Isolation leads to greater clonality and reduced seed production in a temperate seagrass

Many plants have complex mating systems involving sexual and asexual reproduction. Investment in different reproductive strategies can vary among sub-populations and is linked to local ecological conditions, but the key drivers are not well understood. We aimed to use direct estimates of reproductive investment (flowering and seed production), population genetic surveys and a biophysical model to assess the relationship between connectivity and the relative importance of sexual and asexual reproduction in maintaining seagrass populations. We predicted that populations with high levels of connectivity and investment in flowering and seed production will display higher levels of genotypic diversity, while more isolated populations with lower investment in flowering and seed production will display higher levels of clonality.

We combined field surveys of flowering and seed production with population genetic surveys and a biophysical dispersal model to assess reproductive effort and patterns of connectivity in the seagrass Heterozostera nigricaulis across 16 sites in a large embayment in south-eastern Australia.

Estimates of genotypic diversity varied widely between locations, ranging from highly clonal (R = 0.18) to highly diverse (R = 0.91). Genotypic diversity correlated strongly with local seed production and the inflow of propagules derived from the biophysical dispersal model (pseudo-R2 = 0.73). Sites that receive low numbers of propagules and produce few seeds were more clonal than sites with high propagule inflow and seed density.

These results show that isolated populations have higher levels of clonality and invest less in sexual reproduction. This has important consequences for the managing of declining populations of seagrass where fragmentation and loss of key source populations of propagules may lead to declines in genotypic and genetic diversity and the long-term viability of these important habitat-forming species.