Diploidization without Genome Reduction? Insights from Sturgeon

It is generally assumed that following a genome duplication event, duplicated copies of chromosomes and genes diverge over time, eventually leading to re-establishment of a diploid state (diploidization) accompanied by genome reduction via chromosomal rearrangements and neofunctionalization, subfunctionalization, or gene silencing in one copy of each duplicated gene. Karyological evidence suggests that an ancient genome duplication event estimated at over 200 million years before present occurred in the lineage that gave rise to all acipenseriform fishes (sturgeons and paddlefish). Sturgeons in the genus Scaphirhynchus are considered one of the basal sturgeon lineages and possess approximately 120 chromosomes, whereas some sturgeon taxa exhibit additional genome duplications and possess 250 or as many as 500 chromosomes. Thus, sturgeons provide a unique study system to examine the fate and consequences of genome duplication events. It has been suggested that in basal sturgeon lineages, the process of diploidization is nearly complete and has been accompanied by significant genome reduction. However, preliminary evidence from Scaphirhynchus transcriptomes and genetic markers developed in protein-coding genes provide evidence for widespread retention of functional gene duplicates that are unlinked. These data suggest that diploidization has not been accompanied by functional genome reduction following ancient genome duplication in sturgeons.