Genetic characterization of field-evolved resistance to phosphine in the rusty grain beetle, Cryptolestes ferrugineus (Laemophloeidae: Coleoptera)

Abstract

Inheritance of resistance to phosphine fumigant was investigated in three field-collected strains of rusty grain beetle, Cryptolestes ferrugineus, Susceptible (S-strain), Weakly Resistant (Weak-R) and Strongly Resistant (Strong-R). The strains were purified for susceptibility, weak resistance and strong resistance to phosphine, respectively, to ensure homozygosity of resistance genotype. Crosses were established between S-strain x Weak-R, S-strain x Strong-R and Weak-R x Strong-R, and the dose mortality responses to phosphine of these strains and their F 1 , F 2 and F 1 -backcross progeny were obtained. The fumigations were undertaken at 25°C and 55% RH for 72 hours.Weak-R and Strong-R showed resistance factors of 6.3× and 505× compared with S-strain at the LC 50 .Both weak and strong resistances were expressed as incompletely recessive with degrees of dominance of -0.48 and -0.43 at the LC 50 , respectively. Responses of F 2 and F 1 -backcross progeny indicated the existence of one major gene in Weak-R, and at least two major genes in Strong-R, one of which was allelic with the major factor in Weak-R. Phenotypic variance analyses also estimated that the number of independently segregating genes conferring weak resistance was 1 (nE = 0.89) whereas there were two genes controlling strong resistance (nE = 1.2). The second gene, unique to Strong-R, interacted synergistically with the first gene to confer a very high level of resistance (~80×).Neither of the two major resistance genes was sex linked. Despite the similarity of the genetics of resistance to that previously observed in other pest species, a significant proportion (~15 to 30%) of F 1 individuals survived at phosphine concentrations higher than predicted. Thus it is likely that additional dominant heritable factors, present in some individuals in the population, also influenced the resistance phenotype.Our results will help in understanding the process of selection for phosphine resistance in the field which will inform resistance management strategies. In addition, this information will provide a basis for the identification of the resistance genes.

Relative tolerance and expression of resistance to phosphine in life stages of the rusty grain beetle, Cryptolestes ferrugineus

“…We observed incomplete recessivity of phosphine resistance genes in C. ferrugineus. This is typical of adult insects of stored products and has been observed in C. ferrugineus (Jagadeesan et al 2016), R. dominica (Collins et al 2002), T. castaneum (Bengston et al 1999;Jagadeesan et al 2012) and S. oryzae (Daglish et al 2014;Nguyen et al 2015). The expression of resistance in immature stages of insect pests of stored products is poorly understood, however.…”

“…Resistance to phosphine in C. ferrugineus is conferred by two major genes (Jagadeesan et al 2016); therefore, there are nine possible genotypes that could be expected in a population that segregates for resistance. In these experiments, we measured responses in the three of the nine possible genotypes, that is, homozygous resistant (r 1 r 1 ; r 2 r 2 ), F1 hybrid (r 1 s 1 ; r 2 s 2 ) and homozygous susceptible (s 1 s 1 ; s 2 s 2 ).…”

“…These were sexed based on a mandibular projection on males (Rilett 1949) and used to establish a series of reciprocal single-pair crosses, susceptible 9 resistant and resistant 9 susceptible. Bioassays of adults (Jagadeesan et al 2016) as well as our preliminary work on other life stages confirmed that resistance was not sex linked; thus, responses of the reciprocal crosses were pooled for statistical analysis and interpretation.…”

“…Before undertaking crosses, this strain was purified to homozygosity in the laboratory by in-breeding and selection with phosphine. The response to phosphine of the F 1 hybrid between these two strains was linear indicating homozygosity of the parental strains (Jagadeesan et al 2016). …”

“…In adults of C. ferrugineus, strong resistance to phosphine is conferred by two or more autosomal genes and both are expressed as incompletely recessive traits (Jagadeesan et al 2016). Thus, strong resistance is fully expressed only in individuals that are homozygous for at least two of the major resistance genes.…”