Characteristics and taxonomy
Many phylogenetic studies have shown that members of the Family Clavicipitaceae (also known as the “clavicipitalean or ergot fungi”) form a polyphyletic group within the Order Hypocreales in the Ascomycota. At one time the Family was placed in the Order Clavicipitales, but that Order is no longer recognised as valid. Spatafora et al. (2007) provided phylogenetic evidence to suggest that the Clavicipitaceae most likely originated from an animal (arthropod) pathogen through a succession of hosts from different kingdoms.
Recent revisions of the Clavicipitaceae restrict the members of the family to plant- or insect- associated fungi characterised by the sexual state having flask-shaped ascomata (perithecia) immersed in a dark, sessile or stalked (stipitate) stroma. The perithecia are flask shaped and narrowed towards the top into a neck lined by long, sterile cells (periphyses). The neck protrudes above the stroma surface by varying degrees, forming a dome-shaped papilla which has a pore (ostiole) through which ascospores are ultimately ejected. Consequently the surfaces of stromata can vary from finely to coarsely papillate, depending on the species. A cluster of asci develops at the base of the perithecium, within a palisade of sterile paraphyses which usually are inconspicuous or disappear at maturity. Each ascus is cylindrical, composed of a single wall and is apically thickened, either as a distinct hemispherical cap perforated by a fine pore or as a slightly thickened, dome-shaped layer. Eight filiform, multiseptate ascospores develop in each ascus, with the ascospores breaking into part-spores after ejection from the ascus or less commonly within the ascus. Within the Clavicipitaceae, over 25 genera based on the sexual state or asexual state have been recognised as being associated with plants (White 1988; Shivas et al. 1997; Bischoff and White 2003). Species of only Balansia, Cepsiclava, Claviceps, Myriogenospora, Nigrocornus and Parepichloë, and the three asexual genera Corralocytostroma, Ephelis and Neotyphodium,are known to occur in Australia, so just only these species will be discussed further. Although a specimen identified as a species Dothichloë (a subspecies of Balansia according to Bischoff and White, 2003) was collected in 1930 in southern Queensland (BRIP 21053a), it is now of such poor quality that all taxonomic features are no longer discernable.
The sexual states of species of Balansia, Cepsiclava, Myriogenospora, Nigrocornus and Parepichloë are common in nature and their morphological features are predominantly used in identification and taxonomy. By contrast, the sexual reproductive structures of Claviceps species are rarely observed under natural conditions. so the characteristics of the asexual state (Sphacelia) are important in identification. Ephelis is widely accepted as the asexual state of Balansia, Myriogenospora and Nigrocornus (White 1988; Hodge 2003; Ryley 2003), with the conidia being hyaline, aseptate, acicular and developing holoblastically in a sympodial fashion on simple conidiophores either on an effuse hyhal mat (Nigrocornus) (Ryley 2003) or in a compound conidioma of varying size and shape (Balansia, Myriogenospora) (Hodge 2003), or both (Ephelis) (Christensen et al 2001; Ryley 2003). The Ephelis fructifications of Balansia, Myriogenospora, and Nigrocornus are ephemeral and easily overlooked or missed. The conidia of Corralocytostroma develop from phialides lining locules immersed in large, superficial conidiomata and similarly are cryptic Shivas et al. 1997; Pažoutová et al. 2004). No asexual state has been associated with species belonging in Parepichloë (Bischoff and White, 2003). In nature, Neotyphodium species have phialides arranged in small conidial heads in sporodochia (Pažoutová et al. 2004).
