COMPARATIVE MAPPING OF THE BRASSICA S-LOCUS REGION AND ITS HOMEOLOG IN ARABIDOPSIS - IMPLICATIONS FOR THE EVOLUTION OF MATING SYSTEMS IN THE BRASSICACEAE

The crucifer family includes self-incompatible genera, such as Brassic a, and self-fertile genera, such as Arabidopsis. To gain insight into mechanisms underlying the evolution of mating systems in this family, we used a selective comparative mapping approach between Brassica camp estris plants homozygous for the Se haplotype and Arabidopsis. Startin g with markers flanking the self-incompatibility genes in Brassica, we identified the homeologous region in Arabidopsis as a previously unch aracterized segment of chromosome 1 in the immediate vicinity of the e thylene response gene ETR1. A total of 26 genomic and 21 cDNA markers derived from Arabidopsis yeast artificial and bacterial artificial chr omosome clones were used to analyze this region in the two genomes. Ap proximately half of the cDNAs isolated from the region represent novel expressed sequence tags that do not match entries in the DNA and prot ein databases. The physical maps that we derived by using these marker s as well as markers isolated from bacteriophage clones spanning the S -a haplotype revealed a high degree of synteny at the submegabase scal e between the two homeologous regions. However, no sequences similar t o the Brassica S locus genes that are known to be required for the sel f-incompatibility response were detected within this interval or other regions of the Arabidopsis genome. This observation is consistent wit h deletion of self-recognition genes as a mechanism for the evolution of autogamy in the Arabidopsis lineage.