Does Hybridization Increase Evolutionary Rate? Data from the 28S-rDNA D8 Domain in Echinoderms

Influence of the larval phase on connectivity: strong differences in the genetic structure of brooders and broadcasters in the Ophioderma longicauda species complex

Closely related species with divergent life history traits are excellent models to infer the role of such traits in genetic diversity and connectivity. Ophioderma longicauda is a brittle star species complex composed of different genetic clusters, including brooders and broadcasters. These species diverged very recently and some of them are sympatric and ecologically syntopic, making them particularly suitable to study the consequences of their trait differences. At the scale of the geographic distribution of the broadcasters (Mediterranean Sea and northeastern Atlantic), we sequenced the mitochondrial marker COI and genotyped an intron (i51) for 788 individuals. In addition, we sequenced 10 nuclear loci newly developed from transcriptome sequences, for six sympatric populations of brooders and broadcasters from Greece. At the large scale, we found a high genetic structure within the brooders (COI: 0.07 < F(ST) < 0.65) and no polymorphism at the nuclear locus i51. In contrast, the broadcasters displayed lower genetic structure (0 < F(ST) < 0.14) and were polymorphic at locus i51. At the regional scale, the multilocus analysis confirmed the contrasting genetic structure between species, with no structure in the broadcasters (global F(ST) < 0.001) and strong structure in the brooders (global F(ST) = 0.49), and revealed a higher genetic diversity in broadcasters. Our study showed that the lecithotrophic larval stage allows on average a 50-fold increase in migration rates, a 280-fold increase in effective size and a threefold to fourfold increase in genetic diversity. Our work, investigating complementary genetic markers on sympatric and syntopic taxa, highlights the strong impact of the larval phase on connectivity and genetic diversity.

Differential reproductive timing in Echinocardium spp.: the first Mediterranean survey allows interoceanic and interspecific comparisons

Echinocardium cordatum had long been considered as cosmopolitan, but molecular data revealed it is a complex of cryptic species, with two non-hybridizing species (B1 & B2) in the Mediterranean Sea living in syntopy with Echinocardium mediterraneum. Histological analyses of the gonads from a 17-month sampling period revealed a statistically significant time lag between the Maturity Indices of E. cordatum and E. mediterraneum. The main environmental stimulus may be different for the two nominal species, possibly seawater temperature for E. cordatum and chlorophyll a concentration for E. mediterraneum. Within the E. cordatum complex, spawning timing and synchrony are different according to major geographic areas (Atlantic/Pacific/Mediterranean) and/or the corresponding genetic subdivision [A/P/(B1 & B2)]. In contrast, the effects of temperature on the reproductive cycle seem rather to mirror the genetic lineages than environmental similarities of the different localities. Between the sister species (B1 & B2) no differences could be detected, maybe due to small sample sizes.