Identification and pathogenesis of an entomopathogenic fungus isolated from black-legged tick, Ixodes scapularis (Say)
2007
Flor-Weiler, L.B. | Kurtti, T.J. | Munderloh, U.G.
Ticks carry pathogenic and symbiotic microorganisms including fungi. The authors isolated a fungus from a female black-legged tick, Ixodes scapularis, collected from a white-tailed deer. The isolate (SC1s 3-2) formed white colonies that turned creamy when mature, producing smooth-walled conidia and mycelia that bore clustered swollen conidiosphores. Conidia were single-celled, arranged alternately in zigzag rachis and somewhat circular in shape, 2.20 (+- 0.33)um x 2.08 (+- 0.28) um. The authors sequenced PCR amplicons of the ITS, EF1-subunit rRNA gene of SC1s 3-2. The primer set ITS4 and ITS5, that targets the ITS regions of the nuclear rDNA, amplified an approximately 600 bp PCR product. Primers NS5 and NS6 for the nuclear small subunit rRNA gene yielded a 350 bp PCR, product that locked the group 1 intron. Phylogenetic analyses of sequences revealed that isolate SC1s 3-2, is the ITS and EF1- Beauveria cf. bassiana, a species closely related to, but distinct from, B. bassiana sensu stricto. SC1s-2 conidia adhered immediately to the tick cuticle and germinated in 24 to 48 hours. Development of the germ tube was slow but penetration of the fungus into the hard cuticle and intersegmental membrane of legs was observed. There was also infection through the natural openings of the tick, i.e. pores of the spiracular plate. Internal infection was evident 10 to 14 days later. Budding blastospores and hyphae were visible in extirpated tick tissues. Dead and mummified ticks were observed 20-28 days post infection. The fungus grew out of the ticks' cuticle and produced conidia on surface of the cadaver. The cuticle proved to be the main barrier to fungal infection, delaying disease initiation and mortality of immature I. scapularis ticks.
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