Moelleriella, and Samuelsia - CBS

Ch av e r r i e t a l.forests, the spatial separation between conspecific plant individualsmay have given rise to spatially separated insect populationsthat, in turn, coevolved with their fungal parasites (Evans 1988).Successful horizontal dispersal of the pathogens between coccidand whitefly colonies would have decreased proportionally withincreasing adaptation and restriction of their hosts to certain trees(Evans 1988).Epizootics caused by Hypocrella, Samuelsia, and Moelleriellaon coccids and whiteflies in the Tropics are so prominent thatseveral authors have wondered how these insects survived (Petch1925, Evans 1974, 1982, 1988). As mentioned before, dispersal ofthe fungus probably played an important role in the co-evolution withits insect host, and the co-evolution of the insect with its plant host.The anamorphic forms of Hypocrella, Samuelsia, and Moelleriellaproduce slimy masses of conidia that are well adapted for shortdistance,water-borne movement over leaf surfaces (Parkin 1906,Chaverri & Samuels 2003, Hodge 2003). The mucilage that coversthe conidia is high in sugar content making the conidia morehygroscopic (Meekes 2001) and attractive to insects. This modeof short-distance dispersal is apparently so efficient that once acolony of insects becomes infected by the fungus, it is difficult tofind healthy individuals (Evans 1988).Evans (1988) speculated about insect adaptation to the threatfrom entomopathogenic fungi. The short-distance dispersal ofconidia that contributes to epizootics may select for discontinuouspatterns of insect distribution. On the other hand, the teleomorphicforms of Hypocrella, Samuelsia, and Moelleriella are lesscommonly encountered and have dry, discharged ascospores thatare probably wind-dispersed and so better adapted for medium tolong distance dispersal (Evans 1988, 1989). However, the abovehypothesis could be refuted because in related genera, such asSphacelia, Ephelis and Neotyphodium, the discharge of conidia inslime appears to aid dispersal by insects (Loveless 1964, Moweret al. 1973, Mower & Hancock 1975, Samways 1983, Butler et al.2001, Hodge 2003).Scale insects and whiteflies often secrete sticky honeydewthat is attractive to wasps and ants, so it is also possible thatHypocrella, Samuelsia, and Moelleriella evolved to produce slimyconidia that adhere to non-host insect vectors. Infected coccid orwhitefly alates may further transport the fungus, as shown for someaphid-Entomophthorales interactions (Wilding & Perry 1980, Evans1989). Insect vectors and infected alates could disperse the sporesacross long distances; water, such as rain splash and run-off, coulddisperse across short distances (Chaverri et al. 2005a).Plant-insect-fungus specificity is suspected but not wellknown in Hypocrella, Samuelsia, and Moelleriella, and furtherexperimental evidence will be important in developing thesefungi as biological controls. Based on observations made for thepresent study, the majority of the specimens were found on shrubs,trees, palms, and Musaceae leaves, with a few species (e.g. M.libera, M. ochracea, H. disciformis, and H. viridans) found onsmall herbaceous plants. The plant-specificity of the host insectsmay be important in determining where the fungi are found; scaleinsects and whiteflies are themselves poorly understood, with theexception of a handful of species that are crop pests. Samuelsiageonomis, S. chalalensis, S. rufobrunnea, M. gaertneriana and M.schizostachyi have been found only on monocotyledonous plants,such as bamboo culms and palm leaves; and the related speciesM. africana and M. macrostroma have been found only on stemsof dicotyledonous plants (Hywel-Jones & Samuels 1998, Chaverriet al. 2005b). Although most species of Hypocrella, Samuelsia, andMoelleriella are found on leaves, other species such as M. epiphyllaand M. turbinata are found both on leaves and stems (Petch 1921,Mains 1959a, b). A few other species that are more widespreadcan be found both on monocot and dicot leaves (e.g. M. basicystis,M. libera, M. ochracea, and M. phyllogena). The great majority ofthe species of Hypocrella, Samuelsia, and Moelleriella are foundon leaves, with a larger portion occurring on the abaxial surfaceof leaves and a smaller percentage on the adaxial surface (e.g. M.epiphylla, M. turbinata, M. reineckiana) (Petch 1921, Hywel-Jones1998). Whitefly and scale-insect nymphs are mainly present on theabaxial surface of leaves.NutritionThe mechanisms by which Hypocrella, Samuelsia, and Moelleriellaspecies obtain enough nutrients from the host to support the relativelylarge size of the stromata are not well studied. In Moelleriella,several species have especially large stromata: M. gaertneriana,M. africana, M. schizostachyi, and M. macrostroma. Other generain the Clavicipitaceae, including Ascopolyporus A. Möller, DussiellaPat. and Hyperdermium, also parasitise scale insects and haverelatively large stromata. In these genera, the stromatal massgreatly exceeds that of the scale-insect host. Sullivan et al. (2000)hypothesised that the large size of the stromata results from a kindof secondary plant parasitism. Once the fungus has consumedthe scale-insect body, the fungus may continue to access plantnutrients through the insect’s stylet. Another hypothesis suggeststhat the mechanism of nutrient acquisition in fungal speciesparasitic on scale insects and whiteflies is through the living insectthat forms a bridge between the fungus and the plant (Couch 1938,Hywel-Jones & Samuels 1998). Koroch et al. (2006) studied hostnutrient adaptation in Balansia henningsiana (a plant pathogen)and M. phyllogena (a scale-insect pathogen). They observed thatboth fungi exhibit a restricted range of similar nutrient sources thatmay support growth and that those nutrients (such as sucrose) aremost likely from plant sources. This same study suggests that thescale-insect pathogen M. phyllogena obtains some of its nutrientsfrom the host plant rather than exclusively from the insect. In termsof its nutrition, M. phyllogena is more similar to the plant biotrophsthan to entomopathogenic fungi in the same family (Koroch et al.2006). Its close evolutionary relationship with plant pathogens hasbeen supported by previous studies (Bischoff et al. 2004, Chaverriet al. 2005a).Hypocrella, Samuelsia, and Moelleriella species can usually begrown in culture on most standard laboratory media. Growth ratesare relatively slow, but most species will produce conidia in culture.Typical stromata are not formed in culture, and sexual fruitingbodies have not been observed.Life cycle and epidemiologyHost and conidia may meet in two different ways: (1) directcontact, when conidia are released/introduced upon the insect host(dispersed by water, air, or other insects), or (2) indirect contact,when hatching or moulting nymphs (larvae) settle on or nearconidia already present on the leaf surface (Meekes 2001). Thesticky mucilage that covers the conidia permits them to adhere tohydrophobic surfaces, such as the insect cuticle in the presence ofwater (St.-Leger 1991, Meekes 2001). The whitefly or scale insectis infected when germinating spores penetrate the insect cuticle.Germination and appressorium formation in several aschersonialikespecies does not seem to be affected by the specific binding6