor availability of native trophic resources, besides the presence of new competitors. On the other hand, they offer new substrates to be exploited by the colonizer species as feeding and breeding sites, as well as new evolutionary opportunities.
The present study was done in order to contribute to the knowledge of the genetic characteristics of these typically peripheral populations, and of the evolutionary implications of the colonizing potential of D. paulistorum.
MATERIAL AND METHODS
Porto Alegre is a city with 1,500,000 inhabitants (30o02S, 51o14W) in Rio Grande do Sul, the southernmost state of Brazil (Figure 1). At this latitude, the climate is subtropical, with a mean temperature ranging from 11.3 to 20.3oC in winter, although in this season temperatures as low as 0oC are common.
The samples of D. paulistorum were obtained from streets and residential gardens. The urbanization levels were established by the criteria of Ruszczyk (1986/1987). According to this author, three levels of urbanization can be recognized in Porto Alegre city based on the proportions of green and constructed areas. The high urbanization zone has less than 20% plant cover and is occupied mainly by tall buildings; the medium urbanization zone has 20 to 40% vegetation cover and is equally occupied by buildings and singlefamily houses; and the low urbanization zone has more than 40% plant cover, with the rest of the soil mostly covered with single-family houses or low buildings.
Drosophila was sampled using two collection methods: 1) capturing adults with nets over conventional banana or orange bait, and 2) collecting rotting fruit with pre-adult forms that are allowed to complete their cycles in the laboratory in bottles containing culture medium (Marques et al., 1966) under controlled temperature and humidity conditions (25 ± 1oC, 60% relative humidity).
D. paulistorum adult specimens were identified and counted (as also were individuals of the other species detected) and isofemale strains were obtained in order to analyze the chromosomal inversion polymorphism in the salivary glands from one female larva per strain. Third-instar F1 female larvae were dissected and the salivary glands processed according to the technique of Ashburner (1967).
Photomicrographs and drawings of inversions reported in the literature (Burla et al., 1949; Da Cunha et al., 1950; Dobzhansky et al., 1950; Dobzhansky and Pavlovsky, 1962; Kastritsis, 1966, 1967, 1969 and Santos and Valente, 1990) were used to identify the inversions found in the Porto Alegre populations.
The cytologic material of D. paulistorum, however, is very difficult to work with because many of the heterozygous inversions are short and quite often subbasal, making exact determination of their breakpoints difficult. So, we decided to maintain the nomenclature used by Santos and Valente (1990), numbering each inversion on each chromosome arm as 1, 2, 3 and so on. This decision was due to the fact that it is very difficult to compare our inversion configurations with those recorded as drawings and plates by previous authors.
The test employed to evaluate the statistical significance of the differences observed in the frequencies of different inversions on the chromosome arms was the exact permutational test described by Roff and Bentzen (1989). To perform this test, both the number of heterozygotes and homozygotes as well as those of homozygotes versus inversion numbers were considered, in order to do the comparisons. This was done to evaluate the existence of possible spatial, microspatial, seasonal and annual differences among the samples, using data from Tables III, IV, V Table V - Frequencies and absolute number of inversions detected on the left arm of the chromosome 2 (2L) in D. paulistorum population. , VI e VII. This test is very useful when the expected values are very small. The values were obtained after 1,000 randomizations. The adjusted residuals (Haberman, 1973; Everitt, 1992) for each cell of the table of comparisons were individually analyzed and compared with critical values (e.g., Z0.05 = 1.96) of the normal distribution in the cases where the above test showed a statistically significant value.
RESULTS
Table I shows data for two subsequent years of collection, indicating the frequency of the two sibling species D. willistoni and D. paulistorum and of other species occurring in the urban area of Porto Alegre. The resources exploited by the flies and the urbanization level of the samples are also shown.
- Sampling data with sites, resources, urbanization levels and data for D. paulistorum, D. willistoni and other species in the urban area of Porto Alegre.
N: Native fruit; E: exotic fruit; U.L.: urbanization level; Low: low urbanization; High: high urbanization; *same place of Averrhoa carambola sampling; **rind of citric fruit; +adult flies captured with nets.
List of other species of Drosophila captured. D. melanogaster, D. simulans, D. mercatorum, D. cardinoides, D. polymorpha, D. mediosignata, D. mediopicta, D. hyde, D. mediopunctata, D. kikawaii, D. nebulosa, D. immigrans, D. griseolineata.
(1) Fruit of Averrhoa carambola; (2) fruit of Butia eriospatha; (3) fruit of Citrus sinensis; (4) banana baits; (5) fruit of Punica granatum; (6) fruit of Maclura pomifera; (7) Citrus sinensis baits; (8) fruit of Syagrus romanzofiana; (9) fruit of Psidium guajava; (10) fruit of Diospyros kaki.
For street abbreviations see legend to Figure 1.
Table II presents all the heterozygous inversion types found in the urban populations of D. paulistorum at the different sites colonized and the trophic resources used by the species.
Heterozygous inversion types found in populations of Drosophila paulistorum in Porto Alegre city.
See legend to Figure 1 and Table I for street abbreviations and resources, respectively
Twenty-three different arrangements were detected in the populations during the period studied and the samples from Mário Totta Street (M.T.St.), obtained with fruit of Averrhoa carambola, showed the largest number of inversions. The other samples from exotic fruit, Citrus sinensis and Punica granatum, showed also larger number of different chromosomal arrangements than those of native fruit (Butia eriospatha) and banana bait samples. For instance, the samples from Deoclecio Pereira Street (D.P.St.), collected in C. sinensis (exotic citrus plant from Asia), had double the number of inversions found in conventional banana baits, or those obtained from Osvaldo Cruz Street (O.C.St.) in native fruit (B. eriospatha). Despite these differences, the samples of D. paulistorum captured in the exotic fruits showed greater number of chromosome rearrangements than those of native fruit, we did not find statistically significant differences among them by means of a c2 contingency test.
