Target-Site Resistances to ALS and PPO Inhibitors Are Linked in Waterhemp (Amaranthus tuberculatus)

Abstract

It is generally expected that, in the case of multiple herbicide resistance, different resistance mechanisms within a weed will follow Mendel's law of independent assortment. Research was conducted to investigate anecdotal observations suggesting that target site-based resistances to inhibitors of acetolactate synthase (ALS) and protoporphyrinogen oxidase (PPO) did not follow independent assortment in common waterhemp. Cosegregation of the two resistances was observed in backcross lines (population sensitive to both herbicides as recurrent parent). Specifically, whereas 52% of backcross plants were resistant to a PPO inhibitor, this percentage increased to 92% when the backcross plants were preselected for resistance to an ALS inhibitor. Molecular marker analysis confirmed that the corresponding genes (ALS and PPX2) were genetically linked. When data from all plants analyzed were pooled, the genetic distance between the two genes was calculated to be 7.5 cM. The two genes were found to be about 195 kb apart in the recently published grain amaranth genome, explaining the observed genetic linkage. There is likely enough recombination that occurs between the linked genes to prevent the linkage from having significant implications in terms of resistance evolution. Nevertheless, documentation of the happenstance linkage between target-site genes for resistance to ALS and PPO inhibitors in waterhemp is a reminder that one should not assume distinct resistance mechanism will independently assort. Nomenclature: Common waterhemp, Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea and Tardif AMATU; grain amaranth, Amaranthus hypochondriacus L.

Molecular and physiological characterization of six‐way resistance in an Amaranthus tuberculatus var. rudis biotype from Missouri

“…The outcrossing nature of A. tuberculatus leads to extensive genetic diversity, which predisposes this species to evolve resistance to multiple herbicides. The movement of pollen within and among populations can also facilitate the stacking of different traits contributing to multiple herbicide resistances …”

“…The movement of pollen within and among populations can also facilitate the stacking of different traits contributing to multiple herbicide resistances. 5,8,9 Currently, A. tuberculatus populations resistant to herbicides from six sites-of-action (SOA) groups: synthetic auxins, photosystem II (PSII)-, 5-enylpyruvylshikimate-3-phosphate synthase (EPSPS)-, acetoacetate synthase (ALS)-, protoprophyrinogen oxidase (PPO)-, and 4-hydroxphenylpyruate dioxygenase (HPPD)-inhibitors have been reported. 10 The first multiple-resistant A. tuberculatus population resistant to PSII-and ALS-inhibiting herbicides was reported in Bond County, Illinois.…”

Dicamba resistance in kochia from Kansas and Nebraska evolved independently

“…Linkage of herbicide resistance genes has been investigated in several weed species, such as simazine, chlorotoluron and chlorsulfuron resistance in rigid ryegrass (Lolium rigidum), 16 and acetolactate synthase and protoporphyrinogen oxidase resistance in waterhemp (Amaranthus tuberculatus). 17 All these studies were mainly focused on uncovering the linkage of resistance genes of different herbicides in the same species. However, there is no report assessing the linkage of different genes conferring resistance to the same herbicide.…”

Limited fitness costs of herbicide‐resistance traits in Amaranthus tuberculatus facilitate resistance evolution

“…When our study was initiated, there was no known linkage of the five HR traits examined. Linkage was subsequently reported, however, between target‐site resistances to ALS and PPO inhibitors, with a recombination distance of 7.5 cM . Nevertheless, the mean effective population size (approximated from the harmonic mean across generations) averaged 723 plants for the three replicate rooms, providing ample opportunity for recombination to mitigate linkage effects.…”