A clonal legacy? Reproductive mode variation in hard- and soft-bottom Gracilaria vermiculophylla populations

The invasion of the red macroalga Gracilaria vermiculophylla provided an opportunity to investigate the influence of benthic habitats on the reproductive mode, population structure, and colonization dynamics in haploid-diploid life cycles. In hard-bottom habitats, we observed gametophytes and tetrasporophytes fixed by holdfasts to hard substrata, whereas in soft-bottom habitats, we observed free-living tetrasporophytes either drifting or anchored by tube-building polychaetes. We collected thalli from hard- and soft-bottom habitats along the Eastern Shore of Virginia and Maryland to investigate the role of substrate on life cycle and reproductive mode dynamics. We determined the phase and sex using observable reproductive structures and a sex-linked polymerase chain reaction (PCR) assay, followed by genotyping all thalli using nine microsatellite loci. Sexual reproduction prevailed in hard-bottom sites, whereas clonal (asexual) reproduction dominated soft-bottom sites, also accompanied by tetrasporophytic dominance. There was site-specific variation in selfing and clonal rates that were supported by observations of physiological stress and local extirpation, such as at Ape Hole Creek and Fowling Point, respectively. We observed evidence of isolation by distance and the structuring of genetic diversity by habitat type, then by site, and, finally, by year. Although broad patterns have been described across the extant range, we clarified population genetic patterns in hard- versus soft-bottom habitats with patterns not confounded by the invasion history comparing native and non-native thalli. These results have implications for the ongoing spread of this alga and contribute to our understanding of the population genetics of partially clonal taxa.