The most polymorphic chromosome arms were XR (six arrangements), 2L (five), followed by XL, 2R and three chromosomes, all with four inversions.
Figure 2 shows the four different arrangements on the left arm of chromosome X, and their probable breakpoints. The inversions were denominated: inversions XL 1: includes the sections of regions 10 (proximal) to 15 (distal). Inversion XL 5: includes sections 18 (proximal) and half of section 19 (distal). Inversion XL 6: inversion overlapping the XL 5, including sections 14 (proximal) to 19 (distal). Inversion XL 7: small inversion, including sections 10 (proximal) and 11 (distal).
Table III gives the number and percentage of homozygotes (inverted plus standard orders) and heterozygotes for the different inversions found on chromosome X, left arm (XL). Inversion XL 1 was the most frequent and common in all places (frequencies ranging from nine to 33.33%), whereas the other inversions were found in lower numbers and only at certain sites.
Table III - Frequencies and absolute number of inversions detected on the left arm of chromosome X (XL) in Drosophila paulistorum urban populations.
The same as in Table II.
Inversions XL 5 and XL 7 could not be compared with any of the published arrangements, probably being new inversions. Inversion XL 6 is similar to inversion XL II described by Kastritsis (1967) and XL 1 was observed in Porto Alegre populations by Santos and Valente (1990).
All the six different arrangements found on the right arm of chromosome X (XR) can be observed in .Table IV.
Table IV - Frequencies and absolute number of inversions detected on the right arm of chromosome X (XR) in D. paulistorum urban populations.
Sample
Resource
Date
Homozygotes
Total
1
3
4
5
6
7
N
%
N
%
N
%
N
%
N
%
N
%
N
%
M.T.St.
(1)
10/90
12
70.60
3
17.60
-
-
-
-
1
5.90
-
-
1
5.90
17
4/91
74
81.32
10
10.99
-
-
-
-
3
3.30
2
2.20
2
2.20
91
8/91
29
80.55
6
16.67
-
-
-
-
-
-
-
-
1
2.78
36
4/92
34
77.28
9
20.45
-
-
-
-
-
-
-
-
1
2.27
44
B.Garden
(1)
4/91
41
78.85
6
11.54
-
-
1
1.92
1
1.92
3
5.76
1
1.92
52
4/92
35
85.37
5
12.19
-
-
1
2.44
-
-
-
-
-
-
41
(2)
3/91
11
100.00
-
-
-
-
-
-
-
-
-
-
-
-
11
D.P.St.
(3)
5/91
78
88.63
8
9.10
-
-
-
-
2
2.27
-
-
-
-
88
5/92
23
71.88
9
28.12
-
-
-
-
-
-
-
-
-
-
32
(4)
5/91
10
62.50
5
31.25
-
-
-
-
-
-
1
6.25
-
-
16
5/92
32
78.05
9
21.95
-
-
-
-
-
-
-
-
-
-
41
O.C.St.
(2)
3/91
7
87.50
1
12.50
-
-
-
-
-
-
-
-
-
-
08
(5)
5/91
6
60.00
2
20.00
1
10.00
-
-
1
10.00
-
-
-
-
10
5/92
24
92.31
2
7.69
-
-
-
-
-
-
-
-
-
-
26
(3)
5/92
10
100.00
-
-
-
-
-
-
-
-
-
-
-
-
10
F.Park
(6)
3/92
33
89.20
4
10.80
-
-
-
-
-
-
-
-
-
-
37
5/92
16
72.73
5
22.73
-
-
1
4.54
-
-
-
-
-
-
22
The same as in Table II.
The inversions were denoted (Figure 3A and B): inversion XR 1: this large inversion includes part of section 30 (proximal) to section 36 (distal). Inversion XR 3: small inversion, including section 32 (proximal) and 33 (distal). Inversion XR 4: small inversion, including section 34 (proximal) and a half of section 35 (distal). Inversion XR 5: showing breakpoints in section 29 (proximal) and in the distal portion of section 30. Inversion XR 6: terminal inversion which involved nearly one third of the proximal part of section 38 and the beginning of the distal section 40. Inversion XR 7: with breakpoints in sections 25 (proximal) and 26 (distal).
Only the XR 1 inversion was recognized as the XR III by Kastritsis (1967) and was also previously encountered in Porto Alegre samples by Santos and Valente (1990).
All the remaining inversions on this chromosome arm can be considered new inversions. Table IV shows the inversion frequencies on the right arm of the X chromosome. Inversion XR 1 was the most frequent (7.69 to 31.25%) and was only absent in two samples. The other inversions were uncommon in D. paulistorum populations from Porto Alegre.
The inversions found on the left arm of chromosome 2 (2L) and their breakpoints are shown in Figure 4, and their frequencies are presented in Table V Table V - Frequencies and absolute number of inversions detected on the left arm of the chromosome 2 (2L) in D. paulistorum population. . The inversions were described as following: inversion 2L 1: small inversion, with breakpoints in sections 54 (proximal) and 55 (distal). Inversion 2L 4: breakpoints in the proximal region of section 54 and in the distal region of section 57. Inversion 2L 5: inversion involving sections 46 (proximal) and 51 (distal). Inversion 2L 6: breakpoints in the proximal portion of section 57 and in half of section 59. Inversion 2L 7: small inversion involving only one section (55)
Sample
Resource
Date
Homozygotes
Inversions
Total
1
4
5
6
7
N
%
N
%
N
%
N
%
N
%
N
%
M.T.St.
