VECTOR CONTROL

Inheritance of resistance to organophosphorus in Culex tarsalis Coquillet* CHARLES S. APPERSON 1 & GEORGE P. GEORGHIOU 2

The mode ofinheritance of resistance to , , fenthion, chlorpyrfos- methyl, and parathion-methyl was studied in a strain of C. tarsalis. Resistance in the F1 hybrid was partially dominant. Backcross experiments indicated that resistance is derived from more than one gene. This conclusion was confirmed by repeated backcrosses of the F1 to the susceptible strain and selection ofthe backcrossprogeny with a discriminating dosage ofparathion-methyl.

Resistance to organophosphorus insecticides in a in acetone on a w/v basis. The testing procedures for multiresistant strain of C. tarsalis has been shown to larvae have been described by Georghiou et al. (3). be due to a lower rate of absorption and Tests were replicated on at least 7 different days. The increased insecticide detoxification (1). The present data for the R, S, and F1 populations were subjected study concerns the mode of inheritance of this to probit analysis (6). resistance. MATERIALS AND METHODS Genetic techniques Insect strains Crosses were made using 150-200 adults of each The resistant strain (R) was collected from the sex. Reciprocal F1 crosses were made to expose Coachella Valley of California and selected in the maternal effects. laboratory by parathion-methyl pressure to a resis- The mode of inheritance of resistance to organo- tance level of 93.5 x. In this strain high levels of phosphorus insecticides was investigated by means cross resistance at LC95 were shown towards fenitro- of parathion, fenitrothion, fenthion, - thion (49.4 x ), parathion (55.6 x ), fenthion (76.8 x ), methyl, and parathion-methyl-i.e., those com- and chlorpyrifos-methyla (253.8 x) (2). The sus- pounds towards which the strain showed the highest ceptible strain (S) was collected from the San levels of resistance. Joaquin Valley of California about 20 years ago and The degree of dominance or recessiveness ofthe F1 has been maintained in the laboratory under insecti- hybrid was determined with reference to the log cide-free conditions. The rearing procedures used dose-probit mortality lines of the parental strains. were as described by Georghiou et al. (3) with certain To determine the number of genes involved in modifications (4, 5). resistance the F1 hybrid was backcrossed to the Bioassay procedures S strain. An expected curve for the backcross All the insecticides used were of analytical grade. progeny was calculated assuming monofactorial Solutions ofthe desired concentrations were prepared inheritance as already described (7, 8). Observed and expected mortalities were tested for significance * This investigation was supported in part by NIH using X2 analysis. Training Grant No. ES 47 from the National Institute of Environmental Health Sciences to the senior author. Further experiments to test for monofactorial 1 Present address: Lake County Mosquito Abatement inheritance were carried out by repeated backcross- District, 410 Esplanade, Lakeport, CA 95453, USA. ing and selection of the backcross progeny according 'Professor of Entomology, University of California, to the method of Wright (9). These selections were Riverside, CA 92502, USA. a 0,0-dimethyl 0-(3,5,6-trichloro-2-pyridinyl) phosphoro- carried out with 0.02 mg/litre parathion-methyl, thioate. which discriminates between S and F1 phenotypes.

