New insights into the population genetics of partially clonal organisms: when seagrass data meet theoretical expectations

TL;DR

This study empirically validates recent theoretical models on how clonality affects population genetics in seagrass meadows, emphasizing the importance of Hardy-Weinberg equilibrium measures over genotypic diversity indices for assessing clonal reproduction.

Contribution

It provides the first large-scale empirical validation of theoretical predictions linking clonality, genetic diversity, and Hardy-Weinberg deviations in seagrass populations.

Findings

Significant relationship between Fis and R within species.

Fis is a more reliable indicator of clonality than genotypic diversity R.

Results support theoretical models predicting increased clonality effects along a gradient.

Abstract

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.