Abstract
It is becoming increasingly evident that interspecific hybridization is a common event in phytophthora evolution. Yet, the fundamental processes underlying interspecific hybridization and the consequences for its ecological fitness and distribution are not well understood. We studied hybridization events in phytophthora clade 8b. This is a cold-tolerant group of plant pathogenic oomycetes in which six host-specific species have been described that mostly attack winter-grown vegetables. Hybrid characterization was done by sequencing and cloning of two nuclear (ITS and Ypt1) and two mitochondrial loci (Cox1 and Nadh1) combined with DNA content estimation using flow cytometry. Three different mtDNA haplotypes were recovered among the presumed hybrid isolates, dividing the hybrids into three types, with different parental species involved. In the nuclear genes, additivity, i.e. the presence of two alleles coming from different parents, was detected. Hybrid isolates showed large variations in DNA content, which was positively correlated with the additivity in nuclear loci, indicating allopolyploid hybridization followed by a process of diploidization. Moreover, indications of homeologous recombination were found in the hybrids by cloning ITS products. The hybrid isolates have been isolated from a range of hosts that have not been reported previously for clade 8b species, indicating that they have novel pathogenic potential. Next to this, DNA content measurements of the non-hybrid clade 8b species suggest that polyploidy is a common feature of this clade. We hypothesize that interspecific hybridization and polyploidy are two linked phenomena in phytophthora, and that these processes might play an important and ongoing role in the evolution of this genus.
Highlights
Phytophthora is a genus of plant pathogenic filamentous oomycetes containing more than one hundred species
The increasing body of literature that is becoming available on hybridization in Phytophthora suggests that it could play an important role in the evolution of the genus
Natural interspecific hybridization events have been described in clade 1[5,6,7,9,11], clade 6 [10], clade 7 [4,12] and in clade 8
Summary
Phytophthora is a genus of plant pathogenic filamentous oomycetes containing more than one hundred species. All of them are plant pathogens causing many important plant diseases worldwide, such as potato late blight, sudden oak death and forest dieback caused by Phytophthora infestans, Phytophthora ramorum and Phytophthora cinnamomi, respectively. Oomycetes are very similar to the filamentous fungi (kingdom Fungi). They used to be classified as Fungi. During the last few decades, DNA and whole genome sequencing revealed that they have a completely different evolutionary origin but have adopted similar morphology and infection strategies through convergent evolution and horizontal gene transfer. Oomycetes differ from Fungi in some important morphological and biochemical aspects and in the fact that they are diploid in their vegetative lifestyle, whereas most Fungi are monoploid [1]
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