Mitochondrial DNA Haplotype Diversity in Apparent XY Female Fall-Run and Spring-Run Chinook Salmon in California's Central Valley

Abstract

Mitochondrial DNA (mtDNA) haplotype diversity between putative XY females and genetically normal females of fall- and spring-run Chinook salmon Oncorhynchus tshawytscha in California's Central Valley were compared to ascertain whether or not a subset of mtDNA haplotypes are unique to putative XY females. Two Y-chromosome markers, OtY1 and growth hormone pseudogene (GH-Ψ), were used to screen spring Chinook salmon collected on Butte, Deer, and Mill creeks in 2005 for the presence of XY females. Fall-run fish collected from 2002 to 2004 had previously been screened for genotypic sex using the same Y markers. A 237-base-pair region of the mitochondrial D-loop segment was sequenced to determine the mtDNA haplotypes of XY females and randomly selected normal females of both runs. Putative XY females, according to OtY1 and GH-Ψ, were observed in all three newly sampled spring-run populations: Butte (26%), Deer (33%), and Mill (2%) creeks. No significant differences in haplotype distributions between normal and XY females suggest that these fish are equally represented within the separate fall and spring runs. It is possible that XY female Chinook salmon have been present in the Central Valley of California since before the genetic divergence of the fall and spring runs. Furthermore, XY female Chinook salmon may have become established in this region through a founder event that resulted in fewer modes of variation in genotypic and phenotypic sex compared with populations in the interior Columbia River basin.