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Chi-Hing C. Cheng
ancestral trypsinogen gene by itself had no functional consequence, it is
reasonable to suggest that its de novo amplification must be driven by
certain selection pressure to provide a new function required by the
organism, in this case, the novel ice-binding function to survive the
increasingly frigid, icy Antarctic water. By the use of a molecular clock
rate (sequence divergence rate), 0.5-0.9% per million years estimated for
the mitochondrial DNA of another teleost (salmon) [37], and the amount
of sequence divergence between Dm trypsinogen and AFGP genes in their
homologous segments (4-7%) (Fig. 2), we deduced that the conversion of
the ancestral trypsinogen gene to the first AFGP gene occurred about 5-14
million years ago (mya) [29], a relatively recent event in evolutionary
time.
The time frame during which the Antarctic Ocean reached freezing
temperatures has been examined by physical methods (oxygen isotope
ratios of planktonic sediments) extensively by paleo-oceanographers, but
the estimates have not been unequivocal. The generally accepted time
frame in which the Antarctic water approached present-day conditions is
mid-Miocene or about 10-14 mya, subsequent to the establishment of the
Antarctic Circumpolar Current at about 22 my a which lead to the thermal
isolation of Antarctica [1]. This estimate agrees remarkably well with the
estimated time of appearance of notothenioid AFGP gene from our
molecular data. There is some evidence suggesting production of Antarctic
sea ice at the high latitudes as far back as mid-Oligocene (38 - 30 mya)
when glaciation occurred over land, although bottom water temperature
was still above freezing [1]. Could the first emergence of notothenioid
AFGPs have occurred in that time frame [38]1 The estimate from
molecular data indicates not so, that it is in fact a much more recent, midMiocene event.
Another piece of evidence that supports the mid-Miocene time frame of
AFGP evolution comes from recent molecular phylogenetic analyses of the
notothenioid fishes based on mitochondrial rRNA gene sequences, which
showed that the extant notothenioid families emerged during a burst of
rapid phyletic diversification around 7-15 my a [39]. The ancestral
notothenioid stock is believed to be a bottom dweller [3,28], while the
extant families occupy all niches throughout the water column. Thus it is
reasonable to suggest that the ability to radiate into the cryopelagic habitats
must coincide with or closely follow the emergence of the antifreeze
protective function.
Three independent areas of studies, paleoclimatic history of Antarctica,
evolution of notothenioid AFGPs, and molecular phylogeny of
notothenioid fishes, each with its own methodological uncertainties and
limitations, appear to have converged on the same mid-Miocene time
Chi-Hing C. Cheng
ancestral trypsinogen gene by itself had no functional consequence, it is
reasonable to suggest that its de novo amplification must be driven by
certain selection pressure to provide a new function required by the
organism, in this case, the novel ice-binding function to survive the
increasingly frigid, icy Antarctic water. By the use of a molecular clock
rate (sequence divergence rate), 0.5-0.9% per million years estimated for
the mitochondrial DNA of another teleost (salmon) [37], and the amount
of sequence divergence between Dm trypsinogen and AFGP genes in their
homologous segments (4-7%) (Fig. 2), we deduced that the conversion of
the ancestral trypsinogen gene to the first AFGP gene occurred about 5-14
million years ago (mya) [29], a relatively recent event in evolutionary
time.
The time frame during which the Antarctic Ocean reached freezing
temperatures has been examined by physical methods (oxygen isotope
ratios of planktonic sediments) extensively by paleo-oceanographers, but
the estimates have not been unequivocal. The generally accepted time
frame in which the Antarctic water approached present-day conditions is
mid-Miocene or about 10-14 mya, subsequent to the establishment of the
Antarctic Circumpolar Current at about 22 my a which lead to the thermal
isolation of Antarctica [1]. This estimate agrees remarkably well with the
estimated time of appearance of notothenioid AFGP gene from our
molecular data. There is some evidence suggesting production of Antarctic
sea ice at the high latitudes as far back as mid-Oligocene (38 - 30 mya)
when glaciation occurred over land, although bottom water temperature
was still above freezing [1]. Could the first emergence of notothenioid
AFGPs have occurred in that time frame [38]1 The estimate from
molecular data indicates not so, that it is in fact a much more recent, midMiocene event.
Another piece of evidence that supports the mid-Miocene time frame of
AFGP evolution comes from recent molecular phylogenetic analyses of the
notothenioid fishes based on mitochondrial rRNA gene sequences, which
showed that the extant notothenioid families emerged during a burst of
rapid phyletic diversification around 7-15 my a [39]. The ancestral
notothenioid stock is believed to be a bottom dweller [3,28], while the
extant families occupy all niches throughout the water column. Thus it is
reasonable to suggest that the ability to radiate into the cryopelagic habitats
must coincide with or closely follow the emergence of the antifreeze
protective function.
Three independent areas of studies, paleoclimatic history of Antarctica,
evolution of notothenioid AFGPs, and molecular phylogeny of
notothenioid fishes, each with its own methodological uncertainties and
limitations, appear to have converged on the same mid-Miocene time