A locus in Drosophila melanogaster affecting heat resistance

Temperature is an important environmental factor for


Brief report
A locus in Drosophila melanogaster affecting heat resistance L. OUDMAN Department of Genetics, University of Groningen, P. 0. Box 14, 9750 AA Haren, The Netherlands (Received February 7, 1991. Accepted April 9, 1991) Temperature is an important environmental factor for Drosophila melunogaster and polymorphism for temperature resistance is very likely to occur (PAR-SONS 1973).Many studies are published about heat resistance in Drosophilu melunogaster, but most of them concern the heat shock response (e.g., LIND-QUIST 1986).Little attention is paid to the variation in heat resistance itself, though such studies can reveal additional information about the mechanisms of heat resistance and climatic adaptations.Above this, conditional lethal mutations can play a role in agricultural pest control (e.g., HEDRICK 1984).
The first report of variability for heat resistance in Drosophilu melanogaster is from HOSGOOD and PARSONS (1968), who found differences between iso-female lines and ascribed these to (unknown) polymorphic genes.MORRISON and MILKMAN (1 978) succeeded in selection for decreased resistance, and showed that the gene or genes responsible for this decrease could be localized for the greater part at the second chromosome.The present article describes the genetical localization of a natural mutant for decreased heat resistance in Drosophilu melanogaster-: a recessive heat sensitive lethal on the second chromosome, l(2)h.y.

E.\periments
For all experiments the flies were reared at 25°C and about 50 92 R.H. on standard medium (OUDMAN   et al. 1991) and tests were performed at 35°C and about 90 C/c R.H. to prevent desiccation.
The mutant was discovered in the wild type laboratory strain Groningen 83, which was founded in 1983 with 403 females from a fruit market in Groningen, the Netherlands.Fig. 1 shows survival at 35°C of males of a substrain that was extracted from Groningen 83 for other purposes (OUDMAN et al.. in preparation).The figure clearly shows two phases of mortality, one during the first 18 hours and one starting at 48 hours.Other strains tested at 35°C (not shown) did not have two phases of mortality, but had normal curves with mortality starting at about 48 hours.When survival at 25°C was followed from a strain that had two mortality phases at 35"C, a normal survival curve was observed (Fig. 2), as was the fact at 29°C (not shown).Thus, the two phases of mortality at 35°C seemed to be caused by a polymorphism for survival at high temperature that did not influence longevity at lower temperatures.
For further analysis eight inbred strains were derived from the substrain mentioned above by at least four subsequent sister-brother crosses, and tested for survival at 35°C.Table 1 shows that two strains (1 and 7) had strongly decreased heat resistance compared to the other strains, This effect was similar for males and females.In a resistant strain some flies might die during the first day, due to 'normal' mortality, as would be the fact at any other temperature.Because comparison of these strains in later generations gave similar results (not shown), it was concluded that the inbred strains were truebreeding for a genetic factor determining heat resistance.
A cross between a sensitive and a resistant strain (not shown) yielded a resistant strain; thus sensitivity was a recessive trait.Chromosome analysis was performed with the aid of the balanced marker stock for the second and third chromosome SM5 C\: TM3 S c ~r (Li\i)si.wand GKEI-I.1968).The obaervalion that homozygous sensitive adults never survived for IonLger than 16 hours at 35°C. while resistant flies.it' in food condition.normally survived much longer.uas used in the chromosome localizalion 10 infer the resistance of fies.The heat sensitive strain 7 was crossed with the marker strain according to the scheme in Fig. 3. F1 male\ were tested for survival during 16 hours at 35°C.From the survival percentayes in Fig. 3 it can be deduced that heat sensitivity is a recessive character located on the second chromosome.
Localization experiment\ with the recessive mutants (,iumi/x/r ( ( , / I , I1 57.5) (all standard location5 xcordinp to Lt\iisi F\I and GRf:i.i.1968) and hrmiw t h ~. .11 104.5)(not shoun) induced that the character w i l h locatcd just left of < ' / I , Final localization on thc second chromosome was performed %ith the recessive mutants p i w p l ~ (p.I1 53.5) and ( ' / I .Feinalei of the heat wnsitivc strain 7 were crossed u ith males / I / ' C I ~.F2 females iheterozygotes in which recombination could occur) Lverc backcrushed to pr. ('rr males.In the F2 four phenotypes c)ccun-etl: \% ild t) pe.purplc.cinnabar.and purple/ cinnabar (Table 2).In the F2 a recombination percentage brtwc.cnp i -and ( ' 1 1 o f 2.3 C/r ~i i h found.instead of the expected 3 %, which is significantly different (x', = 3.96.0.04<P<0.05).This difference might be caused by different viability of the recombinant genotypes.but there were no other indications for such differences.To determine the frequency of sensitivity in the F2 phenotypes, a number of individual inales from each of the F2 phenotypes were crossed with sensitive females from strain 7. and their progeny were kept apart.These F3 flies were tested for survival at 35°C.If sensitivity was present in an F2 male in hcterozygous state.mortality of F, progeny was expected to be SO %.If sensitivity was not present in the F2 male. 100 % survival of F3 progeny was expected.At least 70 flies per cross were tested at 35°C.The location on a standard second chromosome is 54.5 + (9 + 10)/54*3 = 55.6.The locus is a conditional, heat sensitive, recessive lethal on the second chromosome: l(2)hs.
( PARKER-THORNBURG and BONNER 1987).Studies of the molecular and physiological mechanism of the locus and the temperature range of sensitivity will be necessary.
Up to now l(2)hs is only found in the Groningen 83 population.Because the mutant has not been discovered before it is probably rare in nature, but further studies on the occurrence are necessary.

Fig. 3 .
Fig. 3. Crossing scheme and survival percentages of the adult males in the Fl 01' the chromosome localization experiment.(m = chromomne of heat sensitive strain).

Table 2
Occurrence of heat sensitivity in F3 males from crosses of F2 males with heat sensitive females locus is involved.The locus is located between p r and cn, closer to p r than to cn.