Are responses to artificial selection for reproductive fitness characters consistently asymmetrical?

Abstract

Non-linear offspring-parent regressions and heritabilities are expected for characters showing genetic asymmetry due to directional dominance and/or asymmetrical gene frequencies. Since reproductive fitness characters exhibit these characteristics, they should show consistently nonlinear heritabilities, with greater heritabilities in the direction of lower fitness. As a consequence, responses to bi-directional selection on fitness traits should be asymmetrical in the same direction. This prediction has been tested by an analysis of published bi-directional selection experiments for reproductive fitness traits. Significant asymmetry (24 of 30 studies) in the predicted direction was found. For studies reporting realized heritabilities, the means were 0-173 and 0-259 for lines selected for higher and lower reproductive fitness, respectively, the high lines being 33 % less than the low lines. Asymmetry was evident for studies reporting realized heritabilities and for those with random mating controls of the same size as the selection lines. Consequently, it is argued that the asymmetry results from genetic asymmetries. This asymmetry has important implications in the improvement of reproductive fitness traits in plant and animal breeding.

Dominance Genetic Variance for Traits Under Directional Selection inDrosophila serrata

“…These observations are consistent with predicted patterns of dominance genetic variance for traits that are genetically correlated with fitness (Fisher 1930;Frankham 1990;Crnokrak and Roff 1995). Directional selection on fitnesscorrelated traits is expected to erode the additive genetic variance in these traits (Fisher 1930), resulting in lower heritabilites (Mousseau and Roff 1987;Price and Schluter 1991;Kruuk et al 2000) and a higher proportion of dominance variance contributing to overall phenotypic variance (Merilä and Sheldon 1999).…”

“…The increased frequency of homozygous loci across the genome, caused by inbreeding, exposes recessive deleterious alleles that are typically held in a heterozygous state, resulting in the decrease in trait means, and initial fitness reductions that accompany inbreeding (Lande and Schemske 1985;Lynch 1991;Charlesworth et al 1999). Similarly, selection for recessive deleterious alleles in heterozygotes results in the typically faster responses observed for downward than upward artificial selection, when the dominant allele confers a higher trait mean than the recessive (Falconer and Mackay 1981).Remarkably high levels of inbreeding depression, averaging 50%, have been demonstrated for primary fitness components (e.g., viability, fertility, egg production) in Drosophila, compared to levels of a few percent for morphological traits (Lynch and Walsh 1998), and asymmetric selection responses for fitness components have also been demonstrated in several cases (Frankham 1990). These observations are consistent with predicted patterns of dominance genetic variance for traits that are genetically correlated with fitness (Fisher 1930;Frankham 1990;Crnokrak and Roff 1995).…”

“…Remarkably high levels of inbreeding depression, averaging 50%, have been demonstrated for primary fitness components (e.g., viability, fertility, egg production) in Drosophila, compared to levels of a few percent for morphological traits (Lynch and Walsh 1998), and asymmetric selection responses for fitness components have also been demonstrated in several cases (Frankham 1990). These observations are consistent with predicted patterns of dominance genetic variance for traits that are genetically correlated with fitness (Fisher 1930;Frankham 1990;Crnokrak and Roff 1995).…”

Genetics and genomics of Drosophila mating behavior

“…The response to selection for mating speed was highly asymmetrical, as is often observed for traits that are major components of fitness (72,75), including previous studies selecting for divergent mating speed in Drosophila (76)(77)(78)(79). Asymmetrical responses of fitness traits to selection are generally attributable to directional dominance and͞or genetic asymmetry, such that alleles increasing fitness are at high frequency.…”

In search of genetic constraints limiting the evolution of egg size: direct and correlated responses to artificial selection on a prenatal maternal effector

“…Between generations 2 and 5, egg size did not increase in the maternal high investment lines but rapidly decreased in the maternal low investment lines. Interestingly, such asymmetrical responses to selection are regularly observed in fitness-related traits (Frankham, 1990).…”