(1)
10/90
9
47.40
1
5.30
7
36.80
3
15.80
-
-
-
-
19
4/91
56
60.21
9
9.68
24
25.81
10
10.75
1
1.07
1
1.07
93
8/91
21
58.33
5
13.89
10
27.78
4
11.10
-
-
-
-
36
4/92
42
89.36
-
-
4
8.51
1
2.13
-
-
-
-
47
B.Garden
(1)
4/91
30
58.82
2
3.92
12
23.53
7
13.72
4
7.84
1
1.96
51
4/92
31
70.45
3
6.82
8
18.19
2
4.54
-
-
-
-
44
(2)
3/91
7
63.63
1
9.10
3
27.27
-
-
-
-
-
-
11
D.P.St.
(3)
5/91
50
56.82
4
4.54
30
34.10
7
7.95
2
2.28
1
1.14
88
5/92
23
76.66
-
-
7
23.33
1
3.33
-
-
-
-
30
(4)
5/91
11
78.57
1
7.15
2
14.28
-
-
-
-
-
-
14
5/92
44
93.62
-
-
3
6.38
-
-
-
-
-
-
47
O.C.St.
(2)
3/91
7
77.76
-
-
1
11.12
-
-
-
-
1
11.12
09
(5)
5/91
6
66.00
-
-
2
22.00
1
11.00
1
11.00
-
-
09
5/92
22
84.60
-
4
15.40
-
-
-
-
-
-
26
(3)
5/92
8
80.00
1
10.00
1
10.00
1
10.00
-
-
-
-
10
F.Park
(6)
3/92
35
89.76
1
2.56
1
2.56
1
2.56
-
2.56
-
-
39
5/92
19
86.36
1
4.54
2
9.10
1
4.54
-
-
-
-
22
The same as in Table II.
None of these five inversions could be homologated with other descriptions, except 2L 1, 2L 4 and 2L 5 which have been reported to occur in Porto Alegre by Santos and Valente (1990).
Thus, once again inversions 2L 6 and 2L 7 can be considered as new inversions.
Of the five different inversions detected on this arm, 2L 4 was the most common and frequent, being absent in only two of the samples, whereas inversions 2L 1 and 2L 5 occurred at all sites, but neither in all the resources nor in all sampling dates.
Four arrangements on the right arm of chromosome 2 (2R) were found in our populations (Figure 5). The descriptions of inversions and their breakpoints are as follows: inversion 2R 3: small inversion, with breakpoints in the proximal portion of section 67 and in the distal portion of section 68. Inversion 2R 4: including only sections 70 (proximal) and 71 (distal). Inversion 2R 5: inversion involving the proximal portion of section 72 and the distal portion of section 73. Inversion 2R 6: showing breakpoints in the proximal portion of section 74 and in the distal portion of section 79.
Only the 2R 6 inversion has been described before (Kastritsis, 1967) as 2L IV, characteristic of the Orinocan semispecies, although Burla et al. (1949) showed a similar configuration in a camera lucida drawing but none of the break points were identified by us. The remaining inversions have not been reported previously.
All heterozygous inversions observed on the right arm of chromosome 2 (except 2R 6) were found only once at each place, but they were common to several sites. Inversion 2R 3 was widely spread and was not detected at only one site (Farroupilha Park), while 2R 5 was found only a few times, but at all sites (Table VI).
Table VI - Frequencies and absolute number of inversions detected on the right arm of chromosome 2 (2R) in D. paulistorum populations.
N
%
N
%
N
%
M.T.St.
(1)
10/90
16
84.21
2
10.53
1
5.26
-
-
-
-
19
4/91
90
96.78
2
2.15
1
1.07
-
-
-
-
93
8/91
33
91.67
3
8.33
-
-
-
-
-
-
36
4/92
46
97.87
-
-
-
-
1
2.13
-
-
47
B.Garden
(1)
4/91
41
80.39
5
9.80
3
5.88
2
3.92
1
1.96
51
4/92
40
95.24
2
4.76
-
-
-
-
-
-
42
(2)
3/91
10
91.00
1
9.00
-
-
-
-
-
-
11
D.P.St.
(3)
5/91
86
96.63
2
2.25
1
1.12
-
-
-
-
89
5/92
29
90.63
2
6.25
-
-
1
3.12
-
-
32
(4)
5/91
14
100.00
-
-
-
-
-
-
-
-
14
5/92
46
97.87
1
2.13
-
-
-
-
-
-
47
O.C.St.
(2)
3/91
7
87.50
-
-
1
12.50
-
-
-
-
08
(5)
5/91
8
80.00
2
20.00
-
-
-
-
-
-
10
5/92
26
100.00
-
-
-
-
-
-
-
-
26
(3)
5/92
8
80.00
-
-
-
-
2
20.00
-
-
10
F.Park
(6)
3/92
38
97.44
-
-
1
2.56
-
-
-
-
39
5/92
22
100.00
-
-
-
-
-
-
-
-
22
The same as in Table II.
Four heterozygous inversions were found on the third chromosome of the D. paulistorum populations studied here. Figure 6 presents the configurations of the arrangements and their probable breakpoints, which are described below: inversion 3.1: inversion with breakpoints in the first proximal region of section 93 and in the distal region of section 98. Inversion 3.6: including the end of the proximal portion of section 90 or the beginning of section 91 and section 93 (distal). Inversion 3.7: inversion with breakpoints in sections 93 (proximal) and 96 (distal). This inversion overlaps inversion 3.1, which is only two sections higher. Inversion 3.8: small inversion on the extreme part of the chromosome, including half of the proximal portion of section 83 to the half of the distal portion of section 85.
