This project aimed to provide rapid identification of the phosphine resistance status of any individual R. dominica or T. castaneum collected from grain in storage across Australia. The resistance genes that are directly responsible for phosphine resistance in these insects were identified and used as indicators of resistance status.

Research outcomes

Key outcomes of the research are:

1. Phosphine resistance is mediated by two major genes in both T. castaneum and R. dominica. These two genes have been named rph1 and rph2 (i.e. resistance to phosphine 1 and 2).
2. The two genes are incompletely recessive and individually confer weak resistance when homozygous for the resistance mutation.
3. The two genes are synergistic in effect and confer strong resistance in R. dominica and T. castaneum when both are present and homozygous for the resistance alleles.
4. The two genes are expressed in all insect life stages suggesting a constitutively expressed resistance factor that does not appear to be 'switched off' at any stage of development.
5. The location of the rph1 gene has been narrowed down to a very small number of candidate genes (about six) for T. castaneum. However the location for R. dominica is less clear with a region of about 100 genes identified as the source area.
6. While gene expression profiling was achieved for R. dominica and T. castaneum the results indicate that the technique is not suitable as the basis for development of a diagnostic test for resistance as the known resistance genes are not differentially expressed in resistant and sensitive strains and do not change expression in response to phosphine.

Research implications

Implication 1

The research has shown that there are two major genes that confer resistance in the two insect species studied and that both these genes are expressed in all insect life stages. This means that:

1. Selection for resistance can occur in all life stages of the two studied insects, and
2. There are no vulnerable life-stages that could have been targeted in attempts to manage resistance development

Implication 2

The finding that the two genes are synergistic in effect and confer strong resistance only when both genes are homozygous (for resistance) explains why strong resistance has taken a relatively long time to increase in frequency and appear in enough numbers to be detected. This is because you must have, in one individual, both resistance genes present and both homozygous and the chances of this occurring in random mating events in nature are not high.

Implication 3

It was found that the gene rph2 is highly conserved between R. dominica and T. castaneum. The significance of this finding is that a similar mechanism for resistance could be common across all major grain storage pest species where phosphine is used for control.

Implication 4

The research has shown that mutations in rph2 vary between populations of R. dominica and T. castaneum. This indicates that industry is unlikely to gain a universal molecular diagnostic test for phosphine resistance that could be applied across Australian grain growing areas. 

Acknowledgements

The authors would like to acknowledge the support of the Australian Government’s Cooperative Research Centres Program. We would also like to acknowledge the contributions of the University of Queensland which contributed to two PhD scholarships that worked on this project.