INCIDENCE OF ALBINOS AS A MONITOR FOR INDUCED TRIPLOIDY IN RAINBOW-TROUT

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