Since it was first described as a biological species by Jordan and Gilbert in 1880, Inopsetta ischyra has been a disputed taxonomic unit, generally regarded in recent decades as an intergeneric hybrid between English sole Parophrys vetulus and starry flounder Platichthys stellatus, both of which are common in coastal waters of the eastern North Pacific. Here, we investigate this suspected hybridization with molecular genetic and morphological data. Genotyping at four diagnostic allozyme loci clearly demonstrates that I. ischyra is an intergeneric hybrid between English sole and starry flounder; one individual was homozygous at one locus and may have been a backcross with starry flounder. Diagnostic restriction fragment length polymorphisms of a 464-base-pair region of the cytochrome b gene revealed symmetric, two-way directionality of hybridization in which 50% of maternities are assigned to each of the parental species. Principal components analysis of meristic characters demonstrated that I. ischyra is, an intermediate between the parental species. Based on the low frequency of hybrids and the rarity of backcrosses, these natural hybridization events do not appear to be ecologically or evolutionarily significant.
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