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GENETIC SEXING SYSTEM FOR THE PREFERENTIALELIMINATION OF FEMALES IN CULEX QUINQUEFASCIATUS

N. J. SHETTYI

Departntent of Zoology & Centre for Applied, Genetics, Bangalnre tJniuersity, Bangalnre India

ABSTRACT. A genetic sexing strain T(1M;2)I of Cutex quinquefasciatus was synthesized for thepreferential elimination of females during the larval stage. Translocations were induced which linkedthe-gene for resistance to malathion to the male-determining factor. Mitotic chromosomes were analyzedto determine the precise nature of the translocation.

INTRODUCTION

Culex quinquefascintus Say is the most impor-tant carrier of Bancroftian filariasis in Asia. andthe main urban nuisance mosquito. This speciesis found throughout the tropics and extends intothe temperate regions ofthe northern and south-ern hemispheres (Mattingly 1951). Because ofthe serious problem of insecticide resistance inthis species, alternative insect control metho-dologies based on genetic mechanisms are beingexplored. The success of genetic manipulationinvolving sterile-male releases can be enhancedby developing a method by which males can beeasily separated from females during mass pro-duction. Since the females of this species arepotential vectors, and cause biting nuisance,they should be eliminated during the early de-velopmental stages by genetic methods. Thiswill also help to lower the cost of mass produc-tion of males for release purposes. To achievethis objective, we have used a male-linked trans-location and a malathion resistance gene to syn-thesize a genetic sexing strain of Cx. quinquefas-clatus.

MATERIALS AND METIIODS

The following strains were used in the exper-iment.

1. MlNclLoRE (MNG)-This strain is ho-mozygous for malathion resistance and was orig-inally collected from Mangalore, Karnataka, In-dia during April 1980.

2. MADRAS (MDS)-This strain is homozy-gous for malathion susceptibility and was origi-nally collected from Madras, Tamil Nadu, Indiaduring May 1978.

Our preliminary studies on the inheritance ofmalathion resistance in Cr. quinquefascintusconfirmed the observation of Tadano (1969)that malathion resistance is inherited as a par-tial dominant gene on linkage group 2.

For the induction of translocations. two-to-

I Present address: Insects Affecting Man and Ani-mals Research Laboratory, U.S.D.A., P. O. Box 14565,Gainesville, FL 32604

three-day-old males from MNG strain were ex-posed to 4,500 rads of gamma rays at the rate of140 rads per minute from a 60Co source at theKidwai Memorial Institute of Oncology, Ban-galore. The irradiated males were crossed tothree-day-old, virgin MDS females. The result-ing F1 males were back crossed to MDS females,and after a blood meal. the females were indi-vidually isolated in vials for egg laying. The eggswere held for 72 hours to ensure complete hatch-ing. The total number of eggs laid and the num-ber of larvae from each female was recorded.Egg batches with greater than 35% sterility,indicating the possible presence of a reciprocaltranslocation, were exposed as early fourth in-star larvae to 1.5 ppm malathion for 24 hours.The surviving larvae from each family werereared to the adult stage. Families showing onlymales were saved and were mated with MDSfemales for continuation of the line and furthertesting. Translocation breakpoints were ascer-tained from the mitotic chromosomes of pupaltestes as described by French et al. (1962).

RESULTS AND DISCUSSION

Of the 119 backcross females that laid eggs,87 families showing greater than 307o sterilitywere treated with malathion and the survivorswere reared to adults. Twenty-two familiesshowed a significant sex distortion favoringmales but only one family, later designated asT(1M;2)1, contained no females. This clearlyshowed that in the T(1M;2)1 line a translocationwas induced which linked the male-determiningIocus to the malathion resistance gene. Exami-nation of mitotic chromosomes from the testisof the T(1M;2)I line confirmed a reciprocaltranslocation involving the shortest chromo-some (sex chromosome) and one of the auto-somes. The translocation break points are nearthe distal end of the metacentric chromosome Iand about the middle of the larger arm of chro-mosome 2 (Fig. 1B). In Cx. quinqu.efascintus,Iikeother culicine mosquitoes, there is no hetero-morphic pair of sex chromosomes. Sex is deter-mined by a single pair of alleles, M and rn, for

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Chromosomal translocations can easily be in-duced in Cx. quinquefasciofus. During earlierwork, 30 different translocation stocks were iso-lated. These included 12 male-linked (TM), 8autosomal simple reciprocal translocations and10 male-linked double translocation heterozy-gotes (Shetty 1982, 1986). Of these, one sex-linked double translocation heterozygote, MPL-K (l; 2;3), showed higher mating competitive-ness than the normal males both in the labora-tory and field cages (Shetty 1984, 1986). Thus,this line may well be suitable for mass rearingand field release for the suppression of naturalpopulations of Cx. quinquefasciatus.

ACKNOWLEDGMENTS

The author is grateful to Prof. B. N. Chow-daiah, Director, Centre for Applied Genetics,Bangalore University for encouragement andDr. J. A. Seawright and Dr. S. K. Narang,USDA, Gainesville, Florida for reviewing thismanuscript. This work was supported in part bya grant from the Indian Council of MedicalResearch. New Delhi.

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