NSF Award
2312043
Sex and recombination are integral to life cycles and reproduction across the kingdom of eukaryotes that includes animals, plants, fungi, seaweeds and many single-celled algae and other protists. A hallmark of eukaryotic sex involves genetic systems such as sex chromosomes or mating type genes to ensure that matings involve individuals with two distinct mating types or sexes. However, outside of animals, fungi, and a few other taxa, little is known about how sexes and mating types are determined or how male and female sexes evolved in multicellular species from single-celled ancestors with mating types. Volvocine green algae are a unique group of relatives that include single celled and small colonial species with mating types, and larger more complex species with sexes and sex chromosomes; but they all share a common genetic system for controlling gamete differentiation. Under this proposal volvocine algae will be investigated to understand how the genetic systems governing mating types were modified in the evolutionary transition to sexes. Importantly, the genetic machinery that controls sexes and mating types in volvocine algae is at least partly shared with other groups of green algae and their land plant cousins. Thus, the knowledge obtained from this proposal can impact the understanding of sex determination in a wide range of species in the plant kingdom and may be further leveraged for breeding and strain improvement in emerging algal crop species that are used to make high value bioproducts or biofuels.<br/><br/>Ancestral eukaryotes were single cells with isogamous sexual reproduction. However, in nearly every lineage where complex multicellularity evolved, so did dimorphic sexes—a transition whose origins and mechanisms remain poorly understood. Volvocine green algae (Chlamydomonas reinhardtii, Volvox carteri, and relatives) are a uniquely tractable model for investigating the genetic networks governing sexual differentiation and the evolution of dimorphic sexes. The conserved transcription factor (TF) MID is a dominant specifier of minus or male gamete differentiation in volvocine algae; but the factor(s) responsible for basal sexual differentiation as plus or female have remained unknown. Under this proposal a newly discovered TF, VSR1, that is essential for differentiation of both gamete types and sexes in volvocine algae will be investigated. A new model where competing interactions between VSR1 homodimers and MID-VSR1 heterodimers form a binary switch for sex or mating type determination will be tested and refined, and the mechanisms underlying expansion and divergence of the MID/VSR1 gene regulatory networks (GRNs) during the transition to oogamy will be explored. Specific Aims are designed to 1) Elucidate and compare the gene expression programs for gametogenesis that are governed by MID and VSR1 in key isogamous and oogamous volvocine representatives; 2) Determine the bases for MID-VSR1 physical interactions and how the two TFs co-evolved during diversification of the volvocine lineage; 3) Identify the DNA binding specificity and direct targets of VSR1 and MID in key volvocine representatives to enable modeling of sex-related GRN evolution and testing of hypotheses about GRN expansion in Volvox.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
