Bull. Org. mond. Sante 1963, 29, Suppl., pp. 77-87 Bull. Wld Hlth Org. 19329Sup.p.778 Genetical Aspects of Insecticide Resistance R. MILANI 1 Studies on the mode of inheritance of insecticide resistance have been making rapid progress over the last few years and now cover some twenty-two species and about seventy different strains. These studies include not only the inheritance of particular forms of resistance but also the genetical aspects of cross-resistance and biochemical properties causing or related to resistance. The author reviews 27 instances of resistance recently subjected to genetic analysis; all butfour were considered to be examples ofmonofactorial inheritance. The use of genetic markers has made it possible in some instances to map the genes responsible for resistance and has proved useful in the study ofthe genetic mecha- nisms involved in cross-resistance andmultiple resistance. Correlations have been established between resistance, genetic constitution, and the activity of certain enzymes. Experiments are described illustrating the technique of crossing genetically marked resistant strains with susceptible strains in the study of the mode of inheritance of resistance to DDT and dieldrin. When I reviewed this subject in 1960 at the resistance genetically analysed between 1960 and Eleventh International Congress of Entomology 1962 only 6 are to DDT, 13 to dieldrin, one to endrin (Milani, 1962), the data available on the inheritance (in the boll weevil), and 7 to organophosphorus of resistance related to some 14 species and more compounds (diazinon, malathion, parathion). than 50 independent strains. Now, after little more than two years, it is possible to add at least 8 INHERITANCE OF PARTICULAR FORMS OF RESISTANCE more species-more than one third of the material The great majority of the cases analysed have now available-and about 20 new strains (the real proved, or strongly suggested, that it is possible independence of strains is often difficult to establish) to isolate individual factors causing resistance. It (see table). would be an overstatement to claim that these are The species to be added to the previous list are: all instances of monofactorial inheritance, because Anopheles albimanus, A. stephensi, A. quadrima- the possibility of isolating a main factor with a culatus, Culex tarsalis, Cimex lectularius, Pediculus specific action does not necessarily exclude the humanus humanus, Chrysomyia putoria, Tethra- presence of other ancillary ones. Several years ago nychus pacificus, Anthonomus grandis. The new I criticized the reluctance of many workers to additions indicate that the genetical approach is accept the evidence that single genes can cause being extended to organisms not previously in- physiological differences large enough to bring cluded in studies of this type. about resistance (Milani, 1954). During the inter- Genetical studies on resistance now seem to vening years evidence in favour of this interpretation concentrate mainly on three lines of approach: has been accumulating and it has become obvious (1) inheritance of particular fo rms of resistance; that resistance levels sufficient to give full protection (2) genetical aspects of cross-resistance or to insects very often have a simple genetic basis. multiple resistance; However, it is felt now that categorical assertions (3) genetics of biochemical properties causing or have occasionally been based on rather unsatis- related to resistance. factory experiments. Comments on these aspects of inheritance are included in the general considera- The interest is shifting from DDT to other forms tions in the above-mentioned review (Milani, 1962). of insecticide resistance. Out of 27 instances of A detailed analysis of each of the 27 instances of 1 Acting Director, Institute of Zoology, University of resistance referred to above seems unnecessary for Pavia, Italy. the purpose of the present paper. The significant 1319 -77- 78 R. MILANI C~~~~~~C o CD ' a)Co U C a C C' C~~~~~~~~~0 ~~~~~~~~~~~~~~~~0 0 0 C~~~~~~~~~~~0 0000c0 CDCC0 .0 ~~ 0 U 0 00 .S -~~~~0 -0 - -~~ 4CS0O>=(0 c u u0.0>.E>0 u im~C0 - cos coC 0 . j 0co 0 C *6 -, v-. Coad co 0o 0 0 a)~~~~~~~~~~~~~~~Ea U c 0 C 0 > C c . 