The effects of inbreeding and artificial selection on reproductive fitness

Abstract

The competitive-index method of measurement of over all fitness in Drosophila has been used to measure the effect of inbreeding and of artificial selection for metric characters in a large population of Drosophila melanogaster. The technique itself was examined in detail with particular reference to its repeatability and to the effect on it of the modification of various environmental variables.With continued full-sib mating the decline in the competitive index was very rapid (it was reduced to a half by a single generation of full-sib mating) and there were no indications that interactions between deleterious genes at different loci were important in determining the rate of decline of fitness as inbreeding increased. Other unselected lines with ten pairs of parents in each generation were carried to serve as a control for the lines under artificial selection. At the same theoretical degree of inbreeding the control lines had a much higher average fitness than the lines produced by continued full-sib mating.From the base population lines were selected in both directions for abdominal bristles, sternopleural bristles and for wing length, there being two replicates in all cases. Four control lines were kept with the same number of parents as the selected lines. In all cases the selected lines declined in fitness below the value for the base population. However, in three of the lines the fitness was not significantly below the value for the control lines. The effect of artificial selection on fitness was asymmetrical, the decline being greater with down selection for all characters.The relevance of these results to various theoretical models is discussed. If the variation in these characters is actively maintained in the base population by the selection of heterozygotes then the results are consistent with an average selection disadvantage of homozygotes relative to heterozygotes of about 0·5%.

THE EFFECT OF MUTAGENESIS ON COMPETITIVE ABILITY IN DROSOPHILA

“…The very weak epistasis observed for new mutations in some experiments (Mukai 1967(Mukai , 1969) is consistent with that expected under pleiotropy and multiplicative fitness (Rose, 1982). The quadratic change in competitive fitness as a result of inbreeding is compatible with multiplicative fitness effects at unlinked, nonepistatic loci (Latter and Robertson, 1962).…”

Effectiveness of Regular Cycles of Intermittent Artificial Selection for A Quantitative Character in Drosophila Melanogaster

“…Yet marked declines in fitness have been observed in long-term selection for this character (Mather and Harrison 1949;Clayton and Robertson 1957;Latter and Robertson 1962;Sheldon 1963). These declines could have been due to fixation of genes deleterious to fitness because of inbreeding resulting from the small size of the populations and accentuated by selection (Robertson 1961), or because oflinkage between genes deleterious to fitness and ones responsible for response in bristle number.…”

“…The method of testing was that of Knight and Robertson (1957) as modified by Latter and Robertson (1962), and designated by them as competitive index (female). Twenty-five inseminated females from each of the w strain and the line under test were placed in a 5-oz cream bottle containing 30 ml of a culture medium lightly fortified with dead yeast.…”

Genetic considerations on wolf conservation in Italy

“…In fact the first traits with dominance to be changed are those connected with reproduction, and me might expect inbred lines to have a significant reduction in fertility, fecundity, litter size, and developmental rate. But also behavioural traits are affected by inbreeding and it is interesting to cite the decline of competitive ability, as shown in experiments with Drosophilu (Latter & Robertson, 1962) and with Peroiiziscrrs (Garten, 1976) among others. As inbreeding can lead to fixation of deleterious recessive alleles, but also to their elimination, the question is which of these processes has the faster rate.…”