The taxonomically challenging genus Amaranthus (Family Amaranthaceae) includes important agricultural weed species that are being spread globally as grain contaminants. We hypothesized that the ALS gene will help resolve these taxonomic challenges and identify potentially harmful resistant biotypes. We obtained 153 samples representing 26 species from three Amaranthus subgenera and included in that incorporated ITS , ALS (domains C, A and D) and ALS (domains B and E) sequences. Subgen. Albersia was well supported, but subgen. Amaranthus and subgen. Acnida were not. Amaranthus tuberculatus , A. palmeri and A. spinosus all showed different genetic structuring. Unique SNPs in ALS offered reliable diagnostics for most of the sampled Amaranthus species. Resistant ALS alleles were detected in sixteen A. tuberculatus samples (55.2%), eight A. palmeri (27.6%) and one A. arenicola (100%). These involved Ala 122 Asn, Pro 197 Ser/Thr/Ile, Trp 574 Leu, and Ser 653 Thr/Asn/Lys substitutions, with Ala 122 Asn, Pro 197 Thr/Ile and Ser 653 Lys being reported in Amaranthus for the first time . Moreover, different resistant mutations were present in different A. tuberculatus populations. In conclusion, the ALS gene is important for species identification, investigating population genetic diversity and understanding resistant evolution within the genus Amaranthus .

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