On the Origin and Trigger of the Notothenioid Adaptive Radiation

Adaptive radiation is usually triggered by ecological opportunity, arising through (i) the colonization of a new habitat by its progenitor; (ii) the extinction of competitors; or (iii) the emergence of an evolutionary key innovation in the ancestral lineage. Support for the key innovation hypothesis is scarce, however, even in textbook examples of adaptive radiation. Antifreeze glycoproteins (AFGPs) have been proposed as putative key innovation for the adaptive radiation of notothenioid fishes in the ice-cold waters of Antarctica. A crucial prerequisite for this assumption is the concurrence of the notothenioid radiation with the onset of Antarctic sea ice conditions. Here, we use a fossil-calibrated multi-marker phylogeny of nothothenioid and related acanthomorph fishes to date AFGP emergence and the notothenioid radiation. All time-constraints are cross-validated to assess their reliability resulting in six powerful calibration points. We find that the notothenioid radiation began near the Oligocene-Miocene transition, which coincides with the increasing presence of Antarctic sea ice. Divergence dates of notothenioids are thus consistent with the key innovation hypothesis of AFGP. Early notothenioid divergences are furthermore congruent with vicariant speciation and the breakup of Gondwana

Molecular pedomorphism underlies craniofacial skeletal evolution in Antarctic notothenioid fishes

Background Pedomorphism is the retention of ancestrally juvenile traits by adults in a descendant taxon. Despite its importance for evolutionary change, there are few examples of a molecular basis for this phenomenon. Notothenioids represent one of the best described species flocks among marine fishes, but their diversity is currently threatened by the rapidly changing Antarctic climate. Notothenioid evolutionary history is characterized by parallel radiations from a benthic ancestor to pelagic predators, which was accompanied by the appearance of several pedomorphic traits, including the reduction of skeletal mineralization that resulted in increased buoyancy. Results We compared craniofacial skeletal development in two pelagic notothenioids, Chaenocephalus aceratus and Pleuragramma antarcticum, to that in a benthic species, Notothenia coriiceps, and two outgroups, the threespine stickleback and the zebrafish. Relative to these other species, pelagic notothenioids exhibited a delay in pharyngeal bone development, which was associated with discrete heterochronic shifts in skeletal gene expression that were consistent with persistence of the chondrogenic program and a delay in the osteogenic program during larval development. Morphological analysis also revealed a bias toward the development of anterior and ventral elements of the notothenioid pharyngeal skeleton relative to dorsal and posterior elements. Conclusions Our data support the hypothesis that early shifts in the relative timing of craniofacial skeletal gene expression may have had a significant impact on the adaptive radiation of Antarctic notothenioids into pelagic habitats

A comparison of adaptive radiations of Antarctic fish with those of nonAntarctic fish

Antarctic biologists frequently emphasize the differences between the modern Antarctic environment and its fauna, and aquatic habitats and faunas elsewhere in the world. While it is valid to portray Antarctica as remote and its fauna as endemic and cold adapted, this approach tends to obscure broad scale similarities between Antarctic and non-Antarctic faunas. For example, the Antarctic fish fauna shares an evolutionary response to its habitat with fish in some tropical, temperate and boreal lakes. In this review we compare some well studied lacustrine radiations of fish with the two radiations of marine fish in the Antarctic Region of the Southern Ocean, notothenioids and liparids. We shall first make the case that, unlike other marine habitats, the Antarctic Region fulfills most of the essential parameters of lakes containing radiations of fish and that this large component of the world ocean is equivalent to a closed basin. Therefore in spite of its vastness, the Antarctic Region provides a comparable opportunity for studying evolutionary biology within a confined area. It is likely that notothenioids, and possibly liparids, are the first known examples of species flocks or radiations of marine fish. Thus the high Antarctic shelf and upper slope is an insular evolutionary site, with endemic faunas equally as interesting, but less well known, as those in ancient lakes throughout the world