Natural and genetic engineering of the heat-shock protein Hsp70 in Drosophila melanogaster: consequences for thermotolerance.

INTRODUCTION

Carl Gans (1978) entitled his presidential address to the American Society of Zoologists "All animals are interesting!" In this address, he decried the tendency of some biological scientists to focus on popular model organisms (e.g., Drosophila melanogaster, Escherichia coli, Caenorhabditis elegans, Saccharomyces cerevisiae, mammalian cells in culture, and so on) and to eschew the insights that might be forthcoming from biological diversity in general and exotic species in particular. Today, many members of the Society of Integrative and Comparative Biologists, successor to the American Society of Zoologists and the publisher of American Zoologist, perpetuate the outlook symbolized by Gans's title and have accordingly engendered enormous scientific progress. In so doing, however, many have tended to regard the popular model organisms as mundane, not particularly interesting, or not "real" organisms. My intent here is to emphasize by example the first word in Gans's title. The features that have made the favored model organisms popular are evolutionary solutions to significant environmental problems; these problems and solutions are every bit as interesting as those of more exotic species and deserve the same respect.

Among the most maligned of the standard model organisms is Drosophila melanogaster, the fruit fly. Kohler (1994) has recounted how generations of laboratory culture have stripped away many Drosophila traits that inconvenience laboratory experimentation. Nonetheless, Drosophila flourish outside the laboratory, where it is a human commensal and exploits necrotic fruit in the wild (Ashburner, 1989). The particular challenges of the necrotic fruit habitat may be responsible for some of the distinctive features of Drosophila. Several stages of the life cycle, (eggs, embryos, three larval instars, and perhaps pupae) obligately occur on or within necrotic fruit. Necrotic fruit is a rich but ephemeral food source for many organisms. Unlike "true" fruit flies (Rhagoletis), many other insects, and many non-insect frugivores, however, Drosophila are unable to breach the intact epidermis of necrotic fruit. Drosophila thrives when fruit necrosis is advanced. These characteristics of Drosophila limit their exploitation of any given fruit to the brief time from when other frugivores or mechanical injury breach a fruit's epidermis to when desiccation, dissolution, or consumption by frugivores exhaust the edible fruit. The emphemerality of this habitat may have led to the unusually rapid life cycle of Drosophila. Embryogenesis occurs at the physical limit of replication fork formation and movement (Kart and Mittenthal, 1992), and the time from oviposition to pupation is brief, 5-6 days at 25 [degrees] C. The larvae themselves live in a semi-fluid with high concentrations of alcohols, ketones, and other organic substances, which can be nutritive at low concentration but toxic at high concentrations (McKenzie and McKechnie, 1979; McKechnie and Morgan, 1982). Biochemically, larvae deploy a suite of mechanisms for detoxifying ethanol and utilizing its breakdown products (Geer et al., 1993). Necrotic fruit also supports a rich microbial flora, which can be a significant source of protein to the growing larva (Begon, 1983). When the fruit is sufficiently fluid, the larvae respire intermittently; they remain within the fluid but periodically project a pair of ventral spiracles into the atmosphere to breathe. Eggs, which are deposited on the surface of the necrosis, have a pair of projections that keep the egg from drowning; the projections themselves are especially extensive in Drosophila species that typically lay on more fluid media. Although these and many other features of Drosophila have come to light as by-products of genetic and/or molecular investigations, they are "interesting" (Gans, 1978) in their own right and ripe for future study.

THE THERMAL ECOLOGY OF NON-ADULT DROSOPHILA MELANOGASTER

While adult Drosophila are minuscule, they can travel many kilometers each day (Coyne et al., 1982). With both such locomotor abilities and small size, adult Drosophila may prospectively seek out equable microhabitats in the most hostile of environments. Whether adult Drosophila routinely avoid temperature stress is unknown, for current technology does not permit the long-term monitoring of body temperatures of unrestrained adult Drosophila in the wild. Physiological experimentation (Huey et al., 1992) suggests that sustained flight is not feasible at temperatures much above 35oC (and possibly even at much lower temperatures), which ought to restrict activity at certain times of day and year.

In contrast to the adults, eggs and all but very late larvae are restricted to necrotic fruit. Pupae have not been studied systematically in the wild, although they sometimes occur on the fruit or form some distance away in soil (Sokolowski, 1985), prospectively in relation to thermal and hydric conditions immediately before pupation. When on or in necrotic fruit, non-adult Drosophila have negligible thermal inertia and are thus subject to the prevailing temperatures of the host fruit. This dynamic can and does impose massive high temperature stress on Drosophila living within or on sunlit necrotic fruit (Feder, 1996; Feder, 1997; Feder et al., 1997a; Feder et al., 1997b; Feder and Krebs, 1997):

* On summer days, necrotic fruit can heat to [is greater than] 36 [degrees] after 60mm minutes of insolation. Temperatures [is greater than] 40 [degrees] are not uncommon.

* Fruit color, mass, and water loss affect heating kinetics but are not sufficient to mitigate temperatures harmful to Drosophila.

* Temperatures within a fruit are not sufficiently diverse for larvae to thermoregulate behaviorally. Accordingly, larvae cannot entirely avoid high temperatures that may be present.

* Ovipositing females do not avoid fruit unless it is warm at the time of oviposition, suggesting that at many rimes females oviposit on fruit that may overheat.

* Drosophila larvae and pupae actually infest necrotic fruit that experiences high temperatures on summer days, as is evident from acute measurements of larvae and pupae in situ in an orchard in Indiana (Fig. 1).

[Figure 1 ILLUSTRATION OMITTED]

Such high temperatures can harm nonadult Drosophila (David et al., 1983; Ashburner, 1989; Feder and Krebs, 1997), as the discovery of dead larvae in the field (Fig. 1) and the effects of temperature on physiological function in the species (Fig. 2) suggest. Growth, reproduction, and other physiological processes that underlie fitness occur most rapidly at approximately 27 [degrees] C or slightly below; at higher temperatures, these functions deteriorate in direct proportion to the excess in temperature. 30 [degrees] C is an ecological upper limit; unless Drosophila can be at temperatures [is less than] 30 [degrees] C for some time, their populations cannot persist in nature (Parsons, 1978). Temperatures [greater than] 30 [degrees] C compromise reproduction, and temperatures [greater than] 37-38 [degrees] C can kill quickly.

These features place Drosophila in elite company with respect to natural high temperature stress in animals. Most other animals…