0 0 La o~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~co00~~~~~~~o 2 C C - 0 0~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~oC. ( o~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~c C - 00 0~~~~~~~~~~~~~~~~~~~~~~~~~~~ -% *C it0 0 x In 0 0 0D a 0 o U. 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U F- F- i- F- F- F-~r ~' a VaV a C V ~ ~ ~~~E:2~ 0 UUL 2 2 2 .2 .2 o E o _ 1 CO 0 InC Co C- W a 0o -_ In O N- X _ 0 0 Cc CO CO N- z 'WI n- WI, Ir v- Irl v- l010'1010101N " NJ N10 GENETICAL ASPECTS OF INSECTICIDE RESISTANCE 79 data are collected in the accompanying table; where F2 flies showed only intermediate and high information followed by a question mark or written resistance, without segregation of susceptibility. between brackets has been inferred from the pub- lished descriptions. For the sake of uniformity, the GENETIC MAPPING OF GENES FOR RESISTANCE term " monofactorial " has been used as a convenient synonym for "single gene" and " simple inheri- In the housefly a gene for DDT-resistance (kdr) tance ". is linked with several good genetic markers, one of Resistance is indicated as recessive only twice; which is brown body (Milani & Travaglino, 1957). in both instances the insect was a multiresistant To the same linkage group belong the factors for mosquito and the insecticide was DDT, but this DDT resistance of at least three different strains, was not the selecting factor primarily acting in the two of which are certainly different from kdr. This field. linkage group can occasionally follow male-limited The outstanding feature of resistance to dieldrin inheritance, as has been clearly shown by using (or to endrin) is that it regularly reaches inter- morphological genetic markers; male-limited in- mediate levels in the heterozygotes (with two heritance of a low-level DDT-resistance has been exceptions), whereas resistance to organophosphorus described by Kerr (1961). The gene a (aliesterase compounds tends to be dominant or nearly so. activity) is located on the fifth chromosome, like The low level of male-limited DDT-resistance the mutants aristapedia and carmine (ar cm) reported by Kerr (1961) in a strain of housefly has (Franco & Oppenoorth, 1962). Resistance to been entered in the table as " (intermediate?) ", dieldrin recombines freely with the second and the on the assumption that it may depend on obligatory fifth chromosomes, and is not sex-linked. heterozygosis. This assumption is based upon the In Drosophila two genes for resistance control actual LD50 and upon analogy with other genetic well-known properties, described in the section on factors also showing male-limitation, all of which the genetics of biochemical properties related to have an allele in the second chromosome (Sullivan, resistance. 1958, 1961; Franco et al., 1962). The gene for DDT-resistance found in a European strain of the German cockroach is linked with While 23 out of the 27 instances listed in the table balloon wings and is independent of orange body are examples of simple inheritance, in the remaining (Cochran & Ross, 1962). four instances resistance appears to be due to (a) Dieldrin-resistance and DDT-resistance in Aedes two of additive a main factor plus modifier, (b) pairs are factors on the second and non- aegypti controlled by alleles, (c) multifactorial inheritance, (d) chromosome, giving 25 % cross-overs with the segregation in F2. marker yellow larvae (Khan & Brown, 1961). A main factor plus modifiers seems to be respons- ible for dieldrin resistance in a strain of housefly carrying several genetic markers, and therefore GENETIC ASPECTS OF CROSS-RESISTANCE necessarily of mixed and mainly unknown origin. AND MULTIPLE RESISTANCE Two pairs of additive factors have been assumed Resistance to more than one toxicant can depend by Harris et al. (1961) in order to explain the on a single basic property or on the co-existence in inheritance of malathion resistance in a strain of the same strain of distinct defence mechanisms. The houseflies of fairly recent colonization. The pecu- term cross-resistance is used when a single pro- liarity of this strain is that when crossed with sus- perty ensures cross-protection to various toxicants; ceptible flies it gives in F1 and in F2 both susceptible multiple resistance refers to the co-existence of and resistant flies in equal proportion.