A conserved RWP-RK transcription factor VSR1 controls gametic differentiation in volvocine algae

Abstract

AbstractVolvocine green algae are a model for understanding the evolution of mating types and sexes. They are facultatively sexual, with gametic differentiation occurring in response to nitrogen starvation (-N) in most genera, and to sex inducer hormone (SI) inVolvox. The conserved RWP RK family transcription factor (TF) MID is encoded by theminusmating type (MT) locus or male sex-determining region (SDR) of heterothallic volvocine species and dominantly determinesminusor male gametic differentiation. However, the factor(s) responsible for establishing the defaultplusor female differentiation programs have remained elusive. We performed a phylo transcriptomic screen for autosomal RWP-RK TFs induced during gametogenesis in unicellular isogamousChlamydomonas reinhardtii(Chlamydomonas) and in multicellular oogamousVolvox carteri(Volvox) and identified a single conserved ortho-group we named Volvocine Sex Regulator 1 (VSR1). Chlamydomonasvsr1mutants of either mating type failed to mate and could not induce expression of key mating-type-specific genes. Similarly, Volvoxvsr1mutants in either sex could initiate sexual embryogenesis, but the presumptive eggs or androgonidia (sperm packet precursors) were infertile and unable to express key sex-specific genes. Yeast two-hybrid assays identified a conserved domain in VSR1 capable of self-interaction or interaction with the conserved N terminal domain of MID.In vivoco-immunoprecipitation experiments demonstrated association of VSR1 and MID in both Chlamydomonas and Volvox. These data support a new model for volvocine sexual differentiation where VSR1 homodimers activate expression ofplus/female gamete-specific-genes, but when MID is present MID-VSR1 heterodimers are preferentially formed and activateminus/male gamete-specific-genes.Significance StatementSex and recombination are conserved features of eukaryotic life cycles, but sex determination mechanisms are diverse, and are poorly understood in most major taxa. Our study identified a long-sought regulator of sexual differentiation in volvocine green algae—the RWP-RK family transcription factor (TF) VSR1— leading to the first complete paradigm for mating type or sex determination in this lineage. Our results support a model where gametically expressed VSR1 homodimerizes and activatesplus/female specific genes. When the dominant sex-linkedminus/male RWP-RK family TF MID is present MID-VSR1 heterodimers are preferentially formed and activateminus/male genes. The widespread association of RWP-RK TFs with gamete differentiation in the green lineage suggests that a similar paradigm may operate throughout the plant kingdom.