Genetic and biochemical analysis in microorganisms has been greatly facilitated by the use of conditional lethal mutations that die under "restrictive" conditions but survive in a "permissive" environment." 2 One class of conditional lethals, the temperature-sensitive (ts) mutants that result from a single amino acid substitution that renders a polypeptide inactive at high temperatures,3-5 has been extensively analyzed in Neurospora,6 bacteria,7 T4 phages,"8 and tobacco mosaic virus.3 4 In higher organisms, ts mutants have been detected in Arabidopsis,9 Paramecium,'0 and Drosophila. II The potential usefulness of ts mutants in several types of genetic analyses prompted us to screen for them systematically in Drosophila melanogaster. This paper reports the frequencies of ts mutants among sex-linked recessive lethals and semilethals induced by Py-rays, ethyl methanesulfonate, and mitomycin C. Methods and Materials.-Screening technique: Oregon-R males were collected within 48 hr of eclosion, treated as described below, and then mated separately with three Basc females in vials at room temperature (230 1 20C). The males were transferred without etherization to a fresh set of females at 2-day intervals for several consecutive broods. Each set of females was removed from the vials 3 days after transfer of the males in the experiments with mitomycin C and y-rays. Owing to the poor fertility of males treated with ethyl methanesulfonate, females in these experiments were allowed to lay eggs for up to 5 days after transfer of the males and in some cases were then transferred to a fresh vial. Single F, females from each brood of each male were placed together with one or two Basc males at 290 ± 0.50C. The presence of two or less non-Basc F2 males in a fertile culture suggested that a lethal or semilethal had been induced on the X chromosome. Replicate cultures of the F2 of all possible lethals and semilethals were set at-290C and

T . SUZURI L)epnrt.//zent of Zoology, U7ziversity of Uritisl~ Cohl71zbin, V~~Z C O~L Z~T , B.C.Crossing ovcr was studicd in Drosopbiln 711clnnognster fenlales bet\vcen f -cnr -170' -Dp(y+) on the X chromosome. hlitomycin C and y-rays incrcar.ed crossing over in all ~h r c c regions, increased nondisjunction and affected meiotic as well as gonial cells. Temperature was recombinagcnic only in thc cnr -00' regions and the autosomal inversions (331 and Ubr':' increased crossing over in both regions containing euchromatin (f -cnr and cm--00') . Actinon~ycin D-treatment rcsulted in a slight decrease in crossing orcr. T h e data indicate that there may be two gcncral classes of rccombinagcns, those that affect crossing ovcr in prosimal euchromatin and thosc \\~hich in addition, induce brcalcs in hcterocliromatin. I~ltrocluctionIncreases in crossing over induced in D~osophila 77zela7zogaste~ by recombinagenic agents are invariably most striking in the proximal regions of thc major cl~romosomes. O n the-basis of a similarity in the patterns of increases induced by diverse agents (actinomvcin D, radiafion, chromoson~e aberration) around the centromcre of c h r o m o s o~e 3, it has been suggested that these aoents ? increase crossing over in that region through the same mechnnism (Suzulu and Parry, 1961). T h c common basis of induced proximal increases has been post;latcd to result from an interference with genetic activity of prosimal loci which normally fuilctioll at the time of crossing over (Suzulti, 1963; l965a, c ) . It xvas suggested that consequent structural challges analogous to lateral loop collapse after actinomycin D trcatmcnt or X-irradiation (Izawa, Allfrej? and ALirslty, 1963; fileyer and Hess, 196.5; Hess, 1965) now permit the intimaic synapsis ileccssary fop crossing over to occur.T h e esperiments reportcd here xvcre dcsigncd to determine whether a number of different rccombinagcnic factors do indeed act on all proximal regions in a qualitatively silnilar manner. Since proximal increases induced in thc S cl~rornosome by the prcsencc of autosomal inversions appear to be collfillcd to cuchromatic rcoions adjaccnt to heterochromatin (Roberts, 1965), s recombination was determined in both cucl~ron~atic and heterochromntic segments. Methods a i d MaterialsCrossing over was mensurcd in the X cl~romosome using the followillg inar1;crs (Bridgcs and Urchme, 1944) : y -yclloxv 0.0, f -forlted 56.7, carcaranation 62. j, hb' -bobbcd-lethal 66.0, f i p (l;l)scH, y' -a duplication of 91' to the right of thc ccntromcrc. T h e f-car region was designated as 1, thc car-bb' region as 2 and bb' -y-as 3. Region 1 is completely euchron~atic, rcgion 2 a composite of euchrornatin and l~etcrochron~atin and region 3 is con~plctelv hetcrochron~atic. All fcmalcs tested had an X-cl~ron~osome gcnotx pe of y-+ car + .y+/y f + bb', and werc crosscd to 117 ( I ) p~~1 ' 2 ' , 1~~""/ybb' rnalcs. Only ~nale progeny were scored and it was found that no males carrying 611' on both the S and Y dl~romosomes survived.