Male recombination in Brazilian populations of Drosophila ananassae.

With few exceptions, spontaneous crossing over does not normally occur in male Drosophila. Drosophila ananassae males show considerable amounts of crossing over. In wild males of D. ananassae from Asian (2008) and Brazilian populations (1986 and 2007) variable frequencies of meiotic crossing over, estimated from chiasmata counts, suggested the existence of factors controlling male crossing over in these populations. To corroborate for such prediction, we present data on spontaneous recombination in F1 males of D. ananassae heterozygous for chromosomes of the same Brazilian populations (1986) and marker chromosomes using three testers stocks. Mean recombination value was low, although high variability existed between individual frequencies. Recombination frequencies between lines in each tester stock were not significantly different, excepting when the 3ple-px and 3ple-cy testers were compared (p < 0.05). These two testers differ in respect to the regional distribution of crossovers. The occurrence of recombination in chromosomes 2 and 3 in F1 males tested with e(65) se; bri ru was not related, suggesting they are under independent genetic control. Our data are consistent with proposed genetic factors controlling male crossing over in the tester stocks and to the presence of enhancers and suppressors of male crossing over segregating in the Brazilian populations (1986).

Crossing over does occur in males of Drosophila ananassae from natural populations.

Spontaneous crossing over in males of Drosophila ananassae has been well demonstrated using F(1) individuals from crosses between marker stocks and wild type strains. However, the question of its occurrence in males from natural populations remained open. Here we present the cytological evidence that crossing over does occur in males of D. ananassae from two Brazilian populations, sampled nearly 21 years apart, and in two recently sampled populations, one from Indonesia and one from Okinawa, Japan. Cytological analysis of meiosis in males collected from nature and in sons of females from the same population inseminated in nature revealed the presence of chiasmata, inversion chiasmata, and isosite chromosome breakages in the diplotene cells in all sampled populations. These data demonstrate that reciprocal and nonreciprocal exchanges and chromosome breakages, previously reported as related events of male crossing over, do occur at variable frequencies among males from natural populations.

Polytene chromosomal maps of 11 Drosophila species: the order of genomic scaffolds inferred from genetic and physical maps.

The sequencing of the 12 genomes of members of the genus Drosophila was taken as an opportunity to reevaluate the genetic and physical maps for 11 of the species, in part to aid in the mapping of assembled scaffolds. Here, we present an overview of the importance of cytogenetic maps to Drosophila biology and to the concepts of chromosomal evolution. Physical and genetic markers were used to anchor the genome assembly scaffolds to the polytene chromosomal maps for each species. In addition, a computational approach was used to anchor smaller scaffolds on the basis of the analysis of syntenic blocks. We present the chromosomal map data from each of the 11 sequenced non-Drosophila melanogaster species as a series of sections. Each section reviews the history of the polytene chromosome maps for each species, presents the new polytene chromosome maps, and anchors the genomic scaffolds to the cytological maps using genetic and physical markers. The mapping data agree with Muller's idea that the majority of Drosophila genes are syntenic. Despite the conservation of genes within homologous chromosome arms across species, the karyotypes of these species have changed through the fusion of chromosomal arms followed by subsequent rearrangement events.

Evolution in the Drosophila ananassae species subgroup.

Drosophila ananassae and its relatives have many advantages as a model of genetic differentiation and speciation. In this report, we examine evolutionary relationships in the ananassae species subgroup using a multi-locus molecular data set, karyotypes, meiotic chromosome configuration, chromosomal inversions, morphological traits, and patterns of reproductive isolation. We describe several new taxa that are the closest known relatives of D. ananassae. Analysis of Y-chromosomal and mitochondrial haplotypes, shared chromosome arrangements, pre-mating isolation and hybrid male sterility suggests that these taxa represent a recent evolutionary radiation and may experience substantial gene flow. We discuss possible evolutionary histories of these species and give a formal description of one of them as D. parapallidosa Tobari sp. n. The comparative framework established by this study, combined with the recent sequencing of the D. ananassae genome, will facilitate future studies of reproductive isolation, phenotypic variation and genome evolution in this lineage.

