Genetic homogeneity, lack of larvae recruitment, and clonality in absence of females across western Mediterranean populations of the starfish Coscinasterias tenuispina

Abstract

We here analysed the populations’ genetic structure of Coscinasterias tenuispina, an Atlantic-Mediterranean fissiparous starfish, focusing on the western Mediterranean, to investigate: the distribution and prevalence of genetic variants, the relative importance of asexual reproduction, connectivity across the Atlantic-Mediterranean transition, and the potential recent colonisation of the Mediterranean Sea. Individuals from 11 Atlantic-Mediterranean populations of a previous study added to 172 new samples from five new W Mediterranean sites. Individuals were genotyped at 12 microsatellite loci and their gonads histologically analysed for sex determination. Additionally, four populations were genotyped at two-time points. Results demonstrated genetic homogeneity and low clonal richness within the W Mediterranean, due to the dominance of a superclone, but large genetic divergence with adjacent areas. The lack of new genotypes recruitment over time, and the absence of females, confirmed that W Mediterranean populations were exclusively maintained by fission and reinforced the idea of its recent colonization. The existence of different environmental conditions among basins and/or density-depend processes could explain this lack of recruitment from distant areas. The positive correlation between clonal richness and heterozygote excess suggests that most genetic diversity is retained within individuals in the form of heterozygosity in clonal populations, which might increase their resilience.

Exceptional population genomic homogeneity in the black brittle star Ophiocomina nigra (Ophiuroidea, Echinodermata) along the Atlantic-Mediterranean coast

“…This asymmetric migration pattern reveals a disruptive effect of the Gibraltar Strait or the Almería-Oran front on gene flow. Therefore, the role of the Alboran Sea as a transitional area connecting these two major basins, the Mediterranean Sea and the Atlantic Ocean 3,9,15,22 , is also found in O. nigra.…”

“… 3 , 7 , 18 ), including other brittle stars 18 . Such a pattern of panmixia, or low genetic divergence, between the Mediterranean and Atlantic basins, or within the Mediterranean sub-basins is relatively common in echinoderms with large dispersal potential 15 , 21 , 59 and in those with a predominance of asexual reproduction 9 , 17 . Nevertheless, the absence of divergence found in O. nigra between the Northeast Atlantic and the Mediterranean is an exceptional pattern even for species with large dispersal potential 12 , 14 , 15 , 21 , 59 .…”

“…These two barriers delimit the Alboran Sea, a mixing-water zone between the warmer and saltier Mediterranean Sea and the lower-salinity and colder Atlantic Ocean 8 . Depending on the species, these marine barriers have different effects on the connectivity and gene flow patterns between Atlantic and Mediterranean populations 6,7,[9][10][11][12][13][14] , thus providing different disruptive effects of these two oceanographic barriers 3,9,10,12,13,[15][16][17][18] . Genetic diversity patterns highlighted that large-and short-scale oceanographic circulation interplays with each species life history traits and its evolutionary history to determine its divergence level 3,6,19,20 .…”

“…Although an array of studies on population genetics, phylogeography and speciation have been performed along the Atlantic-Mediterranean coast [9][10][11][12][13][14][15][16][17][21][22][23][24] , some animal groups, such as the ophiuroids (Ophiouroidea, Echinodermata) have been less studied. Ophiuroids, commonly known as brittle stars, are the most diverse echinoderm class, with important ecological roles in marine benthic food webs that occupy a large range of ecological and trophic niches in all oceans around the world [25][26][27] .…”