The inversions 3.1, 3.7 and 3.8 were described before by Kastritsis (1967) as 3I, 3IX and 3VI, respectively. Only the 3.1 inversion was previously observed in populations from Porto Alegre (Santos and Valente, 1990).
The arrangement 3.6 was cited by Dobzhansky et al. (1950) and Da Cunha et al. (1950). The former authors showed this arrangement only as drawings, whereas the latter denominated it as inversion D. This inversion has been reported to be very common in the center of Brazil and rare in southern populations.
Finally, the frequencies of the five types of inversions on the third chromosome are shown in Table VII. In this chromosome, it was observed that the inversion 3-1 was the most frequent and widespread, being absent in only three of the samples.
Table VII - Frequencies and absolute number of inversions detected on the third chromosome in D. paulistorum urban populations.
M.T.St.
(1)
10/90
15
93.70
1
6.30
-
-
-
-
-
-
16
4/91
78
84.78
10
10.87
3
3.26
1
1.09
1
1.09
92
8/91
32
94.10
2
5.90
-
-
-
-
-
-
34
4/92
42
87.50
6
12.50
-
-
-
-
-
-
48
B.Garden
(1)
4/91
42
80.77
5
9.61
4
7.69
1
1.92
-
-
52
4/92
38
92.68
3
7.32
-
-
-
-
-
-
41
3/91
9
82.00
-
-
2
18.00
-
-
-
-
11
D.P.St.
(3)
5/91
66
74.10
20
22.50
3
3.40
-
-
-
-
89
5/92
25
80.64
6
19.36
-
-
-
-
-
-
31
(4)
5/91
14
100.00
-
-
-
-
-
-
-
-
14
5/92
39
84.78
7
15.22
-
-
-
-
-
-
46
O.C.Str.
(2)
3/91
8
100.00
-
-
-
-
-
-
-
-
08
(5)
5/91
6
60.00
3
30.00
1
10.00
-
-
-
-
10
5/92
24
92.31
2
7.69
-
-
-
-
-
-
26
(3)
5/92
7
70.00
3
30.00
-
-
-
-
-
-
10
F.Park
(6)
3/92
34
97.14
1
2.86
-
-
-
-
-
-
35
5/92
19
90.48
2
9.52
-
-
-
-
-
-
21
The same as in Table II.
The inversions 3.6, 3.7 and 3.8 were uncommon in the D. paulistorum populations studied here. The latter inversion was observed only in one sample, and in a single isofemale strain, in fruit of A. carambola, in April 1991 (M.T.St.).
When we consider both the comparisons performed between data from homozygotes versus heterozygotes and from homozygotes versus inversion frequencies through the tests used (Haberman, 1973; Roff and Bentzen, 1989; Everitt, 1992), some differences between samples are recognized.
Despite the variation observed in the frequencies of inversions, we did not find statistically significant differences when we performed the possible comparisons among samples from different seasons, collected at the same places and on the same resources. When the comparisons were made among samples from different sites, during the same period and in the same resources, some differences could be detected according to the test employed. For instance, when only homozygotes versus heterozygotes were compared, the 2R arm presented statistically significant differences (P < 0.01) with respect to the samples of M.T.St. versus Botanic Garden (B.Garden, in April 91, in A. carambola. The analysis of the frequencies of homozygotes versus inversion frequencies of the same samples showed that such differences in the 2R chromosomic arm are due to the contribution of the inversion 3. The excess of homozygotes in the M.T.St. samples could be explained mainly by the lower frequency of such inversion. The second statistically different spatial comparison (M.T.St. versus B.Garden, April 92, in A. carambola) was obtained in the 2L arm, but it was not possible to appoint to a particular arrangement, as responsible by the difference detected among homozygotes versus inversions. It was also observed that in the M.T.St. sample there was an excess of homozygotes in relation to the B.Garden sample.
Only two samples of the D.P.St. were used for the microspatial comparisons. The comparison between C. sinensis and banana baits, in May 91, indicated that in both XR and 3 chromosome arms there were significant differences (P < 0.05), when we considered the frequencies of homozygotes versus heterozygotes. Only in the XR arm, however, was it possible to determine the proportional contribution of particular inversions to this result. They were inversion 1 and 6. In C. sinensis, for instance, inversion 1 is less frequent than in the banana samples, whereas inversion 6 is absent in the former and present in the latter samples.
In the same comparison performed with samples of the next year (May, 1992), the differences between homozygotes versus heterozygotes of the 2L chromosome (P < 0.05) could be explained by the frequency of the inversion 4, that is more frequent in Citrus than in banana baits.Differently from the anterior year, the lack of heterozygosis was detected in the banana sample.
The greater number of significant differences observed in the different arrangements of the chromosomal arms was obtained when we compared samples from different years, at the same sites and on the same resources. In the four collection sites, it was possible to perform such type of analysis, and only one of the comparisons in banana baits (D.P.St., May 91/May 92) showed non-significant differences.
In the first comparison (M.T.St. in A. carambola, April 91 x April 92), the 2L homozygote frequencies were significantly different from those of heterozygotes (P < 0.01). This difference seems to be due to the contribution of the inversions 1 and 4, as revealed by the residue analysis. So, the excess of homozygotes in the 1992 sample was probably a consequence of the absence of the inversion 1 and of the low frequency of the inversion 4.
