Genetic variation for preadult viability in Drosophila melanogaster

The extent of genetic variation in fitness and its components and genetic v ariation's dependence on environmental conditions remain key issues in evol utionary biology. We present measurements of genetic variation in preadult viability in a laboratory-adapted population of Drosophila melanogaster, ma de at four different densities. By crossing flies heterozygous for a wild-t ype chromosome and one of two different balancers (TMI, TM2), we measure bo th heterozygous (TM1/+, TM2/+) and homozygous (+/+) viability relative to a standard genotype (TM1/TM2). Forty wild-type chromosomes were tested, of w hich 10 were chosen to be homozygous viable. The mean numbers produced vari ed significantly between chromosome lines, with an estimated between-line v ariance in log(e) numbers of 0.013. Relative viabilities also varied signif icantly across chromosome lines, with a variance in log(e) homozygous viabi lity of 1.76 and of log(e) heterozygous viability of 0.165. The between-lin e variance for numbers emerging increased with density, from 0.009 at lowes t density to 0.079 at highest. The genetic variance in relative viability i ncreases with density, but not significantly. Overall. the effects of diffe rent chromosomes on relative viability were remarkably consistent across de nsities and across the two heterozygous genotypes (TMI, TM2). The 10 lines that carried homozygous viable wild-type chromosomes produced significantly more adults than the 30 lethal lines at low density and significantly fewe r adults at the highest density. Similarly, there was a positive correlatio n between heterozygous viability and mean numbers at low density, but a neg ative correlation at high density.