OAKLEY, T.H.: Multiple lines of evidence support the independent origin of an arthropod compound eye

One of the most debated issues in arthropod evolution is whether compound eyes evolved once or multiple times. One reason this issue has remained unresolved is that most previous researchers have relied on single lines of evidence. Here I present four separate lines of evidence that the compound eyes of ostracod crustaceans are independently derived. The first evidence is phylogeny. Myodocopids – the only ostracods with compound eyes – are phylogenetically nested within several groups that lack compound eyes. Sequence analyses of 28S and 18S rDNA unequivocally support this result. Second, I have documented a recent duplication of visual pigment genes (opsins) coupled with a change to compound eye-specific gene expression. The timing of these events are coincident with the origin of myodocopids and therefore the putative origin of ostracod compound eyes. The recent origin of a compound eye-specific opsin is especially compelling because I cannot detect any other compound eye-opsins in ostracods with older origins. The ancestral absence of compound eye opsins suggests an ancestral absence of compound eyes themselves because all metazoan eyes express opsin. Third, one would expect a unique structure in independently derived organs. The ultrastructure of the facets of ostracod compound eyes deviates from the common arthropod structure of eight retinular cells and four crystalline cone cells. Ostracods have six and two, respectively. Finally, the oldest known fossil ostracod with a compound eye is also the oldest myodocopid. No fossil compound eyes are known from ostracods older than myodocopids. Taken together, these data provide the strongest case yet for the multiple origin of arthropod compound eyes.