Origin and Mechanism of Evolution of Antifreeze Glycoproteins in Polar Fishes
319
This hybrid gene indicates that tripeptide duplication first and bulk
deletion of trypsinogen later was likely the order of events in the
conversion of trypsinogen to AFGP gene. The origin of the spacer
sequence is presently unknown. Apparently it must have been acquired
early as it was iteratively duplicated along with the ThrAlaAla coding
sequence to produce the polyprotein structure. Further investigations may
solve this remaining mystery in the trypsinogen to AFGP conversion.
How Fast Could the First Notothenioid AFGP Gene Emerge?
The threat of freezing is a matter of life and death, and therefore would be
expected to exert tremendous selection pressure for the emergence of a
freeze-avoidance function, otherwise a lineage would rapidly terminate. In
the notothenioids, the first ThrAlaAla duplication in fact could have
occurred before the selection pressure was in full force, as an accidental
result of replication slippage at the mini satellite sequence (gt)n in intron 1
immediately ahead of the putative ThrAlaAla coding element in the
ancestral trypsinogen gene (Figs. 2,3). A single or two ThrAlaAla repeats
at the N-terminus of a trypsin molecule might not affect trypsin activity
but could endow the hybrid protein with some measure of ice-binding
activity, which would be a selective advantage that became retained as the
seawater approached and hovered around freezing. The destination of
trypsin secretion, the intestinal fluid, where ice crystals could enter
through seawater and food ingestion, provided the logical testing site for
refinement of the emerging antifreeze molecule. The smallest fully
functional AFGP, AFGP8, has only 4 tripeptide repeats, which could be
produced in only two additional rounds of slippage replication after the
first ThrAlaAla duplication depicted in Fig. 3 (a to c). Thus the
"experimental process" whereby the first functional antifreeze gene (and
gene product), likely an AFGP8/trypsinogen hybrid, was generated could
theoretically be very short, a matter of several rounds of DNA replications.
As the environmental pressure increased and persisted, the incipient
AFGP/trypsinogen hybrid gene could rapidly undergo further AFGP
tripeptide expansion and whole gene duplications to escalate the gene
dosage required for large AFGP output to meet the environmental
exigency.
When Did Notothenioid AFGP Gene Arise?
The answer to this question will also answer the question of when the
Antarctic Ocean reached freezing as the two are presumably temporally
linked. Since the partly non-sense 9-nt ThrAlaAla coding element in the
319
This hybrid gene indicates that tripeptide duplication first and bulk
deletion of trypsinogen later was likely the order of events in the
conversion of trypsinogen to AFGP gene. The origin of the spacer
sequence is presently unknown. Apparently it must have been acquired
early as it was iteratively duplicated along with the ThrAlaAla coding
sequence to produce the polyprotein structure. Further investigations may
solve this remaining mystery in the trypsinogen to AFGP conversion.
How Fast Could the First Notothenioid AFGP Gene Emerge?
The threat of freezing is a matter of life and death, and therefore would be
expected to exert tremendous selection pressure for the emergence of a
freeze-avoidance function, otherwise a lineage would rapidly terminate. In
the notothenioids, the first ThrAlaAla duplication in fact could have
occurred before the selection pressure was in full force, as an accidental
result of replication slippage at the mini satellite sequence (gt)n in intron 1
immediately ahead of the putative ThrAlaAla coding element in the
ancestral trypsinogen gene (Figs. 2,3). A single or two ThrAlaAla repeats
at the N-terminus of a trypsin molecule might not affect trypsin activity
but could endow the hybrid protein with some measure of ice-binding
activity, which would be a selective advantage that became retained as the
seawater approached and hovered around freezing. The destination of
trypsin secretion, the intestinal fluid, where ice crystals could enter
through seawater and food ingestion, provided the logical testing site for
refinement of the emerging antifreeze molecule. The smallest fully
functional AFGP, AFGP8, has only 4 tripeptide repeats, which could be
produced in only two additional rounds of slippage replication after the
first ThrAlaAla duplication depicted in Fig. 3 (a to c). Thus the
"experimental process" whereby the first functional antifreeze gene (and
gene product), likely an AFGP8/trypsinogen hybrid, was generated could
theoretically be very short, a matter of several rounds of DNA replications.
As the environmental pressure increased and persisted, the incipient
AFGP/trypsinogen hybrid gene could rapidly undergo further AFGP
tripeptide expansion and whole gene duplications to escalate the gene
dosage required for large AFGP output to meet the environmental
exigency.
When Did Notothenioid AFGP Gene Arise?
The answer to this question will also answer the question of when the
Antarctic Ocean reached freezing as the two are presumably temporally
linked. Since the partly non-sense 9-nt ThrAlaAla coding element in the