Heterozygosity, inbreeding and neonatal traits in Soay sheep on St Kilda

Abstract

We investigated whether birth weight and neonatal survival, a period within which 24% of all mortalities occur, were correlated with levels of inbreeding in St Kilda Soay sheep, using pedigree inbreeding coefficients and four marker-based estimators of inbreeding. None of the inbreeding estimators, either of the offspring, or of their mothers, explained significant variation in a lamb's birth weight or probability of surviving the neonatal period, suggesting low inbreeding depression for these traits. We evaluated the correlation between the marker-based measures of inbreeding and inbreeding coefficients obtained from the Soay pedigree, where paternal links were inferred using the same panel of microsatellite markers. Even when using a relatively complete portion of the pedigree, in which all individuals had known maternal and paternal grandparents, the correlation was found to be weak (r = -0.207, where mean f = 0.0168). These results add support to the recent prediction that when the mean and variance in inbreeding are low in a population, heterozygosity-fitness correlations can be very weak or even undetectable. The pursuit of more detailed pedigrees offers the best prospect for identifying inbreeding depression within this study population.

Development of a Linkage Map and Mapping of Phenotypic Polymorphisms in a Free-Living Population of Soay Sheep (Ovis aries)

“…In this population, maternity is determined by observation, and paternity is inferred through molecular analysis (Overall et al 2005). The mating system is polygynous and promiscuous, such that very few full-sibs occur in the population.…”

“…Some cases of paternity mis-assignment were expected since in the original data set paternity was assigned with only 80% confidence (Overall et al 2005).…”

A quantitative review of heterozygosity–fitness correlations in animal populations

“…As the general effect hypothesis relies on variance in inbreeding within populations, if this hypothesis is correct, the strength of the relationship between heterozygosity and fitness will depend on the variance of f in the population; hence, highly inbred populations with low variance in f would not be expected to exhibit strong HFCs due to general effects any more than outbred populations would (Balloux et al 2004;Slate et al 2004;Overall et al 2005). Similarly, populations that have undergone historical or prolonged bouts of inbreeding may have successfully purged much of their mutational load (Charlesworth & Charlesworth 1987;Crnokrak & Barrett 2002), and the relationship between heterozygosity and fitness attributable to general effects might thus be lower (Reed & Frankham 2003), although the degree of purging will be dependent on the extent to which deleterious alleles result in lowered fitness (Wang et al 1999;Crnokrak & Barrett 2002), and whether the purging occurs via drift or nonrandom mating (Glémin 2003).…”

Wild pedigrees: the way forward

“…The observed correlation between pedigree inbreeding coefficients and marker-based estimates of inbreeding is often low despite good data (Markert et al 2004;Slate et al 2004;Overall et al 2005;Rodriguez-Ramilo et al 2007) and this is largely because the mean and variance of inbreeding coefficients are both low in those natural populations so far studied (Slate et al 2004). Similar conclusions were reached by a simulation study (Balloux et al 2004).…”