Factors eliminating inbreeding depression in a captive herd of speke's gazelle (Gazella spekei)

Abstract

Inbreeding is commonly associated with a lowering of viability and birth weights—a phenomenon known as inbreeding depression. A severe inbreeding depression was encountered in a captive breeding program for Speke's gazelle. Unfortunately, the solution of simply avoiding inbreeding could not be implemented because the entire herd was descended from one import male and three import females. Because of this founder effect, it was impossible to avoid inbreeding. However, laboratory experiments with fruit flies and basic evolutionary theory indicate that animals can rapidly adapt to inbreeding by the selective elimination of the genes responsible for inbreeding depression. These experimental and theoretical results were translated into a breeding program for the Speke's gazelle. The first goal of the breeding program is a demographic goal: increase the total population size as rapidly as possible to the carrying capacity. The other goals all deal with genetic attributes of either parents or offspring: Both parents and offspring should be inbred, and both parents and offspring should have genes from as many different founding ancestors as possible. In this paper, we document that this breeding program does eliminate the inbreeding depression very rapidly, and moreover that the genetic goals of the program aid this elimination exactly as theory predicts. Furthermore, our analysis clearly shows that the gazelles suffered from an inbreeding depression rather than an outcrossing depression. We conclude that inbreeding depressions can be rapidly and effectively reduced by an appropriate breeding program, and hence an inbreeding depression does not constitute an insurmountable barrier to the long‐term maintenance of a species in which inbreeding cannot be avoided.

An investigation of inbreeding depression and purging in captive pedigreed populations

“…We use two similar logistic regression models to analyse the pedigree data, one of which was devised by Ballou (1997) (hereafter known as 'Ballou's model') and the other by Boakes and Wang (2005) (hereafter known as the 'alternative model'). The models are based on the assumption that inbred animals with inbred ancestry will be less susceptible to inbreeding depression than inbred animals with non-inbred ancestry as those inbred ancestors that are able to survive and to reproduce will be less likely to be carriers of deleterious alleles (Templeton and Read, 1984). Historical inbreeding can be measured by the ancestral inbreeding coefficient (f a ), which is defined as the cumulative proportion of an individual's genome that has been previously exposed to inbreeding in its ancestors (Ballou, 1997).…”

“…When this was tried in a zoo population of Speke's gazelle (Gazella spekei), the level of inbreeding depression was reported to be dramatically reduced after only two or three generations (Templeton and Read, 1984), leading to support for this approach (Ralls and Ballou, 1986;Templeton et al, 1986). However, reanalysis of the same data failed to find evidence of genetic improvement .…”

Purging deleterious mutations in conservation programmes: combining optimal contributions with inbred matings

“…A strategy to avoid this problem is to combine optimal contributions with inbred matings, in order to expose deleterious recessive mutations, which will then be eliminated by natural selection in a process known as purging. Purging by inbred matings has been shown in some species to reduce the magnitude of inbreeding depression (Templeton and Read, 1983;Keller and Waller, 2002;Swindell and Bouzat, 2006;Leberg and Firmin, 2008). However, in populations of small census size inbred matings increase their extinction risk (Hedrick, 1994;Wang et al, 1999;Wang, 2000).…”

Analysis of inbreeding depression in the first litter size of mice in a long-term selection experiment with respect to the age of the inbreeding

“…The impact of inbreeding depression (and other phenomena) has frequently been found to depend upon DF, with faster rates having more impact (Ehiobu et al, 1989;Wang et al, 1999;Pedersen et al, 2005). The effect of inbreeding rate may be explained by natural selection: slow inbreeding would increase the number of generations for selection to purge the genetic load, leading to a smaller impact for a given inbreeding coefficient (Templeton and Read, 1984;Lacy and Ballou, 1998). A different distinction, which we consider in this paper, is whether an increment of inbreeding was recent in origin ('new' inbreeding) or occurred further back in the population history ('old' inbreeding).…”