Are Current Aspergillus sojae Strains Originated from a Native Aflatoxigenic Aspergillus Species Population Also Present in California?

Perng-Kuang Chang; Sui Sheng T Hua

doi:10.1080/12298093.2023.2217495

Are Current Aspergillus sojae Strains Originated from a Native Aflatoxigenic Aspergillus Species Population Also Present in California?

Mycobiology. 2023 Jun 8;51(3):139-147. doi: 10.1080/12298093.2023.2217495. eCollection 2023.

Authors

Perng-Kuang Chang¹, Sui Sheng T Hua²

Affiliations

¹ Southern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, New Orleans, LA, USA.
² Western Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, Albany, CA, USA.

Abstract

Aspergillus sojae has long been considered a domesticated strain of Aspergillus parasiticus. This study delineated relationships among the two species and an Aspergillus PWE36 isolate. Of 25 examined clustered aflatoxin genes of PWE36, 20 gene sequences were identical to those of A. sojae, but all had variations to those of A. parasiticus. Additionally, PWE36 developmental genes of conidiation and sclerotial formation, overall, shared higher degrees of nucleotide sequence identity with A. sojae genes than with A. parasiticus genes. Examination of defective cyclopiazonic acid gene clusters revealed that the PWE36 deletion pattern was identical only to those of A. sojae. Using A. sojae SMF134 genome sequence as a reference, visualization of locally collinear blocks indicated that PWE36 shared higher genome sequence homologies with A. sojae than with A. parasiticus. Phylogenetic inference based on genome-wide single nucleotide polymorphisms (SNPs) and total SNP counts showed that A. sojae strains formed a monophyletic clade and were clonal. Two (Argentinian and Ugandan) A. parasiticus isolates but not including an Ethiopian isolate formed a monophyletic clade, which showed that A. parasiticus population is genetically diverse and distant to A. sojae. PWE36 and A. sojae shared a most recent common ancestor (MRCA). The estimated divergence time for PWE36 and A. sojae was about 0.4 mya. Unlike Aspergillus oryzae, another koji mold that includes genetically diverse populations, the findings that current A. sojae strains formed a monophyletic group and shared the MRCA with PWE36 allow A. sojae to be continuously treated as a species for food safety reasons.

Keywords: Aspergillus parasiticus; Aspergillus sojae; aflatoxin gene cluster; cyclopiazonic acid; phylogeny; single nucleotide polymorphism.

This work was authored as part of the Contributor's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.