The evolutionary ecology of reproduction in the red alga Palmaria palmata: Insights from an atypical life cycle.

Unlike other red algae, the perennial species Palmaria palmata exhibits pronounced sexual dimorphism with a microscopic female gametophytic phase that does not produce cystocarps. To investigate how these unique features of its life cycle influence its reproductive mode, we analyzed the relative proportion of macroscopic individuals (tetrasporophytes vs. male gametophytes) across multiple geographic scales and between intertidal and subtidal habitats along the western coast of France (892 individuals collected from 17 locations). Because female gametophytes are microscopic and cannot be directly sampled in the field, their frequencies were inferred under two alternative hypotheses: (H1) assuming an equal sex ratio (i.e., the proportion of females equal to males) and (H2) assuming the proportion of females equals tetrasporophytes, given that tetrasporophytes develop directly on a female gametophyte. Under H1, our results predict a demographic advantage of tetrasporophytes over gametophytes. Moreover, tetrasporophytes were significantly more abundant in intertidal than in subtidal zones, likely reflecting their greater resilience to environmental fluctuations. Conversely, under H2, the sex ratio was significantly biased toward female gametophytes and the estimated proportion of tetrasporophytes was lower than that of gametophytes. In addition, the relative contribution of self-fertilization and clonal reproduction were assessed using 11 microsatellite markers. These findings confirm that this species reproduces mainly sexually and suggest that sexual dimorphism may limit selfing and enhance the survival of the tetrasporophyte, which develops directly on the female, thereby compensating for the absence of cystocarps. Overall, our results provide valuable insights into the reproductive strategies and adaptive potential of this commercially important alga.