Ecology and population structure of a tree wound-infecting fungus in a native South African forest environment

Ceratocystis tsitsikammensis was first isolated from bark harvesting wounds on two indigenous tree species in the Afromontane forests of the Western Cape Province of South Africa. Inoculation studies indicated that it is a potential pathogen of native Rapanea melanophloeos trees. In this study, we investigated the distribution, ecology and biology of C. tsitsikammensis in the Garden Route National Park of South Africa. Isolates were obtained from wounds on R. melanophloeos, three non-native hosts as well as from nitidulid and staphylinid beetles visiting wounds on these trees. The genetic diversity and population biology of the fungus was examined using microsatellite markers. Its mating strategy was also determined by amplifying its mating type genes and the fungus was shown to be homothallic. Despite the homothallic nature of the fungus, high levels of random mating and absence of genetic structure was found in the investigated population, suggesting a strong effect of gene flow, probably linked to insect dispersal. The gene diversity of C. tsitsikammensis was similar to that of a related fungus, Ceratocystis albifundus, that is known to be native in Africa. This, together with the fact that C. tsitiskamensis is not known elsewhere, within or outside South Africa, suggests that it is native and endemic to the Cape Afromontane region.

Redefining Ceratocystis and allied genera

The genus Ceratocystis was established in 1890 and accommodates many important fungi. These include serious plant pathogens, significant insect symbionts and agents of timber degradation that result in substantial economic losses. Virtually since its type was described from sweet potatoes, the taxonomy of Ceratocystis has been confused and vigorously debated. In recent years, particulary during the last two decades, it has become very obvious that this genus includes a wide diversity of very different fungi. These have been roughly lumped together due to their similar morphological structures that have clearly evolved through convergent evolution linked to an insect-associated ecology. As has been true for many other groups of fungi, the emergence of DNA-based sequence data and associated phylogenetic inferences, have made it possible to robustly support very distinct boundaries defined by morphological characters and ecological differences. In this study, DNA-sequence data for three carefully selected gene regions (60S, LSU, MCM7) were generated for 79 species residing in the aggregate genus Ceratocystis sensu lato and these data were subjected to rigorous phylogenetic analyses. The results made it possible to distinguish seven major groups for which generic names have been chosen and descriptions either provided or emended. The emended genera included Ceratocystis sensu stricto, Chalaropsis, Endoconidiophora, Thielaviopsis, and Ambrosiella, while two new genera, Davidsoniella and Huntiella, were described. In total, 30 new combinations have been made. This major revision of the generic boundaries in the Ceratocystidaceae will simplify future treatments and work with an important group of fungi including distantly related species illogically aggregated under a single name.

Analysis of microsatellite markers in the genome of the plant pathogen Ceratocystis fimbriata

Ceratocystis fimbriata sensu lato represents a complex of cryptic and commonly plant pathogenic species that are morphologically similar. Species in this complex have been described using morphological characteristics, intersterility tests and phylogenetics. Microsatellite markers have been useful to study the population structure and origin of some species in the complex. In this study we sequenced the genome of C. fimbriata. This provided an opportunity to mine the genome for microsatellites, to develop new microsatellite markers, and map previously developed markers onto the genome. Over 6000 microsatellites were identified in the genome and their abundance and distribution was determined. Ceratocystis fimbriata has a medium level of microsatellite density and slightly smaller genome when compared with other fungi for which similar microsatellite analyses have been performed. This is the first report of a microsatellite analysis conducted on a genome sequence of a fungal species in the order Microascales. Forty-seven microsatellite markers have been published for population genetic studies, of which 35 could be mapped onto the C. fimbriata genome sequence. We developed an additional ten microsatellite markers within putative genes to differentiate between species in the C. fimbriata s.l. complex. These markers were used to distinguish between 12 species in the complex.

Ceratocystis species, including two new species associated with nitidulid beetles, on eucalypts in Australia

The genus Ceratocystis includes important fungal pathogens of trees, including Eucalyptus spp. Ironically, very little is known regarding the diversity or biology of Ceratocystis species on Eucalyptus species in Australia, where most of these trees are native. The aim of this study was to survey for Ceratocystis spp., and their possible insect associates, on eucalypts in Australia and thus to establish a foundation of knowledge regarding these fungi on the continent. Collections were made in three states of Australia from wounds on trees, as well as from nitidulid beetles associated with these wounds. Ceratocystis spp. were identified based on morphology and multigene sequence comparisons. Of the 54 isolates obtained, two previously unknown species of Ceratocystis were found and these are described here as Ceratocystis corymbiicola sp. nov. and Ceratocystis tyalla sp. nov. Furthermore, the distribution of Ceratocystis pirilliformis is expanded to include Eucalyptus spp. in Tasmania.

