The concept of population in clonal organisms: mosaics of temporally colonized patches are forming highly diverse meadows of Zostera marina in Brittany

Abstract

Seagrasses structure some of the world's key coastal ecosystems presently in decline due to human activities and global change. The ability to cope with environmental changes and the possibilities for shifts in distribution range depend largely on their evolvability and dispersal potential. As large-scale data usually show strong genetic structure for seagrasses, finer-grained work is needed to understand the local processes of dispersal, recruitment and colonization that could explain the apparent lack of exchange across large distances. We aimed to assess the fine-grained genetic structure of one of the most important and widely distributed seagrasses, Zostera marina, from seven meadows in Brittany, France. Both classic population genetics and network analysis confirmed a pattern of spatial segregation of polymorphism at both regional and local scales. One location exhibiting exclusively the variety 'angustifolia' did not appear more differentiated than the others, but instead showed a central position in the network analysis, confirming the status of this variety as an ecotype. This phenotypic diversity and the high allelic richness at nine microsatellites (2.33-9.67 alleles/locus) compared to levels previously reported across the distribution range, points to Brittany as a centre of diversity for Z. marina at both genetic and phenotypic levels. Despite dispersal potential of several 100 m, a significant pattern of genetic differentiation, even at fine-grained scale, revealed 'genetic patchiness'. Meadows seem to be composed of a mosaic of clones with distinct origins in space and time, a result that calls into question the accuracy of the concept of populations for such partially clonal species.

Scaling of processes shaping the clonal dynamics and genetic mosaic of seagrasses through temporal genetic monitoring

“…Using PCR conditions identical to those used by Becheler et al (2010), the nine microsatellite loci used in 2009 were amplified in the present study. PCR products were visualized using the same ABI-3100 FNVR automated sequencer (Applied Biosystems, Life Technologies, SaintAubin, France), and scored using the microsatellite plugin of Geneious v5.6.4 (Biomatters, Auckland, New Zealand).…”

“…Following the same sampling scheme and strategy as in Becheler et al (2010), sampling units of Z. marina were collected in Spring 2009 and Spring 2012 from seven locations in Brittany, France (Supplementary Figure S1), stretching from Saint-Malo to Arradon. Distances between meadows ranged from 33 km (Molène-Roscanvel) to 442 km (Arradon-Saint-Malo).…”

“…Clonal diversity was estimated with three parameters, previously used in Becheler et al (2010) and recommended in Arnaud-Haond et al (2007a), as this set fully describes richness and diversity of the clonal composition. R is the clonal richness, estimated as the ratio of the number of discriminated genets within the sampling set with the number of sampling units (Dorken and Eckert, 2001).…”

“…The extreme and most demonstrative cases of ISR dominance are the monoclonal meadows of some seagrass species, including Z. marina (Reusch et al, 1999;Olsen et al, 2004), C. nodosa (Alberto et al, 2008) or P. oceanica (Arnaud-Haond et al, 2007b. Meadows of the seagrass Z. marina surveyed simultaneously for their demographic evolution and genetic composition in Brittany were shown to be fluctuating mosaics of genetically differentiated patches with variable levels of genotypic and genetic diversity (Becheler et al, 2010). The clonal and genetic composition of meadows suggested that both ISR and RSR strategies may apply in variable proportions as a function of the time elapsed since the last colonization and of the frequency and extent of disturbances (Becheler et al, 2010).…”

“…Meadows of the seagrass Z. marina surveyed simultaneously for their demographic evolution and genetic composition in Brittany were shown to be fluctuating mosaics of genetically differentiated patches with variable levels of genotypic and genetic diversity (Becheler et al, 2010). The clonal and genetic composition of meadows suggested that both ISR and RSR strategies may apply in variable proportions as a function of the time elapsed since the last colonization and of the frequency and extent of disturbances (Becheler et al, 2010). However, the thorough appraisal of the dynamics of these genetic mosaics and of the recruitment strategies operating at different spatial and temporal scales in natural meadows cannot be inferred from a single instantaneous snapshot.…”