Inbreeding and extinction: Effects of purging

Abstract

Deleterious alleles may be removed (purged) bynatural selection in populations undergoinginbreeding. However, there is controversyregarding the effectiveness of purging inreducing the extinction risk due to inbreeding,particularly in conservation contexts. Weevaluated the effects of purging on theextinction risk due to inbreeding in Drosophila melanogaster using two basepopulations, an outbred population (non-purged)and four-way crosses between highly inbredlines derived from the same population(purged). The inbred lines used in the four-waycrosses were previously subjected to 20generations of full-sib mating. The impact offull-sib inbreeding over a further 12generations was compared in 200 populationsfrom each of the two base populations. Therewas a small and non-significant differencebetween the extinction rates at an inbreedingcoefficient of 0.93 in the non-purged (0.74± 0.03) and purged (0.69 ± 0.03)treatments. This is consistent with otherevidence indicating that the effects of purgingare often small. Purging using rapid inbreedingin very small populations cannot be relied uponto eliminate the deleterious effects ofinbreeding.