All species of Claviceps possess macroconidia which vary considerably in dimensions and shape,from oblong to reinform through to triangular depending on the species. The morphology of the macroconidia is a very useful taxonomic criterion which can be used to identify many species (Loveless, 1964; Pažoutová & Parbery, 1999). Macroconidia develop on short simple conidiophores (phialides) in a palisade layer over the surface of sphacelia which are composed of the colonised, undeveloped caryopses of their grass (Poaceae), sedge (Cyperaceae) and rush (Juncaceae) hosts (Pažoutová et al. (2004) Mature macroconidia are immersed in a sugary suspension which exudes from the base of the infected floret/sphacelium, filling the floret and spikelet and ultimately oozing from the spikelet. Small, globose microconidia that are often found in the honeydew of some Claviceps species develop on different phialides to those that produce macroconidia in the palisade but are not known to germinate (Pažoutová et al. (2004)
For some Claviceps species, macroconidia near the surface of the exuded honeydew drop germinate, producing a short sterigma. A single, hyaline, aseptate, obpyriform secondary conidium develops holoblastically at the tip of the sterigma in the air above the honeydew surface. These secondary conidia can be carried in the wind for many hundreds of kilometres. Interestingly, the existence of secondary conidia was not reported in the scientific literature before the publication of Frederickson et al., (1991) who considered it an important criterion in separating C. africana in Africa (secondary conidiation common) from C. sorghi in India (secondary conidiation rare, if at all). Eight (8) of the 15 described Australian Claviceps species are now known to produce secondary conidia quite commonly, so previously the presence of secondary conidia was considered either to be due to sporulation of another fungus that was growing on the honeydew surface (Ryley 1981), or was ignored. Raymond Forbes Newton Langdon (see History in Australia below), who conducted detailed studies on the morphology, development and taxonomy of Australian ergot fungi in the 1940’s and 1950’s, did not report the presence of secondary conidia for any species.
The biology of most of the other genera within the Clavicipitaceae is poorly understood. Some are endophytes, e.g., species of Cepsiclava and Neotyphodium(Bacon and Siegel 1988; Walker 2004), while others are reported as epiphytes, e.g., Myriogenospora and Nigrocornus ( Rykard et al. 1985; Ryley 2003).. For many species, stromata develop on host tissues (including inflorescences, stems and leaves depending on the species) on which asexual fruiting bodies (eg., acervulus, sporodochium) develop, followed soon after by sexual reproductive structures. The majority of genera produce visible stromata on their hosts’ surfaces, while plants infected by Neotyphodium species display no visible symptoms or signs of infection. Members of some genera (Cepsiclava, Ephelis, Nigrocornus and Parepichloe,) are known to survive from season to season in the tillers of their perennial grass hosts (White and Reddy 1998; Ryley 2003; Walker 2004), but the survival, spread and infection of hosts of most of the clavicipitaceous plant-infecting fungi has not been studied.
The largest genus in the Family Clavicipitaceae is the true ergot genus Claviceps with almost 60 valid species being described to date (Pažoutová et al. 2008) The macroconidia and microconidia of Claviceps species are transported in honeydew to the stigmas of flowering grasses, sedges and rushes by insects (especially flies) (Langdon and Champ 1954) or through other forms of mechanical transfer, while the secondary conidia are transported in the wind often for many hundreds of kilometres (Bandyopadhyay et al. 1998).. If the flowers of the hosts have not been fertilised by pollen, macroconidia and/or secondary conidia germinate and the resultant hyphae grow down the styles and colonise the undeveloped embryo, producing a sphacelium (Frederickson and Mantle 1988; Bandyopadhyay et al. 1998). Over time the hyphae of the sphacelium become thicker and compress into a hard-walled structure called a sclerotium, which is capable of surviving from one season to the next. When environmental conditions are conducive, the sclerotia germinate and produce one or more globose-subglobose, dark-coloured hyphal ascostromata (capitula) each at the end of a long, thin stipe. Perithecia are immersed around the outside of the ascostromata and at maturity each ascus in the perithecium elongates up through the neck of the perithecium where the ascospores are forcibly ejected through the pore in the tip of the ascus. Ascospores are capable of infecting unfertilised flowers of their hosts, similar to airborne or insect-borne conidia. The role of sclerotia and the ascal state in the biology of some species, such as Claviceps purpurea, a pathogen of wheat, barley and other temperate grass crops, is well understood, but their importance for other species, such as Claviceps africana, a pathogen of Sorghum and other subtropical/tropical grasses is uncertain.
