National Institute of Communicable Diseases, New Delhi
**Present address: C/o National Institute of Communicable Diseases, Southern India Branch, Coonoor, Madras State.
*The studies were carried out in 1961 under the enquiry on the “Behaviour of vector species and their resistance to insecticides” of the Indian Council of Medical Research.
Laboratory studies were carried out on the inheritance of DDT resistance in the larvae of A. stephensi Liston (type form). The resistant strain was highly resistant to DDT at larval stage only. Both the susceptible and resistant strains originated from Madras state of India.
The allele for susceptibility to DDT was found to be slightly dominant over the allele for resistance. Auxiliary factors associated with the sex-chromosomes of resistant males were found also to contribute towards the resistance of the species.
DDT resistance was found to be controlled by one major nuclear factor in association with other minor factors (dominant polygenic system). The mode of inheritance and dominance of DDT resistance were found to be similar to that reported for A. stephensi of Iraq. The comparison, however, suggested the existence of functional differences in the determinants controlling the ‘ stage-specific ‘ expression of resistance.
Formation of major factor due to slow integration and organization of minor factors into a cluster has been discussed in the Light of the results of selection experiments. The role of minor factors, constituting the cluster, in endowing varying degree of correlated resistance to different insecticides and in evolution of resistant strain has been discussed.
