AVERAGE FITNESS OF POPULATIONS OF DROSOPHILA MELANOGASTER AS ESTIMATED USING COMPOUND-AUTOSOME STRAINS.

Abstract

The purpose of this paper and its companion (Hartl and Jungen, 1979) is to present a novel approach to obtaining an operational measure of average fitness in populations of Drosophila melanogaster. The procedure, which makes use of special strains carrying compound autosomes, yields a measure of population fitness that includes such components of fitness as mating speed, fertility, duration of fertile period, fecundity, egg hatchability, viability, efficiency of food assimilation, and growth rate, each component automatically given weight according to its actual importance under the experimental conditions. Furthermore, the procedure can be used to estimate average fitness of genetically heterogeneous populations, preserving intact whatever polymorphisms, epistasis, and linkage disequilibrium may be present in the original population. Because the average fitness of a population is frequently maximized by evolution in simple theoretical models of selection (Wright, 1937), and because in such models average fitness increases at a rate equal to the additive genetic variance in fitness (Fisher, 1930), the concept of average fitness of a population has played a key historical role in population genetics theory. (See Edwards, 197 7, for a review.) On the other hand, the average fitness of a population is not a readily estimated parameter. Total fitness can be split into its various components, to be sure, and these components can be estimated in individual genotypes, but the actual estimation of fitness components is a tricky business (Prout, 1971). Even so, experimental studies of fitness components have given rise to an enormous literature. (See Lewontin, 1974, and Wright, 1977, for reviews.) Attempting to translate fitness components as estimated in individual genotypes back into an estimate of population fitness runs up against two principal difficulties, however. First, estimation of fitness components does not resolve the question of which components are the key ones in terms of overall fitness; secondly, in estimating fitness components the genome must usually be studied chromosome by chromosome, as in the celebrated Curly-Plum method of viability estimation, and therefore any important nonrandom allelic or chromosomal associations that may exist are disrupted by the experimental procedure itself. Many of the practical problems encountered in trying to estimate average population fitness can be overcome by the use of so-called compound-autosome strains. Such strains have received considerable interest recently because of their potential practical utility in insect population control; compound-autosome strains can, in theory, be used to introduce conditional deleterious mutations into natural populations (Whitten, 1971; Foster et al., 1972). As a consequence, the behavior of compound-autosome strains in laboratory populations has been studied extensively I Supported by NSF grant PCM77-00886. D. L. H. is recipient of Research Career Award GM0002301. This paper is dedicated to the memory of Spencer W. Brown.