Evolution of the internal female reproductive system

The species-specificity of male genitalia has been well documented in many insect groups and sexual selection has been proposed as the evolutionary force driving the often rapid, morphological divergence. The internal female genitalia, in sharp contrast, remain poorly studied.

We conducted an extensive comparative study of the internal reproductive system in sepsid flies. We described the ectodermal parts of the female reproductive tract (see figure) for 41 species representing 21 of the 37 described genera and define 19 morphological characters with discontinuous variation found in eight structures that are part of the reproductive tract.

Using a well-resolved molecular phylogeny based on 10 genes, we reconstructed the evolution of these characters across the family [120 steps; Consistency Index (CI): 0.41]. Two structures, in particular, evolve faster than the rest. The first is the ventral receptacle (B, in figure), which is a secondary sperm storage organ. It accounts for more than half of all the evolutionary changes observed (7 characters; 61 steps; CI: 0.46). It is morphologically diverse across genera, can be bi-lobed or multi-chambered (up to 80 chambers), and is strongly sclerotized in one clade. The second structure is the dorsal sclerite (D, in figure), which is present in all the sepsids studied, except two. It is associated with the opening of the spermathecal ducts and is often distinct even among sister species (4 characters; 16 steps; CI: 0.56). Surprisingly, however, the main sperm storage organs, the spermathecae (C, in figure) were relatively similar across the family.

For details, see Puniamoorthy et al. 2010 BMC Evol.Bio.