Evidence of multiple alleles of the nuclear 18S ribosomal RNA gene in sturgeon (Family: Acipenseridae)

Abstract

Summary Nuclear 18S ribosomal RNA (rRNA)gene sequences were determined during a phylogenetic study of the evolutionary relationships of the North American sturgeon and paddlefish. In the course of this study, intra-individual variation of the 18S rRNA gene was discovered in all nine species of sturgeon examined, while no variation was detected for this gene in the North American paddlefish (Polyodon spathula)or in other non-acipenseriform fish species included in the study (bichirs, reedfish, gar, bowfin and lungfish). The discovery of multiple sequence variants of the 18S rRNA gene in an individual is unexpected; although most eukaryotes possess many copies of the rDNA unit in their genomes, the copies are normally thought to be homogenized within an individual as well as in a species by concerted evolution. The effect of the variation on the determination of species level relationships within the order and of the relationship of the order to other fish groups is examined. The actual cause(s)for the presence of intraindividual variation of the 18S rRNA gene are as yet unknown, however, it is possible that the origin and maintenance of multiple 18S rDNA sequence variants in sturgeon may be related to the polyploid status of the sturgeon genome.