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

doi:10.1073/pnas.251464498

High frequency mitotic gene conversion in genetic hybrids of the oomycete Phytophthorasojae

Department of Plant Pathology, University of California, Davis, CA 95616

Edited by David D. Perkins, Stanford University, Stanford, CA, and approved October 5, 2001 (received for review August 30, 2001)

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 × 10⁻² per locus per nucleus per generation were observed. In other hybrids, rates were of the order of 5 × 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.

Footnotes

↵ * Present address: Research School of Biological Sciences, Australian National University, PO Box 475, Canberra Act 2601, Australia.
↵ † To whom reprint requests should be addressed at: Department of Plant Pathology, One Shields Avenue, University of California, Davis CA 95616. E-mail: bmtyler{at}ucdavis.edu.
This paper was submitted directly (Track II) to the PNAS office.
↵ ‡ We use the term disparity here to indicate that gene conversion occurs primarily in favor of one allele (54). The terms polarity and directionality are used sometimes to describe this phenomenon, but these terms are used also to describe bias in the position of a conversion tract relative to the site of its initiation.
Abbreviation:

RAPD,

random amplified polymorphic DNA