  Seagrass meadows are among the most important coastal ecosystems, in terms of
both spatial extent and ecosystem services, but they are also declining
worldwide. Understanding the drivers of seagrass meadow dynamics is essential
for designing sound management, conservation, and restoration strategies.
However, the poor knowledge of the effect of clonality on the population
genetics of natural populations severely limits our understanding of the
dynamics and connectivity of meadows. Recent modelling approaches have
described the expected distributions of genotypic and genetic descriptors under
increasing clonal rates. Here, in light of these recent theoretical
developments, we revisited population genetic data for 165 meadows of four
seagrass species. Contrasting shoot life span and rhizome turnover led to the
prediction that the influence of asexual reproduction will increase along a
gradient from Zostera noltii to Zostera marina, Cymodocea nodosa and Posidonia
oceanica, which should be reflected by an increasing departure from
Hardy-Weinberg equilibrium (Fis) and decreasing genotypic diversity (R). This
meta-analysis provides a nested validation of this hypothesis at both the
species and meadow scales through a significant relationship between Fis and R
within each species. By empirically demonstrating the theoretical expectations
derived from recent modelling approaches, this work calls for the use of
Hardy-Weinberg equilibrium (Fis) rather than the strongly sampling-sensitive
genotypic index R to assess the importance of clonal reproduction, at least
when the impact of selfing on Fis can be neglected. The results also emphasize
the need to revise our appraisal of the extent of clonality and its influence
on the dynamics, connectivity and evolutionary trajectory of partial asexuals
in general, including in seagrass meadows, to develop the most accurate
management strategies.
