The analysis of polytene chromosomes in 26 strains of seven species in the Drosophila fasciola subgroup, from several locations in Brazil, in addition to strains of two species belonging to the Drosophila mulleri subgroup (D. aldrichi and D. mulleri), enabled us to determine that the 3c inversion found in the latter species differ in one of its break points from that present in the species of the fasciola subgroup. Therefore, a change in the mulleri complex denomination from inversion 3c to inversion 3u is proposed. Accordingly, the fasciola subgroup is no longer a lesser phylogenetic part within the mulleri subgroup. Rather, it is directly related to the likely ancestor of the repleta group, called Primitive I. This information removes the main obstacle to considering the Drosophila fasciola subgroup as an ancestral group within the Drosophila repleta species group, according to the hypothesis of Throckmorton. Our data also support the conclusion that D. onca and D. carolinae are closely related species based on one new inversion in chromosome 4 (4f²), in both species. D. fascioloides and D. ellisoni also form a pair of sister species based on the presence of fusions of chromosomes 2-4 and 3-5. D. rosinae is related only to the likely ancestor of the fasciola subgroup, where the 3c inversion was fixed.
Drosophila; chromosome phylogeny; repleta group; fasciola subgroup; chromosome inversions
Chromosomal phylogeny of the Drosophila fasciola species subgroup revisited (Diptera, Drosophilidae)
Nilda Maria DinizI; Fabio Melo SeneII
IUniversidade de Brasília, Departamento de Genética e Morfologia, Brasília, DF, Brazil
IIUniversidade de São Paulo, Faculdade de Medicina de Ribeirão Preto, Departamento de Genética, Ribeirão Preto, SP, Brazil
The analysis of polytene chromosomes in 26 strains of seven species in the Drosophila fasciola subgroup, from several locations in Brazil, in addition to strains of two species belonging to the Drosophila mulleri subgroup (D. aldrichi and D. mulleri), enabled us to determine that the 3c inversion found in the latter species differ in one of its break points from that present in the species of the fasciola subgroup. Therefore, a change in the mulleri complex denomination from inversion 3c to inversion 3u is proposed. Accordingly, the fasciola subgroup is no longer a lesser phylogenetic part within the mulleri subgroup. Rather, it is directly related to the likely ancestor of the repleta group, called Primitive I. This information removes the main obstacle to considering the Drosophila fasciola subgroup as an ancestral group within the Drosophila repleta species group, according to the hypothesis of Throckmorton. Our data also support the conclusion that D. onca and D. carolinae are closely related species based on one new inversion in chromosome 4 (4f2), in both species. D. fascioloides and D. ellisoni also form a pair of sister species based on the presence of fusions of chromosomes 2-4 and 3-5. D. rosinae is related only to the likely ancestor of the fasciola subgroup, where the 3c inversion was fixed.
Key words:Drosophila, chromosome phylogeny, repleta group, fasciola subgroup, chromosome inversions.
The repleta group of the genus Drosophila is endemic to the Americas. This group comprises more than 95 nominal species (Sturtevant, 1942; Vilela, 1983; Rafael and Arcos, 1989; Vilela and Bächli , 1990; Tidon-Sklorz and Sene, 1995a b, 2001; Bächli and Vilela, 2002), and is divided into six subgroups: fasciola, hydei, inca, mercatorum, mulleri and repleta.
Their species are widely distributed in the New World and mostly found in semiarid regions with open vegetation (Pavan, 1959; Sene et al., 1980; Vilela, 1983; Vilela et al., 1983; Tidon-Sklorz and Sene, 1995c; Tidon-Sklorz et al., 1994). The species in the hydei, mercatorum and repleta subgroups are mostly generalists, while those in the mulleri and inca subgroups use cacti as breeding sites (Pereira et al., 1983; Rafael and Arcos, 1989).
