Incidence of albinos as a monitor for induced triploidy in rainbow trout

Abstract

Albinism is a recessive trait in rainbow trout ( Oncorhynchus mykiss). We tested for complementation at the albino locus among six USA rainbow trout strains and found that albinos from four domesticated strains and one Idaho steelhead strain are mutant at the same locus. These strains apparently are tyrosinase-deficient albinos. An albino steelhead strain from Washington State and an albino brook trout ( Salvelinus fontinalis) strain are apparently mutant at other loci because they produce pigmented progeny when crossed to the common form of albino rainbow trout. We determined using gynogenesis that the albino locus common in domesticated rainbow trout maps very near the end of a chromosome; virtually all the gynogenetic progeny of heterozygous females were pigmented. In contrast, the Golden locus of rainbow trout appears centromere-linked. Female rainbow trout heterozygous for the albino gene have 50% albino offspring when crossed to albino males but a high proportion of pigmented offspring after a heat shock is applied to induce triploidy. The proportion of pigmented offspring would be expected to correlate directly to the proportion of triploid individuals in such crosses. However, pigmented diploids can sometimes be found in such crosses in higher than predicted frequencies, apparently because heat shock can sometimes induce rejection of the sperm. This indicates that diploids observed among lots treated to induce triploidy may result from sperm rejection as well as from failure of second polar body retention.