Molecular ecology of southern elephant seals (mirounga leonina): mating system and population genetics

Abstract

The general aim of this research was the analysis of the mating system and the genetic structure of the southern elephant seal (Mirounga leonina). The specific objectives were to: 1) estimate the distribution of paternity and the relationship between behavioural and genetic indices of male reproductive success; 2) quantify the level of kinship among seals and investigate the presence of any genetic pattern that might be a consequence of the social structure of the population; 3) assess the genetic variation among and within the stocks of the Southern Ocean and analyse the pattern of individual dispersal. The research was conducted at Sea Lion Island (SLI, Falklands), from 1996 to 1998. All breeding males of the colony, and females and pups belonging to seven harems (n = 455) were tissue sampled. The samples were analysed at nine microsatellite loci and likelihood based paternity analyses were conducted. Samples (n = 46) from Elephant Island (EI, South Shetlands) were also characterised at the same microsatellite loci, and the mtDNA control region (299bp) was sequenced in 57 seals from SLI and 30 from EI. The success of the paternity inference was very high, as a father was found for 95.3% of the pups. Out of 183 assigned paternities, 151 (82.5%) were secure at the 95% confidence level and 32 (17.5%) at the 80% level. The distribution of paternities indicated an extremely polygynous system, with the majority of males achieving zero paternities and the harem holders siring up to 96% of the offspring in each harem (mean 78). Paternity was highly correlated with behavioural indices of mating success (R(^2) 0.80-0.99), and predicted individual paternities 60%-100% of the time in each harem. Elevated relatedness values (R) within colony suggested some level of philopatry, though the low F(_ST), indicated female dispersal between SLI and EI. Females from SLI showed a general higher level of relatedness among each other than did males. However, the harem structuring did not lead to any detectable genetic substructure within the population. Genetic differentiation was found both within and among putative colonies of the Southern Ocean. The differentiation at mitochondrial markers was higher than at nuclear markers, pointing to a difference in the pattern of breeding dispersal between sexes. Despite the extensive mitochondrial variation (ɸ(_ST)= 0.5), genetic evidence of male mediated gene flow was also found between SLI and Macquarie Island (MQ), as an adult male on SLI showed the same haplotype as a seal belonging to the very well-defined MQ lineage. Given the pattern of genetic diversity and patterns of fidelity and dispersal in elephant seals, the most parsimonious interpretation is that the male travelled from MQ to SLI. He successfully reproduced in 1996 on SLI, fathering at least 18 offspring. His exceptional migration demonstrates that even populations separated by wide genetic and geographic distances can remain linked, and it is an example of the potential homogenising effect that dispersal and mating system can have on the genetic patterns of a population.