Chiasmata and chromosome breakages are related to crossing over in Drosophila ananassae males.

A cytogenetic analysis of male crossing over in Drosophila ananassae revealed that cytological exchanges resulted in genetic crossing over, and that chiasma frequency and the genetic recombination correlated positively in chromosomes 2 and 3. Furthermore, the frequency of chromosome breakages correlated positively with chiasma frequency. Paracentric inversion heterozygosity had no detectable influence on the chromosome pairing or exchange events within the inversion loop at meiosis. Scoring of the chiasma demonstrated that males homozygous for the previously mapped enhancers of male crossing over had low frequencies of chiasmata, whereas higher frequencies of chiasmata were observed in males heterozygous for enhancers. The results presented here indicate that the genetic factors controlling male crossing over are involved in the origin of chromosome breakages and in exchange events.

Reproductive isolation among geographical populations of Drosophila bipectinata Duda (Diptera, Drosophilidae) with recognition of three subspecies.

Among D. bipectinata Duda, 1923, three subspecies, bipectinata from Southeast Asia (SEA) and Okinawa (OKN), szentivanii stat. nov. from Papua New Guinea (PNG) and pacificiae ssp. nov. from South Pacific Ocean (SPO), are recognized. The external morphology of the reproductive organs and the numbers of teeth per row in the sex combs are different between the three subspecies. Furthermore, the sterility of hybrid males between strains from the different regions confirms the subspecies status of each population from SEA, PNG and SPO, together with different gene arrangements in the geographical populations. Although males of the strains from OKN (Okinawa), the northernmost population, show significant differences in the number of teeth of sex combs from males of SEA (Southeast Asia) strains, hybrid males between them are fertile.

Chromosomal phylogeny and geographical divergence in the Drosophila bipectinata complex.

We have prepared reference polytene photographic maps as a standard sequence for the Drosophila bipectinata complex using structurally homozygous flies derived from a stock of Drosophila parabipectinata from Brunei, Borneo, in 1971. We found 87 inversions in the D. bipectinata complex and described their breakpoints on the reference maps. Only 2 arrangements were shared interspecifically: 2R-AB was shared with 3 species, D. parabipectinata, D. bipectinata, and Drosophila malerkotliana, and 3L-A was found in 2 species, D. parabipectinata and D. malerkotliana. The 2 subspecies of D. malerkotliana and the 2 subspecies of Drosophila pseudoananassae shared half of the total gene arrangements detected in each species. The number of different inversions found between species in the complex ranges from 7 (between D. parabipectinata and D. malerkotliana) to at least 24 (between D. bipectinata and D. pseudoananassae). On the basis of the characteristic differences of their gene arrangements, we propose a reliable chromosomal phylogeny of the D. bipectinata complex.

Genetic analyses of several Drosophila ananassae-complex species show a low-frequency major gene for parthenogenesis that maps to chromosome 2.

Parthenogenetic strains of several species have been found in the genus Drosophila. The mode of diploidization in the eggs of females has been found to be post-meiotic nuclear fusion. The genetic basis for this parthenogenesis is not understood but is believed to be under the control of a complex polygenic system. We found parthenogenetic females in an isofemale strain (LAE345) of D. pallidosa-like collected in 1981 at Lae, Papua New Guinea, and established a parthenogenetically reproducing strain. Parthenogenetic strains of D. ananassae and D. pallidosa collected at Taputimu, American Samoa had also been established by Futch (1972). D. ananassae, D. pallidosa and D. pallidosa-like are very closely related species belonging to the ananassae complex of the ananassae species subgroup of the melanogaster species group. Using these three species, we found that more than 80% of females from parthenogenetic strains produced progeny parthenogenetically and that inter-specific hybrid females also produced impaternate progeny. In the present report, we demonstrate that the mode of parthenogenesis of D. ananassae appears to be the post-meiotic nuclear doubling of a single meiotic product, and that a major gene responsible for the parthenogenesis maps to the left arm of the second chromosome of D. ananassae. We also suggest that the genetic basis for parthenogenesis capacity may be identical among the three closely related species. We discuss the function of the gene required for parthenogenesis and its significance for the evolutionary process.