On the genetic parameter determining the efficiency of purging: an estimate for Drosophila egg‐to‐pupae viability

Abstract

The consequences of inbreeding on fitness can be crucial in evolutionary and conservation grounds and depend upon the efficiency of purging against deleterious recessive alleles. Recently, analytical expressions have been derived to predict the evolution of mean fitness, taking into account both inbreeding and purging, which depend on an 'effective purging coefficient (d e )'. Here, we explore the validity of that predictive approach and assay the strength of purging by estimating d e for egg-to-pupae viability (EPV) after a drastic reduction in population size in a recently captured base population of Drosophila melanogaster. For this purpose, we first obtained estimates of the inbreeding depression rate (d) for EPV in the base population, and we found that about 40% was due to segregating recessive lethals. Then, two sets of lines were founded from this base population and were maintained with different effective size throughout the rest of the experiment (N = 6; N = 12), their mean EPV being assayed at different generations. Due to purging, the reductions in mean EPV experienced by these lines were considerably smaller than the corresponding neutral predictions. For the 60% of d attributable to nonlethal deleterious alleles, our results suggest an effective purging coefficient d e > 0.02. Similarly, we obtain that d e > 0.09 is required to roughly account for purging against the pooled inbreeding depression from lethal and nonlethal deleterious alleles. This implies that purging should be efficient for population sizes of the order of a few tens and larger, but might be inefficient against nonlethal deleterious alleles in smaller populations.

Estimation of genetic purging under competitive conditions

“…This should be ascribed to purging which, during this period, was more efficient in reducing the average frequencies of (partially) recessive deleterious alleles in the lines than in the large laboratory population (Glémin ), although it would be expected to be more efficient preventing long‐term decline for outbred fitness in the large population (Wang et al. ; García‐Dorado ; Bersabé and García‐Dorado ).…”

“…The purging coefficient against nonlethal alleles was estimated by assuming that these alleles contributed half the initial inbreeding load. This supposition is based on the empirical observation, consistent through Drosophila literature, that lethal inbreeding load usually accounts for 40-50% of the overall viability inbreeding load of wild populations (Simmons and Crow 1977;Bersabé and García-Dorado 2013). In our case, the actual contribution of δ L to initial δ could have been somewhat below 50%, both because this value is in the upper end of the observed range and because our trait P includes fecundity components, while the lethal inbreeding load estimated is ascribed just to alleles that have lethal effects on viability.…”

“…Thus, it is necessary to evaluate purging efficiency and to estimate the purging coefficient operating in competitive conditions in experiments designed in the light of theory. So far, the only estimate available for the purging coefficient comes from a Drosophila experiment carefully designed for that purpose, where the consequences of inbreeding and purging on egg to pupae viability were investigated in lines derived from a wild population (Bersabé and García‐Dorado , B&GD hereafter). However, these lines were maintained under low density conditions, with no competition regarding fecundity or mating ability (single mating vials), and with small effective size N e = 6 or 12.…”

On the consequences of ignoring purging on genetic recommendations for minimum viable population rules

“…(2) and (3) produce the corresponding classical neutral predictions. To date, the only empirical estimate of the intensity of purging in the fraction of B not due to recessive lethal alleles, obtained in the lab for a partial measure of fitness in Drosophila (Bersabé and García-Dorado, 2013), suggests 0.02od NL o0.08. However, it is reasonable to assume that, as B estimates in the wild are about four times those for captive and laboratory populations, d NL in the wild should behave similarly (0.08od NL o0.32).…”

Predictive Model and Software for Inbreeding-Purging Analysis of Pedigreed Populations

“…Until now, the only empirical estimates of the purging coefficient d have been obtained from the evolution of fitness average in Drosophila bottlenecked populations (Bersabé and García-Dorado 2013; López-Cortegano et al 2016). However, in conservation practice, fitness data are often available for pedigreed populations.…”