Many of the clavicipitaceous fungi produce mycotoxins, with those produced by species of Claviceps being collectively known as ergot alkaloids. In nature, the ergot alkaloids are produced in the sphacelia and sclerotia, although they can leach into the surrounding honeydew. Different Claviceps species produce different suites of alkaloids. Claviceps purpurea produces several alkaloids including ergotamine, which can cause gangrene, hallucinations and other symptoms in mammals, including cattle, horses and occasionally humans. Outbreaks of St Anthony’s Fire, which occurred several times in European peasant populations from the Middle Ages up to recent times, was caused by the ingestion of bread made with rye flour contaminated with sclerotia of Claviceps purpurea. Claviceps paspali, which produces alkaloids and tremorgens (such as paspalitrem), periodically causes a mycotoxosis known as paspalum staggers in cattle, horses and sheep grazing in ergot-infected paspalum paddocks in Australia and overseas. Claviceps africana, an exotic ergot species first identified in Australia in 1996, produces mainly the alkaloid dihydroergosine, which causes agalactia in cows and sows and weight loss in cattle fed with ergot-contaminated sorghum grain. Endophytic Neotyphodium species produce a range of alkaloids; lolitrems, particularly lolitrem B are responsible for ryegrass staggers in cattle, sheep and other mammals grazing on ryegrass (Lolium perenne) infected by N. lolii, while fescue toxoses including fescue foot and fescue-associated oedema may occur when horses graze on tall fescue (Festuca arundinacea) infected with certain strains of N. coenophialum which produce alkaloids including ergovaline and N-acetyl norloline. Corralocytostroma has been shown to be the cause of Black Soil Blindness in cattle which grazed on infected grasses (species of Astrebla and Dichanthium) after wetter than normal winters in northern Australia. The mycotoxins responsible for the poisoning in grazing cattle have not been fully characterised.
Some ergot alkaloids produced in vitro are used to pharmaceutically treat a range of human ailments, including migraine and uterine contractions, and to control bleeding after childbirth. The psychedelic drug LSD (lysergic acid diethylamide) was first synthesised from the ergot alkaloid ergotamine by the German chemist Albert Hotmann in 1938 and was introduced commercially as a drug with various psychiatric uses in 1947. It has also has widespread use as an illegal, “recreational” drug.
History in Australia
It was not until 1853 that the French mycologist L. R. Tulasne explained the life history of Claviceps purpurea on rye by demonstrating that the sphacelial stage, the sclerotia and the ascostromata were all stages of the same fungus. In the following century, many species of clavicipitaceous fungi were described and named on grasses and sedges. In the 1940s and 1950s, the Australian mycologist Raymond Forbes Newton Langdon (1916–2014) conducted a comprehensive study of the ergot fungi over 2 decades and published a PhD thesis and many papers which amounted to a revision of the genus Claviceps in Australia. He recognised 26 species, eight of which were described by him, including six that were considered to be indigenous to Australia. Alderman (2003) listed 39 species of Claviceps that he considered to be valid.
The first plant–associated clavicipitaceous fungus collected in Australia was a specimen of Claviceps purpurea found on wheat in Victoria in 1884 and deposited in the Herbarium at the Royal Botanical Gardens, Kew (McAlpine, 1894; Langdon, 1960). Daniel McAlpine (1849–1932) reported that the fungus also occurred on perennial ryegrass, Lolium temulentum and a number of native grasses (McAlpine, 1894). Langdon (1950a, 1952) considered that C. purpurea was introduced into Australia soon after white settlement in 1788, probably in cereal or grass seed contaminated with sclerotia. A molecular analysis of European, American and Australian isolates supported Langdon’s hypothesis (Pažoutová & Parbery, 1999). It is interesting to note that neither C. purpurea nor any other Claviceps species is listed in the Handbook of Australian Fungi (Cooke, 1892), although two Cerebella species [Ce. andropogonis and Ce. paspali (=Ce. andropogonis)], which are considered to grow sapropthytically on the honeydew and sphacelia of Claviceps species, are included. However, in Cooke’s (1892) publication, two clavicipitaceous fungi are listed, namely #1489 Epichloë cinerea (now Parepichloë cinerea) and #1495 Hypocrella axillaris (now Nigrocornus scleroticus).
