A CLONAL LEGACY? REPRODUCTIVE MODE VARIATION IN HARD AND SOFT BOTTOM GRACILARIA VERMICULOPHYLLA POPULATIONS

The red macroalga Gracilaria vermiculophylla invasion provides an opportunity to investigate the evolution of biphasic life cycles and of reproductive modes by understanding how they structure and contribute colonizing new environments in natural conditions. In hard bottom habitats, we find gametophytes and tetrasporophytes fixed by holdfasts to hard substrates, whereas in soft bottom habitats, we find 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 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 and was accompanied by tetrasporophytic dominance. There was site-specific variation in selfing and clonal rates that are supported by observations of physiological stress and local extirpation, such as at Ape Hole Creek and Fowling Point, respectively. We found evidence of isolation by distance and the structuring of genetic diversity by habitat type, then site, and finally by year. While broad patterns have been described across the extant range, we clarify population genetic patterns in hard versus soft bottom habitats that are not confounded by the invasion history comparing native and non-native thalli. These results have implications for the on-going spread of this alga and contribute to our understanding of the population genetics of partially clonal taxa. This is a table of supplemental data affiliiated with our manuscript. https://doi.org/10.1101/2025.06.19.660572 on biorxiv and submitted to the Journal of Phycology We thank R. Snyder, S. Fate, P.G. Ross, J. Lewis, and E. Smith for field help and logistics at the Virginia Institute of Marine Science Eastern Shore Laboratory (VIMS ESL). We thank B. Thornton for help with July 2022 collections. We thank C. Amsler, J. McClintock, S. Watts, and R. Snyder for serving on for serving on the dissertation committee of A.P. Oetterer. This work was supported by an International Phycological Society Paul Silva Student Grant (to APO), start-up funds from the University of Alabama at Birmingham (UAB) College of Arts and Science (to SAKH), an NSF CAREER award (DEB-2141971 [UAB] and DEB-2436117 [VIMS|W&M] to SAKH), NSF OIA-1946412 (to SAKH), ANR-18-CE32-0001 (to SS and SAKH), and start-up funds from the Virginia Institute of Marine Science William & Mary (to SAKH). The Department of Biology at UAB provided logistical support. VIMS and VIMS ESL provided additional facilities and logistical support. A.P. Oetterer was supported by the UAB Blazer Fellowship. WHR was supported by the NIH IRACDA MERIT postdoctoral fellowship (NIH K12GM088010). S.A. Krueger-Hadfield was supported by the NSF (CAREER Award DEB-2141971 and EAGER DEB-2113745).