Population Genomics and Lagrangian Modeling Shed Light on Dispersal Events in the Mediterranean Endemic Ericaria zosteroides (=Cystoseira zosteroides) (Fucales)

Abstract

Dispersal is a central process that affects population growth, gene flow, and ultimately species persistence. Here we investigate the extent to which gene flow occurs between fragmented populations of the deep-water brown algae Ericaria zosteroides (Turner) Greville (Sargassaceae, Fucales). These investigations were performed at different spatial scales from the bay of Marseille (western Provence) to Corsica. As dispersal of zygotes is shown to be limited over distances beyond a few meters, we used a multidisciplinary approach, based on Lagrangian modeling and population genomics to test the hypothesis that drifting of fertile parts of thallus (eggs on fertile branches), mediated by ocean currents, enable occasional gene flow between populations. Therefore we assessed the respective contribution of oceanographic connectivity, geographical isolation, and seawater temperatures to the genetic structure of this species. The genetic structure was assessed using 10,755 neutral SNPs and 12 outlier SNPs genotyped by dd-RAD sequencing in 261 individuals of E. zosteroides. We find that oceanographic connectivity is the best predictor of genetic structure, while differentiation in outlier SNPs can be explained by the depth of populations, as emphasized by the minimum seawater temperature predictor. However, further investigations will be necessary for clarifying how depth drives adaptive genetic differentiation in E. zosteroides. Our analyses revealed that local hydrodynamic conditions are correlated with the very high divergence of one population in the Bay of Marseille. Overall, the levels of gene flow mediated by drifting were certainly not sufficient to counteract differentiation by local genetic drift, but enough to allow colonization several kilometers away. This study stresses the need to consider secondary dispersal mechanisms of presumed low dispersal marine species to improve inference of population connectivity.

Comparative population genetics of habitat-forming octocorals in two marine protected areas: eco-evolutionary and management implications

“…In C. rubrum, the clustering analyses supported only one primary discontinuity between the northern part of Cap de Creus and the remaining sampling sites. A formal characterization of these barriers would require dispersal modelling through Lagrangian simulations (e.g., Reynes et al 2021). It is however noteworthy that the only barrier shared between the two species is separating the Northern and Southern parts of Cap de Creus, which is in line with the highly contrasting oceanological conditions in these areas.…”

Population dynamics of Ericaria zosteroides (Ochrophyta, Fucales) in the central Mediterranean

“…Moreover, the dispersal of E. zosteroides zygotes is very reduced (10 m) (Capdevila et al, 2018) and this limit the gene flow between populations. For this reason, E. zosteroides populations are genetically differentiated from each other, even at short distances (Reynes et al, 2021).…”

Thermal vulnerability of the Levantine endemic and endangered habitat-forming macroalga, Gongolaria rayssiae: implications for reef carbon

“…To quantify seasonal patterns of this macroalga, from January 2018 to June 2020, we marked ten 50x50cm 2 quadrats, 2-3 meters apart from one another and deployed in a circle. The quadrats were visited monthly by SCUBA or snorkeling to measure the density of thalli per m 2 (Reynes et al, 2021), their individual lengths (Åberg, 1990) and to take photos for percent cover analysis. Photoquadrats were processed in CoralNet ® , a repository and online annotation tool for benthic survey images (Beijbom et al, 2015), which allowed us to quantify the monthly percent cover of G. rayssiae.…”

“…We decided to continue following the quadrats and counting the number of individuals in each quadrat to have monthly density data for the whole of 2019 (except October due to sea conditions). The population dynamics was described considering the specimens spotted within the quadrats and by measuring the maximum thallus length directly in the field with a ruler (from the holdfast to the top of the longest branchlet) (Medrano et al, 2020;Valdazo et al, 2020;Reynes et al, 2021). Population size structure was defined with the characterization of six different size classes as follows: 1 ≤ 5, 2 = 5.…”