Sex determination in Antarctic notothenioid fish: chromosomal clues and evolutionary hypotheses

In fish, the determination of sex can be controlled by genetic factors, environmental factors or a combination of both. The presence of heteromorphic sex-related chromosomes is widely acknowledged as strongly indicative of genetic control of sex determination (GSD) acting over other sex control systems. Heteromorphic sex-related chromosomes have been observed in a minority of teleosts (approximately 4 %). However, when looking at the fishes of the suborder Notothenioidei the frequency of sex-related chromosomes increases substantially, reaching 26.67 % of the cytogenetically studied species. Noteworthy, sex chromosomes were observed only in cold-adapted species which live in the Antarctic coastal waters, whereas morphologically differentiated sex chromosomes were never observed in the temperate non-Antarctic notothenioid families. Recent evidence suggests that the sex-linked chromosomes across the Antarctic notothenioid families may not share a common origin, but likely originated more than once during notothenioid evolutionary history, thus implying the presence of selection pressures operating toward fixation of GSD system. On the whole, the cytogenetic evidences suggest the Antarctic-specific fixation of differentiated heteromorphic sex-related chromosomes and of a prominent GSD across Antarctic notothenioids that may be an additional manifestation of notothenioid evolution in thermally stable cold environment.