MICROSATELLITES REVEAL HETEROSIS IN RED DEER

The fitness consequences of inbreeding and outbreeding are poorly unde rstood in natural populations. We explore two microsatellite-based var iables, individual heterozygosity (likely to correlate with recent inb reeding) 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 b y molecular methods. Only 14 out of 231 calves (6.1%) had non-zero inb leeding coefficients, and neither inbreeding coefficient nor individua l 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, especial ly following cold Aprils, in which foetal growth is reduced. Low mean d(2) was also associated with low neonatal survival, but this effect w as probably mediated by birth weight because fitting birth weight to t he neonatal survival model displaced mean d(2) as an explanatory varia ble. We conclude that in the deer population fitness measures expresse d early in life do not show evidence of inbreeding depression, but the y do show evidence of heterosis, possibly as a result of population mi xing. The also demonstrate the practical problems of estimating inbree ding via pedigrees compared with a direct marker-based estimate of ind ividual heterozygosity. We suggest that, together, individual heterozy gosity and mean d(2), estimated using microsatellites, are useful tool s for exploring inbreeding and outbreeding in natural populations.