Identification of a two-domain antifreeze protein gene in Antarctic eelpout Lycodichthys dearborni

Type III antifreeze proteins (AFP III) in the Antarctic eelpout Lycodichthys dearborni contain at least two size variants—a 7-kDa protein family and a specific 14-kDa isoform composed of two 7-kDa domains linked in tandem. We report the characterization of a two-domain AFP III gene from L. dearborni, and propose that the two-domain AFP III gene arose from a single-domain AFP III gene through duplication and degeneration. AT-rich regions played an important role in the degeneration of the duplicated AFP III gene that resulted in the concatenation of two originally separated 7-kDa AFP-coding exons into a single gene. We also identified a pseudo-AFP III gene interrupted at an AT-rich coding region, supporting AT-rich regions as hotspots for DNA recombination in AFP III gene evolution. Interestingly, study of AFP III genes in the related Antarctic eelpout Pachycara brachycephalum showed absence of two- and multi-domain AFP III genes, indicating that modes of AFP III gene family evolution are specific within species.