Lack of Host Specialization inAspergillus flavus

Abstract

Aspergillus spp. cause disease in a broad range of organisms, but it is unknown if strains are specialized for particular hosts. We evaluated isolates of Aspergillus flavus, Aspergillus fumigatus, and Aspergillus nidulans for their ability to infect bean leaves, corn kernels, and insects (Galleria mellonella). Strains of A. flavus did not affect nonwounded bean leaves, corn kernels, or insects at 22°C, but they killed insects following hemocoelic challenge and caused symptoms ranging from moderate to severe in corn kernels and bean leaves injured during inoculation. The pectinase P2c, implicated in aggressive colonization of cotton bolls, is produced by most A. flavus isolates, but its absence did not prevent colonization of bean leaves. Proteases have been implicated in colonization of animal hosts. All A. flavus strains produced very similar patterns of protease isozymes when cultured on horse lung polymers. Quantitative differences in protease levels did not correlate with the ability to colonize insects. In contrast to A. flavus, strains of A. nidulans and A. fumigatus could not invade living insect or plant tissues or resist digestion by insect hemocytes. Our results indicate that A. flavus has parasitic attributes that are lacking in A. fumigatus and A. nidulans but that individual strains of A. flavus are not specialized to particular hosts.Aspergillus species are associated with disease in plants, insects, man, and other animals (4, 14, 24). For example, Aspergillus flavus causes disease of agronomically important crops, such as corn and peanuts, is second only to Aspergillus fumigatus as the cause of human invasive aspergillosis, and is the Aspergillus species most frequently reported to infect insects.Aspergillus spp. are generally regarded as opportunistic pathogens that require wounds or otherwise weakened hosts for colonization (24). A. flavus, however, also has limited parasitic abilities and, in some cases, can directly invade seeds and colonize living tissues (20). We found previously (35) that Aspergillus spp. produced a much broader spectrum of protein and polysaccharide-hydrolyzing enzymes than did specialized pathogens, which may be indicative of their less-specialized lifestyle. Shieh et al. (29), however, found that aggressiveness toward cotton depended on production of a specific pectinase isoenzyme (P2c) that facilitates spread between cotton boll locules. Weakly pathogenic strains lack P2c, and their growth is limited to individual locules. Likewise, production of proteinases (elastases) in culture has been correlated with isolate virulence to mice and with isolation from human hosts with aspergillosis (12, 13, 25), although soil isolates also produce proteases (17). Proteases may be required for virulence to plants, as resistance to A. flavus in corn kernels derives from a protease inhibitor (5).Populations of A. flavus are highly polymorphic and complex (40), but we do not know if specialized subspecies or physiological races contribute to the broad host range (1, 6, 9). If specializat...

Nuclear small subunit rRNA group I intron variation among Beauveria spp provide tools for strain identification and evidence of horizontal transfer

“…St Leger et al (1992) and Maurer et al (1997) suggested that insect host range affected genetic similarity among strains, with selection imposed by insect pathogen defense restricting the number of successful genotypes, but this was contradicted by data presented by Coates et al (2001Coates et al ( , 2002 and analogous data from Aspergillus (St Leger et al 2000). Viaud et al (1996) and Couteaudier and Viaud (1997) suggested that vegetative compatibility groups influenced genetic similarity among isolates and likelihood of genetic exchange, especially those from Ostrinia Beauveria group 1 intron sequence data generated in this study are marked with an asterisk, followed by internal transcribed spacer region haplotype (A, C, G, L, O), as determined by Coates et al (2002).…”

“…Supporting data by Viaud et al (1996) and Couteaudier and Viaud (1997) found genetic homogeneity among isolates from Ostrinia nubilalis and implicated vegetative compatibility groups in defining the likelihood of genetic exchange. Two studies by Coates et al (2001Coates et al ( , 2002 indicated a weak association between genotype and insect host preference, which mirrored conclusions from Aspergillus (St Leger et al 2000). Significant genetic variation among haplotypes was defined at a minisatellite locus, BbMin1, but a high level of polymorphism was implicated in the generation of homoplasy between strains from different insect hosts (Coates et al 2002).…”

Methods for usingGalleria mellonellaas a model host to study fungal pathogenesis

“…9 However within 16 to 24 hours A. flavus spores were able form hyphae that penetrated the hemocytes and the larvae defense response was insufficient to combat the fungal pathogen, resulting in a moribund infection for the larvae. Moreover, unlike A. flavus, A. fumigatus cells do not germinate within the live larval host but are able to germinate in dead hosts.…”

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“…Fungal dissemination throughout different organs begins with invasion or inhalation of A. flavus spores, which may involve particular virulence routes such as interaction of the fungus with epithelial receptors and colonization of the digestive tract 23 . A. flavus is not host-specific, and is capable of causing disease in humans, plants, and insects 34 . A. flavus strains differed significantly in virulence on D. melanogaster, implying variability in virulence factors 13 .…”