Tremorgenic indole metabolites and aflatoxins in sclerotia of Aspergillus flavus: an evolutionary perspective

Abstract

Isolates of Aspergillus flavus Link from both cool and warm latitudes were cultured on potato dextrose agar containing yeast extract to identify sclerotia-producing strains. Chloroform–MeOH extracts of sclerotia were analyzed for the presence of aflatoxins and major indole metabolites (e.g., cyclopiazonic acid, aflatrem, and dihydroxyaflavinine). Aflatoxin is reported from sclerotia of A. flavus for the first time. Cyclopiazonic acid was detected primarily in sclerotia of isolates from warmer latitudes. Aflatrem and dihydroxyaflavinine were detected in sclerotia from 85% of the strains examined. These metabolites are associated with the sclerotial stage of the life cycle, because neither were detected in extracts of the culture medium and mycelium of Petri dish cultures from which all the sclerotia were removed. Geographic variation and intrafungal allocation of these toxic compounds in A. flavus are examined from the evolutionary ecologist's perspective of selective forces shaping the chemical defense systems of fungi.

Distribution of sterigmatocystin in filamentous fungi

“…The species were predominantly isolated from soil, crop fields, nuts, figs and insects which might correlate with the hypothesized insecticide properties of aflatoxins and STs (Gloer et al 1988;Wicklow & Cole 1982 2), with branch lengths calculated using the average pathway method (see p. 132 in (Nei & Kumar 2000)) and are in the units of the number of changes over the whole sequence. Nidulantes in Table 2.…”

“…than aflatoxins (13 spp.). Despite 50 y of intensive research, the ecological function of ST and aflatoxin has never been elucidated; however, these toxins have, often in synergy with other toxic extrolites, been demonstrated to be active against fungivorous insects probably as chemical protection for conidia and sclerotia (Gloer et al 1988;Matsumura & Knight 1967;Ohtomo et al 1975;Wicklow & Cole 1982). ST biosynthesis is coupled to conidiation, and might improve fungal fitness for survival (Wilkinson et al 2004).…”

NsdC and NsdD Affect Aspergillus flavus Morphogenesis and Aflatoxin Production

“…High concentrations of AFs may occur in both conidia and sclerotia of A. flavus. Increased animal toxicity has been attributed to the combined activity of AFs and other metabolites present in sclerotia (42). Sclerotia are proposed by some researchers to be a vestige of the sexual structures termed cleistothecia produced by Aspergillus spp.…”

Occurrence of pyrrocidine and dihydroresorcylide production amongAcremonium zeaepopulations from maize grown in different regions

“…Warm humid conditions also favor the development of some maize diseases in the tropics and subtropics (Ahuja and Payak 1981). Aspergillus flavus isolates that produce aflatoxins and cyclopiazonic acid are more frequently recovered from warmer regions, suggesting that geographic variation in selection pressure has impacted the distribution ratio of these fungal toxins (Manabe and Tsuruta 1978;Wicklow 1990;Wicklow and Cole 1982). Dihydroresorcylide displayed phytotoxic activity in a maize leaf needle wound assay (Poling et al 2008) but showed no activity (MIC > 50 µg/mL) against fungal endophytes and pathogens of maize.…”