THE MATING-TYPE LOCUS OF CHLAMYDOMONAS-REINHARDTII CONTAINS HIGHLY REARRANGED DNA-SEQUENCES

The mating-type locus of Chlamydomonas reinhardtii exists as two appar ent alleles (mt+ and mt-) that control mating in haploid gametes and s porulation and meiosis in diploid mt+/mt- zygotes. Twelve genes, seven unrelated to life cycle transitions, are tightly linked to mt, sugges ting that the locus exerts recombinational suppression. A 1.1 Mb chrom osome walk from a gene linked to mt demonstrates that the mt+ and mt- loci carry four intrachromosomal translocations, two inversions, and l arge deletions and duplications within a 190 kb sector, presumably acc ounting for the recombinational suppression that extends through 640 k b of flanking homologous DNA. The rearranged domain also carries block s of mt+- and mt--specific sequences, at least one of which includes a mt+-specific gene. The locus has the properties of an incipient sex c hromosome.