The British mycologists Mordecai Cubitt Cooke (1825–1914) and George Massee (1850–1917) described Cerebella paspali from Queensland, and considered that it belonged in the Order Ustilaginales (Cooke & Massee, 1887). Cerebella andropogonis had been noted by Frederick Manson Bailey (1827-1915) on Themeda australis (now T. triandra) and Ce. paspali on Heteropogon contortus in Queensland (Bailey, 1890). Langdon (1942a, 1952) examined the type specimen of Cerebella paspali listing it as K ex herb. F.M. Bailey no. 260 from Paspalum, Queensland. So, it is apparent that C. purpurea and a Claviceps species on Paspalum sp. had been collected in Australia before 1890. Langdon (1942b) deduced that the Paspalum species was most likely P. orbiculare (now Paspalum scrobiculatum), and later concluded that it was the Australian indigenous C. queenslandica (Langdon, 1954a, 1963). He noted that Claviceps paspali,the only other ergot species that has been recorded on P. scrobiculatum, had not been seen on its preferred host (Paspalum dilatatum) before 1935.
A widespread epidemic of C. paspali occurred on Paspalum dilatatum in the eastern states of Australia over the summer months of 1935–36, which lead paspalum staggers in dairy cattle that grazed ergot–infected paspalum pastures (Noble, 1936; Morwood, 1936; Hindmarsh & Hart, 1938). Paspalum ergot is now endemic on P. dilatatum and other Paspalum species throughout Australia. Langdon (1952) was of the opinion that C. paspali had been introduced to Australia not long before the epiphytotic in 1935–36, but it is possible that the species had been present for many years before. Outbreaks of staggers following ingestion of Paspalum have been recorded in cattle, horses and sheep in eastern Australia for over 100 years (Cawdell–Smith et al., 2007). Claviceps queenslandica, the other Claviceps species recorded on Paspalum in Australia has been rarely collected, and then only on water paspalum, P. scrobiculatum, a species less widely distributed than P. dilatatum. In addition, it is not known if C. queenslandica produces any mycotoxins.
Prior to the work of Langdon in the 1940s and 1950s, the knowledge of the Clavicipitaceae in Australia was confined to intermittent collections, records in lists of fungi, and occasional rare epiphytotics such as that of C. paspali. Langdon’s taxonomic studies were based on herbarium specimens collected in the subtropical and temperate zones of the eastern states of Australia and overseas continents including Africa. Langdon collected prolifically in southern Queensland during those two decades. When he began his research, C. purpurea and C. paspali were the only Claviceps species recorded in Australia, both of which he considered to be introduced. He ultimately recognised 7 indigenous Australian species (C. annulata, C. glabra, C. hirtella, C. inconspicua, C. queenslandica, C. platytricha and C. pusilla), with all but C. pusilla confined to the continent. Langdon also conducted a detailed study of Cerebella, definitively demonstrating that Cerebella is not a parasite of grass florets, and in the process relegated 32 names to synonymy with the type and only species, Cerebella andropogonis (Langdon, 1955). He also showed that insects in 13 Orders, in particular Pyrellia caerulea,were common and effective vectors of C. paspali on Paspalum species in Australia, a finding which Langdon used to explain the apparent rapid spread of the fungus in the mid 1930s (Langdon, 1952; Langdon & Champ, 1954).
Five Claviceps species have been recorded in Australia since Langdon’s studies. Claviceps nigricans was collected on Eleochlaris acuta (Cyperaceae) by L.D. Williams at Meningie, S.A. in 1953, and later at several other localities in S.A. and N.S.W. Ryley (1981) reported an outbreak of Claviceps maximensis on Panicum maximum (= Megathyrsus maximus) around Brisbane in 1981, with the first Australian record being in March 1980 from north Queensland. Since then, C. maximensis has been collected from localities throughout Queensland. It is likely that C. maximensis entered Australia some years before, because sclerotia of the fungus were found in seed imported from Africa in the mid 1970’s (Anonymous, 1974, 1975). Claviceps africana was first recorded in Australia from southern Queensland in April 1996 (Ryley et al., 1996), but was believed to have been present Australia for some years before (Bandyopadhyay et al., 1998); it is now endemic in Australia, having been found wherever grain and forage sorghum is grown (Ryley et al., 2002b). Airborne secondary conidia are considered to be the primary mode of transport of this species, resulting in the rapid spread of C. africana in Australia and overseas (Bandyopadhyay et al., 1998; Ryley et al., 2002a). Pazoutova (2000) used DNA markers to demonstrate that there are two strains of C. africana, the so-called “East strain” in India and Australia and the “West strain” in Africa and north and south America. Similar molecular technology has provided evidence that there have been several introductions of C. africana into Australia, most likely from secondary conidia originating on grasses in S.E. Asia (Komolong et al., 2002).
