Resolving between novelty and homology in the rapidly evolving phallus of Drosophila

Gene regulatory network co-option is sufficient to induce a morphological novelty in Drosophila

Identifying the molecular origins by which new morphological structures evolve is one of the long standing problems in evolutionary biology. To date, vanishingly few examples provide a compelling account of how new morphologies were initially formed, thereby limiting our understanding of how diverse forms of life derived their complex features. Here, we provide evidence that the large projections on the Drosophila eugracilis phallus that are implicated in sexual conflict have evolved through co-option of the trichome genetic network. These unicellular apical projections on the phallus postgonal sheath are reminiscent of trichomes that cover the Drosophila body but are up to 20-fold larger in size. During their development, they express the transcription factor Shavenbaby, the master regulator of the trichome network. Consistent with the co-option of the Shavenbaby network during the evolution of the D. eugracilis projections, somatic mosaic CRISPR/Cas9 mutagenesis shows that shavenbaby is necessary for their proper length. Moreover, mis-expression of Shavenbaby in the sheath of D. melanogaster , a naïve species that lacks these extensions, is sufficient to induce small trichomes. These induced extensions rely on a genetic network that is shared to a large extent with the D. eugracilis projections, indicating its co-option but also some genetic rewiring. Thus, by leveraging a genetically tractable evolutionarily novelty, our work shows that the trichome-forming network is flexible enough that it can be co-opted in a new context, and subsequently refined to produce unique apical projections that are barely recognizable compared to their simpler ancestral beginnings.

A developmental atlas of male terminalia across twelve species of Drosophila

How complex morphologies evolve is one of the central questions in evolutionary biology. Observing the morphogenetic events that occur during development provides a unique perspective on the origins and diversification of morphological novelty. One can trace the tissue of origin, emergence, and even regression of structures to resolve murky homology relationships between species. Here, we trace the developmental events that shape some of the most diverse organs in the animal kingdom-the male terminalia (genitalia and analia) of Drosophilids. Male genitalia are known for their rapid evolution with closely related species of the Drosophila genus demonstrating vast variation in their reproductive morphology. We used confocal microscopy to monitor terminalia development during metamorphosis in twelve related species of Drosophila. From this comprehensive dataset, we propose a new staging scheme for pupal terminalia development based on shared developmental landmarks, which allows one to align developmental time points between species. We were able to trace the origin of different substructures, find new morphologies and suggest possible homology of certain substructures. Additionally, we demonstrate that posterior lobe is likely originated prior to the split between the Drosophila melanogaster and the Drosophila yakuba clade. Our dataset opens up many new directions of research and provides an entry point for future studies of the Drosophila male terminalia evolution and development.

Trichomes on female reproductive tract: rapid diversification and underlying gene regulatory network in Drosophila suzukii and its related species

Background

The ovipositors of some insects are external female genitalia, which have their primary function to deliver eggs. Drosophila suzukii and its sibling species D. subpulchrella are known to have acquired highly sclerotized and enlarged ovipositors upon their shifts in oviposition sites from rotting to ripening fruits. Inside the ovipositor plates, there are scale-like polarized protrusions termed “oviprovector scales” that are likely to aid the mechanical movement of the eggs. The size and spatial distribution of the scales need to be rearranged following the divergence of the ovipositors. In this study, we examined the features of the oviprovector scales in D. suzukii and its closely related species. We also investigated whether the scales are single-cell protrusions comprised of F-actin under the same conserved gene regulatory network as the well-characterized trichomes on the larval cuticular surface.

Results

The oviprovector scales of D. suzukii and D. subpulchrella were distinct in size and spatial arrangement compared to those of D. biarmipes and other closely related species. The scale numbers also varied greatly among these species. The comparisons of the size of the scales suggested a possibility that the apical cell area of the oviprovector has expanded upon the elongation of the ovipositor plates in these species. Our transcriptome analysis revealed that 43 out of the 46 genes known to be involved in the trichome gene regulatory network are expressed in the developing female genitalia of D. suzukii and D. subpulchrella. The presence of Shavenbaby (Svb) or svb was detected in the inner cavity of the developing ovipositors of D. melanogaster, D. suzukii, and D. subpulchrella. Also, shavenoid (sha) was expressed in the corresponding patterns in the developing ovipositors and showed differential expression levels between D. suzukii and D. subpulchrella at 48 h APF.

Conclusions

The oviprovector scales have divergent size and spatial arrangements among species. Therefore, these scales may represent a rapidly diversifying morphological trait of the female reproductive tract reflecting ecological contexts. Furthermore, our results showed that the gene regulatory network underlying trichome formation is also utilized to develop the rapidly evolving trichomes on the oviprovectors of these flies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-022-02046-1.