The Genus Drosophila As A Model System

In addition to referring to the single species D. melanogaster, "Drosophila" can also refer to the entire genus Drosophila, a spectacular radiation of roughly 1500 described species. This genus can be found throughout the world in every conceivable habitat, from tropical rain forests to subarctic regions. Generally, these species are saprophytic, feeding and ovipositing in rotting plant and, sometimes, animal material. Members of this genus have been used as a model system for understanding evolutionary biology. A number of Drosophila groups, such as the obscura, repleta, and virilis species groups, have become prominent model systems in evolutionary biology. Such studies include chromosome and molecular evolution, the mechanisms of species formation, phylogeny, ecology, and behavior.

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Chromosomes • Diptera • Genetic Engineering • Research Tools, Insects as

Further Reading

Adams, M. D., Celniker, S. E., Holt, R. A., Evans, C. A., Gocayne, J. D., Amanatides, P. G., Scherer, S. E., Li, P. W., Hoskins, R. A., Galle, R. F., et al. (2000). The genome sequence of Drosophila melanogaster. Science 287, 2185-2195.

Ashburner, M. (1989). "Drosophila: A Laboratory Handbook." Cold Spring

Harbor Laboratory Press, Cold Spring Harbor, New York. Ashburner, M., Carson, H. L., and Thompson, J. N., Jr. (eds.). (1981— 1986). "The Genetics and Biology of Drosophila." Academic Press, New York. (See especially Wheeler, M. R., pp. 1-84, and Lemeunier et al., pp. 147-256.)

Barker, J. F. S., Starmer, W. T., and MacIntyre, R. J. (eds.). (1990). "Ecological and Evolutionary Genetics of Drosophila." Plenum Press, New York. Kohler, R. E. (1994). "Lords of the Fly: Drosophila Genetics and the

Experimental Life." University of Chicago Press, Chicago. Krimbas, C., and Powell, J. R. (eds.). (1992). "Drosophila Inversion

Polymorphism." CRC Press, Boca Raton, FL. Lawrence P. A. (1992). "The Making of a Fly: The Genetics of Animal Design." Blackwell Scientific, Oxford, U.K.

Patterson, J. T., and Stone, W. S. (1952). "Evolution in the Genus Drosophila." Macmillan, New York.

Powell, J. R. (1997). "Progress and Prospects in Evolutionary Biology: The Drosophila Model." Oxford University Press, New York.

Sullivan, W., Ashburner, M, and Hawley, R. S. (eds.). (2001). "Drosophila Protocols." Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY. (See especially, Pardue, M.-L., pp. 119-129.)

Throckmorton, L. H. (1975). The phylogeny, ecology, and geography of Drosophila. In "Handbook of Genetics," Vol. 3, "Invertebrates of Genetic Interest." (R. C. King, ed.), pp. 421-469. Plenum Press, New York.

James Ridsdill-Smith

Commonwealth Scientific and Industrial Research Organisation, Australia

Dung beetles are specialized to feed and breed on an ephemeral and discrete food resource, namely, the piles of dung produced by herbivorous warm-blooded animals. Adults are strong flyers and can search for some distance to find fresh dung. Most species make tunnels in the soil and remove dung from the pat, which is packed into the tunnel to form a brood mass in which a single egg is laid. Ecosystems contain many coexisting dung beetle species, particularly in tropical grasslands. Intraspecific and interspecific competition for dung is high, and beetles show diverse behavior to reduce its effects. Fecundity of dung beetles is extremely low, the eggs are relatively large, and adult investment in nesting behavior is high, as is illustrated in male/female adult reproductive behavior and in brood care by female beetles.

Dung beetles have a place in history, in which the ball-rolling species, Scarabeussacer (Fig. 1), was sacred to the early Egyptians (Fig. 2). The ability of large ball-rolling beetles to create a perfect sphere, which is then rolled along the soil surface, was taken to be representing Khepri, a great scarab beetle, rolling the globe of the rising sun. The scarabeus, symbol of the sun, is often depicted hovering with outstretched wings.The new beetle emerges from the inactive pupa, representing rebirth or reincarnation. Scarab amulets often were placed over the heart of the dead to simulate rebirth or worn widely by the living to bring good luck.

BIOLOGY Dung as a Resource

When scarabaeine dung beetles are abundant, numbers can be observed swarming upwind in the odor plume from fresh dung. Volatile compounds produced from the fresh dung attract adult dung beetles, and most arrive within the first few hours after the dung is deposited. Up to 16,000 beetles have been recorded at a single elephant dropping, to which

FIGURE 1 Adult S. sacer. Unworn tibiae indicate a newly emerged beetle. (Photograph courtesy of CSIRO Entomology.)

