Linkage Disequilibrium Between Incompatibility Locus Region Genes

• We have studied diversity in Arabidopsis lyrata of sequencesorthologous to the ARK3 gene of A. thaliana. Our main goal wasto test for recombination in the S-locus region. In A. thaliana,the single-copy ARK3 gene is closely linked to the non-functionalcopies of the self-incompatibility loci, and the ortholog inA. lyrata (a self-incompatible species) is in the homologousgenome region and is known as Aly8. It is thus of interest totest whether Aly8 sequence diversity is elevated due to closelinkage to the highly polymorphic incompatibility locus, asis theoretically predicted. However, Aly8 is not a single-copygene, and the presence of paralogs could also lead to the appearanceof elevated diversity. We established a typing approach basedon different lengths of Aly8 PCR products and show that mostA. lyrata haplotypes have a single copy, but some have two genecopies, both closely linked to the incompatibility locus, onebeing a pseudogene. We determined the phase of multiple haplotypesin families of plants from Icelandic and other populations.Different Aly8 sequence types are associated with differentSRK alleles, while haplotypes with the same SRK sequences tendto have the same Aly8 sequence. There is evidence of some exchangeof sequences between different Aly8 sequences, making it difficultto determine which ones are allelic or to estimate the diversity.However, the homogeneity of the Aly8 sequences of each S-haplotypesuggests that recombination between the loci has been very infrequentover the evolutionary history of these populations. Overall,the results suggest that recombination rarely occurs in theinterval between the S-loci and Aly8 and that linkage to theS-loci can probably account for the observed high Aly8 diversity.

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