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

Abstract

Volvocine 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 in <i>Volvox</i>. The conserved RWP-RK family transcription factor (TF) MID is encoded by the <i>minus</i> mating-type locus or male sex-determining region of heterothallic volvocine species and dominantly determines <i>minus</i> or male gametic differentiation. However, the factor(s) responsible for establishing the default <i>plus</i> or female differentiation programs have remained elusive. We performed a phylo-transcriptomic screen for autosomal RWP-RK TFs induced during gametogenesis in unicellular isogamous <i>Chlamydomonas reinhardtii</i> (Chlamydomonas) and in multicellular oogamous <i>Volvox carteri</i> (Volvox) and identified a single conserved ortho-group we named Volvocine Sex Regulator 1 (VSR1). Chlamydomonas <i>vsr1</i> mutants of either mating type failed to mate and could not induce expression of key mating-type-specific genes. Similarly, Volvox <i>vsr1</i> mutants 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 vivo coimmunoprecipitation 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 of <i>plus</i>/female gamete-specific-genes, but when MID is present, MID-VSR1 heterodimers are preferentially formed and activate <i>minus</i>/male gamete-specific-genes.