Differences in orthodenticle expression promote ommatidial size variation between Drosophila species

The compound eyes of insects exhibit extensive variation in ommatidia number and size, which affects how they see and underlies adaptations in their vision to different environments and lifestyles. However, very little is known about the genetic and developmental bases underlying differences in compound eye size. We previously showed that the larger eyes of Drosophila mauritiana compared to D. simulans is caused by differences in ommatidia size rather than number. Furthermore, we identified an X-linked chromosomal region in D. mauritiana that results in larger eyes when introgressed into D. simulans. Here, we used a combination of fine-scale mapping and gene expression analysis to further investigate positional candidate genes on the X chromosome. We found that orthodenticle is expressed earlier in D. mauritiana than in D. simulans during ommatidial maturation in third instar larvae, and we further show that this gene is required for the correct organisation and size of ommatidia in D. melanogaster. Using ATAC-seq, we have identified several candidate eye enhancers of otd as well as potential direct targets of this transcription factor that are differentially expressed between D. mauritiana and D. simulans. Taken together, our results suggest that differential timing of otd expression contributes to natural variation in ommatidia size between D. mauritiana and D. simulans, which provides new insights into the mechanisms underlying the regulation and evolution of compound eye size in insects.

Interspecific introgression reveals a role of male genital morphology during the evolution of reproductive isolation in Drosophila

Rapid divergence in genital structures among nascent species has been posited to be an early-evolving cause of reproductive isolation, although evidence supporting this idea as a widespread phenomenon remains mixed. Using a collection of interspecific introgression lines between two Drosophila species that diverged ∼240,000 years ago, we tested the hypothesis that even modest divergence in genital morphology can result in substantial fitness losses. We studied the reproductive consequences of variation in the male epandrial posterior lobes between Drosophila mauritiana and D. sechellia and found that divergence in posterior lobe morphology has significant fitness costs on several pre-fertilization and post-copulatory reproductive measures. Males with divergent posterior lobe morphology also significantly reduced the life span of their mates. Interestingly, one of the consequences of genital divergence was decreased oviposition and fertilization, which suggests that a sensory bias for posterior lobe morphology could exist in females, and thus posterior lobe morphology may be the target of cryptic female choice in these species. Our results provide evidence that divergence in genitalia can in fact give rise to substantial reproductive isolation early during species divergence, and they also reveal novel reproductive functions of the external male genitalia in Drosophila.

tartan underlies the evolution of male Drosophila genital morphology

Male genital structures are among the most rapidly evolving morphological traits and are often the only features that can distinguish closely related species. This process is thought to be driven by sexual selection and may reinforce species separation. However, while the genetic basis of many phenotypic differences have been identified, we still lack knowledge about the genes underlying evolutionary differences in male genital organs and organ size more generally. The claspers (surstyli) are periphallic structures that play an important role in copulation in insects. Here we show that natural variation in clasper size and bristle number between Drosophila mauritiana and D. simulans is caused by evolutionary changes in tartan (trn), which encodes a transmembrane leucine-rich repeat domain protein that mediates cell-cell interactions and affinity differences. There are no fixed amino acid differences in trn between D. mauritiana and D. simulans but differences in the expression of this gene in developing genitalia suggest cis-regulatory changes in trn underlie the evolution of clasper morphology in these species. Finally, analysis of reciprocal hemizyotes that are genetically identical, except for which species the functional allele of trn is from, determined that the trn allele of D. mauritiana specifies larger claspers with more bristles than the allele of D. simulans. Therefore we have identified the first gene underlying evolutionary change in the size of a male genital organ, which will help to better understand the rapid diversification of these structures and the regulation and evolution of organ size more broadly.Significance StatementThe morphology of male genital organs evolves rapidly driven by sexual selection. However, little is known about the genes underlying genitalia differences between species. Identifying these genes is key to understanding how sexual selection acts on development to produce rapid phenotypic change. We have found that the gene tartan underlies differences between male Drosophila mauritiana and D. simulans in the size and bristle number of the claspers - genital projections that grasp the female during copulation. Moreover, since tartan encodes a protein that is involved in cell affinity, this may represent a new developmental mechanism for morphological change. Therefore, our study provides new insights into genetic and developmental bases for the rapid evolution of male genitalia and organ size more generally.

Loss of cytoplasmic incompatibility and minimal fecundity effects explain relatively low Wolbachia frequencies in Drosophila mauritiana

ABSTRACTMaternally transmitted Wolbachia bacteria infect about half of all insect species. Many Wolbachia cause cytoplasmic incompatibility (CI), reduced egg hatch when uninfected females mate with infected males. Although CI produces a frequency-dependent fitness advantage that leads to high equilibrium Wolbachia frequencies, it does not aid Wolbachia spread from low frequencies. Indeed, the fitness advantages that produce initial Wolbachia spread and maintain non-CI Wolbachia remain elusive. wMau Wolbachia infecting Drosophila mauritiana do not cause CI, despite being very similar to CI-causing wNo from D. simulans (0.068% sequence divergence over 682,494 bp), suggesting recent CI loss. Using draft wMau genomes, we identify a deletion in a CI-associated gene, consistent with theory predicting that selection within host lineages does not act to increase or maintain CI. In the laboratory, wMau shows near-perfect maternal transmission; but we find no significant effect on host fecundity, in contrast to published data. Intermediate wMau frequencies on the island Mauritius are consistent with a balance between unidentified small, positive fitness effects and imperfect maternal transmission. Our phylogenomic analyses suggest that group-B Wolbachia, including wMau and wPip, diverged from group-A Wolbachia, such as wMel and wRi, 6–46 million years ago, more recently than previously estimated.

A bend, flip and trap mechanism for transposon integration

Cut-and-paste DNA transposons of the mariner/Tc1 family are useful tools for genome engineering and are inserted specifically at TA target sites. A crystal structure of the mariner transposase Mos1 (derived from Drosophila mauritiana), in complex with transposon ends covalently joined to target DNA, portrays the transposition machinery after DNA integration. It reveals severe distortion of target DNA and flipping of the target adenines into extra-helical positions. Fluorescence experiments confirm dynamic base flipping in solution. Transposase residues W159, R186, F187 and K190 stabilise the target DNA distortions and are required for efficient transposon integration and transposition in vitro. Transposase recognises the flipped target adenines via base-specific interactions with backbone atoms, offering a molecular basis for TA target sequence selection. Our results will provide a template for re-designing mariner/Tc1 transposases with modified target specificities.

Maternal effect and speciation: maternal effect contributes to the evolution of hybrid inviability between Drosophila simulans and Drosophila mauritiana

Haldane’s rule has been the basis of speciation research during the last 30 years. Most studies have focused on the nature of incompatibilities in the hybrid male, but not much attention has been given to the genetic basis of fertility and inviability in hybrid females. Hybridizations between Drosophila simulans and Drosophila mauritiana produce fertile females and sterile males. Here, we re-examined the level of fertility in reciprocal F1 females of these two species and looked for the presence of maternal effects. Our results show that the reciprocal F1 females of D. simulans and D. mauritiana hybridizations are fully fertile and in fact show a significant level of heterosis in the rate of oviposition but display reduced egg hatching in one direction. Reduced egg hatching was observed in the progenies of F1 hybrid females with D. mauritiana as mother, the same cross that showed a stronger negative effect on F1 male fertility. A review of the literature on the hybridizations in Lepidoptera also showed a maternal effect on inviability when reciprocal crosses produced asymmetric results. Our findings point to the importance of maternal effects in the evolution of embryo inviability and thus enhancing the process of speciation through the evolution of hybrid inviability.