270
L. Bargelloni and G. Lecointre
From this common ancestor, according to our estimate, the major
radiation ofnotothenioids probably occurred 16-10 Ma, when a consistent
sea ice formation, drop in water temperatures and expansion of ice sheet
played an important role in modifying the Antarctic climate [1].
These major changes in the Antarctic environment might have created
empty niches to which very few nonnotothenioid fish were able to adapt.
If antifreeze glycopeptides (AFGPs) evolved only once in notothenioid
evolution, after the separation of B. variegatus and E. maclovinus and
before the diversification of all the other notothenioids, as proposed by
Bargelloni et al. [9], AFGPs might have played a key role, among other
features, in the adaptive radiation of notothenioid fishes. Recently, this
hypothesis has found direct confirmation, with the evidence of a recent,
common evolution of the ancestral AFGP gene from a trypsinogenencoding gene [32].
Conclusive Remarks
Molecular studies seem to corroborate the hypothesis that notothenioids
underwent an adaptive radiation, possibly quite recently (after the
formation of the PFZ). The ecological plasticity and the ability to survive
the freezing conditions through AFGPs might well have been the key
factors which allowed the remarkable diversification of the notothenioid
fish.
Regarding the notothenioids from the taxonomic point of view, it should
be noticed that molecular phylogenies helped to define new morphological
synapomorphies supporting some nodes of the notothenioid tree.
According to Patterson et al. [33J, such interactions are not so common.
Most of molecular phylogenies limit their conclusions in a statement of
disagreement, but will rarely reconsider morphological characters. The
clade rendering bovichtids paraphyletic (Pseudaphritis plus the rest of
nonbovichtid notothenioids) is highly supported by two molecular studies
based on different genes extracted from different structures of the cell.
Such strong evidence eliminates hesitations in reconsidering
morphological characters. Lecointre et al. [10J proposed that this node
could be supported by six synapomorphies: (1) the second (the uppermost
free) pectoral radial in connection both with the scapula and the coracoid
(the first being fused with the coracoid), (2) last second dorsal fin ray
divided to the base, (3) the ventral process of the hyomandibula shortened,
(4) the loss of the connection between the ectopterygoid and the
metapterygoid, (5) appearance in the opercular bones of connective
tissues, (6) major RbI component in the hemoglobin, loss of all
hamoglobins being a more derived state. In the future, if Bovichtus and
L. Bargelloni and G. Lecointre
From this common ancestor, according to our estimate, the major
radiation ofnotothenioids probably occurred 16-10 Ma, when a consistent
sea ice formation, drop in water temperatures and expansion of ice sheet
played an important role in modifying the Antarctic climate [1].
These major changes in the Antarctic environment might have created
empty niches to which very few nonnotothenioid fish were able to adapt.
If antifreeze glycopeptides (AFGPs) evolved only once in notothenioid
evolution, after the separation of B. variegatus and E. maclovinus and
before the diversification of all the other notothenioids, as proposed by
Bargelloni et al. [9], AFGPs might have played a key role, among other
features, in the adaptive radiation of notothenioid fishes. Recently, this
hypothesis has found direct confirmation, with the evidence of a recent,
common evolution of the ancestral AFGP gene from a trypsinogenencoding gene [32].
Conclusive Remarks
Molecular studies seem to corroborate the hypothesis that notothenioids
underwent an adaptive radiation, possibly quite recently (after the
formation of the PFZ). The ecological plasticity and the ability to survive
the freezing conditions through AFGPs might well have been the key
factors which allowed the remarkable diversification of the notothenioid
fish.
Regarding the notothenioids from the taxonomic point of view, it should
be noticed that molecular phylogenies helped to define new morphological
synapomorphies supporting some nodes of the notothenioid tree.
According to Patterson et al. [33J, such interactions are not so common.
Most of molecular phylogenies limit their conclusions in a statement of
disagreement, but will rarely reconsider morphological characters. The
clade rendering bovichtids paraphyletic (Pseudaphritis plus the rest of
nonbovichtid notothenioids) is highly supported by two molecular studies
based on different genes extracted from different structures of the cell.
Such strong evidence eliminates hesitations in reconsidering
morphological characters. Lecointre et al. [10J proposed that this node
could be supported by six synapomorphies: (1) the second (the uppermost
free) pectoral radial in connection both with the scapula and the coracoid
(the first being fused with the coracoid), (2) last second dorsal fin ray
divided to the base, (3) the ventral process of the hyomandibula shortened,
(4) the loss of the connection between the ectopterygoid and the
metapterygoid, (5) appearance in the opercular bones of connective
tissues, (6) major RbI component in the hemoglobin, loss of all
hamoglobins being a more derived state. In the future, if Bovichtus and