The diversity of nuclear cycle in microcyclic rust fungi (Uredinales) and its ecological and evolutionary implications

First European records of Puccinia modiolae and P. platyspora, two native South American rust fungi, and new observations on their life cycle and morphology

This paper reports the South American rust fungi Puccinia modiolae and P. platyspora (Pucciniales/Uredinales) as new alien species of the European rust funga. Puccinia modiolae is presently known from Switzerland and Germany, P. platyspora from Switzerland, Germany, and France. The records of P. platyspora are the first ones from outside South America. The specimens were identified by teliospore characters and sequences of the nuclear ribosomal DNA (internal transcribed spacer 2 and domains D1-D2 of the nuclear ribosomal large subunit) and the mitochondrial CO3 (cytochrome c oxidase III) gene. Puccinia modiolae and P. platyspora have been recorded so far in Europe on members of the genera Alcea, predominantly on Alcea rosea, Althaea, and Malva of the Malvaceae, subfam. Malvoideae. Alcea rosea is host of both species and shared also with the common mallow rust, P. malvacearum, allowing for mixed infections. The plant is commonly grown as an ornamental and may play a major role for the spread of the alien Malvaceae rust fungi. It was observed for the first time that P. platyspora can produce spermogonia and aecidium-type aecia, suggesting phenotypic plasticity regarding the formation of spore states. The observed spermogonia mainly remained closed and did not liberate spermatia. They produced telio- and aeciospores besides spermatia in their cavity and eventually converted entirely into telia or, rarely, into aecidium-like sori. Small clusters of aeciospores and peridial cells were commonly found hidden in the telial plectenchyma, and well-developed aecidium-type aecia provided with a peridium developed rarely in the center of mature telia. Spermogonia belonging to group V type 4 were found in P. malvacearum, which is generally supposed to lack spermogonia. Some spermogonia produced only spermatia in their cavity; others formed spermatia and teliospores, and some eventually converted into telia.

Phylogenetics of the rust fungi (Pucciniales) of South Africa, with notes on their life histories and possible origins

South Africa has an indigenous rust (Pucciniales) funga of approximately 460 species. This funga was sampled with species from as many genera as possible. The nuclear ribosomal large subunit (28S) region was amplified from samples representing 110 indigenous species, as well as the small subunit (18S) region and the cytochrome c oxidase subunit 3 (CO3) in some cases, and these were used in phylogenetic analyses. One new species is described, 12 new combinations made, six names reinstated, and two life history connections made. The life histories of this funga were summarized; it is dominated by species with contracted life histories. The majority of species are autoecious, with a small proportion being heteroecious. Of the autoecious species, many will likely be homothallic with no spermagonia. A shortened life history with homothallism allows for a single basidiospore infection to initiate a local population buildup under the prevailing unpredictable climatic conditions. Suggestions are made as to the possible origin of this funga based on the development of the modern South African flora. It is postulated that the rusts of South Africa are of relatively recent origin, consisting of three groups. Firstly, there is an African tropical element with members of the Mikronegerineae (Hemileia), the Sphaerophragmiaceae (Puccorchidium, Sphaerophragmium), and certain Uredinineae (Stomatisora). Their immediate ancestors likely occurred in the tropical forests of Africa during the Paleogene. Secondly, there is a pantropical element including the Raveneliaceae (e.g., Diorchidium, Maravalia, Ravenelia sensu lato, Uropyxis). This likely diversified during the Neogene, when the mimosoids became the dominant trees of the developing savannas. Thirdly, the Pucciniaceae invaded Africa as this continent pushed northward closing the Tethys Sea. They diversified with the development of the savannas as these become the dominant habitat in most of Africa, and are by far the largest component of the South African rust funga.

Multinuclei Occurred Under Cryopreservation and Enhanced the Pathogenicity of Melampsora larici-populina

Melampsora larici-populina is a macrocyclic rust, and the haploid stage with two nuclei and the diploid of mononuclear sequentially occur annually. During the preservation of dry urediniospores at −80°C, we found that one isolate, ΔTs₀₆, was different from the usual wild-type isolate Ts₀₆ at −20°C because it has mixed polykaryotic urediniospores. However, the other spores, including the 0, I, III, and IV stages of a life cycle, were the same as Ts₀₆. After five generations of successive inoculation and harvest of urediniospores from the compatible host Populus purdomii, the isolate ΔTs₀₆ steadily maintained more than 20% multiple nucleus spores. To test the pathogenesis variation of ΔTs₀₆, an assay of host poplars was applied to evaluate the differences between ΔTs₀₆ and Ts₀₆. After ΔTs₀₆ and Ts₀₆ inoculation, leaves of P. purdomii were used to detect the expression of small secreted proteins (SSPs) and fungal biomasses using quantitative real-time PCR (qRT-PCR) and trypan blue staining. ΔTs₀₆ displayed stronger expression of five SSPs and had a shorter latent period, a higher density of uredinia, and higher DNA mass. A transcriptomic comparison between ΔTs₀₆ and Ts₀₆ revealed that 3,224 were differentially expressed genes (DEGs), 55 of which were related to reactive oxygen species metabolism, the Mitogen-activated protein kinase (MAPK) signaling pathway, and the meiosis pathway. Ten genes in the mitotic and meiotic pathways and another two genes associated with the “response to DNA damage stimulus” all had an upward expression, which were detected by qRT-PCR in ΔTs₀₆ during cryopreservation. Gas chromatography–mass spectrometry (GC-MS) confirmed that the amounts of hexadecanoic acid and octadecadienoic acid were much more in ΔTs₀₆ than in Ts₀₆. In addition, using spectrophotometry, hydrogen peroxide (H₂O₂) was also present in greater quantities in ΔTs₀₆ compared with those found in Ts₀₆. Increased fatty acids metabolism could prevent damage to urediniospores in super-low temperatures, but oxidant species that involved H₂O₂ may destroy tube proteins of mitosis and meiosis, which could cause abnormal nuclear division and lead to multinucleation, which has a different genotype. Therefore, the multinuclear isolate is different from the wild-type isolate in terms of phenotype and genotype; this multinucleation phenomenon in urediniospores improves the pathogenesis and environmental fitness of M. larici-populina.

