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Neotrop. Entomol. vol.34 no.2 Londrina Mar./Apr. 2005
SYSTEMATICS, MORPHOLOGY AND PHYSIOLOGY
Karyotype characterization of Anastrepha fruit flies (Diptera: Tephritidae)
Caracterização de cariótipos de moscas-das-frutas do gênero Anastrepha (Diptera: Tephritidae)
Denise SelivonI; André L.P. PerondiniI; Lincoln S. RochaI, II
IDepto. Biologia, Instituto de Biociências, Universidade de São Paulo 05508-900 São Paulo, SP
IIPresent address: Lab. Parasitologia, Instituto Butantã, Av. Vital Brazil, 1500, 05503-900 São Paulo, SP
The study comprises the description of the C-banded mitotic chromosomes of five species of Anastrepha, A. amita Zucchi, A. turpiniae Stone, A. zenildae Zucchi, A. grandis (Macquart) and A. leptozona Hendel , and a reanalysis of the chromosomes of three other species, A. distincta Greene, A. obliqua (Macquart) and A. sororcula Zucchi. The species have a diploid number of 12, while A. leptozona has 2n = 10. Heteromorphism of the sex chromosomes is present in all of the species, which have a XY/XX sex chromosome system. A. grandis and A. leptozona have meta and submetacentric autosomes while all of the chromosomes in the other species are acrocentric. The species can be characterized unequivocally by the length and the pattern of blocks of heterochromatin in the sex chromosomes.
Key words: Heterochromatin, mitotic chromosomes, sex chromosomes, C-band
O estudo apresenta a descrição dos cromossomos mitóticos, através do bandamento-C, de cinco espécies de Anastrepha, A. amita Zucchi, A. turpiniae Stone, A. zenildae Zucchi, A. grandis (Macquart) and A. leptozona Hendel e uma reanálise dos carótipos de três outras espécies, A. distincta Greene, A. obliqua (Macquart) and A. sororcula Zucchi, segundo a mesma técnica. As espécies apresentam número diplóide igual a 12 cromossomos, exceto A. leptozona com 2n = 10. Os cromossomos sexuais são heteromórficos e com sistema XX/XY. Os autossomos são meta ou submetacêntricos em A. grandis and A. leptozona e acrocêntricos nas demais espécies. Os cromossomos sexuais apresentam comprimento e padrão de blocos de heterocromatina característicos para cada espécie.
Palavras-Chave: Heterocromatina, cromossomo sexual, cromossomo mitótico, banda-C
The genus Anastrepha of frugivorous Tephritidae is endemic to tropical and subtropical regions of the American Continent occurring from southern United States to Northern Argentina, and in the Caribbean islands (Aluja 1994). It comprises about 200 species, distributed into 18 intrageneric groups (Norrbom et al. 1999). Of these, 94 species occur in Brazil, infesting native and introduced fruits of 31 different families (Norrbom et al. 1999, Zucchi 2000a, b). The taxonomy of fruit flies is usually based on the analysis of adult females, more specifically on the morphology of the aculeus tip (Zucchi 2000a). Structures of adult males, 3rd-instar larvae and eggshell morphology have been indicated as useful characters for species identification but these studies are incipient (Aluja 1994; Selivon & Perondini 1998, 1999; Norrbom et al. 1999).
Despite the studies so far done, there are some problems to ascertain the taxonomic status of some species. One example is the nominal species A. fraterculus (Wied.) that congregates a complex of cryptic species, as suggested by Baker et al. (1944) and Bush (1962), and evidenced by Morgante et al. (1980), Steck (1991), Steck & Sheppard (1993), McPheron et al. (1999) and Smith-Caldas et al. (2001). This complex of cryptic species is being characterized by Selivon (1996) Selivon & Perondini (1998) Selivon et al. (1999, 2004, 2005). The precise knowledge of the taxonomic status of these and other species is fundamental for the development of strategies oriented to control or supress these pests, since these methods are species-specific.
