Comparative antifreeze trait analyses of Antarctic and temperate sister species pairs shed light on tempo of adaptive trait loss under relaxed selection

• Main Author: Wilson, Loralee
• Other Authors: Cheng-DeVries, Chi-Hing Christina, Fuller, Becky, Catchen, Julian, Tan, Milton
• Format: Thesis
• Language: English
• Published: 2022
• Subjects: notothenioid; antifreeze glycoprotein; Antarctic; secondarily temperate; relaxed selection; Harpagifer; Champsocephalus; Southern Ocean; The Antarctic; Antarc*; antarcticus; Icefish
• Online Access: http://hdl.handle.net/2142/108725

Antarctic notothenioid fishes endemic to the ice-laden, freezing (-1.9°C) Southern Ocean survive by having evolved antifreeze glycoproteins (AFGP). Over evolutionary time several lineages have escaped the isolated frigid Southern Ocean to north of the Antarctic Polar Front and successfully colonized temperate sub-Antarctic waters. This study investigates the predicted reduction or loss of the prior adaptive AFGP trait in select secondarily temperate notothenioid species when selective pressure no longer exists. The magnitude of the change in AFGP trait expression could inform on the evolutionary history of these species since the AFGP gene evolved only once in notothenioids. We determined the evolutionary status of the AFGP trait in the South American notothenioid Harpagifer bispinis and compared it with its Antarctic sister species Harpagifer antarcticus at the genotypic and phenotypic levels. We found the AFGP trait persists at considerable levels in H. bispinis. AFGP coding sequences remained in its DNA and are transcribed and expressed into functional AFGP proteins. We contrasted this AFGP trait status with another sister notothenioid species pair, the S. American icefish Champsocephalus esox and its Antarctic sister C. gunnari. C. esox co-occurs geographically with H. bispinis, and while AFGP coding sequences also persist in its genome, they are not transcribed or expressed into protein. We constructed a time calibrated Bayesian inferred phylogeny and estimated similar divergence times of H. bispinis/H. antarcticus and C. esox/C. gunnari. Thus, the disparate tempo of AFGP trait change in these two species pairs indicates that alteration of a prior adaptive trait must involve other factors in addition to time in the absence of the apparent selective modality. Limited Author requested closed access (OA after 2yrs) in Vireo ETD system