northern and southern limits of distribution of D. .... Paltarno strain, the between-line differences are great ..... northern and southern groups of lines correspond.
Hereditas 86: 129-142 (1977)
The genetic basis of the geographically variable photoperiodic diapause in Drosophila littoralis JAAKKO LUMME A N D AILA OIKARINEN Department of Genetics, University of Oulu, Finland
LUMME,J . and OIKARINEN, A. 1977. The genetic basis of the geographically variable photoperiodic diapause in Drosophila littoralis. -- Hereditus 86: 129-142. Lund, Sweden. ISSN 0018-0661. Received March 24, 1977 The variation and inheritance of the photoperiodic adult diapause in females was studied in eight Drosophila littoralis strains originating from latitudes ranging from 42" N to 69" N. The extent of genetic variation was from photoperiodic neutrality (no diapause) to complete short-day diapause with critical daylengths between 13.0 to 17.6 hours light per day. Local populations share a small proportion of the total variation within the species, but are not genetically uniform. Genetic analysis shows that the variation in hybrids indicates a quantitative mode of inheritance. Long critical daylength (northern characteristic) is due to incompletely dominant alleles. The variation corresponds to the segregation of a single, autosomal mendelian unit, indicating at least linkage between loci responsible for the daylength measurement. There is enough variation in this unit to form a continuous latitudinal cline in photoperiodic reaction. Jaukko Lumme, Department of Genetics, University of Oulu, SF-90100 Oulu 10, Finland
Drosophila littoralis MEICENis a polycyclic insect, i.e. it has principally several generations a year. The egg-laying period of overwintered adults is quite long, and the emergence of the following generations extends over a long time. Adult flies emerging in the summer measure the daylength for obtaining information on the calendar time (LUMME et al. 1974, in press). After a locally varying critical time, the probability of successful reproduction is smaller than the probability of survival over the winter, and hence the adults stay in diapause, i.e. a reproductively immature stage until the following spring. Natural selection adjusts the photoperiodic response of local populations to maximize the long-term growth of the population (COHEN1970). There must be geographical variation in the critical point of time, in which the reproduction must cease. Most insect populations follow this in their photoperiodic reaction by forming clines in the critical daylengths across (and also along) latitudes (DANILEVSKY et al. 1970). Recently, BRADSHAW (1976) gave a beautiful example of the accuracy of the latitudealtitude cline in Wyeomyia smithii. The genetic basis of variation allowing for optimal local adaptation, and thus clines, is in most studies explained to be polygenic. In an earlier study on the genetics of dia-
pause in D . littoralis we have concluded that a single mendelian unit controls photoperiodic variation in this species (LUMME et al. 1975). The present study is directed to test this hypothesis. In addition, we try to quantify this variation: is it continuous or stepwise along the geographical change in the environment.
Material and methods Flies. - The eight strains of Drosophila littoralis MEIGENused in this study were the following: Kilpisjarvi (latitude of origin 69" 10' N, Finland), an old isofemale strain reared in laboratory since 1970. Rovaniemi (66" 10' N, Finland), an old strain, several founders, 1969. Oulu (65" 1' N, Finland), five separate isofemale lines collected from wild population in the summer of 1973, just before the start of the experiments. Paltamo (64" 17' N, Finland), five fresh isofemale lines. Kuopio (62" 55' N, Finland), an old laboratory strain, several founders, 1970. Zurich (47" 15' N, Switzerland) and Ticino (46" 10' N, Switzerland), the strains were taken into culture in 1970 by Dr. G. Bachli, University of Zurich. Caucasus (appr. 42" N, USSR), the strain was ob-
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tained from Dr. M. Evgen’ev, the USSR Academy of Science, Moscow (in Russian literature the synois used for this strain). nyme D . imeretensis SOKOLOV The number of founders of the three last-mentioned strains is unknown. In the laboratory, the strains have been reared in continuous illumination since 1970, in ordinary bottle cultures. Thus, in old strains some inbreeding and loss of variability may have occurred before the start of the experiments in September 1973. - The extreme trapping localities are very close to the northern and southern limits of distribution of D. littoralis. - In the following, the names of the original strains are written in roman letters. When the same name is used to indicate an inbred line derived from a strain, it is written in italics.
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experimental conditions. After three weeks of exposure, the reproductive status of females was checked by dissecting their ovaries. After this age, the percentage of diapausing females does not change. Females without any sign of development in ovaries were classified as diapausing (GEYSPITSand SIMONENKO 1970; LUMME et al. 1974; KAMBYSELLIS and HEED1974). In the individual level, there are only two possible phenotypes with regard to the reproductive stage (in the classification used here). The percentage of diapausing females out of all females studied is used to quantify the photoperiodic response of a population (“phenotype of a population”). When necessary, the diapause percentage is symbolized by small letters instead of capitals to denote parents and offspring of crosses in the usual fashion (i.e. fi is the diapause percentage in an F2 generation).
