Opsin transcripts of predatory diving beetles: a comparison of surface and subterranean photic niches

The regressive evolution of eyes has long intrigued biologists yet the genetic underpinnings remain opaque. A system of discrete aquifers in arid Australia provides a powerful comparative means to explore trait regression at the genomic level. Multiple surface ancestors from two tribes of diving beetles (Dytiscidae) repeatedly invaded these calcrete aquifers and convergently evolved eye-less phenotypes. We use this system to assess transcription of opsin photoreceptor genes among the transcriptomes of two surface and three subterranean dytiscid species and test whether these genes have evolved under neutral predictions. Transcripts for UV,long-wavelength and ciliary-type opsins were identified from the surface beetle transcriptomes. Two subterranean beetles showed parallel loss of all opsin transcription, as expected under ‘neutral’ regressive evolution. The third species Limbodessus palmulaoides retained transcription of a long-wavelength opsin (lwop) orthologue, albeit in an aphotic e...