Within-island differentiation and between-island homogeneity: non-equilibrium population structure in the seaweed Cladophoropsis membranacea (Chlorophyta) in the Canary Islands

Abstract

The seaweed Cladophoropsis membranacea forms extensive mats of coalesced thalli on coral reefs and along rocky coastlines throughout the tropics and subtropics. In a previous study on Gran Canaria, small-scale dispersal distances were found to be limited to < 5 m and significant differentiation (F ST) was detectable at distances as small as 5 km. Such strong, small- scale differentiation led to the prediction that strong isolation by distance (IBD) would be found under a stepping stone model at larger spatial scales. In the present survey, 23 sites were sampled in the Canary Islands and one in the Cape Verde Islands. Using eight microsatellite loci analysed in an AMOVA framework, we determined that approximately 75% of the variation occurred within sites and approximately 25% between sites separated by 1–125 km. In a three-level AMOVA, only 6% of the variation was accounted for between islands (≈100–300 km). Moderately strong IBD was found within islands and Mantel tests revealed significant correlation for Gran Canaria and Tenerife but not for Fuerteventura. In contrast, there was no detectable IBD among the Canary Islands regardless of how geographic distances were computed. Only when the Canary Islands were compared with the Cape Verde Islands was strong IBD detected. Our seemingly paradoxical results of strong differentiation and IBD at small distances and weak to absent IBD at large distances reflect non-equilibrium conditions. In addition, the wide scatter of points we observed over all degrees of geographic separation is consistent with isolation in which drift dominates over gene flow. The lack of equilibrium in present-day populations of C. membranacea is probably mainly due to the fact that they are only thousands or even hundreds of years old. Therefore, population structure should be interpreted in terms of history rather than gene flow.