ORIGINAL ABSTRACT

BACKGROUND: Glyphosate [N-(phosphonomethyl) Glycine], commonly known as Roundup, is one of the most widely used herbicides for controlling crop weeds. However, its extensive use has led to the rapid evolution of glyphosate-resistant (GR) weed populations, often via amplification of the target gene EPSPS (5-enolpyruvylshikimate-3-phosphate synthase), sometimes accompanied by chromosomal abnormalities. OBJECTIVE: This study investigates the occurrence and characteristics of aneusomaty in GR Amaranthus palmeri and its interspecific hybrids with A. tuberculatus. METHODS: Fluorescence in situ hybridization (FISH) was used to cytologically characterize glyphosate resistance and aneusomaty in A. palmeri. RESULTS: Aneusomaty was observed in GR A. palmeri and its hybrids with A. tuberculatus. Root tips from a single GR A. palmeri plant exhibited two distinct chromosome numbers: 2n = 34 and 2n = 68. Similarly, F1 hybrid root tips [A. tuberculatus (2n = 32) x A. palmeri (2n = 34)] displayed 2n = 33 and 2n = 66. The pseudo F2 plants, apomictically derived from F1 hybrids, consistently showed 2n = 50 chromosomes. FISH targeting 5 S rDNA loci, was used to distinguish the genomes of A. palmeri and A. tuberculatus in the F1 and pseudo F2 plants. The pseudo F2 plants (2n = 50) contained the diploid chromosome set of A. palmeri (2n = 34) and the haploid chromosome set of A. tuberculatus (n = 16). Notably, these sporadic chromosome duplications occurred only in glyphosate-resistant (GR) plants and were absent in glyphosate-sensitive (GS) plants. CONCLUSION: Our results suggest that aneusomaty-related increases in chromosome numbers during the sporophytic phase may facilitate EPSPS gene duplication, offering a previously unrecognized mechanism contributing to glyphosate resistance in A. palmeri.