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Nuclear rDNA phylogeny in the fungal genus Verticillium and its relationship to insect and plant virulence, extracellular proteases and carbohydrases

Department of Biology, Trent University, Peterborough, Ontario, Canada K9J 7B8
Department of Entomology, University of Maryland, College Park, MD 20742, USA
Department of Zoology, University of Toronto, Toronto, Ontario, Canada M5S 1A6

Author for correspondence: Michael J. Bidochka. Tel: +1 705 748 1021. Fax: +1 705 748 1205. e-mail: mbidochka@trentu.ca

ABSTRACT

Summary: Phylogenetic relationships among 18 isolates in the genus Verticillium, representing 13 species of diverse econutritional groups (pathogens of insects, plants, mushrooms, nematodes and spiders, and sparobes), were examined by using sequences from the internal transcribed spacer (ITS) and small nuclear (NS) rRNA regions. The isolates were also assessed for their abilities to infect insect larvae (Galleria mellonella) and to cause necrosis in alfalfa (Medicago sativa), and for their proteolytic, chitinolytic and pectinolytic activities. The phylogenetic data suggested that Verticillium is polyphyletic in origin and is therefore a form genus. However, the phylogenetic tree supported the plant pathogens (V. dahliae, V. albo-atrum and V. nigrescens) as a clade. The alfalfa isolate of V. albo-atrum (isolate 595) was an interesting outlier to the main body of plant pathogens as it clustered with the insect pathogen V. indicum. Strains of V. lecanii and V. indicum were able to infect insects and are present in divergent groups in the consensus tree, suggesting that the ability to infect insects may have evolved independently many times. Similarly, the nematophagous Verticillium species appear to have evolved independently along several different routes and one isolate, V. chlamydosporium, was able to infect insects. V. albo-atrum, V. nigrescens and V. dahliae all produced high levels of enzymes capable of degrading pectin, a major component of plant cell walls. The ability to excrete pectinase was a broad indicator of the ability to produce lesions on alfalfa. In the plant pathogens, the functions of a broad-spectrum protease were assumed by trypsins which degrade Bz-AA-AA-Arg-NA substrates (Bz, benzoyl; AA, various amino acids; NA, p-nitroanilide). The insect pathogens and mushroom pathogen (V. fungicola) were characterized by production of high levels of subtilisin-like proteases active against a chymotrypsin substrate (succinyl-Ala₂-Pro-Phe-NA) and the inability to clear pectin. The insect and mushroom pathogens, and several nematode pathogens, were distinguishable from the plant pathogens in their ability to produce chitinases.

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