However, when we compared the samples collected in A. carambola (B.Garden, April/91 versus April/92) the difference (P < 0.05) detected between homozygotes versus heterozygotes in the 2L chromosome could not be attributed to any of the inversions. The only fact that was possible to observe was that the 1991 sample was more polymorphic than that of the next year.
The small frequency of the inversion XR 1 seems to be the responsible for the excess of homozygotes observed in the 1991 sample when compared with that of May/92 in the Citrus of D.P.St. (P < 0.05). Finally, both XR and 3 chromosomic arms seem to contribute to the differences observed in the comparison between the samples from Punica collected in May/91 and in May/92 at O.C.St. None of the inversions of both chromosomic arms, however, was capable of explaining the great heterozygosity found in the 1991 sample.
The degree of polymorphism of D. paulistorum populations was evaluated by the average number of inversions per female and chromosome (Table VIII). The higher values were obtained on the left arm of chromosome 2, whereas on the right arm a low average number of inversions was detected. The same average was obtained for both arms of chromosome X and the estimate for the third chromosome was 0.14. Yet, the average estimate obtained for the populations from Porto Alegre city was 0.91, considering the average number of inversions per female. Since the variances of the values of the average number of inversions of each chromosome arm were very high the nonparametric Kruskal-Wallis test was used to evaluate the significance of the differences among them. This test showed that these differences were highly significant (P < 0.001). Only the right arm of the second chromosome was not statistically different from the chromosome 3. The results present wide variation in the average number of inversions (1.42 to 0.61) per site, or per resource exploited on Mário Totta Street. The highest values were detected in samples from fruit that were more successfully exploited by D. paulistorum, such as P. granatum, C. sinensis and in A. carambola. Beyond the above differences found in the average number of inversions per female related with places and resource exploited, no more significant differences were encountered.
Table VIII - Mean number of inversions per chromosome and per female in urban samples of D. paulistorum from Porto Alegre.
Same as in Table II.
DISCUSSION
In spite of being an extremely marginal population and of the unusual ecological characteristics faced by D. paulistorum in the urban environment, the populations studied presented a considerable number of chromosomal variants. This finding, taken together with those of Santos and Valente (1990), is not in a strict accordance with the majority of the data about patterns of central-marginal populations of Drosophila.
According to several authors, marginal Drosophila populations present as a general characteristic a considerable decrease in chromosomal polymorphism in relation to central populations (Townsend, 1952; Dobzhansky, 1957; Da Cunha et al., 1959; Carson, 1955, 1956, 1959; Prevosti, 1964; Sperlich, 1971). Yet considering data from protein electrophoresis, the decrease of variability that occurs in peripheral populations should be differentially considered when both allelic and inversion variants are analyzed. So, whereas the chromosomal polymorphism is diminished in the edge of the range, the allozyme variability usually is as high on the border as in the center of the distribution (review in Brussard, 1984).
Fourteen of the 23 inversions detected as heterozygotes on all chromosome arms can be considered as new rearrangements, not homologous with any of the 85 arrangements previously reviewed by Kastritsis (1966, 1967, 1969) for all semispecies of the D. paulistorum complex. Those studies, however, were done with samples from very different latitudes when compared to our populations.
It was noticeable, nevertheless, that our samples only shared six arrangements of the 18 observed in the same city seven years ago (Santos and Valente, 1990). In contrast, our samples and those of the above authors presented 21 differences, overall. Such findings could be interpreted as follows: there are some inversions that seem to be still endemic (as XL 1), whereas several others (as XR 3, 4, 5, 6, and 2L 6, 7) probably correspond to inversions that are currently being formed under the influence of some kind of genomic disturbance. Their establishment, however, should be dependent of natural selection and/or of the action of genetic drift.
In fact, the characteristics of the chromosomal polymorphism of our populations resemble those of Drosophila species which are in the process of colonizing new territories. For example, Fontdevila et al. (1981) observed an unusually high polymorphism in colonizer populations of D. buzzatti than in others previously analyzed by Wasserman (1954, 1962), Mather (1957), and Carson and Wasserman (1965). Those findings were corroborated by the results of a subsequent study by Fontdevila et al. (1982) who observed major geographic variability in populations from central Argentina, not explained by the level of polymorphism.
According to Fontdevila et al. (1981, 1982) and later by Safriel and Ritter (1983) and Carson (1987), the primary colonizer populations are derived directly from individuals from the central populations, with high chromosomal polymorphism. The secondary colonizer populations, however, are descendents of primary colonizers or of other secondary populations. As a consequence, their chromosomal polymorphism tends to be low. The characteristics of the polymorphism encountered in colonizer populations can thus be suggestive of their origin, as already proposed by Powell et al. (1972) and Prevosti et al. (1988). A similar hypothesis to explain the origin of the Porto Alegre colonizer populations, however, is not yet valid due to the lack of information about other natural wild populations and about their dispersion ability.
If, as shown by Coyne et al. (1982) and by Crumpacker and Williams (1973) for D. melanogaster, D. simulans and D. pseudoobscura, flies are able to fly across great distances under hard pressure to find appropriate substrates, and if D. paulistorum is equally a good spreading species, it is possible to speculate that they came from Osório County, 100 km from Porto Alegre. In this location, considered to be the extreme southern limit of D. paulistorum geographic distribution (Spassky et al., 1971), the populations inhabit the remnants of the Atlantic Forest, which is an environment compatible with the original one of the species. These populations, however, have never been studied.
