No abstract

Lateral gene transfer (LGT) can facilitate the acquisition of new functions in recipient lineages, which may enable them to colonize new environments. Several recent publications have shown that gene transfer between prokaryotes and eukaryotes occurs with appreciable frequency. Here we present a study of interdomain gene transfer of cutinases -well documented virulence factors in fungi -between eukaryotic plant pathogens Phytophthora species and prokaryotic bacterial lineages. Two putative cutinase genes were cloned from Phytophthora brassicae and Northern blotting experiments showed that these genes are expressed early during the infection of the host Arabidopsis thaliana and induced during cyst germination of the pathogen. Analysis of the gene organisation of this gene family in Phytophthora ramorum and P. sojae showed three and ten copies in tight succession within a region of 5 and 25 kb, respectively, probably indicating a recent expansion in Phytophthora lineages by gene duplications. Bioinformatic analyses identified orthologues only in three genera of Actinobacteria, and in two distantly related eukaryotic groups: oomycetes and fungi. Together with phylogenetic analyses this limited distribution of the gene in the tree of life strongly support a scenario where cutinase genes originated after the origin of land plants in a microbial lineage living in proximity of plants and subsequently were transferred between distantly related plant-degrading microbes. More precisely, a cutinase gene was likely acquired by an ancestor of P. brassicae, P. sojae, P. infestans and P. ramorum, possibly from an actinobacterial source, suggesting that gene transfer might be an important mechanism in the evolution of their virulence. These findings could indeed provide an interesting model system to study acquisition of virulence factors in these important plant pathogens.

In a survey of Phytophthora associated with alder decline in Poland, several isolates of a homothallic Phytophthora sp., which could not be assigned to other taxa including Phytophthora alni subspecies, were consistently recovered from rhizosphere soil samples. Their morphology and pathogenicity, as well as sequence data for three nuclear regions (internal transcribed spacer rDNA, elongation factor-1alpha and beta-tubulin) and a coding mitochondrial DNA region (nadh1), were examined. The new Phytophthora species is characterized by the moderate to slow growth rate of its colony in carrot agar at 20 degrees C, high optimal (c. 30 degrees C) and maximum (c. 38 degrees C) growth temperatures, formation of catenulate, often lateral, hyphal swellings, large chlamydospores in agar media and in soil extract, persistent, ovoid to ellipsoid nonpapillate sporangia and large oogonia with paragynous and sometimes amphigynous antheridia. Phytophthora polonica was slightly pathogenic to alder twigs and not pathogenic to trunks of several tree species. In a phylogenetic analysis using either Bayesian inference or maximum likelihood methods, P. polonica falls in clade 8 'sensu Kroon et al. (2004)' of Phytophthora.

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Sixteen Pythium isolates from diverse hosts and locations, which showed similarities in their morphology and sequences of the internal transcribed spacer (ITS) region of their rRNA gene, were investigated. As opposed to the generally accepted view, within single isolates ITS sequence variations were consistently found mostly as part of a tract of identical bases (A-T) within ITS1, and of GT or GTTT repeats within the ITS2 sequence. Thirty-one different ITS sequences obtained from 39 cloned ITS products from the 16 isolates showed high sequence and length polymorphisms within and between isolates. However, in a phylogenetic analysis, they formed a cluster distinct from those of other Pythium species. Additional sequencing of two nuclear genes (elongation factor 1 alpha and beta-tubulin) and one mitochondrial gene (nadh1) revealed high levels of heterozygosity as well as polymorphism within and between isolates, with some isolates possessing two or more alleles for each of the nuclear genes. In contrast to the observed variation in the ITS and other gene areas, all isolates were phenotypically similar. Pythium mercuriale sp. nov. (Pythiaceae) is characterized by forming thin-walled chlamydospores, subglobose to obovoid, papillate sporangia proliferating internally and smooth-walled oogonia surrounded by multiple antheridia. Maximum likelihood phylogenetic analyses based on both ITS and beta-tubulin sequence data place P. mercuriale in a clade between Pythium and Phytophthora.