3329 97- BULL. WORLD HEALTH ORGAN., Vol. 52, 1975 98 C. S. APPERSON & G. P. GEORGHIOU

LC5o values for various insecticides tested on Table 1. SF F1 7 Fi hybrids of reciprocal crosses of C. tarsalis 95 RcTxS? R?xST 90 o Insecticide LC5o (mg/litre) LC50 (mg/litre) 80 FENTHION6 (fiducial limits) (fiducial limits) 70 -expected*backcross 60 - , 50 .0 Z 40' CLjJ4 J. parathion 0.051 (0.049-0.053) 0.07 (0.068-0.073) 30 -530 fenitrothion 0.084 (0.075-0.095) 0.093 (0.082-0.11) 20 - ~~~~~~backcross 4 fenthion 0.033 (0.032-0.034) 0.039 (0.038-0.041) 10 observed chlorpyrifos- 5 methyl 0.098 (0.084-0.11) 0.12 (0.11-0.13) 3 0.001 0.01 0.1 parathion-methyl 0.043 (0.041-0.045) 0.051 (0.048-0.054) dosage (mg/litre) WHO 75304 Fig. 2. Dosage-mortality relationships of larvae treated with fenthion. RESULTS AND DISCUSSION The data in Table 1 reveal a slight matroclinous effect in resistance. This is evident from the higher 5 FIO R resistance to insecticides shown by the F1 hybrid from the R ? x Sc3 cross. The log dose-probit mortal- ity lines for the reciprocal crosses are significantly 80 backcross 1 6 *70 expected 7 different with the exception of that for fenitrothion. >V 60 a Fig. 1-4 reveal that the F1 hybrid (R5 x is ST) 40 2 partially dominant in resistance to parathion, feni- 30 trothion, fenthion, and chlorpyrifos-methyl. The F1 20 hybrid is more intermediate in resistance to para- thion-methyl, as shown in Fig. 5. In other studies on the inheritance of organophosphorus resistance in mosquitos the F1 hybrids have exhibited partial dominance towards in C. tarsalis (10), C. pipiensfatigans (11), and C. p. pallens (12). Partial Fig.3.Dosage dostaget (mglitre)ip flavetrae dominance has been reported for resistance with fenitrothion. .~~~~~~~~~~~~~~~~ S Fe 95 - 95 90 / 90 6 80 80 CHLORPYRIFOS-bMETHYL 004 6 70 - backcross / 6 t 70 -. 6 0 11 backcross >60 - /expected 60 - expected "40-T50~~~~~~~~~~~~~~ 30 - ~~~~~PARATHION 20*o backcross 10 /~^observed 4 10 ,backcross / observed 5 . ., l .I.I... .,,. 3 *0.001 0.01 0.1 1.0 0.001 0.01 0.1 1.0 dosage7 (mg/litre) dosage (mg/litre) WHO 75303 WHO 75306 Fig. 1. Dosage-mortality relationships of larvae treated Fig. 4. Dosage-mortality relationships of larvae treated with parathion. with chlorpyrifos-methyl. ORGANOPHOSPHORUS INSECTICIDE RESISTANCE 99

