Section 14 — Animal Genetics and Breeding 



14.17. The Genetics of Insect Resistance to Insecti- 

 cides. R. Milani (Pavia, Italy). 



About seventy independent strains of resistant 

 insects belonging to twenty species have already 

 been studied genetically. 



Toxicological and genetical studies agree 

 indicating specificity of resistance mechanisms 

 for groups of chemically related toxicants and, 

 to some extent, within groups. 



Most of the information on inheritance comes 

 from resistant strains developed under pest- 

 control condition rather than under controlled 

 selective pressure on laboratory populations. 

 For many species the studies on the inheritance 

 of resistance are the only property genetically 

 analyzed. 



Resistance and susceptibility generally are 

 discrete properties, inherited as simple genetic 

 differences; resistance is almost always dominant 

 or partly dominant on susceptibility, as would be 

 expected for a property selected through 

 survival to sudden extreme adverse factors. The 

 very few instances of recessive resistance known to 

 the present author through the literature refer 

 to forms of resistance developed to toxicants 

 different from those which have been exerting 

 pressure and genetically independent from the 

 resistance primarily caused by selection. 



Selective pressure with Organophosphorous 

 (OP) insecticides on the housefly and on mosqui- 

 toes appears to cause resistance also to DDT; it 

 has been shown for a strain of flies and one of 

 Culex tarsalis that independent genes are 

 involved. Other instances of multiple resistance 

 have been genetically resolved in their com- 

 ponents in various species. Linkage relations of 

 various genes for resistance and genetic markers 

 are on records for Musca domestica, Aedes 

 aegypty, Anopheles albimanus, various Droso- 

 phila species, Blattella germanica. 



High dechlorinating activity is always joined 

 by DDT-resistance, both properties being 

 jointly inherited as a simple mendelian character. 



Low-aliesterase activity is often coupled with 

 OP-resistance in the housefly; they are then 

 inherited as a unit. 



Cholinesterase activity is inherited jointly 

 with DDT-resistance in Culex tarsalis, but, like 

 aliesterase activity, independently from mala- 

 thion resistance; in the same species carboxyes- 

 terase activity and malathion resistance have 

 been inherited jointly. 



The full text has been accepted for publication 

 in a special issue of the Bulletin of the World 

 Health Organization. 



14.18. Artificial Allotetraploids (2n Bombyx moii 

 L. + 2n B. mandarina Moore) in Silkworm and 

 Their Bisexual Reproduction during Three 

 Successive Generations. B. L. Astaurov and 

 V. N. Vereiskaya (U.S.S.R.). 



As reported earlier (Proc. X Inter. Congr. 

 Genet., 1958), by crossing mixoploid partheno- 

 genetic females (3« + 6n) with normal 2/7-males 

 autotetraploids could be easily obtained in the 

 silkworm B. mori L. Females An display normal 

 fertility, whereas autotetraploid males are 

 sterile. Repeated attempts to obtain a bisexual 

 autotetraploid race failed. The usual presence 

 of several tetra- or polyvalents in 4/7-males 

 meiosis suggests the aneuploidy of spermatozoa 

 to be a cause of sterility. Consequently am- 

 phidiploids produced by crossing domestic 

 B. mori L. (//= 28) with wild B. mandarina 

 Moore (Ussury and Shanghai races, n= 28) 

 could be expected to be fertile. 220 crosses of 

 partheno-females An-mori x <§ 2n mandarina 

 gave many offspring 3// (2« mori+ln mandarina) 

 of both sexes. Analysis of 595 males proved 

 their and their sisters triploidy. By heat acti- 

 vation of unfertilized ova extracted from 532 

 3/7-females many parthenogenetic 3/7-daughters 

 were produced in 52.9 per cent of batches. 

 Percentage of hatching made 6.4 per batch, 

 reaching 63.0 per cent. A portion of partheno- 

 females was expected to be mixoploids 3/? + 6//. 

 After reduction of 6/?-oocytes (An mori -(-2// 

 mandarina) in case of preferential homologous 

 pairing, 3/7-pronuclei (2/7 mori-\- In mandarina) 

 must be formed. When fertilized with mandarina 

 haplosperm, amphidiploid (2« mori + 2n man- 

 darina) progeny of both sexes is expected. 

 1049 crosses of hybrid partheno-females 3/7 and 



3/7 



- x $ 2/7 mandarina were performed and 1439 



larvae obtained. Percentage of batches with 

 hatched larvae (17.9) and total percentage of 

 hatching (0.36) are close to those observed in 

 matings of partheno-females 3// mori x <§ 

 2n mori, yielding autotetraploids. Sex ratio in 

 235 V-instar caterpillars was 1.00$ : 1.05^. 

 Cytogenetican alysis proved their tetraploidy and 

 presumably — amphidiploidy. Phenotypically they 

 resemble 2/7 mori x mandarina Fi-hybrids. 103 

 males were repeatedly mated and 51 of them 

 (49.5 per cent) proved to be partially fertile. 

 Hatching percentage fluctuates from to 35.8 

 per batch, total one being 1.1 per cent. Poor 

 fertility indicates the probable segmental allo- 

 tetraploidy. Up to 1963 three successive gener- 

 ations of allotetraploids were reared: 1° — 

 primary "amphidiploids" (Ai); 2° — "amphidi- 

 ploids" A2 and /?i — hybrids from back crosses 

 $4/z mori x <§ An "amphV; 3° — progeny of 



255 