A more recent hypothesis has been proposed by Fontdevila (1992) to explain the high degree of chromosomal polymorphism in marginal populations of Drosophila. According to this hypothesis, marginal populations are more subject to genomic stresses capable of promoting mobilization of transposable elements, recognized by their ability to yield chromosome rearrangements. The idea that these elements can play an important role in the generation of spontaneous inversions in natural populations due to genome "disturbances" has been consistently suggested by several authors (review in Krimbas and Powell, 1992). Particularly interesting is the fact that a lot of new arrangements were observed in our samples and that several inversions detected a few years ago by Santos and Valente (1990) appeared to be lost. Whether these findings reflect the generation "bursts" of inversions by transposable elements, stress-induced mobilization followed by selection or drift is a matter for further study, but this phenomenon should be considered as theoretically possible. Once again, we emphasize that we have no information about the repertory of transposable elements in these populations.
Previous studies performed by our staff, regarding chromosomal polymorphism of other wild species in the same urban and control places here reported, presented quite different results, suggesting species-specific peculiarities of the inversion systems. For instance, the D. willistoni urban populations showed a clear tendency to lose chromosomal variability, as more urbanized is the collection place (Valente et al., 1993). The analysis of the behavior of each inversion detected in 19 samples showed that only the IILE inversion was associated with greater vegetation cover. The chromosomal polymorphism of the D. nebulosa Porto Alegre populations (Bonorino et al., 1993) could not be so strictly associated with the degree of urbanization of the collecting site, but a certain tendency of samples to come from more similar microenvironments seemed clearer.
Data on the association between chromosomal polymorphism with geographic variation, latitudinal and seasonal clines have been reported in populations of many species of Drosophila. This variation in the polymorphism is the result of the ability of those species to exploit several different environments and plays an adaptive role to different aspects of individual survival and reproduction (review in Sperlich and Pfriem, 1986).
As our samples of D. paulistorum mainly originated from sites with a low urbanization level, it was not possible to test the hypothesis of association of their chromosomal polymorphism with urbanization levels. However, the existence of some significant results between certain other comparisons deserves mention.
For instance, the statistically significant differences in the inversions of the chromosomic arms XR and 3 between samples of banana baits and Citrus (both, in 1991 and 1992) suggest different microenvironment adaptabilities of their carriers related to the choice of trophic resources. This is partially comparable with the results obtained by Bonorino et al. (1993) with D. nebulosa and with those of Valente and Araújo (1985) with D. willistoni.
Whereas no seasonal differences among the D. paulistorum samples were found, a significant tendency to lose chromosomal polymorphism was observed when the samples of 1991 and of 1992 were compared. The biological meaning of such variability loss is unclear, since the climate variation of data available was not sufficient to explain this phenomenon, and so, the existence of stochastic factors, probable to occur in an unpredictable environment as the city, cannot be ruled out.
Some inversions, however, like XL 1, XR 1, 3.1 and 2L 4, reported by Santos and Valente (1990) and now by us, were found during seven years with very similar frequencies. The last inversion also showed an increment in its frequency as well as it seems to be shared by all the samples. This allows us to suggest that either these inversions confer general adaptations to their carriers in such environment or they have already occurred in high frequency in the Porto Alegre founder population of D. paulistorum.
In a broad sense our findings could be explained by some of the statements of Krimbas and Powell (1992). They appointed to the identification of three stages in the evolutionary history of an inversion as: "shortly after its origin as a unique event, the inversion will last or survive due largely to stochastic events. This is true for all newly arisen mutants, the majority of which are lost early regardless of population size. If the inversion survives this early stage to be present in several copies in the population, selective processes may begin to control its fate, along with stochastic ones especially in small populations. In the third stage, a balance may be achieved in establishing a stable polymorphism due to some form of selection. Alternatively, the inversion may continue to rise in frequency and become fixed due to directional selection or genetic drift".
Finally, it deserves comments the distribution of inversions among the chromosomic arms. When we compared the D. paulistorum inversion records from Porto Alegre samples obtained in the period comprised between 1985 and 1987 (Santos and Valente, 1990) with those between 1991 and 1992 (present report) we observed that the third chromosome presented always the highest number of variants. This is in accordance with Kastritsis (1967), who found the same for all the samples of semispecies studied, although only one of the inversions (3.1) seems to be ubiquitous among the Porto Alegre samples. In spite of this, it was also observed that, in the last years, the chromosome 3 showed a tendency to lose polymorphism, which is expressed by a decrease in the average number of inversions per female. This observation is suggestive that adjustments linked to inversions in this chromosome, effective in the wild populations, should be lost in the urban environment.
All the findings obtained until now suggest that the urbanization process of D. paulistorum is recent. A clear tendency of niche separation expressed by preferential use of exotic fruit by D. paulistorum and of the native ones by D. willistoni (Santos and Valente, 1990; Valiati and Valente, 1996) is already established. This fact probably could explain the higher number of chromosomic variants (including the rare inversions) in samples coming from exotic fruit in which the number of individuals sampled is bigger. So, the more common inversions seem to be equally represented both in samples coming from exotic and native fruit. Probably these inversions were still present in the original founder populations.
ACKNOWLEDGMENTS
We are grateful to Dr. Alan R. Templeton for helpful suggestions, criticism and comments. We also thank Ms. Nena B. Morales for technical assistance. This research was supported by grants from CNPq, FAPERGS, FINEP and PROPESP-UFRGS.
