The Detection of Nonhybrid, Trisomic, and Triploid Offspring in Sexual Progeny of a Mating of Phytophthora infestans

Eighty single-oospore offspring of Phytophthora infestans from a mating of isolates, which had previously been analyzed for segregation of avirulence/virulence, were assessed for the inheritance of 20 RFLP markers. Three offspring were triploid; they inherited three alleles at all loci where this could be detected and when heterozygous, showed unequal intensities of hybridization with most probes. Twenty-four offspring were trisomic, as each had three doses of one or a few markers, evident from their inheritance of three alleles or from unequal hybridization to one probe. Coinheritance of the extra allele(s) and mitochondrial haplotype in the majority of trisomic offspring suggested that meiosis in oogonia was more aberrant than in antheridia. Linkage analysis was performed on 50 offspring, which were assumed to be euploid; six small linkage groups were detected and several avirulence loci were found to be linked. The origins of aberrant offspring are discussed.     

A Dual Role for Microbial Pathogen-Derived Effector Proteins in Plant Disease and Resistance

Many proteins from plant pathogens affecting the interaction with the host plant have dual functions: they promote virulence on the host species and they function as avirulence determinants by eliciting defense reactions in host cultivars expressing the appropriate resistance genes. In viruses all proteins encoded by the small genomes can be expected to be essential for viral development in the host. However, in different plants surveillance systems have evolved that are able to recognize most of these proteins. Bacteria and fungi have specialized pathogenicity and virulence genes. Many of the latter were originally identified through the resistance gene-dependent elicitor activity of their products. Their role in virulence only became apparent when they were inactivated or transferred to different microbes or after their ectopic expression in host plants. Many microbes appear to maintain these genes despite their disadvantageous effect, introducing only few mutations to abolish the interaction of their products with the plant recognition system. This has been interpreted as been indicative of a virulence function of the gene products that is not impaired by the mutations. Alternatively, in particular in bacteria there is now evidence that pathogenicity was acquired through horizontal gene transfer. Genes supporting virulence in the donor organism's original host appear to have traveled along. Being gratuitous in the new situation, they may have been inactivated without loss of any beneficial function for the pathogen.     

Positive selection in AvrP4 avirulence gene homologues across the genus Melampsora

Pathogen genes involved in interactions with their plant hosts are expected to evolve under positive Darwinian selection or balancing selection. In this study a single copy avirulence gene, AvrP4, in the plant pathogen Melampsora lini, was used to investigate the evolution of such a gene across species. Partial translation elongation factor 1-alpha sequences were obtained to establish phylogenetic relationships among the Melampsora species. We amplified AvrP4 homologues from species pathogenic on hosts from different plant families and orders, across the inferred phylogeny. Translations of the AvrP4 sequences revealed a predicted signal peptide and towards the C-terminus of the protein, six identically spaced cysteines were identified in all sequences. Maximum likelihood analysis of synonymous versus non-synonymous substitution rates indicated that positive selection played a role in the evolution of the gene during the diversification of the genus. Fourteen codons under significant positive selection reside in the C-terminal 28 amino acid region, suggesting that this region interacts with host molecules in most sequenced accessions. Selection pressures on the gene may be either due to the pathogenicity or avirulence function of the gene or both.