CHENG, C.-H.C.: Antifreeze glycoprotein genes in temperate water Notothenioid fish infer an Antarctic origin of speciation

The perciform suborder of otothenioid fishes is indigenous to the Southern Ocean and largely endemic to the Antarctic region. Currently 8 families encompassing ~130 species are recognized. Twelve species makes up 3 basal, non-Antarctic families inhabiting the temperate regions of S. America, Australia, and New Zealand. The majority of the species make up the other 5 families, most of which are confined to the Antarctic region; only about 14 species of these have a sub-Antarctic and south temperate distribution. The Antarctic water is isolated from other southern oceans by the Antarctic Circumpolar Current (ACC), a major oceanographic barrier which also prevents dispersal of fish in either direction. The 3 basal families presumably diverged and became along the south temperate continents before Antarctica was fully isolated (22-25mya). The 5 endemic Antarctic families are thought to have arisen in situ within the Antarctic water, the result of an adaptive radiation underwent by the ancestral notothenioid stock which evolved antifreeze glycoproteins (AFGP) to avoid freezing as the Antarctic water chilled to subzero temperatures. Given the ACC barrier to dispersal, the origin of the extant south temperate notothenioids has been an enigma. Since AFGP gene evolved once before the notothenioid radiation, and its evolution was directly linked to the freezing of the Antarctic water, the presence of AFGP genes in the extant temperate water notothenioids can be used as a marker to infer their geographic origin of speciation. Several species were found to have AFGP genes, two even express the protein at low levels, which would argue for an Antarctic origin of speciation where and when antifreeze protection was needed, and a subsequent escape to the south temperate regions.