The Consequences Of Partial Clonality On The Spatiotemporal Genetic Structure Of The Red Alga Chondria Tumulosa (Ceramiales, Rhodophyta)

The relative rates of sexual versus clonal reproduction partition genetic diversity but the quantification of these rates remains limited in natural populations. This knowledge gap is particularly acute among tropical macroalgae. Chondria tumulosa is a red macroalga that has been documented at high abundance in the three northernmost atolls in the Papah[a]naumoku[a]kea Marine National Monument. We sampled thalli at Manawai in 2021 and 2023, at Kuaihelani in 2022 and 2023, and at H[o]lanik[u] in 2023 to characterize the reproductive mode and patterns of gene flow. Tetrasporophytes overwhelmingly dominated at all sites, atolls, and years, although eight cystocarpic female gametophytes (<1%) were documented for the first time. We observed evidence of very high clonal rates, accompanied by an increase in homozygosity. Surprisingly, our temporal analyses suggested site-specific trends in the predominance of selfing, outcrossing, or clonality. A very small haploid proportion combined with high genetic drift likely rendered temporal analyses of genotypic frequencies challenging to implement and interpret whereby point estimates were a better reflection of reproductive mode variation. Genetic differentiation increased with geographic distance, with strong temporal and spatial structure. The patterns of genetic diversity and structure combined with the dominance of tetrasporophytes suggest that C. tumulosa thalli were introduced to this region, possibly via marine debris. Future studies should focus on identifying the native range to document the ecological and evolutionary shifts that have facilitated the rapid increase and spread of C. tumulosa and its potential similarities to other invasive haploid-diploid algae.