In order to understand an organism as a whole, it is essential to have a full representation of the genes used in its entire life cycle. The genome assemblies generated by JGI for Chlamydomonas reinhardtii and Volvox carteri came from a single mating type, so a large region of sequence from the opposite mating type is missing from both species. The genetic programming that induces sexual differentiation, mating, and zygote development is controlled by the mating loci, and in both species this locus is a large, recombinationally suppressed region. Studies in C. reinhardtii have contributed to our understanding of eukaryotic sexual differentiation and fertilization, and obtaining the entire sequence from both mating locus alleles will promote further understanding of eukaryotic sexual cycles. Sexuality in the multicellular alga V. carteri is poorly understood and likely to be more complicated than in C. reinhardtii. A complete sequence of both mating loci is, therefore, a prerequisite for understanding sex determination in V. carteri.
A second and even more compelling reason for obtaining mating locus sequence from both C. reinhardtii and V. carteri is the synergistic potential for evolutionary genomic studies. Whereas C. reinhardtii is unicellular and makes morphologically similar gametes, V. carteri is multicellular and exhibits sexual dimorphism, with egg-forming females and sperm-forming males. Despite their major phenotypic differences, C. reinhardtii and V. carteri are separated by only ~50 million years, and their genomes share a great deal of similarity, including substantial nucleotide identity in coding regions. Their close phylogenetic relationship but highly divergent morphologies make this pair of model organisms an ideal combination for investigating the origins of multicellularity and sexually dimorphic development.
Principal Investigators: James G. Umen (Salk Inst.) and Patrick J. Ferris (RIKEN Inst., Japan)