because of the independent assortment of modifiers 95 -PARATHION and the major gene in the backcross. Thus, mortality PMETHYL t 90 1 greater than expected would be manifested when only the major gene is present-i.e., in the absence of kcross i modifiers. Where higher resistance existed in the R strain, as with chlorpyrifos-methyl and parathion- - expcted* :50 O methyl, the plateau in the observed backcross backcross was 30 A :expected not as distinctly evident (Fig. 4 and 5). In this case, 20 / / 4 X levels of mortality at the higher dosages were closer to the expected levels. This may indicate that a 10C. p. moberved(3 a 0305 ~~ ~ ~~~~~~~Hbackcross -S0 greater number of modifier genes are involved, some obsvred 3 0.001 0.01 .0.1 1.0 of which may be closely linked with the major gene. dosage (m/litre) In other studies, tolerance to malathion (17) and WHO 75307 parathion (18) in Aedes aegypti was atributed to polygenes. Resistance to fenthion in Fig. 5. Dosage-mortality relationships of larvae treated C. p. fatigans with parathion-methyl. was attributed to a single major gene plus modi- fiers (16). Wright (9) has proposed a method for confirming in C. p. molestus (13), and for fenthion (14), polyfactorial inheritance. Repeated backcrosses are temephos,a3 and fenitrothion in C. p. pallens (15). made to the S strain, and the progeny are selected at However, resistance to fenthion in C. p. fatigans was the midpoint of their distribution at a dosage that found to be slightly recessive (16). eliminates all susceptible phenotypes but is not lethal Fig. 1-5 reveal that the observed and expected to the F1 hybrid. If monofactorial inheritance is mortalities in the backcrosses (F1 [R ? x SS3] 9 x SS) involved the backcross progeny should consistently were significantly different, by x2 analysis (P = 0.05), fall into two numerically equal classes-i.e., there at the majority of dosage levels tested. Observed should be 50% mortality with each backcross mortalities at the critical points corresponding to the selection. However, if more than one gene is 50% level of expected mortality are plotted with associated with resistance then the percentage of their calculated 95 % confidence intervals (Fig. 1-5). mortality should increase with each selected back- These computations confirm the significance of the cross generation, since minor factors for resistance differences between the observed and expected curves are progressively eliminated through independent of the backcrosses. assortment. This procedure was used to confirm the Since there was no overlap between the para- finding of polyfactorial inheritance in the R strain. thion-methyl log dose-probit mortality lines of Table 2 presents the effect of repeated backcrossing the F1 hybrid and R strain, a backcross and selection on the stability of parathion-methyl (Fl [R Y x SS] ?x R&) was made between the two resistance. An increase in mortality from 51.4 % to populations to test further the results of the other 75 % was observed over three backcross generations backcrosses. Significant deviation of the observed with a dosage of 0.02 mg/litre of parathion-methyl. from the expected mortality was again found upon These data and the results of the above-mentioned chi square analysis (Fig. 5). Because the results of backcross experiments are consistent with a poly- each backcross differed significantly from the ex- genic mode of inheritance of organophosphorus pected for monofactorial inheritance, it is concluded resistance in C. tarsalis. that organophosphorus resistance in the R strain is derived from more than one gene. For parathion, Table 2. Effect of repeated backcrossing and selection fenitrothion, and fenthion a depressed plateau is by parathion-methyl (0.02 mg/litre) on stability of evident in the curve of the observed backcross. This resistance in C. tarsalis larvae may indicate that a major gene plus additional Backcross No. larvae Percentage modifiers are involved in resistance. At higher generation selected mortality observed exceeded the calcul- dosages, mortalities 1 1500 51.4 ated ones in most cases. This was to be expected 2 1500 60.7 a O,O'-(thiodi-4,1-phenylene) O,O,O',O'-tetramethyl phos- 3 1500 75.0 phorothioate (" Abate "). 100 C. S. APPERSON & G. P. GEORGHIOU

ACKNOWLEDGEMENTS

The authors thank Professor D. M. Yermanos, Department of Plant Sciences, University of California, Riverside, USA for suggestions in connection with this study.

RtSUME

TRANSMISSION HERiDITAIRE DE LA RJSISTANCE AUX INSECTICIDES ORGANOPHOSPHORLS CHEZ CULEX TARSALIS COQUILLET

Le mode de transmission hereditaire de la resistance croisements repetes entre l'hydride F1 et la souche sen- aux insecticides organophosphores a ete etudie sur une sible et s6lection des descendants au moyen d'une dose souche de Culex tarsalis Coquillet possedant les degres constante de parathion-m6thyl tuant tous les phenotypes de resistance suivants (par rapport a la souche sensible): sensibles sans etre letale pour les hybrides F1. Dans ces parathion-methyl, 93,5 x; fenitrothion, 49,4 x; para- conditions, la mortalite des descendants obtenus par thion, 55,6 x; fenthion, 76,8 x et chlorpyrifos-methyl, retrocroisement passait de 51,4% a 75% en trois gene- 253,8 x . Chez l'hybride F1 la resistance etait partielle- rations. La presence d'un plateau en creux sur la courbe ment dominante pour le parathion, le fenitrothion, le de regression dose-mortalite correspondant aux descen- fenthion et le chlorpyrifos-methyl, et pratiquement inter- dants obtenus par retrocroisement est interpretee comme mediaire pour le parathion-methyl. Des retrocroisements montrant que la resistance est li6e a un gene majeur mais avec les parents S et R ont montre que la r6sistance etait fortement influence par des genes modificateurs. liee a plus d'un gene, ce qui a ete confirme par retro-

REFERENCES

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