Sexual hybridization experiments and phylogenetic relationships as inferred from Rubisco spacer sequences in the genus Alaria (Phaeophyceae)

Abstract

Alaria (Alariaceae, Phaeophyceae) is a common genus of kelps generally found in the lower intertidal and shallow subtidal regions of rocky shores subject to strong wave exposure. Fourteen species are currently recognized, of which 11 are found in the coldtemperate North Pacific Ocean. Alaria esculenta (L.) Greville, the type species described from the North Atlantic, exhibits a range of biogeographically correlated morphotypes indicating the possibility of multiple species, subspecies, and/or hybrids. This has led to an unstable taxonomy. We compared five species from the Atlantic and Pacific, including six North Atlantic isolates of A. esculenta. Phylogenetic analyses based on Rubisco spacer sequences resulted in a well-resolved topology of these five species, but did not distinguish between the six biogeographic isolates of A. esculenta. Laboratory hybridization experiments among four A. esculenta isolates showed partial intrafertility. Among five tested Alaria species, interfertility as well as fertility barriers were encountered, inconsistent with reproductive isolation. The data reject both a biological and morphological species concept and support only a phylogenetic species concept for Alaria, demonstrating that morphological variation has evolved independently of molecular variation in the genes under investigation in the species of the genus Alaria.