The rearing offlies and environmental conditions during the experiments. - Flies were cultured in 250 ml bottles with 50 ml of the malt medium (LAKOVAARA The genetic analysis. - Five lines were founded from 1969). After the adult emergence, the flies were aged six old laboratory strains and two wild populations. up to three weeks in 50 ml tubes with about 5 ml of They were full-sib inbred for five generations, premedium, which was replaced each fifth day. The venting multiple mating. Variation in photoperiodic possible effect of varying microbial flora was ranresponse was studied in the sixth generation by taking domized through culturing the flies of each group in five pairs from each line (= 8 x 5 x 5 pairs planned) at least two bottles (also in pair crosses) and keeping and determining the diapause percentages in their only 5 to 20 flies (males and females together) in progenies at 16”C, L D 1637.5 h. ageing tubes, so that each sample consists of several One inbred line from each strain was then chosen subsamples. - Before crosses, the flies were always on the basis of observed variation (as homogeneous, a aged for at least two weeks in constant light (LL) at line as possible). The inbreeding was relaxed by 1 9 T , for breaking the diapause, and the sexes were starting ordinary bottle cultures. These eight lines separated during this time. Generation time in represent their geographical origin in crossing expericultures is, under our conditions, about 5--6 weeks. ments. Individuals for crosses were taken from these All cultures were kept in a windowless room at mass cultures from undefined generations. The 19$1”C and RH about 50%. Continuous illuminaphotoperiodic response of the lines was controlled tion of 400-1000 lux was maintained by fluorescent one and a half year after the relaxation of inbreeding tubes. Photoperiodic experiments were made in and complete photoperiodic response curves were incubators at 16+1”C, R H about 70-90%, and determined for them, too. the light was given by fluorescent tubes controlled All lines were crossed as pairs to an F, diallel with by switch clocks, the light intensity being 1000--1500 reciprocals and a varying number (more than two) lux on the outside of plastic culture tubes. of replicates; the progenies were studied at LD In most of the experiments, a light-dark cycle (LD) 1657.5. The Ticino line was crossed (mass) with all 16.5:7.5 hours was administered. In this regime, the other lines and photoperiodic response curves in phenotypic variation of the material was maximal. F,-hybrid populations were determined. -F, generaComplete photoperiodic response curves were estabtions and first backcrosses were raised from a half lished for some populations of flies. diallel (no reciprocals). The backcrosses were made reciprocally to the original female parental line (two The determination of the photoperiodic response. - replicates) for detecting the influence of the X-chroIn the species studied, the developmental stages mosome. Flies from one F, progeny (Oulu x Zurich) before adult eclosion are insensitive to daylength were backcrossed to males from both parental lines (LUMME et al. 1974). Therefore, the flies were always for ascertaining the segregation. cultured in LL, 19°C for optimizing the adult production. Within two days after the adult eclosion Statistics. - For statistical analyses, arc sin transthey were collected and transferred to the actual formation of diapause percentages was used. The
Hiwdita.7 86 (1977)
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GENETICS OF DIAPAUSE IN DROSOPHILA
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Fig. I . Photoperiodic reaction curves of inbred lines at 16’C. Symbols: T: Ticino; C : Caucasus; Z: Zurich; Ku: Kuopio; P: Paltarno; R: Rovunierni; Ki: Kilpisjarvi; 0: Oulu.
F, -diallel table was partially analyzed following MATHER and JINKS (1971).
Results 1. Geographical and intra-populationvariation in photoperiodic response
Five inbred lines from each original sample were tested at 16”C, LD 16.5:7.5 for their photoperiodic response. For partitioning the variance into interstrain (“geographical”), intra-strain and intra-line components, five pairs were taken from each line, but progeny was not obtained from all. The results are presented in Table 1. The number of pairs with a progeny belonging to each phenotype class is given: intervals the classes are divided with 10” urc sin for obtaining a normally distributed measure. For each line, weighted mean of diapause percentages, as well as the total number of females studied is also given. Results of an analysis of variance based on the results are represented in Table 2. The differences between strains are highly significant, and 91.3% out of the total variance observed is “geographical”. The remaining 8.7%, only, accounts for intra-strain variation. In this calculation, there are two sources of bias, with opposite influence: the use ofold laboratory strains obviously can decrease the intra-strain variation; and the variation between (and within) southern strains is “hidden” in these conditions.
In spite of the shortness of inbreeding (five generations), five out of eight strains studied show statistically significant differentiation between inbred lines. In the total material, differences between inbred lines are highly significant (P
