324
Chi-Hing C. Cheng
Convergent Evolution of Notothenioid and Cod AFGPs
Although the origin of the Arctic cod AFGP genes remains to be
determined, the above molecular evidence is consistent with and strongly
supports a separate ancestry. Thus the very similar AFGPs of the orthern
cods and Antarctic notothenioids are the products of convergent evolution.
Protein sequence convergence is very rare, and difficult to establish
unequivocally [43]. A priori, the probability for two very similar complex
proteins to arise separately is very low if not nil. What made it possible in
the case of the AFGPs is very likely the simple primary structure, repeats
of just three amino acids or nine nucleotides. The apparently complex
AFGP polyprotein gene structure as well as the protein heterogeneity
encoded in these genes in both fish, seemingly improbable to have risen
separately by chance, could in fact arise quite readily from the tendency
for repetitive sequences to undergo expansion through slippage replication
and unequal crossing over [34-36]. A prime example of the occurrence of
such a process is the variable lengths of short tandem repeats, or length
polymorphism, in mini- and micro satellite DNA prevalent in eukaryote
genomes [44-46], which forms the basis for population studies [46,47].
Indeed the 9-nt AFGP tandem repeats in both fish could be regarded as
mini satellite DNA-like, which could have and had undergone similar
expansion process as mini satellite DNA. The immense selection pressure
from freezing water temperatures drove intra gene expansion of the repeats
by slippage replication or unequal crossing over, as well as whole gene
duplications, resulting in the two similar and apparently complex AFGP
families in these two unrelated fish.
Future Studies
Our recent work on the AFGP genes demonstrates the power of molecular
analyses in answering several long-standing biological questions in the
field of fish antifreeze research - where did the Antarctic notothenioid
AFGPs come from (trypsinogen), how did they arise (partial recruitment
and de novo amplification), when did they appear (mid-Miocene), and how
did the unrelated northern cods arrive at the same AFGPs (through
convergent evolution).
The use of molecular techniques will undoubtedly lead to the answers for
the many other remaining or new questions regarding antifreeze protein.
Besides the northern cods, the origin of type I AFP of flat fishes and
sculpins, and of type III AFP of eel pouts and wolffish, has yet to be
determined. New antifreezes are being discovered in addition to these well
known types. A recent report describes the presence of an AFP in longhorn sculpin that differs in primary sequence from all the known
Chi-Hing C. Cheng
Convergent Evolution of Notothenioid and Cod AFGPs
Although the origin of the Arctic cod AFGP genes remains to be
determined, the above molecular evidence is consistent with and strongly
supports a separate ancestry. Thus the very similar AFGPs of the orthern
cods and Antarctic notothenioids are the products of convergent evolution.
Protein sequence convergence is very rare, and difficult to establish
unequivocally [43]. A priori, the probability for two very similar complex
proteins to arise separately is very low if not nil. What made it possible in
the case of the AFGPs is very likely the simple primary structure, repeats
of just three amino acids or nine nucleotides. The apparently complex
AFGP polyprotein gene structure as well as the protein heterogeneity
encoded in these genes in both fish, seemingly improbable to have risen
separately by chance, could in fact arise quite readily from the tendency
for repetitive sequences to undergo expansion through slippage replication
and unequal crossing over [34-36]. A prime example of the occurrence of
such a process is the variable lengths of short tandem repeats, or length
polymorphism, in mini- and micro satellite DNA prevalent in eukaryote
genomes [44-46], which forms the basis for population studies [46,47].
Indeed the 9-nt AFGP tandem repeats in both fish could be regarded as
mini satellite DNA-like, which could have and had undergone similar
expansion process as mini satellite DNA. The immense selection pressure
from freezing water temperatures drove intra gene expansion of the repeats
by slippage replication or unequal crossing over, as well as whole gene
duplications, resulting in the two similar and apparently complex AFGP
families in these two unrelated fish.
Future Studies
Our recent work on the AFGP genes demonstrates the power of molecular
analyses in answering several long-standing biological questions in the
field of fish antifreeze research - where did the Antarctic notothenioid
AFGPs come from (trypsinogen), how did they arise (partial recruitment
and de novo amplification), when did they appear (mid-Miocene), and how
did the unrelated northern cods arrive at the same AFGPs (through
convergent evolution).
The use of molecular techniques will undoubtedly lead to the answers for
the many other remaining or new questions regarding antifreeze protein.
Besides the northern cods, the origin of type I AFP of flat fishes and
sculpins, and of type III AFP of eel pouts and wolffish, has yet to be
determined. New antifreezes are being discovered in addition to these well
known types. A recent report describes the presence of an AFP in longhorn sculpin that differs in primary sequence from all the known