The Volvox carteri mating locus

Patrick Ferris1, Ichiro Nishii2, Hisayoshi Nozaki3, David Kirk4 and James G. Umen1

1Plant Biology Laboratory, Salk Institute, La Jolla CA 92137 USA 2Frontier Research System, RIKEN, Wako Japan 3Dept. of Biological Sciences, University of Tokyo, Japan 4Dept. of Biology, Washington University, St. Louis MO 63130 USA.

In Chlamydomonas, gametes differentiate in response to nitrogen starvation and are of equal size in the two sexes (plus and minus). Volvox, by contrast, is oogamous; females produce non-motile eggs whereas males produce tiny motile sperm, and sexual differentiation is triggered by a sex-inducing hormone. C. reinhardtii and V. carteri thus provide a model system in which key steps in the evolution of oogamy may be open to experimental analysis. We have begun by characterizing the Volvox mating (MT) locus. Chromosome walking and sequencing has now provided an almost complete description of both male and female MT loci, which were found to share common ancestry with the Chlamydomonas MT locus. The C. reinhardtii MT locus is a non-recombining, 200 kb, dimorphic region within which are 14 housekeeping genes shared by both sexes, as well as sex-specific mating-related genes (e.g. MID, MTD1, FUS1). The genes common to both loci have only minor sex-specific polymorphisms. By contrast, the V. carteri MT locus is about 1.5 Mb and has a central region of 1.2 Mb that contains ~35 genes whose alleles differ by 10-15% at the amino acid level, and which have been evolving independently since well before the emergence of the V. carteri species. This situation creates the potential for rapid evolution of these housekeeping genes and their possible cooption for sex-specific functions. The MAT3 gene is one such example that is being investigated.

e-mail address of presenting author: ferris@salk.edu