A Comparison of Adaptive Radiations of Antarctic Fish with those of NonAntarctic Fish
21
fish. It is likely that antifreeze glycopeptides are a key innovation in the
Antarctic notothenioids [67,83]. Given their origin from a transformed
trypsinogen gene, they are unique and distinct from the antifreeze
glycopeptides in Arctic gadids. Their contribution to fitness or survival is
obvious since diversification into subzero ice-laden water would have been
impossible without them. A novelty or key innovation is responsible for a
new evolutionary direction in a lineage. When mapped on the notothenioid
cladogram, antifreezes have a single origin at the node for the Antarctic
clade (the sister group of Eleginops). Based on ages inferred from the
divergence of molecular sequences between trypsinogen and antifreeze, the
appearance of genes responsible for the synthesis of antifreeze in the basal
nototheniid (Dissostichus) is coincident with the cooling and appearance of
ice in the Southern Ocean 5-14 Ma [67]. This trait allowed considerably
more diversification in the Antarctic clade (95 species) than in the
nonAntarctic (26 species) clade.
As an independent test of antifreeze glycopeptides as a key innovation in
notothenioids, we could ask whether the absence of high levels of
antifreeze in Antarctic liparids has hindered their ecological diversification
into shallow ice-laden waters. This may not be completely valid since
Paraliparis and Careproctus are deep water genera and may have been
unable to occupy niches already filled by the notothenioids which arrived
on the Antarctic shelf and diversified before the liparids [4]. It is also
interesting that whereas the Antarctic fish fauna is dominated by a single
group of fish in which antifreeze capability evolved only once, the Arctic
has a range of lineages in which antifreeze has evolved several times.
Gadoids, for example possess an antifreeze glycoprotein (unrelated to that
in notothenioids) and yet they have not radiated in the Southern Ocean.
In conclusion, the Antarctic shelf and upper slope is an insular
evolutionary site, with radiations or species flocks of fish, equally as
interesting as but less well known than the radiations in ancient lakes
throughout the world. In spite of its vastness, the Antarctic Region
provides an excellent opportunity for retrospective study of the evolution
of an unusual fish fauna within a confined area during the latter part of the
Tertiary.
Acknowledgments
J.T.E. would like to thank the European Science Foundation and the Coordination
Committee of the Fishes of the Antarctic Ocean Network, especially Prof. Guido
di Prisco and Dr. Eva Pisano, for the invitation to participate and for organizing
the final network meeting. J.T.E.'s research is funded by National Science
Foundation grant OPP 94-16870. A.C.'s research is funded by the British Antarctic
Survey (Natural Environment Research Council). It is part of the Nearshore
21
fish. It is likely that antifreeze glycopeptides are a key innovation in the
Antarctic notothenioids [67,83]. Given their origin from a transformed
trypsinogen gene, they are unique and distinct from the antifreeze
glycopeptides in Arctic gadids. Their contribution to fitness or survival is
obvious since diversification into subzero ice-laden water would have been
impossible without them. A novelty or key innovation is responsible for a
new evolutionary direction in a lineage. When mapped on the notothenioid
cladogram, antifreezes have a single origin at the node for the Antarctic
clade (the sister group of Eleginops). Based on ages inferred from the
divergence of molecular sequences between trypsinogen and antifreeze, the
appearance of genes responsible for the synthesis of antifreeze in the basal
nototheniid (Dissostichus) is coincident with the cooling and appearance of
ice in the Southern Ocean 5-14 Ma [67]. This trait allowed considerably
more diversification in the Antarctic clade (95 species) than in the
nonAntarctic (26 species) clade.
As an independent test of antifreeze glycopeptides as a key innovation in
notothenioids, we could ask whether the absence of high levels of
antifreeze in Antarctic liparids has hindered their ecological diversification
into shallow ice-laden waters. This may not be completely valid since
Paraliparis and Careproctus are deep water genera and may have been
unable to occupy niches already filled by the notothenioids which arrived
on the Antarctic shelf and diversified before the liparids [4]. It is also
interesting that whereas the Antarctic fish fauna is dominated by a single
group of fish in which antifreeze capability evolved only once, the Arctic
has a range of lineages in which antifreeze has evolved several times.
Gadoids, for example possess an antifreeze glycoprotein (unrelated to that
in notothenioids) and yet they have not radiated in the Southern Ocean.
In conclusion, the Antarctic shelf and upper slope is an insular
evolutionary site, with radiations or species flocks of fish, equally as
interesting as but less well known than the radiations in ancient lakes
throughout the world. In spite of its vastness, the Antarctic Region
provides an excellent opportunity for retrospective study of the evolution
of an unusual fish fauna within a confined area during the latter part of the
Tertiary.
Acknowledgments
J.T.E. would like to thank the European Science Foundation and the Coordination
Committee of the Fishes of the Antarctic Ocean Network, especially Prof. Guido
di Prisco and Dr. Eva Pisano, for the invitation to participate and for organizing
the final network meeting. J.T.E.'s research is funded by National Science
Foundation grant OPP 94-16870. A.C.'s research is funded by the British Antarctic
Survey (Natural Environment Research Council). It is part of the Nearshore