Adaptation to contrasting habitats and heterozygosity-fitness-correlations in eelgrass (Zostera marina)
Eelgrass Zostera marina is an ecosystem-engineering species in coastal soft sediment habitats that lives in widely diverging habitats. I aimed to detect divergent selection and habitat adaptation comparing 3 pairs of permanently submerged versus intertidal populations using genome scans, a genetic marker based approach. 3 different statistical approaches for outlier identification revealed divergent selection at 6 loci among 46 markers (6 SNPs, 29 EST microsatellites and 11 anonymous microsatellites). These outlier loci were repeatedly detected in parallel habitat comparisons, suggesting the influence of habitat-specific selection. Functionally interesting loci were linked to genes involved in osmoregulation, water balance and reproductive processes. The identified outlier loci are valuable candidates for further investigation into the genetic basis of natural selection. I also aimed to investigate how fitness and multilocus heterozygosity (MLH) are related in Z. marina. Using 37 microsatellites, I assessed the correlation of fitness measured as clone size and MLH in two Z. marina populations from the Baltic Sea. I showed for the first time that the intermediate heterozygosity principle, stating that offspring from parents with an intermediate level of relatedness will be have the highest fitness, applies in a clonal plant. In ecosystems that are structured and maintained by only one species, like seagrass beds, heterozygosity represents an important component of genetic variation that has rarely been assessed before.