Cellular Basis and Developmental Timing in a Size Cline of Drosophila me lunogw

We examined 20 Drosophila melanogaster populations collected from a 2600-km northsouth transect in Australia. In laboratory culture at constant temperature and standard larval density, a genetic cline in thorax length and wing area was found, with both traits increasing with latitude. The cline in wing area was based on clines in both cell size and cell number, but was primarily determined by changes in cell number. Body size and larval development time were not associated among populations. We discuss our results in the context of selection processes operating in natural and experimental populations. B ODY size is strongly associated with fitness in various species of animals (ROFF 1992; STEARNS 1992). In laboratory studies on Drosophila mlanogaster, adult life history characteristics such as female fecundity (ROBERTSON 1957; TANTAWY and RAKHA 1964), adult longevity (TANTAWY and RAKHA 1964; PARTRIDGE and FOWLER 1992; but see HILLESHEIM and STEARNS 1992) and male mating success (EWING 1961, 1964) have shown positive genetic correlations with body size. In contrast, negative genetic correlations of body size with larval development rate (PARTRIDGE and FOWLER 1993), and consequently with larval competitive ability and larval viability ( SANTOS et al. 1992; PARTRIDGE and FOWLER 1993), have been found. Thus, body size may display an intermediate optimum and be under stabilising selection, as a result of conflicting selection on the preadult and adult periods. In natural populations of Drosophila, geographic variation in body size has been shown to be in part genetic (e.g., REED and REED 1948; MCFARQUHAR and ROBERTSON 1963; DAVID et al. 1977; ROBERTSON 1987). Genetic body size clines have been found in various species of Drosophila and on different continents in D. melanogaster and D. simulans (STALKER and CARSON 1947, 1948; PREVOSTI 1955; MISRA and REEVE 1964; LOUIS et al. 1982; COYNE and BEECHAM 1987; CAFY et al. 1993; IMASHEVA et al. 1994; but see SOKOLOFF 1965, 1966); body size increases with latitude and altitude. The repeatability of these clines implies that they are caused by natural selection. Climatic factors that vary with both latitude and altitude such as temperature, rainfall and relative humidity may be involved in the selective processes shaping the clines. The evolution of replicated laboratory populations of Curresponding author: Avis C. James, Department of Genetics and Biometry, Galton Laboratory, University College London, Wolfson House, 4 Stephenson Way, London N W 1 ZHE, United Kingdom. Genetics 140: 659-666 (June, 1995) Drosophila in different thermal regimes has established the importance of temperature as a selective agent. Adaptation to lower temperatures has resulted in increased body size (ANDERSON 1966, 1973; PARTRIDGE et al. 1994a). Other studies reported similar results, but could not discriminate between the effects of natural selection and genetic drift because of lack of replication (POWELL 1974; CAVICCHI et al. 1985, 1989; LINTS and BOURGOIS 1987). Adult body size could be the target of thermal selection if the fitness advantage of larger size is greater at lower temperatures. At present there is little evidence to support or refute this hypothesis. Alternatively large adult body size may be equally favored by selection at different temperatures, but lower temperatures may select for more efficient growth during the preadult period (NEAT et al. 1995). The plausibility of these alternatives can be tested by examining the duration of preadult development. Selection at lower temperatures in the laboratory leads to faster preadult development (ANDERSON 1966, 1973; HUEY et al. 1991) as a result of faster larval development (PARTRIDGE et al. 1994b;Jm~s and PARTRIDGE 1995). These results imply that events during larval development are indeed important for the thermal evolution of body size. The pattern of genetic correlation between adult body size and rate of larval development produced by thermal selection is the opposite to that observed with artificial selection at a single temperature, where large adult size is associated with extended preadult development (ROBERTSON 1957, 1960, 1963), again as a result of an extended larval period (PARTRIDGE and FOWLER 1993). In Drosophila, the hormonal events leading to pupariation are initiated early in the third larval instar when a critical size is reached, after which there is a fixed period of postcritical feeding and growth before pupariation occurs (BEADLE et al. 1938; BAKKER 1959, 1961; 660 A. C. James, R. B. R. Azevedo and L. Partridge ROBERTSON 1963). Variation in larval critical size and feeding rate during the postcritical period leads to differences in adult body size (SEWELL et al. 1975; BURNET et al. 1977). Temperature throughout preadult development can affect adult body size (WRY and ROBERTSON 1979; DAVID et al. 1983), suggesting that it must affect both critical weight and growth in the postcritical period. Laboratory thermal selection was shown to lead to adaptation in larval critical weight at the temperature of selection (PARTRIDGE et al. 199413). Variation in adult body size in Drosophila can be a result of altered cell size or cell number. Each epidermal cell in the wing blade secretes a cuticular trichome so that the average cell size for a given region of the wing can be measured directly and used to produce an index of the total number of cells in the wing (DOBZHANSKY 1929). During development, temperature affects predominantly cell size (ROBERTSON 1959a; CAVICCHI et al. 1985; PARTRIDGE et al. 1994a), whereas food abundance and composition affect predominantly cell number (ROBERTSON 1959a). The body size divergence among populations during laboratory thermal selection is attributable primarily to changes in cell size (PARTRIDGE et al. 1994a), in contrast to the response to artificial selection for body size at a single temperature, which is achieved mainly through changes in cell number (ZARAPKIN 1935; ROBERTSON 1959b, 1962; L. PARTRIDGE, R. E. LANCELAN, K. FOWLER and V. FRENCH, unpublished results). Temperature is therefore unusual in changing body size through altered cell size. In this study, we investigate a latitudinal cline in body size in Australian D. melanogaster. These populations show a cline in development time, with more rapid larval development at higher latitudes (JAMES and PARTRIDGE 1995). We test for a latitudinal association between rapid larval development and large adult body size, as found in laboratory thermal evolution, which would suggest that temperature is a selective agent responsible for latitudinal clines in these traits in nature. We also examine the relative importance of cell size and cell number in causing size variation in wing area. If the latitudinal clines are a consequence solely of thermal selection, we might expect the cellular basis of their altered body size to be the same as for laboratory thermal lines. MATERIALS AND METHODS The geographic stocks originated from flies collected at sites on a 2600-km transect along the eastern coast of Australia during February, 1993. The collections were sampled from 13 latitudes, seven of these with two replicate sites (Figure 1; Table 1). Population cages were initiated with 25 males and 25 females from each of 30 isofemale lines from each site and kept at 16.5”. The experiments described here were conducted at 16.5” and <9 mo after the flies were collected. In the first experiment, egg collections were made on yeasted grape juice and agar plates in each population cage for an 8-hr period. As first instar larvae hatched (-48 hr after CI