RESISTANCE OF THE SIBLING SPECIES DROSOPHILA MELANOGASTER AND DROSOPHILA SIMULANS TO HIGH TEMPERATURES IN RELATION TO HUMIDITY: EVOLUTIONARY IMPLICATIONS.

Abstract

The interphase of genetics and ecology is assuming increasing importance at the present time in evolutionary biology as shown by recent studies on natural populations of Drosophila. Two main methods are being used: (1) the study of flies in natural environments in order to find correlations of species, genotypes within species, and (more rarely) their behavior with environmental factors; (2) the study of populations in the laboratory to assess factors of likely ecological importance under controlled conditions. Work on these two methods has until recent years proceeded rather independently; indeed most early studies are laboratory based. It is, however, essential to link both approaches (Parsons, 1973), and recent examples include detailed studies of cactus-breeding Drosophila (Fellows and Heed, 1972; Johnston and Heed, 1976; Starmer et al., 1977), Hawaiian Drosophila (Kaneshiro et al., 1973; Richardson and Johnston, 1975) and Australian cosmopolitan Drosophila (Parsons, 1975a). In contrast with the enormous variety of resources used by breeding and feeding Drosophila (Carson, 1971) the physical limitations imposed by the environment for such activities are relatively uniform across the genus, at least for Australian temperate zone species (Parsons, 1978). The success of a population depends on its adaptation to climatic conditions; the annual cycle of the temperate zone provides a great range of stresses, but short-lived stress periods occur diurnally in addition. Especially in temperate regions, the major density-independent environmental stresses to which Drosophila is exposed are: (1) a combination of high temperature and desiccation stress and (2) low temperature. For both, it is important to distinguish between conditions for resource utilization (feeding, breeding) and survival. Considering high temperatures, it is normally difficult to breed the cosmopolitan species D. melanogaster above about 29 C although flies can survive at this temperature for 10 days or more, while the sibling species D. simulans normally survives this extreme less well (see Parsons, 1975a). There are, however, some species of Drosophila where adults necessarily must survive at least some hours of the day at temperatures in excess of 30 C. For example, many forms of the North American repleta group encounter very high temperatures in deserts where they characteristically live in damp rot pockets of cactus (Fellows and Heed, 1972). Similar exposure to high temperatures occurs for a recently discovered species D. (Scaptodrosophila) hibisci (Cook et al., 1977), which utilizes the flowers of endemic Hibiscus species as a resource in open forests in the northern half of Australia where temperatures frequently exceed 30 C. In this case the base of the corolla tube apparently provides a humid microhabitat allowing flies to survive in a climate that is otherwise extreme for Drosophila. In addition McKenzie (pers. comm.) found D. melanogaster in winery wastes at high humidities where temperatures exceeded 30 C, and Arlian and Eckstrand (1975) have argued for humid microhabitat selection in D. pseudoobscura. These observations lead to the Dossible