Genomic impacts of chromosomal inversions in parapatric Drosophila species

Chromosomal inversions impact genetic variation and facilitate speciation in part by reducing recombination in heterokaryotypes. We generated multiple whole-genome shotgun sequences of the parapatric species pair Drosophila pseudoobscura and Drosophila persimilis and their sympatric outgroup ( Drosophila miranda ) and compared the average pairwise differences for neutral sites within, just outside and far outside of the three large inversions. Divergence between D. pseudoobscura and D. persimilis is high inside the inversions and in the suppressed recombination regions extending 2.5 Mb outside of inversions, but significantly lower in collinear regions further from the inversions. We observe little evidence of decreased divergence predicted to exist in the centre of inversions, suggesting that gene flow through double crossovers or gene conversion is limited within the inversion, or selection is acting within the inversion to maintain divergence in the face of gene flow. In combination with past studies, we provide evidence that inversions in this system maintain areas of high divergence in the face of hybridization, and have done so for a substantial period of time. The left arm of the X chromosome and chromosome 2 inversions appear to have arisen in the lineage leading to D. persimilis approximately 2 Ma, near the time of the split of D. persimilis–D. pseudoobscura–D. miranda , but likely fixed within D. persimilis much more recently, as diversity within D. persimilis is substantially reduced inside and near these two inversions. We also hypothesize that the inversions in D. persimilis may provide an empirical example of the ‘mixed geographical mode’ theory of inversion origin and fixation, whereby allopatry and secondary contact both play a role.

Encapsulation ability: Are all Drosophila species equally armed? An investigation in the obscura group

Unable to form cellular capsules around large foreign bodies, the species Drosophila subobscura Collin in Gordon, 1936 was previously shown devoid of lamellocytes, the capsule-forming hemocytes in Drosophila melanogaster Meigen, 1830. This unusual case of deficiency in encapsulation ability was remarkable enough to motivate further investigations in phylogenetically related species of the obscura group. Like D. subobscura, the species Drosophila azteca Sturtevant and Dobzhansky, 1936, Drosophila bifasciata Pomini, 1940, Drosophila guanche Monclus, 1976, Drosophila miranda Dobzhansky, 1935, Drosophila persimilis Dobzhansky and Epling, 1944, and Drosophila pseudoobcura Frovola and Astaurov, 1929 were found to be unable to encapsulate large foreign bodies and also to lack lamellocytes. Surprisingly, Drosophila affinis Sturtevant, 1916, Drosophila tolteca Patterson and Mainland, 1944, and Drosophila obscura Fallen, 1823 were capable of mounting cellular capsules, although their encapsulation abilities remained weak. These three species were free of lamellocytes but possessed small pools of never before described “atypical hemocytes” present in the hemolymph when capsules were formed.

Learning decreases heterospecific courtship and mating in fruit flies

Recent theory and data suggest that adaptive use of learning in the context of sexual behaviour could contribute to assortative mating. Experiments examining this issue indicated that male Drosophila persimilis that experienced courtship and rejection by heterospecific females exhibited significantly lower levels of heterospecific courtship and mating compared with those of inexperienced males. These results indicate that experience in the context of sexual behaviour in fruit flies could reduce gene flow between diverging populations, which may contribute to incipient speciation.

Unusual pattern of single nucleotide polymorphism at the exuperantia2 locus of Drosophila pseudoobscura

We have investigated the pattern of DNA sequence variation at the exuperantia2 locus in Drosophila pseudoobscura. This adds to the increasing dataset of genetic variation in D. pseudoobscura, a useful model species for evolutionary genetic studies. The level of silent site nucleotide diversity and the divergence from an outgroup Drosophila miranda are comparable with those for other X-linked loci. One peculiar pattern at the exu2 locus of D. pseudoobscura is a complete linkage disequilibrium between two SNPs, one of which is a replacement site. As a result, there are two distinct haplotype groups in our dataset. Based upon the comparisons with the outgroup sequences from D. miranda and Drosophila persimilis, we show that the newly derived haplotype group has lower diversity than the ancestral haplotype group. The pattern of protein evolution at exu2 shows some deviation from the neutral model. Together, these and other characteristics of the exu2 locus suggest the action of selection on the pattern of SNP variation, consistent with a partial selective sweep associated with the newly derived haplotype.