ABSTRACT: While most parasites primarily reproduce asexually, they can transition to sexual reproduction to promote genetic diversity and maintain their infectiousness. Nonetheless, documentation of sexual reproduction is seldom observed in most parasitic species. In this study, we demonstrate the existence of specialized spores dedicated to sexual reproduction in the marine protist parasite Amoebophrya, which parasitizes planktonic dinoflagellates. These sexual spores exhibit distinct characteristics compared to infective spores, including larger size, unique swimming behavior, shorter lifespan, distinct metabolite production, and an inability to infect a new host. Transcriptome analysis of these spores, isolated through cytometry, reveals a significant expression of genes associated with meiosis and DNA replication, highlighting their involvement in sexual reproduction. The production of these sexual spores exhibits intergenerational variability and remains unaffected by host populations, including factors such as density, age, and the initial host-parasite ratio. Strikingly, a given infected host cell generates one of the two spore types, indicating a predetermined mechanism that is activated before release from the host. Through implementing a decision-support hierarchical model, we identify the density of infective spores before infection, both within the culture prepared for the inoculum (n-1 generation) and within the inoculum itself, as critical explanatory variables for understanding sexual spore production. We suggest that a density-dependent signal, operating among infective free-living spores of one generation, may induce sexual reproduction during the host infection, producing sexual spores in the subsequent generation.