RESUMO
Populações de Drosophila paulistorum colonizadoras da área urbana de Porto Alegre, no sul do Brasil, foram estudadas com o objetivo de caracterizar o seu polimorfismo cromossômico neste novo ambiente. A despeito de serem geograficamente periféricas e ecologicamente marginais, e do processo de colonização ser um evento recente, as populações apresentaram um grande número de inversões em todos os braços cromossômicos. Diferenças nas freqüências de algumas inversões e na porcentagem de heterozigose foram encontradas quando se compararam as amostras quanto aos aspectos geográfico, microambiental e temporal. Tais diferenças, entretanto, podem ser atribuídas tanto a fatores seletivos como estocásticos.
(Received April 22, 1997)
- Sampling sites with their respective urbanization levels (high, medium and low) according to the criterion of Ruszczyk (1986/1987). The dots plotted on the map correspond to sampling of D. paulistorum over a period of seven years (1985 to 1993), and the numbers correspond to the present sampling sites. 1. Mario Totta Street (M.T.St.), 2. Botanic Garden (B.Garden), 3. Deoclecio Pereira Street (D.P.St.), 4. Osvaldo Cruz Street (O.C.St.), 5. Farroupilha Park (F.Park).
- Heterozygous inversions of chromosome arm XL of Drosophila paulistorum populations from Porto Alegre. The bar represents 10 mm. (a) Inversion XL 6; (b) inversion XL 1; (c) inversion XL 7; (d) inversion XL 5.
- Heterozygous inversions of chromosome arm XR of Porto Alegre populations of Drosophila paulistorum. The bar represents 10 mm.A, (a) Inversions XR 3 and XR 5; (b) inversions XR 1; B, (a) inversion XR 4; (b) inversion XR 7; (c) inversion XR 6.
Figure 4 - Heterozygous inversions of chromosome arm 2L of Drosophila paulistorum populations from Porto Alegre. The bar represents 10 mm. (a) Inversion 2L 4; (b) inversion 2L 7; (c) inversion 2L 1; (d) inversion 2L 6; (e) inversion 2L 5.
Figure 5 - Heterozygous inversions found on chromosome arm 2R of Drosophila paulistorum populations from Porto Alegre. The bar represents 10 mm. (a) Inversion 2R 4; (b) inversion 2R 3; (c) inversion 2R 5; (d) inversion 2R 6.
- Ashburner, M. (1967). Patterns of puffing activity in the salivary gland chromosomes of Drosophila I. Autosomal puffing patterns in a laboratory stock of Drosophila melanogaster. Chromosoma 27: 398-428.
- Bonorino, C.B.C., Valente, V.L.S. and Callegari-Jacques, S.M. (1993). Urbanization and chromosomal polymorphism of Drosophila nebulosa. Rev. Bras. Genet. 16: 59-70.
- Burla, H., da Cunha, A.B., Cordeiro, A.R., Dobzhansky, Th., Malogolowkin, C. and Pavan, C. (1949). The willistoni group of sibling species of Drosophila. Evolution 3: 300-314.
- Brussard, P.F. (1984). Geographic patterns and environmental gradients: The central-marginal model in Drosophila revisited. Ann. Rev. Ecol. Syst. 15: 25-64.
- Carson, H.L. (1955). The genetic characteristics of marginal populations of Drosophila. Cold Spring Harb. Symp. Quant. Biol. 20: 276-287.
- Carson, H.L. (1956). Marginal homozygosity for gene arrangement in Drosophila robusta. Science 123: 630-631.
- Carson, H.L. (1959). Genetic conditions which promote or retard the formation of species. Cold Spring Harb. Symp. Quant. Biol. 24: 87-105.
- Carson, H.L. (1987). Colonization and speciation. In: Colonization, Succession and Stability (Crawley, A.J. and Edwards, P.J., eds.). Blackwell Sci. Publ. Oxford, pp. 187-206.
- Carson, H.L. and Wasserman, M. (1965). A widespread chromosomal polymorphism in a widespread species Drosophila buzzatii. Am. Nat. 99: 111-115.
- Coyne, J.A., Boussy, I.A., Pront, T., Bryant, S.M., Jones, J.S. and Moore, J.A. (1982). Long distance migration of Drosophila. Am. Nat. 119: 589-595.
- Crumpacker, D.W. and Williams, J.S. (1973). Density, dispersion and population structure in Drosophila pseudoobscura. Ecol. Monogr. 43: 449-538.
- da Cunha, A.B., Burla, H. and Dobzhansky, Th. (1950). Adaptive chromosomal polymorphism in Drosophila willistoni Evolution 4: 212-235.
- da Cunha, A.B., Dobzhansky, Th., Pavlovsky, O. and Spassky, B. (1959). Genetics of natural populations. XXVIII. Supplementary data on the chromosomal polymorphism in Drosophila willistoni in its relation to the environment. Evolution 13: 389-404.
- Dobzhansky, Th. (1957). Genetics of natural populations. XXVI. Chromosomal variability in island and continental populations of Drosophila willistoni from Central America and the West Indies. Evolution 11: 280-293.
- Dobzhansky, Th. and Pavlovsky, O. (1962). A comparative study of the chromosomes in the incipient species of Drosophila paulistorum complex. Chromosoma 13: 196-218.
- Dobzhansky, Th. and Spassky, B. (1959). Drosophila paulistorum a cluster of species in statu nascendi Proc. Nat. Acad. Sci. 45: 419-428.
- Dobzhansky, Th., Burla, H. and da Cunha, A.B. (1950). A comparative study of chromosomal polymorphism in sibling species of the willistoni group of Drosophila. Am. Nat. 817: 229-246.
- Ehrman, L. and Powell, J.R. (1982). Drosophila willistoni species group. In: The Genetics an Biology of Drosophila Vol. 3b (Ashburner, M., Carson, H.L. and Thompson, J.R., eds.). Academic Press, New York, pp. 193-225.