Previous cytological studies indicated that chromosome morphology may be useful for taxonomic purposes. However, the karyotype of only 10 percent of the species of Anastrepha has been described. These studies showed that similar acrocentric autosomes and a 2n = 12 are found in the karyotypes of the nominal A. fraterculus (Solferini & Morgante 1987, Basso & Manso 1998, Selivon et al. 2005), in A. distincta Greene, A. ludens (Loew), A. obliqua (Macquart), A. sororcula Zucchi, A. zuelaniae Stone (Bush 1962, Solferini & Morgante 1987, 1990), all of them belonging to the fraterculus group (Norrbom et al. 1999). On the contrary, large differences do exist in the karyotypes of Anastrepha species from other intrageneric groups, as to chromosome number and autosome morphology. The diploid number is 2n = 8 in A. pickeli Lima and A. montei Lima (Solferini & Morgante 1987, Morgante et al. 1996), while a 2n = 12 is found in A. aphelocentema Stone, A. barnesi Aldrich, A. bistrigata Bezzi, A. pseudoparalella (Loew), A. serpentina (Wied.), A. spatulata Stone and A. striata Schiner (Bush 1962, Solferini & Morgante 1987, 1990, Morgante et al. 1996). In all of these species most of the autosomes have a median or submedian centromere. Differences are also found in the sex chromosomes system, that is X1X2Y/ X1X1X2X2 in A. serpentina and A. bistrigata (Solferini & Morgante 1987, 1990), while all of the other studied species have a XY/XX system, therefore, heteromorphism of the sex chromosomes seems to be the rule.
In relation to the nominal A. fraterculus, Mendes (1958) found a karyotype having small Y chromosome in southern Brazil, while Bush (1962) described a large isomorphic pair of sex chromosome in Mexican populations. Later on, Solferini & Morgante (1987) described four karyotypes in Brazilian samples, one with a small Y in southern Brazil and the others with a large Y chromosome in northeastern region. However, it was shown later on that one of these karyotypes belong indeed to A. sororcula (Morgante et al. 1993). Polymorphism specially of the sex chromosomes was found by Basso & Manso (1998) and Basso et al. (2003), in nominal A. fraterculus collected in Argentina and southern Brazil. Further karyotypic analysis of nominal A. fraterculus samples from Brazilian populations, have shown the existence of three karyotypes, two of them with small Y and one with a large Y chromosome, as well as of a 4th type with a large Y chromosome occurring in a sample from Guayaquil, Ecuador (Selivon 1996; Selivon et al. 2004, 2005).
Further chromosome analyses of Anastrepha species are still necessary in order to help in taxonomic problems and to understand karyotypic evolution in this group of flies. The present report contains a more detailed analysis of the chromosomes of A. distincta, A. obliqua and A. sororcula, previously studied, respectively, by Bush (1962), Solferini & Morgante (1987) and Morgante et al. (1993). It includes measeruments of chromosome length and/ or heterochromatic blocks evidenced by C-banding, as well as a description of the mitotic chromosomes of five other Anastrepha species , A. amita Zucchi, A. turpiniae Stone, A. zenildae Zucchi, A. grandis (Macquart) and A. leptozona Hendel.
Material and Methods
Collection sites of host fruits infested with fruit flies are shown in Table 1. In the laboratory the species were maintained in population cages according to Selivon & Perondini (1998). Guavas or papayas were furnished as ovipositon sites for A. obliqua, A. sororcula, A. turpiniae and A. zenildae, and pumpkins for A. grandis. From these substrates, 3rd-instar larvae of the first generation were used for the chromosome analyses. For A. amita, A. distincta and A. leptozona, species not usually maintained in the laboratory, the chromosomes were analysed from larvae taken directly from the collected fruits. Although only adults of these three species emerged from the respective collected host fruit, to assure that the analysed larvae belonged to the same species, mitotic chromosomes where also analysed in samples prepared from adult gonads. Since the larval tissues furnished higher number of mitosis than the adult ones, samples of 16 to 20 larvae comprising males and females were analysed for each species.
For chromosome preparations neural ganglia and adjacent imaginal disks were employed as described by Selivon & Perondini (1997). After removal of the coverslip the preparations were air dried, and stored in a sealed box containing silica gel. Heterochromatic regions of the chromosomes were evidenced by the C-banding technique according to Selivon & Perondini (1997). The slides were examined in an Olympus BX60 microscope, and chromosome images were digitalized by a Leica DC100 CCD device coupled to the microscope. They were edited by the AdobePhotoshop 6.0 software, and chromosome length measurements were done using the ProImage IPWIN32 software. The length of each chromosome was expressed as percentage of the total diploid complement length, according to Robertson (1957). Measurements were done in 10 mitosis of each sex from different individuals.
