Abstract:
Genetic structure and reproductive mode of the homothallic fungal pathogen
Sclerotinia sclerotiorum have been widely studied using linkage disequilibrium
(LD) tests with putatively unlinked molecular markers. We previously
observed random association between linked loci in S. sclerotiorum
populations suggesting intrachromosomal recombination or high mutation
rates at these loci. This study was aimed at testing intrachromosomal recombination
using 12 microsatellite loci distributed over four chromosomes. Two
hundred thirty isolates sampled from seven populations in the USA and China
from a variety of crops were genotyped. Each isolate carried a single allele for
each of the 12 loci suggesting the isolates were haploid and homokaryotic.
Pairwise LD tests of all the intrachromosomal loci showed relationship ranged
from linked to random association, and in many cases LD declined with
increasing physical distance between loci. Thus the random associations of
alleles cannot be simply attributed to random mutation. Majority of the
isolates were mycelially incompatible, likely minimizing the possibility of
heterokaryon formation and mitotic recombination. Thus the observed high
intrachromosomal recombination is most likely due to meiotic recombination
following outcross in these populations.