Genetic, physiological and molecular interactions of rice and its major dipteran pest, gall midge

The gall midge, Orseolia oryzae, is a major dipteran pest of rice affecting most rice growing regions in Asia, Southeast Asia and Africa. Chemical and other cultural methods for control of this pest are neither very effective nor environmentally safe. The gall midge problem is further compounded by the fact that there are many biotypes of this insect and new biotypes are continuously evolving. However, resistance to this pest is found in the rice germ plasm. Resistance is generally governed by single dominant genes and a number of non-allelic resistance genes that confer resistance to different biotypes have been identified. Genetic studies have revealed that there is a gene-for-gene interaction between the different biotypes of gall midge and the various resistance genes found in rice. This review discusses different aspects of the process of infestation by the rice gall midge and its interaction with its host. Identification of the gall midge biotypes by conventional methods is a long and tedious process. The review discusses the PCR-based molecular markers that have been developed recently to speed up the identification process. Similarly, molecular markers have been developed for two gall midge resistance genes in rice - Gm2 and Gm4t - and these markers are now being used for marker-assisted selection. The mapping, tagging and map-based gene cloning of one of these genes - Gm2 - has also been discussed.