DIFFERENCES IN INBREEDING COEFFICIENTS DO NOT EXPLAIN THE ASSOCIATION BETWEEN HETEROZYGOSITY AT ALLOZYME LOCI AND DEVELOPMENTAL STABILITY IN RAINBOW TROUT

META-ANALYSES OF THE ASSOCIATION BETWEEN MULTILOCUS HETEROZYGOSITY AND FITNESS

Meta-analyses of published correlation coefficients between multilocus heterozygosity (MLH) and two fitness surrogates, growth rate and fluctuating asymmetry, suggested that the strength of these correlations are generally weak. A variety of plants and animals was included in the meta-analyses. A statistically homogeneous group of MLH-growth rate correlation coefficients that included both plants and animals yielded a common correlation of r_z = 0.133. A common correlation of r_z = -0.170 was estimated for correlations between MLH and fluctuating asymmetry in three species of salmonid fishes. These results suggest that selection, including overdominance, has at most a weak effect at allozyme loci and cast some doubt on the widely held notion that heterozygosity and individual fitness are strongly correlated.

The apparent selection on neutral marker loci in partially inbreeding populations

Deterministic computer calculations were used to investigate the effects on the fitnesses of genotypes at neutral loci that are caused by associations with several linked or unlinked selected loci, in partially self fertilizing populations. Both mutation to partially recessive alleles and heterozygote advantage at the selected loci were studied. In the heterozygote advantage models, either arbitrary linkage between all loci was modelled, with a single neutral locus, or many unlinked selected and neutral loci were modelled. Large apparent overdominance could be generated in all types of model studied. As has previously been suggested, these types of effect can explain the observed associations between fitness and heterozygosity in partially inbreeding populations. There were also apparent fitness differences between the genotypes at the neutral locus among the progeny produced by selfing, especially with linkage between the neutral and selected loci. There is thus no genotype-independent fitness value for these progeny. Marker based methods for estimating the relative fitness of selfed and outcrossed progeny assume equality of these fitnesses, and will therefore be inaccurate (with in most cases a bias towards overestimating the degree of inbreeding depression) when there is linkage between the neutral marker loci and loci determining fitness.

Selection for heterozygosity gives hope to a wild population of inbred wolves

Recent analyses have questioned the usefulness of heterozygosity estimates as measures of the inbreeding coefficient (f), a finding that may have dramatic consequences for the management of endangered populations. We confirm that f and heterozygosity is poorly correlated in a wild and highly inbred wolf population. Yet, our data show that for each level of f, it was the most heterozygous wolves that established themselves as breeders, a selection process that seems to have decelerated the loss of heterozygosity in the population despite a steady increase of f. The markers contributing to the positive relationship between heterozygosity and breeding success were found to be located on different chromosomes, but there was a substantial amount of linkage disequilibrium in the population, indicating that the markers are reflecting heterozygosity over relatively wide genomic regions. Following our results we recommend that management programs of endangered populations include estimates of both f and heterozygosity, as they may contribute with complementary information about population viability.

On the correlation between heterozygosity and fitness in natural populations

Three primary hypotheses currently prevail for correlations between heterozygosity at a set of molecular markers and fitness in natural populations. First, multilocus heterozygosity-fitness correlations might result from selection acting directly on the scored loci, such as at particular allozyme loci. Second, significant levels of linkage disequilibrium, as in recently bottlenecked-and-expanded populations, might cause associations between the markers and fitness loci in the local chromosomal vicinity. Third, in partially inbred populations, heterozygosity at the markers might reflect variation in the inbreeding coefficient and might associate with fitness as a result of effects of homozygosity at genome-wide distributed loci. Despite years of research, the relative importance of these hypotheses remains unclear. The screening of heterozygosity at polymorphic DNA markers offers an opportunity to resolve this issue, and relevant empirical studies have now emerged. We provide an account of the recent progress on the subject, and give suggestions on how to distinguish between the three hypotheses in future studies.

Conditions for positive and negative correlations between fitness and heterozygosity in equilibrium populations

The past decades have witnessed extensive efforts to correlate fitness traits with genomic heterozygosity. While positive correlations are revealed in most of the organisms studied, results of no/negative correlations are not uncommon. There has been little effort to reveal the genetic causes of these negative correlations. The positive correlations are regarded either as evidence for functional overdominance in large, randomly mating populations at equilibrium, or the results of populations at disequilibrium under dominance. More often, the positive correlations are viewed as a phenomenon of heterosis, so that it cannot possibly occur under within-locus additive allelic effects. Here we give exact genetic conditions that give rise to positive and negative correlations in populations at Hardy-Weinberg and linkage equilibria, thus offering a genetic explanation for the observed negative correlations. Our results demonstrate that the above interpretations concerning the positive correlations are not complete or even necessary. Such a positive correlation can result under dominance and potentially under additivity, even in populations where associated overdominance due to linked alleles at different loci is not significant. Additionally, negative correlations and heterosis can co-occur in a single population. Although our emphasis is on equilibrium populations and for biallelic genetic systems, the basic conclusions are generalized to non-equilibrium populations and for multi-allelic situations.

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

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 (25 females x 25 males). 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.

Disease resistance and enzyme heterozygosity in rainbow trout

The relationship between heterozygosity at nine polymorphic enzyme loci and disease resistance was examined in 373 individually identified rainbow trout (Oncorhynchus mykiss) from 12 full-sib families and a pooled gamete cross. These fish were challenged with bacterial gill disease, a potentially lethal epizootic in freshwater fishes. The 213 surviving fish had significantly greater numbers of heterozygous loci per fish and were significantly larger than the 160 individuals that died. Survivors, on average, had higher heterozygosity at six out of nine loci than non-survivors. The differences were significant at three of these loci. These findings suggest that more heterozygous rainbow trout have superior disease resistance than less heterozygous fish.