Development, validation, and evidence of frequent exchange of mating type HD alleles of Puccinia striiformis f. sp. tritici provide an insight into highly genetic diversity

Wheat stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is a worldwide destructive wheat disease. It possesses the potential to generate novel virulence races through sexual reproduction, which is the primary cause of its extensive epidemics. Mating type (MAT) of Pst lacks direct experimental evidence to account for the role of sexual recombination on high genetic diversity in evolution. Herein, we developed 11 pairs of HD alleles of Pst MAT genes, and 2 pairs HD alleles of P. striiformis f. sp. hordei (Psh, the causal agent of barley stripe rust) MAT genes. HD alleles in Pst are different from those in Psh. We tested that mating system of Pst is tetrapolar responsible for sexual genetic recombination via HD genes by experiments of selfing and sexual hybridization on alternate (secondary) hosts. Multiple HD gene types and HD gene recombination types were detected in Chinese natural Pst populations of 200 Pst isolates and 13 Psh isolates from 92 sampling sites in 11 provinces, supporting that sexual reproduction accounts for frequent emergence of new races and high genetic diversity of Pst population in northwestern oversummering variable region (hotspot) than other stripe rust-occurring regions in China. Overall, these results provide an insight into diverse types and frequent exchange of Ps MAT alleles, revealing the important role of sexual recombination via MAT alleles in virulence variation, high genetic diversity and epidemics of Pst, and also provide a basis for wheat stripe rust control in those regions where sexual reproduction occurs. Author summaryHeteroecism of the wheat stripe rust pathogen Puccinia striiformis f. sp. tritici (Pst) was discovered a decade ago. The significance of sexual reproduction in maintaining genetic diversity has emerged as a pressing concern in contemporary scientific discourse. In this study, we elucidated the presence of multiple HD gene types (HD1 linked to HD2) in Puccinia striiformis f. sp. tritici (Pst), distinguishing it from P. striiformis f. sp. hordei. Through biological experiments on sexual reproduction, we demonstrated that the mating system of Pst is tetrapolar. Furthermore, we observed that sexual reproduction, particularly selfing (individual race) and crossing (between two races), facilitates sexual recombination via HD genes, leading to pathogenic variation and a high level of genetic diversity. We established a model illustrating sexual reproduction of Pst by selfing and crossing, respectively. In China, we have observed that the Pst population exhibits a diverse range of HD gene types and HD gene recombination types. Notably, northwestern oversummering variable regions (hotspot) have a higher number of HD genes compared to other epidemic region. Population structure and cluster analyses based on HD genes indicate that Pst populations in Tibet and Xinjiang are distinct from those in other inland regions. These findings provide valuable insights into comprehending the intricate interplay between sexual reproduction and genetic diversity of Pst, thereby elucidating the reasons behind the high genetic diversity of the Chinese Pst population.