348
G. di Prisco
modifications (possibly adaptive) only to a limited extent. If sequence
mutations in Antarctic fish are indeed related to the development of cold
adaptation, this may imply divergence during the first stages of the cooling
process, and in any case before the event which gave origin to antifreeze
glycoproteins. In contrast, the sequence of Hb 1 and Hb 2 of the nonAntarctic nototheniid Notothenia angustata [37], as well as the finding that
its genome contains antifreeze genes (Cheng, personal communication)
suggest that this fish was cold adapted prior to its quite recent migration
from Antarctic to temperate waters.
Globin Genes in Channichthyidae
Channichthyidae, the most phyletically derived notothenioid family, have
Hb-Iess blood. These unique vertebrates carry oxygen in physical solution
at approx. 10% of the carrying capacity of red-blooded notothenioids.
They developed physiological adaptations that maintain adequate tissue
oxygenation, e.g. enhanced gas exchange by highly vascularized gills and
skin, and increased cardiac output, circulatory volume and heart size.
Using cDNAs that encode the a and 13 globins of Hb 1 of red-blooded
Notothenia coriiceps, we have compared the hybridization patterns of
genomic DNAs from three species representing primitive and advanced
genera (Chaenocephalus aceratus, Champsocephalus gunnari, and
Chionodraco rastrospinosus) with those from four red-blooded
notothenioids, the nototheniids N. coriiceps, Gobionotothen gibberifrons
(Antarctic) and N. angustata (temperate) and the bathydraconid
Parachaenichthys charcoti. As expected, the genomes of the four redblooded fish yield strong hybridization signals for both a- and 13-globin
cDNA probes. The channichthyid genomes show high-stringency
hybridization signals when probed with a-globin cDNA, but fail to
hybridize at moderate stringency to the 13-globin probe, suggesting that
icefish genomes share retention of DNA sequences closely related to the
a-globin gene of red-blooded notothenioids and loss of the 13-globin locus
[38-40). The assessment of steady-state globin mRNA levels in
hematopoietic and nonhematopoietic tissues (including the cellular
component of blood) of C. aceratus reveals neither a- nor 13-g10bin
transcripts. Thus, the a-globin-related icefish sequences are not
transcriptionally active.
In the light of these results, the most plausible mechanism leading to the
Hb-Iess phenotype might be deletion of the 13-globin locus in the ancestral
channichthyid; the a-globin genes, no longer under positive selection
pressure, would have accumulated mutations which caused loss of gene
expression without complete loss of sequence information.
G. di Prisco
modifications (possibly adaptive) only to a limited extent. If sequence
mutations in Antarctic fish are indeed related to the development of cold
adaptation, this may imply divergence during the first stages of the cooling
process, and in any case before the event which gave origin to antifreeze
glycoproteins. In contrast, the sequence of Hb 1 and Hb 2 of the nonAntarctic nototheniid Notothenia angustata [37], as well as the finding that
its genome contains antifreeze genes (Cheng, personal communication)
suggest that this fish was cold adapted prior to its quite recent migration
from Antarctic to temperate waters.
Globin Genes in Channichthyidae
Channichthyidae, the most phyletically derived notothenioid family, have
Hb-Iess blood. These unique vertebrates carry oxygen in physical solution
at approx. 10% of the carrying capacity of red-blooded notothenioids.
They developed physiological adaptations that maintain adequate tissue
oxygenation, e.g. enhanced gas exchange by highly vascularized gills and
skin, and increased cardiac output, circulatory volume and heart size.
Using cDNAs that encode the a and 13 globins of Hb 1 of red-blooded
Notothenia coriiceps, we have compared the hybridization patterns of
genomic DNAs from three species representing primitive and advanced
genera (Chaenocephalus aceratus, Champsocephalus gunnari, and
Chionodraco rastrospinosus) with those from four red-blooded
notothenioids, the nototheniids N. coriiceps, Gobionotothen gibberifrons
(Antarctic) and N. angustata (temperate) and the bathydraconid
Parachaenichthys charcoti. As expected, the genomes of the four redblooded fish yield strong hybridization signals for both a- and 13-globin
cDNA probes. The channichthyid genomes show high-stringency
hybridization signals when probed with a-globin cDNA, but fail to
hybridize at moderate stringency to the 13-globin probe, suggesting that
icefish genomes share retention of DNA sequences closely related to the
a-globin gene of red-blooded notothenioids and loss of the 13-globin locus
[38-40). The assessment of steady-state globin mRNA levels in
hematopoietic and nonhematopoietic tissues (including the cellular
component of blood) of C. aceratus reveals neither a- nor 13-g10bin
transcripts. Thus, the a-globin-related icefish sequences are not
transcriptionally active.
In the light of these results, the most plausible mechanism leading to the
Hb-Iess phenotype might be deletion of the 13-globin locus in the ancestral
channichthyid; the a-globin genes, no longer under positive selection
pressure, would have accumulated mutations which caused loss of gene
expression without complete loss of sequence information.