Vertical Resistance to Insects

Gene-for-gene relationships are common among plant pathogens but they are relatively rare among the insect parasites of crops. Indeed, the only well-known examples are the brown plant hopper (Nilaparvata lugens) of rice, the Hessian (Mayetiola destructor) fly of wheat, and various species of aphid. It is thought that this discrepancy occurs because asexual reproduction is so much more common among the plant pathogens than the insects. For this reason, it is probable that many undiscovered gene-forgene relationships exist in the alternating aphid pathosystems (see 8) and, possibly other insect groups that have an asexual, r-strategist reproduction.

However, other explanations are possible. For example, plant breeders and plant pathologists are all botanists, while entomologists are zoologists. It is possible that the co-operation between different disciplines within botany was closer than between botanists and zoologists. The study of gene-for-gene relationships in the insects would then be less frequent than in the plant pathogens. There are also technical reasons for the discrepancy. Genetic studies in the aphids, for example, present extreme technical difficulties.

Singh (1986) has listed the following gene-for-gene relationship in insect pests of crops:

Apple woolly aphid (Eriosoma lanigerum)

Bean aphid (Aphis fabae)

Pea aphid (Acyrthosiphon pisum)

Raspberry aphid (Amphorophora rubi) Corn aphid (Rhopalosiphum maidis) Wheat aphid (Schizaphis graminum) Alfalfa aphid (Theriopaphis maculata) Rice brown plant hopper (Nilaparvata lugens) Wheat Hessian fly (Mayetiola destructor)

In addition, Gallun & Khush (1980) have reviewed reports of apparent vertical resistance against the following insect pests of crops:

Rice green leaf hopper (Nephotettix impicticeps). Rice white-backed plant hopper (Sogatella furcifera). Rice gall midge (Pachydiplosis oryzae).

Chapter Six

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