The fasciola subgroup comprises an assemblage of 21 nominal species (Wasserman, 1962a; Vilela, 1983; Vilela and Bächli, 1990; Bächli and Vilela, 2002), which inhabit mostly forests. They are associated with various substrates: for instance, D. fulvalineata was collected on fungi (Patterson and Wheeler, 1942); D. fasciola emerged from flowers and fruits such as Aphelandra micans (Acanthaceae), Erythrina berteroana (Fabaceae), Heliconia latispatha (Musaceae) and aroid (Araceae) (Pipkin et al., 1966). In forest environments, besides these substrates, these flies use epiphytic cacti (Rhypsalis sp.) as breeding sites (Sene et al., 1977, Morais et al., 1995). Moreover, in open vegetation, D. rosinae emerged from columnar cacti (Cereus sp.) (Wasserman, 1962a; Pereira et al., 1983; Tidon-Sklorz and Sene, 1995c). Apparently, the morphology of the testicles and of the seminal receptacle of the species in this subgroup is intermediate between that in the mulleri and repleta subgroups (Wasserman, 1962a).
The origin and adaptive radiation of the cactophilic species in the repleta group probably occurred in the Oligocene and Miocene (Throckmorton, 1975, 1982). This group is likely to have originated in the transition zone between the Nearctic and Neotropical biogeographic regions in Mexico (Wasserman, 1954).
Cytologically, the ancestral species of all fasciola subgroup would have evolved from the Primitive I, a hypothetical sequence of polytene chromosomal bands, suggested by Wharton (1942), that differs from the standard arrangement of the Drosophila repleta by the presence of the Xabc;2ab;3b inversions, and by the fixation of the 2o3, 2e3 and 2l3 inversions. Accordingly, the basic chromosomal composition of the subgroup would be the Primitive VII (Xabc; 2abo2e3l3; 3b; Wasserman 1960, 1962a, 1992).
According to Wasserman (1982), the existence of the 3c inversion in species of the mulleri complex (included in the mulleri subgroup) and of the fasciola subgroup sustains the hypothesis of a common ancestry, with the fasciola subgroup stemming from the mulleri subgroup. The fact that the species of the fasciola subgroup are forest dwellers could be an indicator of reinvasion of the forests by desert-adapted species.
Throckmorton (1982), and in earlier works (Throckmorton 1962, 1975), discusses the problem of the origin of the repleta group and states: "whether the ancestor of the repleta group itself was a forest species which "became" a "repleta", began diversifying in the forest, and subsequently moved into arid habitats, or whether it first moved into arid habitats and became a repleta there, is difficult to determine. Its closest relatives, the castanea, canalinea, dreyfusi and mesophragmatica groups, are forest forms, for the most part, and apparently primitive members of the repleta group that are at least facultative forest forms breeding in fallen fruit. Parsimoniously, this permits the inference that the founder of the repleta group was a forest form, not necessarily of the wet forest, which "became" a repleta while still associated with forest habitats. At the present time, and on anatomical grounds especially, the major separation within the repleta group is between the hydei subgroup on the one hand and the remaining subgroups on the other, with the fasciola subgroup being the most primitive among the latter forms". Supporting this idea, Morais et al. (1995) proposed the possibility of the repleta group ancestor having inhabited the forests, and, based in composition studies of yeasts, associated to these flies, suggesting that the fasciola subgroup represents the oldest lineage from which the South American species of the repleta group may have evolved. This statement is in agreement to the ecological data mentioned above.
Even though Throckmorton's hypothesis (1962, 1975 and 1982) rests upon morphologic and ecological data, it does not explain the presence of the 3c inversion in both fasciola and mulleri subgroups, favoring the hypothesis proposed by Wasserman (1962a and b; 1963;1982).
Material and Methods
We analyzed 26 isofemale strains of seven species in the fasciola subgroup established from specimens collected (Tidon and Sene, 1988; Tidon-Sklorz and Sene, 1992) in different locations (Table 1).
Polytene chromosomes from the salivary glands of third instar larvae were prepared by squashing techniques in 2% lacto-aceto-orcein, fixed in acetic acid and perchloric acid. They were then compared with the maps depicted by Wharton (1942) and Wasserman (1962a).
In order to define the presence of the 3c inversion, the polytene chromosomes of two species of Drosophila belonging to the mulleri complex of the mulleri subgroup (D. aldrichi and D. mulleri) were also analyzed.
Of the seven Drosophila species in the fasciola subgroup under study, five came from humid coastal and inland Brazilian forests, one from Panamanian forests and the remaining one from the Caatinga domain (Table 1).
