QUANTITATIVE GENETIC ANALYSIS OF DIVERGENCE IN MALE SECONDARY SEXUAL TRAITS BETWEENDROSOPHILA SIMULANSANDDROSOPHILA MAURITIANA

Abstract

Abstract.-The sibling species Drosophila simulans and D. mauritiana differ significantly in a number of male secondary sexual traits, providing an ideal system for genetic analysis of interspecific morphological divergence. In the experiment reported here, F 1 hybrids from a cross of two inbred lines were backcrossed in both directions and about 200 flies from each backcross were scored for several traits (bristle numbers and cuticle areas), as well as 18 markers distributed throughout the genome. Each trait was analyzed by composite interval mapping to identify quantitative trait loci (QTL) and estimate their effects. For each trait, from one to eight loci were detected, with more divergent traits showing evidence for greater numbers of QTL. Estimates of additive effects varied widely, with a range of 0.4 to 4.1 environmental standard deviation units and an average of 2.2 units. There was substantial evidence for nonadditive effects, since the magnitude of estimates often differed significantly between the two backcrosses. The sign of the estimated effect differed among QTL for bristle traits, but not for cuticle area traits, suggesting that these two types of trait may have undergone different types of selection. Finally, several similarities were found between different traits in the estimated positions of QTL, suggesting that pleiotropy and/or linkage of QTL may have been important in the evolution of these traits.

Pleiotropic Quantitative Trait Loci Contribute to Population Divergence in Traits Associated With Life-History Variation in Mimulus guttatus

“…However, a more useful measure for understanding adaptive divergence in the wild may be to estimate QTL effect in terms of the difference between parental populations or relative to the phenotypic variation within populations. For example, True et al (1997) uses a fairly stringent criterion by defining a major QTL as one for which the distributions of alternative homozygotes for a particular QTL show little overlap, so that the probability of misclassification of phenotype is ,5%, equivalent to 3.28 environmental standard deviations. For this study, we have represented QTL effects in terms of both the mean difference between parents and relative to the ESD, as we are most interested in whether substitution of alternative QTL alleles generates visible differences in phenotype relative to the two parents.…”

The genetic architecture necessary for transgressive segregation is common in both natural and domesticated populations

“…Several authors have theorized that the ratio of 1/2 QTLs reflects the history of selection or genetic drift at the trait in question (Coyne 1996;Laurie et al 1997;True et al 1997;Orr 1998). If most or all QTL alleles for a given trait have effects in the same direction within a line, this would imply that the trait has experienced fairly constant directional selection.…”

Variation and Evolution of Male Sex Combs in Drosophila: Nature of Selection Response and Theories of Genetic Variation for Sexual Traits

“…Previous studies have investigated sex comb bristle number variation within and between species of the melanogaster complex using quantitative trait loci (QTL) mapping and gene expression analyses (Coyne 1985;True et al 1997;Macdonald and Goldstein 1999;Nuzhdin and Reiwitch 2000;Kopp et al 2003;Tatsuta and Takano-Shimizu 2006;Graze et al 2007). Despite a large amount of work in this field, there remains a gap in our basic understanding of sex comb bristle number inheritance and evolution.…”