The Ectocarpus UV system has clearly had a distinct evolutionary trajectory not only to the well-studied XY and ZW systems but also to the UV systems described so far. Nonetheless, some striking similarities exist, indicating remarkable universality of the underlying processes shaping sex chromosome evolution across distant lineages.

The extent of changes in genetic diversity and life-history traits associated with farming was investigated in the haploid-diploid red alga, Gracilaria chilensis, cultivated in Chile. This alga belongs to one of the most frequently cultivated seaweed genera around the world. Fifteen farmed populations, 11 wild populations, and two subspontaneous populations were sampled along the Chilean coast. The frequency of reproductive versus vegetative individuals and of haploid versus diploid individuals was checked in each population. In addition, the distribution of genetic variation in wild and cultivated populations was analyzed using six microsatellite markers. Our results first demonstrated that farmed populations are maintained almost exclusively by vegetative propagation. Moreover, the predominance of diploid individuals in farms showed that farming practices had significantly modified life-history traits as compared to wild populations. Second, the expected reduction in genetic diversity due to a cultivation bottleneck and subsequent clonal propagation was detected in farms. Finally, our study suggested that cultural practices in the southern part of the country contributed to the spread of selected genotypes at a local scale. Altogether, these results document for the first time that involuntary selection could operate during the first step of domestication in a marine plant.

In this study, we compared the genotypes obtained at a microsatellite locus using two methods of amplification and detection of variation in a set of individuals belonging to the red alga haplo‐diploid species, Gracilaria gracilis. The methods varied in their capacity to detect longer alleles in heterozygotes, resulting in an apparent heterozygote deficiency. We attributed this bias in favour of short alleles to competition leading to the preferential amplification of shorter alleles (short allele dominance). To test this hypothesis, we created artificial heterozygotes (mixtures of two haploid DNA samples) and showed that long alleles already less intense than short alleles, ‘suffer’ more from being in association.

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Connectivity among populations determines the dynamics and evolution of populations, and its assessment is essential in ecology in general and in conservation biology in particular. The robust basis of any ecological study is the accurate delimitation of evolutionary units, such as populations, metapopulations and species. Yet a disconnect still persists between the work of taxonomists describing species as working hypotheses and the use of species delimitation by molecular ecologists interested in describing patterns of gene flow. This problem is particularly acute in the marine environment where the inventory of biodiversity is relatively delayed, while for the past two decades, molecular studies have shown a high prevalence of cryptic species. In this study, we illustrate, based on marine case studies, how the failure to recognize boundaries of evolutionary-relevant unit leads to heavily biased estimates of connectivity. We review the conceptual framework within which species delimitation can be formalized as falsifiable hypotheses and show how connectivity studies can feed integrative taxonomic work and vice versa. Finally, we suggest strategies for spatial, temporal and phylogenetic sampling to reduce the probability of inadequately delimiting evolutionary units when engaging in connectivity studies.

Understanding how abiotic factors influence the spatial distribution of genetic variation provides insight into microevolutionary processes. The intertidal seascape is characterized by highly heterogeneous habitats which probably influence the partitioning of genetic variation at very small scales. The effects of tidal height on genetic variation in both the haploid (gametophytes) and diploid (tetrasporophytes) stages of the red alga Chondrus crispus were studied. Fronds were sampled every 25 cm within a 5 m × 5 m grid and along a 90-m transect at two shore heights (high and low) in one intertidal site in France. The multilocus genotype of 799 fronds was determined (Nhaploid  = 586; Ndiploid  = 213) using eight microsatellite loci to test the following hypotheses: (i) high and low shore fronds belong to genetically differentiated populations, (ii) gene flow is restricted within the high shore habitat due to tidal-influenced isolation and (iii) significant FIS values are driven by life history characteristics. Pairwise FST estimates between high and low shore levels supported the hypothesis that high and low shore fronds were genetically differentiated. The high shore was characterized by the occurrence of within-shore genetic differentiation, reduced genetic diversity and increased levels of intergametophytic selfing, suggesting it is a marginal environment. These results suggest at fine scales within the intertidal seascape the same mechanisms as those over the species' distributional range are at work with core and marginal population dynamics.

Laminaria digitata is the dominant species of the dense, continuous kelp stands in the English Channel and on the Atlantic coasts of France, where it is harvested for its high quality alginates. However, in spite of its ecological and economic importance, our knowledge of the level and organisation of genetic diversity in this species is scant. Here, using comprehensive hierarchical sampling and 7 microsatellite loci, we explored the roles of dispersal strategies, current regimes and habitat discontinuities in shaping genetic structure of L. digitata populations. Our results show that continuous, non-fragmented forests of L. digitata were genetically differentiated at distances greater than 10 km, despite the absence of clear population boundaries. Furthermore, a pattern of isolationby-distance indicated that gene flow occurred preferentially among adjacent populations following a stepping-stone model. In addition, we analysed the direction of migration using assignment tests and found that currents appeared to play a minor role in orienting gene flow, except in the Gulf of Saint Malo gyre. In contrast, habitat discontinuities were found to accentuate genetic differentiation and resulted in reduced genetic variation of isolated stands. In the context of a potential over-exploitation of kelp stands in Brittany, this study suggests that the existence of neighbouring populations can be vital to maintaining high levels of gene flow and thus, genetic diversity in this species.

The impact of haploid-diploidy and the intertidal landscape on a fine-scale genetic structure was explored in a red seaweed Gracilaria gracilis. The pattern of genetic structure was compared in haploid and diploid stages at a microgeographic scale (o5 km): a total of 280 haploid and 296 diploid individuals located in six discrete, scattered rock pools were genotyped using seven microsatellite loci. Contrary to the theoretical expectation of predominantly endogamous mating systems in haploid-diploid organisms, G. gracilis showed a clearly allogamous mating system. Although withinpopulation allele frequencies were similar between haploids and diploids, genetic differentiation among haploids was more than twice that of diploids, suggesting that there may be a lag between migration and (local) breeding due to the long generation times in G. gracilis. Weak, but significant, population differentiation was detected in both haploids and diploids and varied with landscape features, and not with geographic distance. Using an assignment test, we establish that effective migration rates varied according to height on the shore. In this intertidal species, biased spore dispersal may occur during the transport of spores and gametes at low tide when small streams flow from high-to lower-shore pools. The longevity of both haploid and diploid free-living stages and the long generation times typical of G. gracilis populations may promote the observed pattern of high genetic diversity within populations relative to that among populations.