Abstract
Maerl beds are one of the world’s key coastal ecosystems and are threatened by human activities and global change. In this study, the genetic diversity and structure of one of the major European maerl-forming species, Phymatolithon calcareum, was studied using eight microsatellite markers. Two sampling scales (global: North East Atlantic and regional: Galicia) were investigated and fifteen maerl beds from Atlantic Europe were sampled. At the regional-scale the location of sites outside and within four estuaries allowed to test for the influence of coastal configuration on population connectivity and genetic diversity. Results suggested that clonal reproduction plays an important role in the population dynamics of P. calcareum maerl beds. Clonality was variable among populations, even within the same region. At the European scale, these differences in clonality cannot be explained by the geographic or latitudinal distribution of the populations studied. A significant genetic differentiation was found among almost all population pairs and a positive correlation between geographic and genetic distances showed the limited dispersal capacity of P. calcareum. Moreover, a very clear pattern of genetic structure was revealed at the regional scale between populations located within and at the mouth of the estuaries. Genetic differentiation among estuaries was less marked for the sites located in outer-zones compared to those located in the inner-zones. In addition, variation in level of clonality linked to seascape was also observed: populations situated in the outer-zones of the estuaries were less clonal than those in the inner-zones. Finally, populations from the same estuary generally shared one or several mutilocus genotypes.
Highlights
The majority of plants involve both sexual and asexual/clonal reproduction (Halkett et al, 2005; Vallejo-Marín et al, 2010)
We aim to answer the following questions: (i) what is the level of clonality of P. calcareum populations and its variation among populations?; (ii) what is the effect of clonality on the genetic diversity and the genotypic assemblage at a large scale, and is there any geographical trend linked to latitude?; (iii) at the local scale of the NW Iberian Peninsula (Galicia), how is the seascape shaping the patterns of genetic and genotypic differentiation within and among estuaries?
Because of the existence of various cryptic species in the maerl beds of the North European (NE) Atlantic (Pardo et al, 2014a), Phymatolithon calcareum specimens cannot be identified using taxonomical criteria based on morphology
Summary
The majority of plants involve both sexual and asexual/clonal reproduction (Halkett et al, 2005; Vallejo-Marín et al, 2010). Within these populations, increasing competition among genets for space or resources through time could lead to an overwhelming dominance of one or a few clonal lineages: the more ecologically competent excluding the less fit lineage (Eriksson, 1993; Arnaud-Haond et al, 2010; Becheler et al, 2014) By combining both sexual and clonal mode of reproduction, partially asexual organisms can potentially generate new recombinant genets at each generation avoiding the accumulation of deleterious mutations while being able to maintain the best performing genotypes over time scales far exceeding sexual generation times (Otto and Lenormand, 2002; Arnaud-Haond et al, 2012). These drifting fragments (mature or vegetative) are considered as the major mechanism for long distance dispersal in macroalgae (review in Macaya et al, 2016)
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