Effects of a change in the level of inbreeding on the genetic load

Abstract

"The effects of inbreeding may not be as noticeable in the first generation as the invigoration immediately apparent after crossing". This statement, published in 1919, has received little attention, and has apparently never been tested empirically, although the reduction of the genetic load of populations by inbreeding is well known in theoretical terms. Because inbreeding increases homozygosity, and hence the effectiveness of selection against recessive or partially recessive detrimental alleles, changes in levels of inbreeding can lead to a reduction in the frequencies of such mutant alleles. This results in equilibration at higher population mean fitness and is referred to as 'purging' populations of their genetic load. Severe inbreeding can also reduce genetic load due to overdominant alleles, provided selection coefficients are not symmetrical at all loci, because alleles giving lower fitness will be reduced in frequency at equilibrium. With either fitness model, however, reduction in genetic load takes time, and the initial effect of an increase in inbreeding is reduced fitness due to homozygosity. There are few data relating to the extent to which fitness is reduced during inbreeding in a set of lines and to how long the reduction lasts before increasing again to the initial level, or higher. Inbreeding experiments involving sib mating in mice and Drosophila subobscura, and successive bottlenecks in house flies have yielded some evidence consistent with the purging hypothesis. Here, we report results of an experiment demonstrating a prolonged time-course of recovery of mean fitness under self-fertilization of a naturally outcrossing plant, and also compare our results with expectations derived by computer calculations. Our results show that the genetic load present in an outcrossing population can be explained only with a high mutation rate to partially recessive deleterious alleles, and that inbreeding purges the population of mutant alleles.

Analysis of the effects of inbreeding on lifespan and starvation resistance in Drosophila melanogaster

“…Which of the two hypotheses underlies the cause of inbreeding depression is still open to debate, although the partial dominance hypothesis has replaced the once more popular overdominance hypothesis, and is now the most favored in explaining inbreeding depression at least for most fitness traits Charlesworth 1987, 1999;Barrett and Charlesworth 1991;Dudash and Carr 1998;Roff 2002;Wright et al 2008 but see e.g. Karkkainen et al 1999;Li et al 2001; see also Willis 1999;Frankham et al 2001;Crnokrak and Barrett 2002;Radwan 2003;Swindell and Bouzat 2006c, d).…”

CORRELATED EVOLUTION OF SELF-FERTILIZATION AND INBREEDING DEPRESSION: AN EXPERIMENTAL STUDY OF NINE POPULATIONS OFAMSINCKIA(BORAGINACEAE)

“…Polyploidy is associated with high levels of self-fertilization, especially among annual plants. This association probably results from the fact that a newly arisen polyploid in a diploid population can form successful zygotes only by mating with itself (Stebbins 1971, p. 130;Grant 1981, p. 317; see also Barrett 1989). It is also possible, however, that polyploidy promotes the evolution of self-fertilization by directly decreasing inbreeding depression.…”

An investigation of inbreeding depression and purging in captive pedigreed populations

“…Byers and Waller, 1999;Hedrick and Kalinowski, 2000;Keller and Waller, 2002) and threatens the survival of small populations, both wild and captive. However, since inbreeding increases the frequency of the deleterious homozygous genotype, selection against deleterious alleles is also increased during inbreeding (Crow and Kimura, 1970), thereby providing an opportunity for a population to be 'purged' of its mutational load (Lande and Schemske, 1985;Barrett and Charlesworth, 1991). Following purging, the fitness levels of the population may increase, possibly returning to or even exceeding those of a large, randomly mating population at mutation-selection balance (Lande and Schemske, 1985;Hedrick, 1994).…”

“…Ballou, 1997), changes in fitness components of inbred lines relative to the original outbred line (e.g. Barrett and Charlesworth, 1991), founder-flush experiments (e.g. Miller and Hedrick, 2001) and comparison of populations with the same level of inbreeding but different histories of rates of inbreeding (e.g.…”