Species from within the Phytophthora cryptogea complex and related species, P. erythroseptica and P. sansomeana, readily hybridize

During a study on the phylogenetic relationships between species in the Phytophthora cryptogea complex and related species, Phytophthora erythroseptica and Phytophthora sansomeana, 19 hybrid isolates with multiple polymorphisms in the nuclear sequences were observed. Molecular characterization of hybrids was achieved by sequencing three nuclear (internal transcribed spacers, β-tubulin (TUB), heat shock protein 90) and two mitochondrial (cytochrome c oxidase subunit I (coxI), NADH dehydrogenase subunit I (NADH)) gene regions and cloning of the single-copy nuclear gene, TUB. Based on the molecular studies the hybrid isolates belonged to six distinct groups between P. cryptogea, P. erythroseptica, Phytophthora pseudocryptogea, P. sansomeana, and Phytophthora sp. kelmania. In all cases, only a single coxI and NADH allele was detected and nuclear genes were biparentally inherited, suggesting that the hybrids arose from sexual recombination events. Colony morphology, growth rate, cardinal temperatures, breeding system, and morphology of sporangia, oogonia, oospores, and antheridia were also determined. Some morphological differences between the hybrids and the parental species were noted; however, they were not sufficient to reliably distinguish the taxa and DNA markers from nuclear and mitochondrial genes will to be necessary for their identification. The parental species are all important pathogens of agricultural fields that have been transported globally. With the apparent ease of hybridization within this group there is ample opportunity for virulent hybrids to form, perhaps with extended host ranges

Re-evaluation of the Phytophthora cryptogea species complex and the description of a new species, Phytophthora pseudocryptogea sp. nov

Many studies over recent years have recognised Phytophthora cryptogea as a species complex, with several distinct lineages perhaps representing as yet undescribed species. Additionally, the taxonomic status of the related species P. erythroseptica is also a matter of controversy. In this study, phylogenetic relationships were clarified using nuclear (internal transcribed spacers, ß-tubulin, heat shock protein 90, elicitin) and mitochondrial (cytochrome c oxidase subunit I, NADH dehydrogenase subunit I) gene regions. Our results showed three distinct phylogenetic lineages within P. cryptogea in combined nuclear and mitochondrial gene trees. The molecular divergence observed among these three phylogenetic lineages justifies their re-description as separate species. The first group, including the type isolate, is P. cryptogea sensu stricto, the second group corresponds to the undescribed taxon P. sp. kelmania and the third group is described here as P. pseudocryptogea sp. nov. Although some morphological differences between P. cryptogea and P. pseudocryptogea were notable, they were not sufficient to reliably distinguish these species. Moreover, in all of our phylogenetic trees (with the exception of elicitin), P. erythroseptica isolates were in a strongly supported monophyletic clade. This clade shares a recent common ancestor with P. cryptogea sensu stricto, but is clearly distinct from P. cryptogea. Our results therefore confirmed the position of P. erythroseptica as a distinct species and a close relative to the P. cryptogea species complex

Potential host range of four Phytophthora interspecific hybrids from Clade 8a

In recent years several interspecific hybrids have been reported in the plant pathogenic oomycete genus Phytophthora . Due to the large genotypic and phenotypic changes, these hybrids might have broader or more limited host ranges compared with their parental species. It is crucial to understand the host range of Phytophthora hybrids to minimize the economic losses caused by their infection. The potential host range of four hybrids belonging to Clade 8a of the Phytophthora phylogenetic tree was investigated in this study. Thirty species of herbaceous plants as well as eight species of woody plants were inoculated and monitored for any symptom of infection. In addition, the detached twigs of 32 tree species, fruits of six plant species, tubers of potato, and roots of carrot and sugar beet were investigated for susceptibility to these hybrids. Almost all hybrids caused severe rot on all tested fruits, tubers, and roots, although different isolates showed different pathogenicity on detached tree twigs. All hybrids tested had a different host range compared with their parental species: they were able to infect plants outside the host range of their parents, infect hosts of both parental species, although these parents did not have overlapping hosts, or, in some cases, they were not able to infect hosts infected by the parents