Comparative genomics of the mating type locus in the isogamous colonial volvocalean alga Gonium pectorale
Takashi Hamaji1, Patrick J. Ferris2, Hiroko Kawai-Toyooka1, James Umen2, Ichiro Nishii3, and Hisayoshi Nozaki1
1) Department of Biological Sciences, University of Tokyo, Tokyo 113-0033, Japan
2) The Salk Institute for Biological Studies, La Jolla, California 92037, USA
3) Department of Biological Sciences, Nara Women's University, Nara 630-8506, Japan
The colonial volvocalean lineage is well-suited for a comparative genomic approach to the evolution of sexual dimorphism for several reasons: 1) Volvocine algae have a range of species that include isogamous through oogamous clades (Nozaki et al. 2000, MPE). 2) Genome sequences of oogamous Volvox carteri and isogamous, unicellular Chlamydomonas reinhardtii are available. 3) The genetic and molecular basis of mating type differentiation in C. reinhardtii has been well-studied. Key genes involved in mating type differentiation are encoded on mating type loci (MT) in C. reinhardtii (Ferris and Goodenough 1994 Cell; Ferris et al. 2002 Genetics), the isogamous 8- or 16-celled colonial Gonium pectorale (Hamaji et al. 2008 Genetics; 2009 J. Phycol.), oogamous Pleodorina starrii (Nozaki et al. 2006 Curr. Biol.) and V. carteri (Ferris et al. 2010 Science in press). Here, we have sequenced BAC clones identified by chromosomal walking through the MT locus of G. pectorale, an intermediate species closest to oogamous organisms in the colonial Volvocales. So far, possible contigs of >400 kb from MT- and >300 kb from MT+ show that the rearranged domains in G. pectorale MT are potentially larger than those in C. reinhardtii, and suggest that MT expansion may have been an early event in the evolution of multicellular volvocalean algae.
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