SUMMARY. The long-term goal of this work is to understand the molecular genetic mechanisms that underlie developmental complexity. Multicellularity has evolved multiple times in the history of life and has led to an explosion of eukaryotic diversity, including novel reproductive strategies. Sexual dimorphism and multicellularity have coevolved in several lineages, but the mechanisms that led to their co-evolution in animals and plants lie in the distant past and are unknown. Volvox carteri is a multicellular green alga that shares recent common ancestry with its closest unicellular relative, Chlamydomonas reinhardtii. From a single-celled Chlamydomonas ancestor V. carteri has evolved many metazoan-like features including complete germ-soma separation and sexually dimorphic development. During sexual differentiation V. carteri females produce eggs and males produce sperm, a trait that is controlled by a mating type (mt) locus with two alleles, mtf (female) and mtm (male). The genomic region that encodes V. carteri mt has been identified and partially characterized, leading to the following Aims: (i) Clone and sequence both alleles of mt;(ii) Use annotation and transcriptional profiling to identify key regulators of sex determination;(iii) Characterize the genes that are responsible for sexually dimorphic development including vMATS, a mt-linked retinoblastoma (RB) tumor suppressor homolog that is highly polymorphic between V. carteri males and females;(iv) Directly test the role of female and male vMATS isoforms and other potential sex determining genes in regulating sexual differentiation by using ectopic expression and genetic inactivation experiments. The discovery of the V. carteri mating locus provides an unprecedented opportunity to dissect the molecular genetic changes that underlie the evolution of sexual dimorphism and to understand how sex chromosomes evolve to influence developmental programming. RELEVANCE. Germ cells are critical for human reproductive health and are also associated with some forms of human cancer, but their biology is poorly understood. The work proposed here is expected to reveal the underlying genetic architecture that leads to the production of eggs and sperm in a simple model system, and has already uncovered a potentially conserved link between a human tumor suppressor and germ cell formation.