The six analysed species of the fraterculus group (A. amita, A. distincta, A. obliqua, A. sororcula, A. turpiniae, A. zenildae), and A. grandis (grandis group), presented a diploid number of 12 chromosomes, while A. leptozona (leptozona group) had 2n = 10. The sex chromosomes were XY/XX in all of these species as shown in Figs. 1 to 8.
In previous publications on the mitotic chromosomes of Anastrepha species (Solferini & Morgante 1987, 1990) the sex chromosomes were considered the first pair (I), and the autosomes numbered according to an ascending order, the smallest being number II (or 2). However, in karyological studies of Anastrepha (Basso et al. 2003) and other genera of fruit flies [e.g. Dacus cucurbitae Coquillet (Gopalan 1972), Ceratitis capitata (Wiedemann) (Zacharopoulou 1986), Rhagoletis pomonella (Walsh) (Procunier & Smith 1993), Bactrocera oleae Gmelin (Canovai et al. 1996 )], the autosomes are numbered in a descending order, the largest being number 2 (or II). The latter system will be used in the present report.
The autosomes of the species of the fraterculus group are acrocentric and no specific structural differences were found in any of them. Discrete heterochromatic blocks are usually seen at the centromeric ends of the autosomes except in A. amita (Fig. 1). In A. grandis and A. leptozona, of other intrageneric groups, the autosomes have a submedian centromere, as shown in Figs. 7 and 8 and in the ideograms of Fig. 9. Moreover, an ANOVA test revealed no significant differences in the length of corresponding autosomes of these species. A. leptozona was excluded since it was not possible to establish correspondence among its autosomes to those of other species (Table 2).
Significant differences among the species, including A. leptozona, were observed for the X and the Y chromosomes (Table 2). Details of the sex chromosomes revealed by C-banding will be described below.
In A. amita (Figs. 1 and 9), the X and Y chromosomes are large, the X chromosome presenting about the same length of the largest autosome (II), while the Y chromosome is similiar in size to autosome IV. The X to Y ratio is 1.28, and the X chromosome has a large block of heterochromatin at the centromeric end, and in which a constriction is frequently visible. The Y has also a heterochromatic block at the centromeric end.
The X chromosome of A. distincta (Figs. 2 and 9) presents the length of the smallest autosome, and has two blocks of heterochromatin, one at each tip, the block at the distal end being larger than the block at the proximal, centromeric tip. The Y chromosome, larger than the X (X/Y ratio = 0.83) has also two blocks of heterochromatin, similarly to the X chromosome. The distal block is larger than the proximal one, and also larger than the corresponding block of the X chromosome. Fig. 3 and Fig. 9, show that the X chromosome of A. obliqua has a large heterochromatic region at the centromeric end, usually divided into two blocks by a constriction. The Y chromosome also presents a block of heterochromatin at the proximal tip. The X has approximately the length of autosome III, while the Y is smaller than the smallest autosome. The X/Y ratio is 1.33.
A. sororcula (Figs. 4 and 9) has a small Y chromosome that seems to be entirely heterochromatic presenting a constriction at about 1/3 of its length. The X/Y ratio is 1.89, and the X chromosome has a length similar to autosome III. The X shows a conspicuous block of heterochromatin near its median region, and a smaller block at the centromeric end.
A median constriction is present in the small Y chromosome of A. turpiniae, one half being constituted by a block of heterochromatin (Figs. 5 and 9). The X chromosome has one block of heterochromatin at the proximal end and another one at a median region. The X has approximately the length of autosomes IV or V, and the X/Y ratio is 3.25 .
In A. zenildae (Figs. 6 and 9), the sex chromosomes are almost isomorphic, the X/Y ratio being 1.11, and exhibit a similar length to autosomes IV and V. The Y has a large block of heterochromatin at the centromeric end, and in the X two blocks are visible, one at the proximal tip, the other near the median region.
The sex chromosomes of A. grandis are the smallest in the karyotype, the X/Y ratio being 1.56 (Figs. 7 and 9). Both, the X and the Y chromosomes, exhibit a conspicuous block of heterochromatin at their proximal ends. The autosomes present unequal arms, the arm ratio (S/L) being 0.55, 0.62, 0.61, 0.62, 0.58 respectively for chromosomes II, III, IV, V and VI.
A. leptozona has very small sex chromosomes which represent about 4.5% of the length of the diploid complement (Figs. 8 and 9). The X/Y ratio is 0.45, and both present a small block of heterochromatin at the proximal end. The S/L arm ratios of the autosomes are 0.76, 0.75, 0.85 and 0.69, respectively for chromosomes II, III, IV and V.
