High frequency mitotic gene conversion in genetic hybrids of the oomycete Phytophthora sojae

Abstract

Microbial populations depend on genetic variation to respond to novel environmental challenges. Plant pathogens are notorious for their ability to overcome pesticides and host resistance genes as a result of genetic changes. We report here that in particular hybrid strains of Phytophthora sojae, an oomycete pathogen of soybean, high frequency mitotic gene conversion rapidly converts heterozygous loci to homozygosity, resulting in heterokaryons containing highly diverse populations of diploid nuclei. In hybrids involving strain P7076, conversion rates of up to 3 x 10(-2) per locus per nucleus per generation were observed. In other hybrids, rates were of the order of 5 x 10(-5). Independent gene conversion was observed within a selected linkage group including loci as close as 0.7 kb apart and in unlinked markers throughout the genome. Gene conversions continued throughout vegetative growth and were stimulated by further sexual reproduction. At many loci, conversion showed extreme disparity, with one allele always being lost, suggesting that conversion was initiated by allele-specific double-stranded breaks. Pedigree analysis indicated that individual loci undergo multiple independent conversions within the nuclei of a vegetative clone and that conversion may be preceded by a heritable "activation" state.

Figure 1

Analysis of mitotic changes by isolation of zoospores. The design of the experiment is shown together with anticipated changes in genetic markers. Open and filled circles denote nuclei homozygous for a given polymorphic marker. Half-filled circles denote heterozygous nuclei. Oospores are diploid sexual spores and result from the fusion of haploid gametangia. Zoospores are mononucleate vegetative spores, the nuclei of which are generated by mitosis.

Figure 2

Genetic and physical maps of the linkage group analyzed. (A) Genetic map. The map was constructed by analysis of 100 F₂ progeny from a cross of P6497 × P7064 using MAPMAKER 1.0. All molecular markers are RAPD markers except 121P1, 74P8, and 74P9, which are restriction fragment length polymorphisms. Codominant alleles of the underlined markers were used for this study. Avr1b is a phenotypic marker. Distances are in centimorgans. (B) Physical locations of markers on a 200-kb bacterial artificial chromosome contig spanning Avr1b. Underlined markers were used in this study. Marker 8127 lies within Avr1b. The expansion lines show the position and orientation of the contig on the genetic map.