DNA sequence comparisons of Ophiostoma spp., including Ophiostoma aurorae sp. nov., associated with pine bark beetles in South Africa

BARK BEETLES (COLEOPTERA: Scolytinae) are well-recognized vectors of Ophiostoma species. Three non-native bark beetle species infest various Pinus species in South Africa, and they are known to carry at least 12 different species of ophiostomatoid fungi. Some of these fungi have not been identified to species level. The aim of this study was to determine or confirm the identities of Ophiostoma species associated with bark beetles in South Africa using comparisons of DNA sequence data. Identities of Ophiostoma ips, O. floccosum, O. pluriannulatum, O. quercus and O. stenoceras were confirmed. Ophiostoma abietinum, O. piliferum and Pesotum fragrans are recognised for the first time and the new species, O. aurorae sp. nov., is described from pine-infesting bark beetles in South Africa.

Ceratocystis omanensis, a new species from diseased mango trees in Oman

Mango (Mangifera indica) sudden decline is an important disease in Oman, which is closely associated with infections by Ceratocystis fimbriata and Lasiodiplodia theobromae. Another Ceratocystis species has also been found associated with symptoms on diseased trees. In this study, we identify that Ceratocystis based on morphology and DNA sequences. Morphological comparisons showed that the fungus from dying mango trees in Oman is similar to C. moniliformis. Both fungi have distinct hat-shaped ascospores, disc-shaped plates at the bases of the ascomatal necks and spines on the ascomatal bases. However, comparison of DNA sequences for ITS1-2, the 5.8S RNA gene, the beta-tubulin gene, and Transcription Elongation Factor (EF1-alpha) gene, confirmed that the fungus from Oman is distinct from C. moniliformis and other related species. Phylogenetically, this fungus formed one of four strongly supported sub-clades. The other sub-clades included isolates of C. bhutanensis, C. moniliformis and C. moniliformopsis, respectively. Based on morphological characteristics and differences in DNA sequences for three gene regions, we conclude that the Ceratocystis sp. from wounds on mango in Oman is a new species, for which we provide the name Ceratocystis omanensis sp. nov.

Pathogenicity, Internal Transcribed Spacer-rDNA Variation, and Human Dispersal of Ceratocystis fimbriata on the Family Araceae

ABSTRACT Ceratocystis fimbriata is a complex of many cryptic, host-specialized species that causes wilt and canker of woody species and rot diseases of storage roots and corms of many economically important plants worldwide. With the exception of the family Araceae, all confirmed hosts of C. fimbriata are dicotyledonous plants. We hypothesized that the isolates from members of the family Araceae would form a monophyletic lineage specialized to infect these unique hosts. Analyses of sequences of the internal transcribed spacer (ITS) region of nuclear rDNA indicate that isolates and herbarium specimens of C. fimbriata from the family Araceae represent three different groups: an Xanthosoma/Syngonium group on corms of Xanthosoma spp. from the Caribbean region and on ornamental S. podophyllum from greenhouses in Florida, Hawaii, Australia, and Brazil; an inhame group on corms of Colocasia esculenta in Brazil; and a distantly related taro group on Colocasia esculenta in Hawaii and China and on X. sagittifolium in Fiji. Inoculations of three species of Araceae (Caladium bicolor, S. podophyllum, and Colocasia esculenta) showed that isolates from all three groups are pathogenic to these three hosts. Brazilian isolates from Mangifera indica and Ficus carica were only weakly pathogenic to Caladium and Syngonium sp. and were not pathogenic to Colocasia sp. Syngonium plants appeared to be most susceptible to isolates of the Xanthosoma/Syngonium group, and Colocasia plants were least susceptible to isolates from Syngonium spp. Thus, it appears that adaptations to the family Araceae have evolved more than once in the C. fimbriata complex. It is hypothesized that the three groups of C. fimbriata on the family Araceae are native to the Caribbean, Brazil, and Asia, respectively, but they have been spread elsewhere by humans.