Boule and the Evolutionary Origin of Metazoan Gametogenesis: A Grandpa's Tale

Figure 3

Evolutionary distribution of motile sperm and members of the DAZ family among major lineages of animals adapted from [7, 8]. Motile sperm is found in all major phyla of metazoan animals (evolutionary origin indicated by a blue box). The ancient gene Boule in the common ancestor of bilateria is indicated by a purple box in the tree topology, likely originated during the evolution of eumetazoans. Its function was spermatogenesis specific based on the predominance of a testis-biased expression in diverse bilaterian lineages as well as in the conservation of male reproductive function in the fly and in mice. DAZL arose from the ancestral BOULE through gene duplication events, most likely in the ancestral lineage of bony fishes after the divergence from cartilaginous fishes (indicated by a red box) but before the emergence of tetrapods, and is lacking in protostomes. DAZ arose from DAZL in the primate lineage (pink box) becoming integrated in the Y chromosome later on during primate evolution, after the divergence between New World monkeys and Old World monkeys, approximately 30–40 MYA. Testis expression/functional requirement (T) or ovary expression/functional requirement (O) for Boule in different metazoans is indicated in the right-hand side of the tree in purple background. Low levels of Boule expression in the ovary are indicated by the lowercase letter “o”, in order to distinguish them from abundant expression referred to as “O”.