Enzyme heterozygosity and growth in rainbow trout: genetic and physiological explanations

Abstract

The possibility of whether the association between enzyme heterozygosity and body size (fork length) is consistent among rainbow trout ( Oncorhynchus mykiss) with different degrees of relationship and is affected by fish age was determined. Six-month-old full-sibs and progeny groups from five and 13 parents of each sex do not show the positive associations between multilocus heterozygosity and fork length which are detectable in larger pooled gamete matings (259 x 25 ci). Moreover, the relationship between heterozygosity at single loci and fork length is inconsistent among families. These data suggest that chromosomal segments, marked by the enzyme loci, are responsible for the phenotypic effects (associative overdominance). Fish age affects both the strength and direction of the association between multilocus heterozygosity and fork length; the association is positive in 6-month-old rainbow trout but is weaker, negative, or differs between the sexes in fish at 1 year. Moreover, the strength of the relationship changes over time in repeated measurements on the same fish. A decline in growth of larger and more heterozygous fish, because of precocial sexual maturation, may partially explain the changing relationship between multilocus heterozygosity and fork length. Sexually mature males are significantly more heterozygous than immature males and show significantly reduced recent growth rates as measured by white muscle RNA concentration. However, females, who rarely mature at 1 year, show significantly higher recent growth rates. Thus, heterozygosity is differentially associated with the allocation of energy resources into somatic and reproductive tissue in male and female rainbow trout.

The independence of enzyme heterozygosity and life-history traits in natural populations of Salvelinus fontinalis (brook trout)

“…Declines in growth rate of larger and more heterozygous fish during maturation can account for the age-dependent weakening of the relationship between multilocus heterozygosity and ($ = partial regression coefficient). body size observed in many salmomds (Ferguson, 1990(Ferguson, , 1992. A positive correlation between heterozygosity and growth rate early in life, and the absence of such an association later in life, reflects differential selection pressures on growth rate at different stages of life.…”

Multilocus heterozygosity and size: a test of hypotheses using triploid Mytilus edulis

“…Salmonids as a group have undergone a tetraploidization event relatively recently in their evolutionary history and several studies have demonstrated significant positive correlations between multilocus heterozygosity and physiological parameters in salmonids (e.g. Leary et a!., 1983;Ferguson, 1992). However, there has been functional diploidization in the salmonid lineage with the formation of divalent rather than tetravalent sets of chromosomes at meiosis with duplicate loci either diverging from each other (producing electrophoretically distinct proteins) or becoming silenced (Ferguson, 1980).…”

“…Koehn & Shumway, 1982;Foltz & Zouros, 1984;Koehn & Gaffney, 1984;Zouros & Foltz, 1987;Gentili & Beaumont, 1988;Koehn et a!., 1988;Zouros et a!., 1988;Hawkins et a!., 1989;Volckaert & Zouros, 1989;Gaffney et a!., 1990; although other groups of organisms have also been studied (e.g. fish -McAndrew et a!., 1986; Ferguson, 1992;salamanders -Pierce & Mitton, 1982;frogs -McAlpine, 1993;beetles -Whitlock, 1993; deer -Leberg eta!., 1990; trees - Bush eta!., 1987).…”

Family heterozygosity and progeny body length in pink salmon Oncorhynchus gorbuscha (Walbaum)

“…However, as shown in Table 4, this value could reflect only the effect of heterozygosity on the between-family component of variance. To test this by excluding the differences between families, the body length of each fry was transformed to z (x -x)/a1, where x is the body length The associations between heterozygosity and phenotypic characters has been studied in families of rainbow trout (Leary et a!., 1987;Ferguson, 1992) and marine bivalves (Beaumont et al, 1983;Gaffney & Scott, 1984;Foltz & Chatry, 1986;Mallet et al, 1986).…”

“…In some studies an association between quantitative traits and heterozygosity at polymorphic loci has been demonstrated while other authors have failed to find it (see Mitton & Grant, 1984;Zouros, 1987, for a review). A number of theoretical models has been proposed to explain these results (Lerner, 1954;Livshits & optimum, isozymes, Oncorhynchus gorbuscha, studies on the progeny of individual crosses have failed to find any correlations between heterozygosity and phenotypic characters (Beaumont eta!., 1983;Gaffney & Scott, 1984;Foltz & Chatry, 1986;Mallet et al, 1986;Ferguson, 1992). These results might suggest that these associations are largely the consequence of linkage disequilibrium between allozyme loci and other loci that affect the traits being studied (Zouros & Foltz, 1987).…”