By comparing chromosome 3 of species in the mulleri (D. aldrichi and D. mulleri) and fasciola subgroups, we verified that one of the breakpoints of the 3c inversion present in the species of the fasciola subgroup is not the same as determined for the 3c inversion present in the species of the mulleri group. That is, there are two overlapped inversions sharing one breakpoint, and not one single inversion as previously thought. The 3c inversion was described by Wasserman (1962a), by analyzing species from the fasciola subgroup, as having the E4a and G1c breakpoints. However, we observed that the inversion present in the species of the mulleri subgroup differs by one of the two break points: E5b G1c. Thus, we suggest that this inversion in the mulleri subgroup, as it is still undescribed, should be renamed as 3u. (Figures 1 and 2; Table 1). In addition, two new inversions fixed in chromosome 4 were observed along with the inversions reported in the literature (Table 1; Figure 1). Their breakpoints are shown in Figure 3.
The dozen lineages of D. coroica (2n = 12) that were analyzed have the previously described sequence 3p (Wasserman, 1962a).
In D. ellisoni and D. fascioloides (2n = 8), chromosomes 2 and 4 (2-4F) and 3 and 5 (3-5F) are fused. These fusions were described earlier by Dobzhansky and Pavan (1943), Wasserman (1962a) and Kuhn et al. (1995), studying metaphase chromosomes. The presence of the 2p2 and 2d3 inversions was confirmed. The 2d3 inversion described by Wasserman (1962a) as polymorphic in D. ellisoni (cited as fascioloides) was found to be fixed in our strains.
D. moju, from Panama, has the same sequence previously described by Wasserman (1962a).
D. onca (2n = 12) has a fixed inversion on chromosome 4 (4f2, breakpoints A1c B3b). D. carolinae (2n = 12) presents the 4n2 inversion (break points A3a E3i) overlapping the 4f2 arrangement (breakpoints A1c B3b) (Figure 3).
D. rosinae (2n = 12) shows the standard primitive sequence of the fasciola subgroup.
Most of the information obtained in this study is in accordance with the literature (review in Wasserman, 1992). Fixed inversions on chromosome 4 were found in D. onca as well as in D. carolinae.
What does not match previous findings is the fact that the 3c inversion found in fasciola does not have the same 3c breakpoints as described in the mulleri complex by Wasserman (1962a, b). Accordingly, we propose that the denomination of the inversion present in the species of the mulleri complex be changed from 3c to 3u. This observation changes the previously proposed phylogenetic relationships among the species in the mulleri and fasciola subgroups as well as the relationships of the species in the fasciola subgroup within the repleta group. The fasciola subgroup becomes derivative of Primitive I in the repleta group and is no longer a derivative of the mulleri complex, as proposed by Wasserman (63) (Figure1). The present hypothesis was required for the subgroup fasciola to be considered ancestral of the repleta group with a forest origin, as proposed by Throckmorton (1975, 1982) and "supported" by morphological and ecological data (Pipkin, 1965; Pipkin et al., 1966; Sene et al., 1977; Pereira et al., 1983; Morais et al., 1995). This new phylogenetic hypothesis, based on chromosomal inversions, offers a better perspective to understanding the relationships within the subgroup to be inferred from other markers as in Costa and Sene (2002).
Furthermore, based on cytological data, we propose two new species complexes within the fasciola subgroup: the fascioloides complex, comprising D. ellisoni and D. fasciloides which share one inversion (2d3), two centric fusions (2-4F and 3-5F), and a great karyotype similarity regarding the X chromosomes (Kuhn et al., 1995); and the onca complex, comprising D. carolinae and D. onca, which share one inversion (4f2) in addition to the similarity in the morphology of their aedeagi as shown by Vilela (1983). The species D. rosinae directly derives from Primitive VII, the hypothetic ancestral sequence to the fasciola subgroup (Figure1), after fixation of the 3c inversion.
The authors wish to thank Prof Marwin Wasserman for the time NM Diniz spent in his laboratory, CR Vilela for providing the stock of D. moju, MH Manfrin for a critical reading of the manuscript, MAR Alves for assistance in obtaining the chromosome preparations, and PR Epifânio for assistance in the maintenance of stocks. Financial support from CAPES, CNPq, FAPESP, FINEP and USP made this study possible.
Received: March 28, 2003; Accepted: October 13, 2004
Associate Editor: Angela Maria Vianna-Morgante
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Publication in this collection
14 Jan 2005
Date of issue
13 Oct 2004
28 Mar 2003