The Architecture of Sexual Systems in Mosses and its Ecological and Evolutionary Consequences

Sexual systems in mosses depend on how antheridia and archegonia are arranged in the gametophyte, and this organization directly shapes how fertilization takes place. In this review, we bring together current knowledge and recent conceptual advances on the origin, function, and evolution of sexual systems in mosses. We first place moss reproduction in the broader context of land plant evolution, showing how gametophyte dominance, dependence on free water for fertilization, and strong desiccation tolerance influence life histories. We then survey the diversity of sexual systems, from dioicy to several forms of monoicy, and examine how the spatial distribution of antheridia and archegonia constrains fertilization processes, sexual expression, and mating opportunities. Empirical studies consistently report female-biased sex ratios, low male sex expression, and strong environmental control of phenology, particularly in dioicous species. Recent work also shows that reproductive allocation and sexual architecture covary, with higher investment in male function in systems that favour cross-fertilization over longer distances, such as rhizautoicy. Using the genus Fissidens as a case study, we illustrate how phylogenomic analyses can be related to habitat features, genomic evolution, and functional traits. We also discuss how climate change is likely to affect different sexual systems in distinct ways, with dioicous species and some monoicous architectures showing lower resilience and greater projected niche loss, whereas rhizautoicous systems appear less impacted among monoicous systems. Finally, we highlight key gaps in our understanding of developmental genetics, physiological costs, and macroevolutionary dynamics of sexual systems, and outline research directions to connect processes at the scale of individual buds to global patterns of biodiversity.