The overarching goal of the Tethys project is to decipher the mechanisms and ultimate forces shaping major transitions between sexual systems during the evolution of the brown algae. First, we are exploiting genomic resources and the amenability of brown algae to genetic approaches to uncover the genetic causes, the mechanisms and evolutionary forces shaping transitions between sexual systems across this group. We focus on transitions between separate and combined sexes during the haploid (dioicy to monoicy) and diploid phases (monoecy and dioecy) but also on a major evolutionary transition from haploid to diploid sex determination. We expect to reveal the causal genes and downstream effectors involved in the diversity of sexual systems and to elucidate the forces driving switches between systems. Second, we investigate the regulation and evolution of genes involved in sexual reproduction in the context of sexual system transitions, by delineating a comprehensive transcriptional and chromatin state roadmap for sexual development across key brown algal species. These analyses is uncovering the evolution of the genetic programs involved in sexual system transitions and revealing their higher-order regulation via chromatin remodelling. Third, we examine patterns of correlated evolution between sexual, morphological and genomic traits and incorporate ecological information in a phylogenetic context, in order to link ecological factors with reproductive transitions and evaluate the consequences of shifts in sexual systems on genome structure and species diversification. Collectively, our multilevel analyses is revealing the fundamental forces shaping major transitions in sexual systems with wide relevance to all eukaryotes, but also providing insights into the biology of an important but virtually unexplored eukaryotic supergroup.
