Genetic and morphological diversity in sympatric kelps with contrasting reproductive strategies

The evolution of asexual reproduction is considered a response to environmental conditions where it incurs less cost than sexual reproduction, maintains adapted genotypes and allows rapid proliferation into new areas. In rare circumstances, some species have evolved distinct asexual morphs or lineages in response to ubiquitous environmental conditions. Understanding the implications of, and the mechanisms underpinning, such reproductive strategies will be important for assessing the vulnerability of populations to environmental change. We examined morphological and genetic variation between 2 morphs of the kelp Ecklonia radiata, with sympatric haplodiplontic and vegetatively reproducing individuals growing side by side in Western Australia. Using 6 microsatellite markers, we show that vegetative morphs had a great propensity for asexual reproduction, with all attached haptera (8-20 per plant) being genetically identical to their parent plant. Moreover, for 8 multilocus genotypes (MLGs), Psex (probability that each MLG had clonal origins) scores were significant, suggesting clonal origins, and members of these MLGs were overwhelmingly dominated by individuals of the vegetative morph. Vegetative morphs were morphologically distinct, less morphologically variable, had lower genetic diversity and an excess of heterozygotes relative to haplodiplontic morphs. Nevertheless, vegetative morphs still produced and released zoospores at the same densities as haplodiplontic individuals, suggesting that they still complete an alternation of generations life history strategy. This likely accounted for weak genetic differentiation between morphs and suggests ongoing gene flow. Given that genetic diversity often confers adaptive capacity through change, low diversity may have implications for the vulnerability of this unique vegetative morph to local climatic and environmental stressors.