Phylogenetic studies and distinction of aflatoxin-producing Aspergillus species in section Flavi, Ochraceorosei and Nidulantes: A review.

Abstract

Aspergillus species produce polyketides, which form the basis of aflatoxins, some of the most significant mycotoxins in agriculture. Aflatoxins contaminate cereals, oilseeds, and nuts, both in the field and during storage. Of the 13 naturally occurring aflatoxins, the most potent are aflatoxins B₁, B₂, G₁, and G₂. The primary aflatoxigenic species are A. flavus, A. parasiticus, and A. nomius, while A. arachidicola, A. minisclerotigenes, and A. saccharicola also documented. These aflatoxin producers belong to three sections- 'Flavi', 'Ochraceorosei', and 'Nidulantes.' Aspergillus flavus, within section Flavi, shows morphological diversity, classified into Group I (S- and L- strains) and Group II (S- strains), with S-strains producing higher levels of aflatoxins. Aflatoxin biosynthesis is primarily regulated by the aflR gene, though other genes like aflS, aflP, aflQ, aflC, and aflM are also associated. However, presence of the aflR gene does not guarantee aflatoxin production across species. Sterigmatocystin serves as a precursor molecule within the pathway leading to aflatoxin production. Phylogenetic assessment, using ITS, BenA, CaM, and RBP2 gene sequences, reveals distinct clusters within Aspergillus sections and highlights the co-evolution of aflatoxigenic and non-aflatoxigenic species. Aspergillus ochraceoroseus and A. rambellii diverged out of aflatoxin-producing species earlier in evolutionary history, before splitting from a shared ancestor with A. fumigatus, which neither produces aflatoxins nor sterigmatocystin. Non-aflatoxigenic species like A. oryzae may evolve from aflatoxigenic species like A. flavus due to variations in evolutionary rates, telomere deletions, and mutations in aflatoxin biosynthesis genes. Comparative genomic analysis of AF, AF/ST and ST gene cluster shows that A. flavus has a larger aflatoxin gene cluster, while A. ochraceoroseus lacks the genes aflP and aflQ. Additionally, A. ochraceoroseus and A. rambellii possess a smaller genome, suggesting that genetic drift and deletions have refined their genomes for more efficient aflatoxin production.