- Everitt, B.S. (1992). The Analysis of Contingency Tables 2 edn. Chapman, Hall, London.
- Fontdevila, A. (1992). Genetic instability and rapid speciation: are they coupled? Genetica 86: 247-258.
- Fontdevila, A., Ruiz, A., Alonso, G. and Ocaña, J. (1981). The evolutionary history of Drosophila buzzatii I. Natural chromosomal polymorphism in colonized populations of the Old World. Evolution 35: 148-157.
- Fontdevila, A., Ruiz, A., Ocaña, J. and Alonso, G. (1982). Evolutionary history of Drosophila buzzatii II. How much has chromosomal polymorphism changed in colonization? Evolution 36: 843-851.
- Haberman, S.J. (1973). The analysis of residuals in crossclassified tables. Biometrics 29: 205-220.
- Kastritsis, C.D. (1966). A comparative chromosomal study in the incipient species of the Drosophila paulistorum complex. Chromosoma 19: 208-222.
- Kastritsis, C.D. (1967). A comparative study of the chromosomal polymorphism in the incipient species of Drosophila paulistorum complex. Chromosoma 20: 180-202.
- Kastritsis, C.D. (1969). A cytological study on some recently collected strains of Drosophila paulistorum. Evolution 23: 663-675.
- Krimbas, C.B. and Powell, J.R. (1992). Historical note. In: Drosophila inversion polymorphism (Krimbas, C.B. and Powell, J.R., eds.). CR Press, Boca Raton, Florida, pp. 2-52.
- Marques, E.K., Napp, M., Winge, H. and Cordeiro, A.R. (1966). A corn meal, soybean flour, wheat germ medium for Drosophila. Drosophila Information Service 41: 187.
- Mather, W.B. (1957). Genetic relationships of four Drosophila species from Australia. Univ. Texas Publ. 5721: 221-225.
- Powell, J.R., Levene, H. and Dobzhansky, T. (1972). Chromosomal polymorphism in Drosophila pseudoobscura used for diagnosis of geographic origin. Evolution 26: 553-559.
- Prevosti, A. (1964). Chromosomal polymorphism in Drosophila populations from Barcelona. Genet. Res. 5: 27-38.
- Prevosti, A., Ribó, G., Serra, L., Aguadé, M., Balaña, J., Monclus, M. and Mestres, F. (1988). Colonization of America by Drosophila subobscura: experiment in natural populations that supports the adaptive role of chromosomal-inversion polymorphism. Proc. Natl. Acad. Sci. USA 85: 5597-5600.
- Roff, D.A. and Bentzen, P. (1989). The statistical analysis of mitochondrial DNA polymorphisms: c2 and the problem of small samples. Mol. Biol. Evol. 6: 539-545.
- Ruszczyk, A. (1986/1987). Análise da cobertura vegetal da cidade de Porto Alegre, RS. Rev. Bras. Bot. 9: 225-229.
- Safriel, V.N. and Ritte, V. (1983). Universal correlates of colonizing species. In: The Ecology of Animal Movement (Swingland, J.R. and Greenwood, P.J., eds.). Clarendon Press, Oxford, pp. 213-239.
- Santos, R.A. and Valente, V.L.S. (1990). On the occurrence of Drosophila paulistorum Dobzhansky & Pavan (Diptera, Drosophilidae) in urban environment: ecological and cytological observations. Evol. Biol. 4: 253-268.
- Soulé, M. (1973). The epistasis cycle. A theory of marginal populations. Ann. Rev. Ecol. Syst. 4: 65-187.
- Spassky, B., Richmond, R.C., Pérez Salas, S., Pavlovsky, O., Mourão, C.A., Hunter, A.S., Hoenigsberg, H., Dobzhansky, Th. and Ayala, F.J. (1971). Geography of the sibling species related to Drosophila willistoni, and the semispecies of the Drosophila paulistorum complex. Evolution 25: 129-143.
- Sperlich, D. (1971). Experimentelle Ergebnisse über die Bedeutung der Heterosis bei Drosophila. Ann. Genet. Sel. Anim. 8: 35-41.
- Sperlich, D. and Pfriem, P. (1986). Chromosomal polymorphism in natural and experimental population. In: The Genetics and Biology of Drosophila (Ashburner, M., Carson, H.L. and Thompson Jr., J.N., eds.). Vol. 3c. Academic Press, Inc., London, pp. 257-309.
- Townsend, J.I. (1952). Genetics of marginal populations of Drosophila willistoni. Evolution 6: 428-442.
- Valente, V.L.S. and Araújo, A.M. (1985). Observations on the chromosomal polymorphism of natural populations of Drosophila willistoni and its association with the choice of feeding and breeding sites. Rev. Bras. Genet. 8: 271-284.
- Valente, V.L.S., Ruszczyk, A. and Santos, R.A. (1993). Chromosomal polymorphism in urban Drosophila willistoni. Rev. Bras. Genet. 16: 307-319.
- Valiati, V.H. and Valente, V.L.S. (1996). Observations on ecological parameters of urban populations of Drosophila paulistorum Dobzhansky & Pavan (Diptera, Drosophilidae). Rev. Bras. Ent. 40: 225-231.
- Wasserman, M. (1954). Cytological studies of the repleta group. Univ. Texas Publ. 5422: 130-152.
- Wasserman, M. (1962). Cytological studies of the repleta group of the genus Drosophila V. The mulleri subgroup. Univ. Texas Publ. 6205: 85-117.
Publication Dates
-
Publication in this collection
06 Oct 1998 -
Date of issue
Dec 1997
History
-
Received
22 Apr 1997
