Linking Spatial Patterns of Adult and Seed Diversity Across the Depth Gradient in the Seagrass Zostera marina L.

Abstract

Sexual reproduction is the primary source of genotypic diversity in seagrass meadows, with important implications for both the evolutionary potential and ecological function of these habitats. However, relatively little is known about seagrass pollination dynamics or how genetic diversity in life history stage may influence genetic diversity in another. We used microsatellite DNA markers to assess how genetic diversity of adult shoots and seeds covaried across the depth gradient in two Zostera marina L. (eelgrass) meadows in Massachusetts (USA). Despite strong spatial patterns in the density of reproductive shoots and genetic evidence for limited pollen dispersal, we found modest corresponding variation in outcrossing rates and paternal diversity. Across spatial scales from quadrats to sites, there was a positive association between the density of flowering shoots and clonal richness; however, other measures of diversity (e.g., allelic richness and heterozygosity) showed weak or opposing patterns with reproductive effort, and neither flowering effort nor local genotypic richness of adults were good proxies for seed diversity. Self-pollinated seeds were significantly smaller than their outcrossed half-siblings, and our data suggest that selection against selfed seeds varies spatially within and among meadows, possibly due to high competition with vegetative shoots in the dense shallow margin. Understanding the processes that shape genetic diversity in seagrass populations is key to their conservation and management; our study highlights the challenges of inferring the contribution of sexual reproduction to population structure using a single metric or life history stage.