Molecular phylogenetic analysis of Peronosclerospora (Oomycetes) reveals cryptic species and genetically distinct species parasitic to maize

Abstract

Downy mildews are amongst the most widespread and economically important pathogens of cultivated grasses in the tropics and subtropics. Despite their importance, molecular methods, particularly DNA sequence analysis, have rarely been applied to either species identification or to the determination of phylogenetic relationships between species. Here we report the presence of several cryptic species in the genus Peronosclerospora. Further we confirm that maize can be parasitised by several species of Peronosclerospora, including P. eriochloae, which has not been reported previously as a pathogen of maize. The presence of 14 distinct phylogenetic lineages, including three that are parasitic to maize, highlights the current fragmentary knowledge on the diversity and classification of species within Peronosclerospora. Species identification in Peronosclerospora has been traditionally based on the host genus and a set of variable morphological characteristics, which has meant that the identification of species is often unreliable. This situation is primed for the application of molecular techniques for the identification of species. One of the lineages parasitic to maize in Australia has not yet been formally described and its distribution is not known. Future investigation including a broad sampling of downy mildews from maize and other cultivated and native grasses on a world-wide basis is a prerequisite to a re-evaluation of quarantine regulations aimed at restricting or limiting their spread.

Peronosclerospora australiensis sp. nov. and Peronosclerospora sargae sp. nov., two newly recognised downy mildews in northern Australia, and their biosecurity implications

“…Molecular phylogenetic reconstruction based on partial cox2 (Cytochrome c oxidase subunit II gene) and nrLSU (nuclear ribosomal large subunit) sequences showed two distinct clades of undescribed species of Peronosclerospora infecting Sorghum species that are native to northern Australia (Telle et al 2011). In this present study, we have morphologically re-examined those specimens and described two distinct novel species.…”

“…Conidiophores and conidia not seen on the type host. Previous DNA sequence analysis (Telle et al 2011) showed that P. australiensis also infected S. plumosum and maize. Sorghum timorense and S. plumosum are closely related and difficult to differentiate because both have leaves that senesce during the dry season.…”

“…DNA sequence analysis showed that two specimens (BRIP 46815, BRIP 46527) of downy mildew on maize from the Northern Territory were part of a clade that included the type of P. australiensis (Telle et al 2011). These two specimens contained only a few conidia and conidiophores, and lacked oospores.…”

“…Four of these, P. eriochloae, P. maydis, P. noblei and P. sacchari, have been recorded in Australia (Ryley and Langdon 2001). Peronosclerospora eriochloae is known to occur on Eriochloa pseudoacrotricha and maize in subtropical south-eastern Queensland (Ryley and Langdon 2001;Telle et al 2011). Peronosclerospora noblei is endemic to Australia and only known on Sorghum leiocladum (Ryley and Langdon 2001), which is distributed within 200 km of the eastern coast of Australia, from northern Victoria to southern Queensland (Spangler 2003).…”

“…Recent molecular phylogenetic analysis has shown there are cryptic species amongst Australian specimens of Peronosclerospora Telle et al 2011). Molecular phylogenetic reconstruction based on partial cox2 (Cytochrome c oxidase subunit II gene) and nrLSU (nuclear ribosomal large subunit) sequences showed two distinct clades of undescribed species of Peronosclerospora infecting Sorghum species that are native to northern Australia (Telle et al 2011).…”