Microsatellites reveal heterosis in red deer

The fitness consequences of inbreeding and outbreeding are poorly understood in natural populations. We explore two microsatellite-based variables, individual heterozygosity (likely to correlate with recent inbreeding) and a new individual-specific internal distance measure, mean d(2) (focusing on events deeper in the pedigree), in relation to two measures of fitness expressed early in life, birth weight and neonatal survival, in 670 red deer calves (Cervus elaphus) born on the Isle of Rum between 1982 and 1996. For comparison, we also analyse inbreeding coefficients derived from pedigrees in which paternity was inferred by molecular methods. Only 14 out of 231 calves (6.1%) had non-zero inbleeding coefficients, and neither inbreeding coefficient nor individual heterozygosity was consistently related to birth weight or neonatal survival. However, mean d(2) was consistently related to both fitness measures. Low mean n(2) was associated with low birth weight, especially following cold Aprils, in which foetal growth is reduced. Low mean d(2) was also associated with low neonatal survival, but this effect was probably mediated by birth weight because fitting birth weight to the neonatal survival model displaced mean d(2) as an explanatory variable. We conclude that in the deer population fitness measures expressed early in life do not show evidence of inbreeding depression, but they do show evidence of heterosis, possibly as a result of population mixing. The also demonstrate the practical problems of estimating inbreeding via pedigrees compared with a direct marker-based estimate of individual heterozygosity. We suggest that, together, individual heterozygosity and mean d(2), estimated using microsatellites, are useful tools for exploring inbreeding and outbreeding in natural populations.