¹Laboratoire de Génétique et Evolution des Populations Végétales, CNRS UPRESA 8016, Bât. SN2, Université de Lille I, Villeneuve d'Ascq 59655, France

Correspondence: M Valero, Laboratoire de Génétique et Evolution des Populations Végétales, CNRS UPRESA 8016, Bât. SN2, Université de Lille I, Villeneuve d'Ascq 59655, France. E-mail: valero@sb-roscoff.fr

²Current address: Evolution et Génétique des Populations Marines (EGPM), UMR CNRS-UPMC 7127, Station Biologique de Roscoff, BP 74, Place Georges Teissier, 29680 Roscoff Cedex, France.

Received 29 November 2002; Accepted 10 September 2003; Published online 17 December 2003.

Abstract

The impact of haploid–diploidy and the intertidal landscape on a fine-scale genetic structure was explored in a red seaweed Gracilaria gracilis. The pattern of genetic structure was compared in haploid and diploid stages at a microgeographic scale (<5 km): a total of 280 haploid and 296 diploid individuals located in six discrete, scattered rock pools were genotyped using seven microsatellite loci. Contrary to the theoretical expectation of predominantly endogamous mating systems in haploid–diploid organisms, G. gracilis showed a clearly allogamous mating system. Although within-population allele frequencies were similar between haploids and diploids, genetic differentiation among haploids was more than twice that of diploids, suggesting that there may be a lag between migration and (local) breeding due to the long generation times in G. gracilis. Weak, but significant, population differentiation was detected in both haploids and diploids and varied with landscape features, and not with geographic distance. Using an assignment test, we establish that effective migration rates varied according to height on the shore. In this intertidal species, biased spore dispersal may occur during the transport of spores and gametes at low tide when small streams flow from high- to lower-shore pools. The longevity of both haploid and diploid free-living stages and the long generation times typical of G. gracilis populations may promote the observed pattern of high genetic diversity within populations relative to that among populations.