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

Abstract

The current study investigates the fitness cost associated with phosphine resistance in the rusty grain beetle, Cryptolestes ferrugineus (Stephens), a problematic pest in the stored commodities that has developed strong resistance to fumigant phosphine. Three characterised insect strains: the susceptible (Ref-S), the strongly resistant (Ref-R), the introgressed resistant (Intro-R) and a segregating population (F 25 ) derived from crossing the Ref-S and Ref-R strains were used in this study. Intro-R was developed by introgressing two phosphine resistance genes, cf_rph1 and cf_rph2 into Ref-S, aimed to reduce the influence of background genetic factors. Intro-R exhibited 592 × resistance to phosphine and homozygous for strong resistance allele, cf_rph2 (L73N). Two key fitness cost criteria, developmental time and fecundity, were assessed under optimal and suboptimal conditions (less favourable diet and low temperature). There was no significant difference in developmental time and fecundity between Ref-S and either Intro-R strain or F 25 under optimal conditions. When challenged with a less favourable diet, cracked wheat + cracked sorghum (CW + CS), or exposed to a low temperature (22 °C), both Intro-R and Ref-S had similar developmental time and total numbers of F 1 progeny, confirming the absence of significant fitness effects expressed in these conditions. Therefore, we conclude that strongly phosphine resistant C. ferrugineus are unlikely to incur potential fitness costs. This finding will have implications towards developing strategies to manage this pest.

Fitness costs associated with chlorantraniliprole resistance in Spodoptera frugiperda (Lepidoptera: Noctuidae) strains with different genetic backgrounds

“…No fitness cost associated with near‐isogenic resistant strains was reported in Plutella xylostella resistant to Cry1Ac toxin, 33 S. frugiperda resistant to Cry1F toxin 34 and, Helicoverpa armigera resistant to indoxacarb insecticide, 40 though the genetic background of strains used in those reports were not monitored. On the other hand, experiments with controlled genetic background in Cryptolestes ferrugineus 28 and Tribolium castaneum 41,42 resistant to phosphine showed fitness cost in the resistant strain but not in the near‐isogenic resistant strain. Therefore, the importance of the genetic background of strains used in fitness cost experiments observed in other studies 29,30 is also valid for S. frugiperda resistant to chlorantraniliprole insecticide.…”

“…The identification of modifier genes is challenging since they can only be comprehended in a network context 54 . Pleiotropic effects associated with adaptation to a new environment, a field where an insecticide is constantly sprayed, for example, can be found in the literature 28,55–57 . However, pleiotropic effects are not always granted to resistance mutations, they may be a consequence of other natural occurring mutations whose frequency depends on the environment 58 .…”

“…31 One approach to obtain strains with close genetic background is to establish near-isogenic strains following a gene introgression through a series of backcrosses and reselection. 32,33 A near-isogenic strain can be obtained within a few generations for some pests, 28 but it is usually a longer process. A near-isogenic Cry1F resistant strain of S. frugiperda took up to eight generations to be established.…”

“…Such strain is usually reared in artificial diet and controlled condition in the laboratory for many years. The genetic differences between laboratory insects and field insects could also influence the results of fitness cost studies due to the reduced genetic variability of the laboratory susceptible strain and/or the presence of modifier genes in resistant strains selected from the field 26–28 …”

“…The genetic differences between laboratory insects and field insects could also influence the results of fitness cost studies due to the reduced genetic variability of the laboratory susceptible strain and/or the presence of modifier genes in resistant strains selected from the field. [26][27][28] The importance of the genetic background is commonly considered in genetics of pest management programs 29 and fitness costs associated to Bt resistance, 30 but it is often overlooked in studies of insecticide resistance. Therefore, a growing body of research endorses fitness cost studies with strains that present similar genetic backgrounds.…”