Spontaneous mutation for a quantitative trait inDrosophila melanogaster. II. Distribution of mutant effects on the trait and fitness

Abstract

SummaryStarting from a completely homozygous population of Drosophila melanogaster, lines have been derived and subjected to 47 generations of divergent selection for abdominal bristle number (20 lines selected in each direction) or to 60–67 generations of inbreeding (100 B lines maintained by a single brother-sister mating, 100 C lines maintained by two double first cousin ma tings). In the selected lines, 25 were identified carrying at least 30 mutations affecting bristle number. A large fraction of these mutations (42 %) were lethals. Non-lethal mutations had smaller effects on the trait, were predominantly additive and had no detectable pleiotropic effects on fitness. In the inbred lines, 21 mutations affecting bristles were individually analysed. Deleterious mutations had the largest effects on the trait (irrespective of sign) and showed recessive gene action (complete or incomplete). The rest were predominantly additive and had smaller effects. Thus, both procedures identify a quasi-neutral class of additive mutations which should be close to that responsible for standing variation in natural populations. Moreover, the results indicate a leptokurtic distribution of mutant effects, consistent with a model of natural selection acting on bristles through pleiotropic effects of pertinent loci on fitness. Consequently, neutral additive alleles of considerable effect can be found segregating at intermediate frequencies in natural populations.

The relevance of outcrossing for the dynamic management of genetic resources in predominantly selfing Triticum aestivum L. (bread wheat)

“…David et al (1993) showed that, in a finite population under selection, the (Mukai et al, 1972;Lopez and Lopez-Fanjul, 1993;Keightley, 1996). Such empirical data combined with theoretical considerations (Charlesworth et al, 1993) [Muller, 1964!…”

Mapping Multiple Quantitative Trait Loci by Bayesian Classification

“…This incorporates the empirical observation that the distribution of genetic effects well as genotypes and include all markers as random variables in a single model. The ability to incorporate is heavy tailed (Lopez and Lopez-Fanjul 1993;Keightley 1994;Keightley and Ohnishi 1998). These paravailable information into QTL mapping and update with newly observed data is an advantage provided tially informative prior distributions not only shrink the estimates of the QTL effects toward zero to avoid the uniquely by Bayesian analysis.…”

Gene Action of New Mutations in Arabidopsis thaliana

“…Our choice to limit the number of lines studied allowed us to employ a crossing design and degree of replication that together provided sufficient power to detect effects that are relatively small (e.g., ,2% for earlier bolting and 3.5% for enhanced fruit production), although of an evolutionarily considerable magnitude. Moreover, only 4 of the 8 lines were chosen at random; the remaining 4 evaluated in set A were chosen on the basis of suggestive (not statistically significant) evidence that they might be extreme with respect to one or more morphological traits, a common practice in studies of this kind (e.g., Lopez and Lopez-Fanjul 1993;Peters et al 2003). Both because the number of lines is small and because some were included on the basis of preliminary findings, our study, like others of this kind, cannot yield generalizations about the dominance of single mutations.…”

“…In contrast, lines 17-69 and 17-76 produced fewer leaves, and those of 17-76 were smaller than those of other lines (R. G. Shaw and D. L. Byers, unpublished results). Studies of gene action of mutations have typically focused on effects in lines chosen according to phenotype (e.g., Lopez and Lopez-Fanjul 1993;Peters et al 2003). The second crossing block (set B) involved lines sampled at generation 24 of line advancement, including 24-3, 24-23, 24-39, and 24-102 and lines 0-7 and 0-18.…”

“…From these data, we have estimated homozygous effects of mutations on these traits, as well as the degree of dominance of mutations relative to progenitor alleles. Whereas, ultimately, the effect of individual mutations in homozygous and heterozygous state is of evolutionary importance, experiments of this kind (e.g., Lopez and Lopez-Fanjul 1993;Chavarrias et al 2001;Peters et al 2003) generally estimate the composite effect of the mutations in an MA line, rather than the effect of each mutation singly (for an exception, see Szafraniec et al 2003). In this study, crosses were conducted considerably earlier in the course of line advancement than is often the case; it is therefore reasonable to expect that each line carries few mutations (0-5) affecting a given trait and estimates of mutational effect represent the composite effect of very few mutations.…”