Single-cell transcriptomics highlights sexual cues among reproductive life stages of uncultivated Acantharia (Radiolaria)

As an innate property of life, the ability to reproduce is a key process for the perpetuation of organisms. Along the evolution of protist reproductive strategies, the molecular machinery of sexual recombination is estimated to have been inherited from the last eukaryotic common ancestor (LECA). Nevertheless, unraveling the sexual cycles of extant free-living protist lineages remains challenging, given the enigmatic roles of many uncultivated life stages. Among the uncultivated planktonic group of Acantharia (Radiolaria), a hypothetical sexual cycle has been proposed since the late 19th century, including the existence of a gamete-like life stage of undetermined ploidy, referred to as swarmers. In order to investigate the sexual nature of acantharian reproductive stages, we conducted single-cell transcriptomic analysis across various acantharian life stages. Our results show distinct functional profiles for reproductive and vegetative life stages, while revealing the expression of the reference eukaryotic genes involved in gamete fusion, HAP2/GCS1 and GEX1-KAR5, in swarmers and pre-swarmer stages. Annotation of differentially expressed life stage-specific genes, also highlights putative meiosis-related functions among swarmers, while suggesting the existence of a potential swarmer/vegetative intermediate stage expressing putative growth-related genes. This original life stage-specific genetic data is coherent with morphological evidence supporting the existence of an acantharian sexual cycle, with swarmers acting as gametes. Moreover, it paves the way for a deeper understanding of radiolarian cell biology and ecology at a single-cell scale.