The present results together with data obtained before for other species show a karyotypic constancy among species of the fraterculus group, in regard to the diploid number of 12, and to the size and morphology of the autosomes, which are acrocentric in the species so far studied. In other species groups, variation in the diploid number associated or not to differences in the morphology of the autosomes seems to be frequent. Since the karyotypes of Anastrepha species have a similar total length regardless the variation in chromosome number (data not shown), simple chromosome rearrangements could be invoked to explain the karyotypic modifications either of chromosome morphology (centromeric positions) or changes in the diploid number, as postulated by Solferini & Morgante (1987, 1990).
Contrary to the relative autosomes homogeneity, the acrocentric sex chromosomes of the fraterculus group, exhibit an expressive variation in length, as well as in size and location of heterochromatin blocks. Bush (1962), studying samples from Mexican populations, described the sex chromosomes of A. distincta and A. obliqua as being isomorphic. However, Solferini & Morgante (1987) showed that a heteromorphism does exist for the sex chromosomes in A. obliqua sample from southeastern Brazil, data that were confirmed by the present analysis. The present results on A. distincta collected in southeastern Brazil differ from those of Bush (1962) by showing sex chromosomes heteromorphism. Further analyses of A. distincta are required to interpret such variation since Steck (1991), through an isozyme analysis also recognized genetic differences in samples from Brazil and Venezuela.
Species of Anastrepha belonging to intrageneric groups other than the fraterculus group (Norrbom et al. 1999, Norrbom 2002), A. aphelocentema (mucronota group), A. barnesi and A. leptozona (leptozona group), A. grandis (grandis group), A. pseudoparallela (pseudoparallela group), A. spatulata, A. pickeli and A. montei (spatulata group), and A. striata (serpentina group), have small Y chromosomes, which are dot shaped in some or having from 1/4 to 1/3 of the length of the X chromosme (Bush 1962; Solferini & Morgante 1987, 1990; Morgante et al. 1993, 1996; present report). A. serpentina and A. bistrigata of group serpentina, have a X1X2Y sex chromosome system, the neo-Y being a large chromosome, possible resulting from a translocation of an original Y to an autosome. Considering this hypothesis the original Y must have been a small chromosome having about 1/4 to 1/3 of the original X, the actual X1 chromosome (Solferini & Morgante 1990).
The available data on the karyotypes of the fraterculus group (Mendes 1958, Bush 1962, Solferini & Morgante 1987, Selivon 1996, Basso & Manso 1998, Selivon et al. 2004, 2005, present results), show that the species present sex chromosome heteromorphism and that, according to the size of the Y chromosomes, they can be arranged into two clusters: one exhibiting a small Y chromosome having 1/4 to 1/3 of the length of the X, as in ludens, turpiniae, sororcula, fraterculus complex (sp.1 and sp.3), and the other in which the Y is 2/3, equal or longer than the X chromosomes, as in A. amita, A. distincta, A. fraterculus complex (sp. 2 and sp. 4), A. obliqua, A. zenildae, A. zuelaniae. Heteromorphism of the sex chromosomes is thought to evolve through processes of chromosome degeneration that change the genetic content as well as the morphology of the chromosome that carries the sex determinant factor or genes related to other specific characteristics of the heterogametic sex, for example the Y chromosome (Bull 1983, Charlesworth 1990). If one assumes that the sex determinant for maleness in Anastrepha is located on the Y chromosome, as it occurs in another tephritid, Ceratitis capitata (Willhoeft & Franz 1996), it may be postulated that similar mechanisms of Y chromosome degenaration is involved in the evolution of the sex chromosomes in this genus, producing the small highly heterochromatic Y chromosome exhibited by some species. In this scenario, the differences in the Y chromosome size may be the result of peculiarities occurred during the degenaration process or that the chromosomes are at different stages of this process. For example, in populations of Megaselia scalaris (Loew) (Willhoeft & Traut 1990) and Drosophila miranda Dobzhanski (Steinmann et al. 1993), Y chromosomes at different stages of degeneration process were found. Further studies encompassing a more expressive number of species is still required to provide new insights into the chromosome evolution in Anastrepha species.
This study was supported by FAPESP (grants 98/10701-4 and 03/02698-3). DS and ALPP are fellows of CNPq and LSR was recipient of a Postgraduate scholarship from FAPESP (02/02106-6).
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Received 12/VIII/04. Accepted 19/I/05.