The Gene Introgression Approach and the Potential Cost of Genes that Confer Strong Phosphine Resistance in Red Flour Beetle (Coleoptera: Tenebrionidae)

Abstract

Resistance in pest insects to the grain fumigant phosphine (PH3) poses a threat to trade and food security. The possible pleiotropic effects of PH3 resistance on development and reproduction were investigated in the red flour beetle, Tribolium castaneum (Herbst), by introgressing two genes known to be major contributors to strong resistance (tc_rph1 and tc_rph2) into a susceptible background. The tc_rph2 allele was the G135S variant, whereas the identity of tc_rph1 allele was unknown but could have been one of the three known variants (L119W, V123F, or S349G). The introgressed resistant strain was 288× more resistant than the susceptible strain, based on mortality after a 20 h fumigation with PH3. Molecular screening confirmed that the introgressed strain was homozygous for the resistance genes, but was otherwise indistinguishable from the susceptible strain based on screening with 12 neutral DNA markers. We found no differences of consequence in developmental time between the susceptible and introgressed resistant strains. Similarly, the number of F1 adults produced by these strains was more or less equal, as was the weight of individual F1 adults. The conclusions remained the same regardless of whether the experiments were conducted on a flour-based medium or wheat. Thus, we found no evidence that being fully strongly PH3 resistant (i.e., homozygous for tc_rph1 and tc_rph2) has major consequences in terms of development or reproduction in T. castaneum.

Gene introgression in assessing fitness costs associated with phosphine resistance in the rusty grain beetle

“…While we conclude that phosphine resistant insects do not incur fitness costs in C. ferrugineus, in alignment with previous reports on three major stored product pest species, R. dominica, T. castaneum and S. oryzae (Schlipalius et al 2008;Daglish et al 2014Daglish et al , 2015Daglish et al , 2020, the observed decline in rph2 allele frequency in unselected segregating populations of T. castaneum (Jagadeesan et al 2013a) and S. oryzae (Nguyen et al 2016) needs further examination. Unfortunately, both these latter studies could not continue assessing the decline in allele frequency beyond the F 20 filial generations, and did not correlate the observed change in allele frequency in terms of developmental or reproductive traits.…”

“…Our results supported the importance of controlling for background genetic effects, without which the significant differences observed between the Ref-R and Ref-S in developmental time and body weight could have been interpreted that phosphine resistance genes are associated with fitness costs. Daglish et al (2020) also observed significant difference in beetle weight between susceptible and resistant strains in T. castaneum, but not when the phosphine resistance trait was compared in an isogenic background, supporting the conclusion of this study. Similar results of insecticide susceptible strain having fitness advantage over the resistant strain but not with the near-isogenic resistant strain were reported in resistance to malathion in C. ferrugineus (White and Bell 1990) and Bt toxins (Cry1F) in the fall army worm, Spodoptera frugiperda (J.E.…”

“…Although many studies had adopted the gene introgression approach to investigate the fitness cost (Arnaud et al 2005;Horikoshi et al 2016), very few of them in fact validated the success of this approach phenotypically (White and Bell 1990) or genotypically (Berticat et al 2002). Recently, Daglish et al (2020) employed the gene introgression approach to produce a near-isogenic resistant strain of T. castaneum and confirmed the gene introgression through phenotypic and genotypic characterisation similar to what was used in the current study. In general, the success of introgression approach relies on the number of recurrent backcrosses (F 1 -BC), followed by phosphine selection on progeny (F 2 -BC).…”

“…Data on the cumulative number of adults emerged from all the tested strains over a period of time were first converted to cumulative proportions and then fitted to a 3-parameter logistic equation Y = c/{1 + exp(-b*[X-m])}, using a regression analysis with X as days, which predicted the days to 50% emergence (m), cumulative emergence (Y), maximum emergence of progeny (c) and the shape parameter (b) (Daglish et al 2020). If a significant difference was found among the curves, they were refitted in a pairwise manner to determine whether there was significant difference between the individual pairs.…”

“…Furthermore, the above-mentioned studies had no complete control for inherent genetic variations. Recently, Daglish et al (2020) utilised the gene introgression approach aiming to achieve complete control for genetic background by establishing an isogenic resistant T. castaneum and found no evidence for fitness costs for being strongly resistant to phosphine.…”