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Galápagos sea lion; major histocompatibility complex; survival; reproductive success; overdominance; sequence divergence
Abstract:
It is still debated whether main individual fitness differences in natural
populations can be attributed to genome-wide effects or to particular loci
of outstanding functional importance such as the major histocompatibility
complex (MHC). In a long-term monitoring project on Gala´pagos sea lions
(Zalophus wollebaeki), we collected comprehensive fitness and mating data
for a total of 506 individuals. Controlling for genome-wide inbreeding, we
find strong associations between the MHC locus and nearly all fitness
traits. The effect was mainly attributable to MHC sequence divergence
and could be decomposed into contributions of own and maternal genotypes.
In consequence, the population seems to have evolved a pool of
highly divergent alleles conveying near-optimal MHC divergence even by
random mating. Our results demonstrate that a single locus can significantly
contribute to fitness in the wild and provide conclusive evidence for the
‘divergent allele advantage’ hypothesis, a special form of balancing selection
with interesting evolutionary implications.
