Understanding the genetic composition and mating systems of edge populations provides important insights into the environmental and demographic factors shaping species’ distribution ranges. We analysed samples of the mangrove Avicennia marina from Vietnam, northern Philippines and Australia, with microsatellite markers. We compared genetic diversity and structure in edge (Southeast Asia, and Southern Australia) and core (North and Eastern Australia) populations, and also compared our results with previously published data from core and southern edge populations. Comparisons highlighted significantly reduced gene diversity and higher genetic structure in both margins compared to core populations, which can be attributed to very low effective population size, pollinator scarcity and high environmental pressure at distribution margins. The estimated level of inbreeding was significantly higher in northeastern populations compared to core and southern populations. This suggests that despite the high genetic load usually associated with inbreeding, inbreeding or even selfing may be advantageous in margin habitats due to the possible advantages of reproductive assurance, or local adaptation. The very high level of genetic structure and inbreeding show that populations of A. marina are functioning as independent evolutionary units more than as components of a metapopulation system connected by gene flow. The combinations of those characteristics make these peripheral populations likely to develop local adaptations and therefore to be of particular interest for conservation strategies as well as for adaptation to possible future environmental changes.

Aim  A central question in evolutionary ecology is the nature of environmental barriers that can limit gene flow and induce population genetic divergence, a first step towards speciation. Here we study the geographical barrier constituted by the transition zone between the Atlantic Ocean and the Mediterranean Sea, using as our model Cymodocea nodosa, a seagrass distributed throughout the Mediterranean and in the Atlantic, from central Portugal to Mauritania. We also test predictions about the genetic footprints of Pleistocene glaciations.Location  The Atlantic–Mediterranean transition region and adjacent areas in the Atlantic (Mauritania to south‐west Portugal) and the Mediterranean.Methods  We used eight microsatellite markers to compare 20 seagrass meadows in the Atlantic and 27 meadows in the Mediterranean, focusing on the transition between these basins.Results  Populations from these two regions form coherent groups containing several unique, high‐frequency alleles for the Atlantic and for the Mediterranean, with some admixture west of the Almeria–Oran Front (Portugal, south‐west Spain and Morocco). These are populations where only one or a few genotypes were found, for all but Cadiz, but remarkably still show the footprint of a contact zone. This extremely low genotypic richness at the Atlantic northern edge contrasts with the high values (low clonality) at the Atlantic southern edge and in most of the Mediterranean. The most divergent populations are those at the higher temperature range limits: the southernmost Atlantic populations and the easternmost Mediterranean, both potential footprints of vicariance.Main conclusions  A biogeographical transition region occurs close to the Almeria–Oran front. A secondary contact zone in Atlantic Iberia and Morocco results from two distinct dispersal sources: the Mediterranean and southernmost Atlantic populations, possibly during warmer interglacial or post‐glacial periods. The presence of high‐frequency diagnostic alleles in present‐day disjunct populations from the southernmost Atlantic region indicates that their separation from all remaining populations is ancient, and suggests an old, stable rear edge.

The dwarf seagrass Zostera noltii is an important primary producer in Atlantic coastal ecosystems from Mauritania to southern Norway and the Mediterranean Sea. Sessile intertidal organisms existing at the interface between marine and terrestrial environments may be particularly vulnerable to environmental change. In this study, we asked how near to thermal tolerance limits natural populations of Z. noltii are in the Ria Formosa coastal lagoon system in southern Portugal. We recorded the maximum temperatures in the Ria Formosa during the 2007 summer, and conducted experiments to determine the sub-lethal temperature of Z. noltii shoots sampled at two sites located at different tidal heights. Mortality rates and photosynthetic performance were recorded within a range of heat shock temperatures between 35 and 41°C. Survival was recorded B37°C, while higher temperatures led to a sudden drop in photosynthetic capacity followed by mortality (shoot loss) that occurred more rapidly with increasing temperatures. At 39°C and above, the rate of shoot mortality in both sites was close to 100%, occurring between 5 and 13 days after the heat shock. Survival was ca. 95 and 90% at 35 and 37°C, respectively. From these results for Z. noltii populations in the Ria Formosa we estimated sub-lethal temperature to be approximately 38°C for Z. noltii, close to the maximum of 36°C recorded in the summer 2007. Considering predicted trajectories in the coming decades, these results raise concern as to the future viability of intertidal Z. noltii populations near the southernmost edge of their distribution.

BackgroundThe relationship between species diversity and components of ecosystem stability has been extensively studied, whilst the influence of the genetic component of biodiversity remains poorly understood. Here we manipulated both genotypic and allelic richness of the seagrass Zostera noltii, in order to explore their respective influences on the resistance of the experimental population to stress. Thus far intra-specific diversity was seldom taken into account in management plans, and restoration actions showed very low success. Information is therefore needed to understand the factors affecting resistance and resilience of populations.ResultsOur results show a positive influence of both allelic and genotypic richness on the resistance of meadows to environmental perturbations. They also show that at the low genotypic (i.e. clonal) richness levels used in prior experimental approaches, the effects of genotypic and allelic richness could not be disentangled and allelic richness was a likely hidden treatment explaining at least part of the effects hitherto attributed to genotypic richness.ConclusionsAltogether, these results emphasize the need to acknowledge and take into account the interdependency of both genotypic and allelic richness in experimental designs attempting to estimate their importance alone or in combination. A positive influence of allelic richness on resistance to perturbations, and of allelic richness combined with genotypic richness on the recovery (resilience) of the experimental populations is supported by differential mortality. These results, on the key species structuring of one of the most threatened coastal ecosystem worldwide, seagrass meadows, support the need to better take into account the distinct compartments of clonal and genetic diversity in management strategies, and in possible restoration plans in the future.

Predicted global climate change threatens the distributional ranges of species worldwide. We identified genes expressed in the intertidal seagrass Zostera noltii during recovery from a simulated low tide heat-shock exposure. Five Expressed Sequence Tag (EST) libraries were compared, corresponding to four recovery times following sub-lethal temperature stress, and a non-stressed control. We sequenced and analyzed 7009 sequence reads from 30min, 2h, 4h and 24h after the beginning of the heat-shock (AHS), and 1585 from the control library, for a total of 8594 sequence reads. Among 51 Tentative UniGenes (TUGs) exhibiting significantly different expression between libraries, 19 (37.3%) were identified as 'molecular chaperones' and were over-expressed following heat-shock, while 12 (23.5%) were 'photosynthesis TUGs' generally under-expressed in heat-shocked plants. A time course analysis of expression showed a rapid increase in expression of the molecular chaperone class, most of which were heat-shock proteins; which increased from 2 sequence reads in the control library to almost 230 in the 30min AHS library, followed by a slow decrease during further recovery. In contrast, 'photosynthesis TUGs' were under-expressed 30min AHS compared with the control library, and declined progressively with recovery time in the stress libraries, with a total of 29 sequence reads 24h AHS, compared with 125 in the control. A total of 4734 TUGs were screened for EST-Single Sequence Repeats (EST-SSRs) and 86 microsatellites were identified.