Mutant Alleles of Small Effect Are Primarily Responsible for the Loss of Fitness With Slow Inbreeding in Drosophila melanogaster

Corresponding author: B. D. H. Latter, 43 Camerons Road, McLeans Ridges, Lismore, NSW 2480, Australia, blatter{at}nor.com.au (E-mail).

Communicating editor: D. CHARLESWORTH

Multilocus simulation is used to identify genetic models that can account for the observed rates of inbreeding and fitness decline in laboratory populations of Drosophila melanogaster. The experimental populations were maintained under crowded conditions for ~200 generations at a harmonic mean population size of N_h ~65–70. With a simulated population size of N = 50, and a mean selective disadvantage of homozygotes at individual loci ~1–2% or less, it is demonstrated that the mean effective population size over a 200-generation period may be considerably greater than N, with a ratio matching the experimental estimate of N_e/N_h ~1.4. The buildup of associative overdominance at electrophoretic marker loci is largely responsible for the stability of gene frequencies and the observed reduction in the rate of inbreeding, with apparent selection coefficients in favor of the heterozygote at neutral marker loci increasing rapidly over the first N generations of inbreeding to values ~5–10%. The observed decline in fitness under competitive conditions in populations of size ~50 in D. melanogaster therefore primarily results from mutant alleles with mean effects on fitness as homozygotes of s_m 0.02. Models with deleterious recessive mutants at the background loci require that the mean selection coefficient against heterozygotes is at most hs_m ~0.002, with a minimum mutation rate for a single Drosophila autosome 100 cM in length estimated to be in the range 0.05–0.25, assuming an exponential distribution of s. A typical chromosome would be expected to carry at least 100–200 such mutant alleles contributing to the decline in competitive fitness with slow inbreeding.

This article has been cited by other articles:

				N. Bierne, A. Tsitrone, and P. David An Inbreeding Model of Associative Overdominance During a Population Bottleneck Genetics, August 1, 2000; 155(4): 1981 - 1990. [Abstract] [Full Text]

				J. Wang and W. G. Hill Effect of Selection Against Deleterious Mutations on the Decline in Heterozygosity at Neutral Loci in Closely Inbreeding Populations Genetics, November 1, 1999; 153(3): 1475 - 1489. [Abstract] [Full Text]