High connectivity and directional gene flow in European Atlantic and Mediterranean populations of Ciona intestinalis sp. A

AbstractUnderstanding the factors that cause population divergence has long been ofinterest to marine biologists in their attempts to interpret the effect of human-mediated vectors. Broadcast-spawning species with limited dispersal capabilityare excellent candidates to measure the present-day patterns of genetic diversity.The tunicate Ciona intestinalis (Ascidiacea) is comprised of a complex ofmorphologically cryptic species that form vigorous aggregates in eutrophic habi-tats (harbors, gulfs and lagoons) where they can compete with the epibenthiccommunity and cause biofouling problems. This study investigated biogeograph-ic variability and migration patterns of C. intestinalis sp. A along NortheastAtlantic and Mediterranean coasts using microsatellite markers. Data presentedhere on 371 specimens collected from 17 populations reveal high genetic poly-morphism, but with a deficit of heterozygote deficiency. Absence of evidence forisolation by distance suggests that the genetic patterns do not reflect the geo-graphic distribution of sampled populations. Substantial gene flow and artificialpotential for dispersal boost high levels of within-population genetic variabilityand prevent genetic differentiation within and between seas. A predominant east-ward migration pattern was revealed by the data set, with very limited opportu-nity for C. intestinalis sp. A to travel westward. This directional movementindicates that other properties (e.g. habitat quality, genetic traits, mating system,life cycle) may cause adaptive divergence at a large biogeographic scale.IntroductionDispersal plays an important role in the biogeographicdistribution of marine organisms, influencing the coloni-zation of new habitats, the connectivity of populationsand ultimately species persistence (Bilton et al. 2001;Grantham et al. 2003). At sea, physical barriers are oftenabsent and water currents may accelerate larval dispersal,determining population homogeneity over large geo-graphic areas (Knutsen et al. 2003; Cowen & Sponaugle2009). In benthic invertebrates, large larval output,increased larval duration and long-distance dispersal mayallow high genetic connectivity thereby mitigating theeffect of genetic drift and inbreeding (Hedgecock 1986),and eventually leading to panmixia. Hydrologic (e.g. pat-terns of marine currents, existence of different watermasses) and ecological barriers (e.g. habitat discontinuity,larval behavior) may hinder long-distance movements(Palumbi 1994; Riginos & Nachman 2001; Bierne et al.2003). Once barriers have been established, internal (e.g.life strategy and larval survival) and external (e.g. envi-ronmental parameters) factors may favor, together with