Genetic evidence for sexual reproduction and multiple infections of Norway spruce cones by the rust fungus Thekopsora areolata

Rust fungi are obligate parasites, of plants, with complex and in many cases poorly known life cycles which may include host alteration and up to five spore types with haploid, diploid, and dikaryotic nuclear stages. This study supports that Thekopasora areolata, the causal agent of cherry-spruce rust in Norway spruce, is a macrocyclic heteroecious fungus with all five spore stages which uses two host plants Prunus padus and Picea abies to complete its life cycle. High genotypic diversity without population structure was found, which suggests predominantly sexual reproduction, random mating and a high gene flow within and between the populations in Fennoscandia. There was no evidence for an autoecious life cycle resulting from aeciospore infection of pistillate cones that would explain the previously reported rust epidemics without the alternate host. However, within cones and scales identical multilocus genotypes were repeatedly sampled which can be explained by vegetative growth of the fertilized mycelia or repeated mating of mycelium by spermatia of the same genotype. The high genotypic diversity within cones and haplotype inference show that each pistillate cone is infected by several basidiospores. This study provides genetic evidence for high gene flow, sexual reproduction, and multiple infections of Norway spruce cone by the rust fungus T. areolata which expands the general understanding of the biology of rust fungi.

The genus Ravenelia (Pucciniales) in South Africa

The genus Ravenelia represents the third largest genus of rust fungi and parasitizes a great number of leguminous shrubs and trees, mainly in the subtropics and tropics. Molecular phylogenetic analyses of this genus using nc 28S rDNA and CO3 sequences are presented with a special focus on South African representatives of Ravenelia. Many of the specimens had been collected by us in recent years, mainly from acacia species of the genera Vachellia and Senegalia. Morphological characters were extensively studied using light microscopy and scanning electron microscopy. The analyses resolved several well-supported phylogenetic groups. By linking these groups to their morphology and life cycle characteristics, it was possible to interpret the outcomes in terms of their evolutionary ecology and biogeography. Several characters previously used to define subgeneric groups within Ravenelia were found to be misleading because of assumed convergent evolution. However, host associations, the ability to induce aecial galls as well as the development of two-layered probasidial cells emerged as useful criteria for inferring monophyletic groups. Six novel Ravenelia species were discovered and described. Furthermore, five species represent new reports for South Africa, species descriptions were emended for two taxa, and a new host report emerged for R. inornata.

Morphology of Puccinia horiana, Causal Agent of Chrysanthemum White Rust, Sampled From Naturally Infected Plants

Chrysanthemum white rust (CWR), caused by Puccinia horiana, is pathogenic on many Chrysanthemum spp. and close relatives, and infects commercially important florist chrysanthemum cultivars (Chrysanthemum × morifolium) throughout the world. Due to regulations, most research and observations with CWR are done in vitro with symptomatic plants. In contrast, research presented herein is based on microscopic examination of symptomatic and asymptomatic plants collected from natural outbreaks in the field. We observed scattered (not in a linear pattern) telial sori on infected chrysanthemum leaves, stems, and flowers that coalesced at high infection levels. Teliospores were mainly two-celled but occasionally one- or three-celled. Promycelia arose from the apical teliospore cell, the basal cell, or both. The number of basidiospores on promycelia varied from one to four. Germ tubes, arising from P. horiana basidiospores, penetrated the host epidermis directly without appressoria. A mucilaginous exudate formed at the site of attachment and penetration of leaf and stem tissue, as well as on internal cell walls. P. horiana colonization was systemic, with intercellular mycelium and intracellular M-haustoria in both symptomatic and asymptomatic infected host tissue. Hyphal anastomosis was observed within infected plants, suggesting that asexual fusion between different P. horiana pathotypes or genotypes might occur.

Genetic relationships in an international collection of Puccinia horiana isolates based on newly identified molecular markers and demonstration of recombination

The obligate biotrophic pathogen Puccinia horiana is the causal agent of chrysanthemum white rust. Although P. horiana is a quarantine organism, it has been able to spread to most chrysanthemum-producing regions in the world since the 1960s; however, the transfer routes are largely obscure. An extremely low level of allelic diversity was observed in a geographically diverse set of eight isolates using complexity reduction of polymorphic sequences (CRoPS) technology. Only 184 of the 16,196 contigs (1.1%) showed one or more single-nucleotide polymorphisms (SNPs). Thirty-two SNPs and one simple-sequence repeat were translated into molecular markers and used to genotype 45 isolates originating from North and South America, Asia, and Europe. In most cases, phylogenetic clustering was related to geographic origin, indicating local establishment. The European isolates mostly grouped in two major populations that may relate to the two historic introductions previously reported. However, evidence of recent geographic transfer was also observed, including transfer events between Europe and South America and between Southeast Asia and Europe. In contrast with the presumed clonal propagation of this microcyclic rust, strong indications of marker recombination were observed, presumably as a result of anastomosis, karyogamy, and somatic meiosis. Recombination and transfer also explain the geographic dispersal of specific markers. A near-to-significant correlation between the genotypic data and previously obtained pathotype data was observed and one marker was associated with the most virulent pathotype group. In combination with a fast SNP detection method, the markers presented here will be helpful tools to further elucidate the transfer pathways and local survival of this pathogen.