Claviceps cynodontis was described in 1954 from African and Indian specimens of Cynodon dactylon (Langdon, 1954a), but was not found in Australia until 1998. Since then it has been recorded at only three locations in Queensland. A Claviceps species with falcate conidia was found on Pennisetum glaucum in central Queensland in 1998 and was tentatively identified as Claviceps fusiformis (Ryley et al., 2002b), but a re–examination of that specimen has concluded that it is referable to Claviceps hirtella, a species found on a wide variety of grasses in Australia.
Douglas Parbery (1966- present), an Australian mycologist and plant pathologist who has had a long interest in the ergot fungi, co-authored a paper on the taxonomy and phylogeny of Claviceps with Sylvie Pažoutová, a prominent Czechoslavakian mycologist (Pažoutová & Parbery, 1999).
Roger Graham Shivas (1956–) and others described Corallocytostroma ornithocopreoides (as ‘ornicopreoides’) from several grasses (species of Astrebla and Dichanthium) in northern Western Australia, at localities where over 500 unexplained cattle mortalities had occurred in 1994 (Shivas et al., 1997). It differed from the only other species in that genus, C. oryzae, by the shape of its conidia, the known host range (Oryzae sativa only for C. oryzae) and its geographic distribution (China). These authors believed that C. ornithocopreoides belongs in the Clavicipitaceae based on the discovery of a single Claviceps-like ascostroma. Pažoutováet al. (2004) reported that the asexual state of this species was identical to that of Claviceps and demonstrated, using molecular techniques, that it belonged in the Clavicipitaceae.
The first Australian record of Myriogenospora atramentosa was on lemongrass (Cymbopogon citratus) growing in southern Queensland in 1994 (Shivas et al., 1999). It was found 5–6 years later on lemongrass at Maryborough and on sugarcane near Eumundi, both several hundred kilometres from the first locality, but has not been discovered since.
The genus Nigrocornus was erected by Malcolm John Ryley (1953–), with N. scleroticus as the type and only species (Ryley, 2003, 2006). This epiphytic balansioid fungus produces corniform, black ascostromata which surround the axillary buds at the nodes of its grass hosts, the dimensions of the ascostromata varying considerably from host to host. Its asexual state (referable to Ephelis) is an ephemeral, thin hyphal layer on the upper surfaces of expanding leaf blades on infected tillers, from which conidia develop on short conidiophores. Nigrocornus differs from Balansia and other members of the Clavicipitaceae on the location, morphology and spatial separation of its sexual and asexual states. It is widespread on mostly panicoid and andropogonoid grasses in the subtropical and tropical zones of Australia, Asia, India and Africa.
John Walker (1930–) described Claviceps phalaridis from ascostromata produced by germinating sclerotia found as contaminants in commercial Phalaris aquatica seed harvested in southern NSW and Victoria (Walker, 1958). It differed from C. purpurea by the morphology of the sclerotia, the longer, thinner conidia, and the longer asci and ascospores. Walker (2004) studied the fungus for over 35 years, and concluded that it did not belong in any currently recognised genus in the Clavicipitaceae, erecting the monospecific Cepsiclava (an anagram of Claviceps) to accommodate Cepsiclava phalaridis. It is characterised by being endophytic within its hosts, producing a sclerotium containing mummified floral organs, and two hyphomycetous anamorphs, primary unicellular condia produce holoblastically from enteroblastically proliferating condiogenous cells on the sclerotium apex, and secondary, 2–3 celled apically appendaged conidia produced holoblastically from germinating ascospores or primary conidia, or from the base of other secondary conidia.
Notes on descriptions
The descriptions of fungi in the following section are based primarly on examination of specimens deposited in Australian herbarium (BRIP = Queensland Department of Agriculture, Fisheries and Forestry, Brisbane; DAR = New South Wales Department of primary Industries, Orange) and oversease herbaria (IMI, PREM, etc). The original and other descriptions of taxa were used where herbarium material was scant or unsuitable for collection of data of the relevant taxonomic features.
References | Glossary