Regeneration of the variance of metric traits by spontaneous mutation in aDrosophilapopulation

Abstract

In the C1 population of Drosophila melanogaster of moderate effective size ( approximately 500), which was genetically invariant in its origin, we studied the regeneration by spontaneous mutation of the genetic variance for two metric traits [abdominal (AB) and sternopleural (ST) bristle number] and that of the concealed mutation load for viability, together with their temporal stability, using alternative selection models based on mutational parameters estimated in the C1 genetic background. During generations 381-485 of mutation accumulation (MA), the additive variances of AB and ST approached the levels observed in standing laboratory populations, fluctuating around their expected equilibrium values under neutrality or under relatively weak causal stabilizing selection. This type of selection was required to simultaneously account for the observed additive variance in our population and for those previously reported in natural and laboratory populations, indicating that most mutations affecting bristle traits would only be subjected to weak selective constraints. Although gene action for bristles was essentially additive, transient situations occurred where inbreeding resulted in a depression of the mean and an increase of the additive variance. This was ascribed to the occasional segregation of mutations of large recessive effects. On the other hand, the observed non-lethal inbreeding depression for viability must be explained by the segregation of alleles of considerable and largely recessive deleterious effects, and the corresponding load concealed in the heterozygous condition was found to be temporally stable, as expected from tighter constraints imposed by natural selection.

Estimation of genetic purging under competitive conditions

“…2006; Amador et al. ). Therefore, FM predictions for the evolution of the inbreeding load in our large populations ( δ t ) could in principle be substantially different from IP predictions.…”

The consequences on fitness of equating family contributions: inferences from a drosophila experiment

“…Mean W was predicted as the product of the means predicted for EF and V scaled to the initial mean for W. Table 1 gives the mean for early fecundity (EF) and viability (V) in the base population, and the corresponding inbreeding depression rates (d), which are within the range of values commonly estimated in natural populations (Amador et al 2010) and imply d = 2.678 for fitness (W).…”

“…Under this model, the inbreeding depression rate (d) for fitness at the mutation selection drift (MSD) balance is mainly ascribed to few segregating alleles with important and largely recessive deleterious effects. This mutational model is supported by available estimates from several mutation accumulation experiments (García-Dorado et al 2004) and by the genetic analysis of populations at the MSD balance (Amador et al 2010). To simulate this situation we used the parameter of the T' model defined in García-Dorado 2003, which gives average selection coefficient E(s) = 0.224 and average degree of dominance E(h) = 0.2.…”

“…A Curlymarked (In(2LR) CyO, Cy/L 2 ) balancer stock was used to assay competitive fitness. Flies were reared in the standard conditions of this laboratory (Amador et al 2010), either in bottles for mass breeding or in 20 mm [ vials for single pair matings. The following traits were scored at different moments:…”

The fuel of evolution

“…For metric traits, the per-generation input of variance due to mutation is usually scaled by the environmental variance to give the mutational heritability, a parameter ranging from 10 À2 to 10 À4 and clustering around an average value of 10 À3 for a number of different traits and species. Thus, in an initially invariant population, a few hundred generations will be enough for mutation to regenerate the level of genetic variability for metric traits usually observed in standing populations of similar effective size (Amador et al, 2010). The distribution of effects of new spontaneous mutations on metric traits can only be assessed from MA studies, subjected to the limitations mentioned above.…”