Abstract
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.