Through the eyes of bat flies: Behavioral, phylogenetic, and histological analyses of compound eye reduction in bat flies (Streblidae) provide evidence for positive selection.

摘要

It is often presumed that evolutionary reduction is tantamount to deconstruction, or even destruction, because relaxed selective forces have been insufficient to maintain the organ in its original state. However, studies on reduction are often limited by a lack of diversity, both of related species exhibiting reduction and of the reduced form itself. There have also been very few studies on the reduction of compound eyes, despite the fact that their near ubiquity among arthropods alone makes them perhaps the most common type of eye. Bat flies (Streblidae and Nycteribiidae) are a group of dipterans that exhibit variable degrees of compound eye reduction, and therefore provide the opportunity to study reduction of this organ in a phylogenetic context. The first chapter of this work reports on behavioral experiments demonstrating that the eyes of one bat fly species, Trichobius frequens, are functional, and that they neither exhibit phototaxis typical of other dipteran species, nor move toward a light source. The second chapter uses molecular phylogenetics to identify a correlation between eye and wing morphology. The results also suggest that secondary to their eye reduction, bat flies (at least in the case of New World specie, including Trichobius spp.) have secondarily experienced a shift in the structure of their facets that is convergent with other insects whose eyes have been selected for increased sensitivity. In the final chapter, histological and optical analyses of T. frequens eyes are used to reveal significant structural changes to the microstructure of its ommatidia that increase sensitivity at the expense of acuity. Many of these changes are also convergent with similar adaptations that have been demonstrated to increase sensitivity in organisms that function in reduced light environments. The results of these analyses suggest that reduction in T. frequens eyes may have been part of an active remodeling process resulting from a shift in the relative importance of sensitivity and acuity. As this is a process of reduction not generally considered, the findings here turn our attention to alternative hypotheses that should be considered when studying evolutionary reduction of any organ.