4000 beetles were attracted in 15 min. Beetles leave when most of the dung is buried or when feeding activity has removed most of the moisture from the dung (referred to as shredding). Adult beetles feed on the liquid "soup" in the dung. The incisor lobe of the adults is flattened and fringed for handling soft food, and the particulate components of the food are filtered out before being ingested. The galae and laciniae of the adults have special brushes for collecting food. Dung produced by herbivorous animals is highly variable in size and consistency and may range from mounds weighing over 1 kg from an elephant to pellets of about 1 g from a rabbit. The water content of dung is high at deposition (90% water), but dung dries out quickly, the rate depending on both temperature and the size of the dung pat. Typically, dung is used as a resource by dung beetles for 1 to 4 weeks, although small sheep pellets may dry out in 3 h in summer and are relatively little used by dung beetles after this time.

FIGURE 2 Egyptian sacred scarabs. Top center is a winged scarab pectoral in blue faience with holes for attachment. Bottom left is a heart scarab in blue faience. Bottom right is an inscribed heart scarab in felspar. (Photograph courtesy of Ashmolean Museum, University of Oxford.)

Dung Burial for Brood Masses

The behavior for which the dung beetle is best known is the removal of dung from the pat and burial in the ground as provisioning for their offspring. Three groups are distinguished based on their behavior in creating a brood mass, the compacted dung in which a single egg is laid. In teleocoprids, a sphere is made from pieces of dung at the dung pat or from pellets of dung (Fig. 3A). Beetles roll the ball away from the dung pat, usually with the hind legs. It is buried in the soil, and a single egg is laid in a small cavity. In paracoprids, the beetle digs a tunnel in the soil under a dung pat, cuts off pieces of dung using its front legs, head, and body, and carries them down the tunnel where they are packed into the end to form a compacted brood mass. As each brood mass is completed, a single egg is laid. Soil is then placed over the brood mass and another brood mass is made. Branching tunnels may be made containing many brood masses with eggs (Fig. 3B). The size and shape of the brood mass, and the depth of the brood mass in the soil, are characteristic for each species. These will be affected by soil moisture and soil hardness. Endocoprid species construct brood balls in cavities within the dung pat.

Life Cycle

When a female beetle is ready to lay her eggs, she constructs a chamber in the top of the brood mass and lays an egg on a small pedestal, which prevents it from coming into contact with the surrounding dung (Fig. 3C). The larva hatches after a week or two and feeds on the dung of its brood mass (Fig. 3D). Larvae have biting mouthparts, unlike the adults, and can use the fiber content of the dung. They typically complete three instars over about 12 weeks and then undergo a pupal stage (Fig. 3E) 1 to 4 weeks before turning into an adult (Fig. 3F). The adults emerge from the brood shells, dig their way to the soil surface, and then fly off and find fresh dung on which to feed. Depending on the biology of the individual species, there may be periods of diapause or quiescence by mature larvae, pupae, or adults during development. Such adaptations are usually related to enhancing survival over, for example, a dry summer or a cold winter and can delay the completion of the life cycle by several months. Dung beetle species are univoltine, completing one generation a year, or multivoltine, completing several.

FIGURE 3 Dung beetle reproduction in cattle dung. (a) Dung pat with a teleocoprid species removing a ball of dung and burying it. (b) Dung pat with a paracoprid species producing brood masses in tunnels beneath the pat. Brood mass containing: (c) egg, (d) larva, (e) pupa, and (f) young adult. (Illustration by Tom Prentis from Waterhouse, 1974; reproduced with permission.)

FIGURE 3 Dung beetle reproduction in cattle dung. (a) Dung pat with a teleocoprid species removing a ball of dung and burying it. (b) Dung pat with a paracoprid species producing brood masses in tunnels beneath the pat. Brood mass containing: (c) egg, (d) larva, (e) pupa, and (f) young adult. (Illustration by Tom Prentis from Waterhouse, 1974; reproduced with permission.)

Dung Quality

Beetle egg laying is very sensitive to seasonal changes in dung quality. This quality is influenced by several factors, including rainfall, which affects the growth of plants on which grazing animals are feeding and hence the quality of dung they produce, and the plant species grazed upon. Under favorable laboratory conditions, the rate of egg production of Onthophagus binodis on dung collected from cattle grazing on dry summer annual pasture in winter rainfall regions of Australia is 7% of that on dung from cattle grazing on green spring annual pasture. Onitis alexis, a larger species, is somewhat less affected by the same seasonal changes in dung quality. The rate of egg production of Euoniticellus intermedius on dung collected from cattle grazing on dry winter pasture in summer rainfall regions of Australia is 30% of that on dung from green summer pastures.

Egg Laying

The adult female reproductive system of Scarabaeinae has only a single ovary, consisting of a single ovariole. Newly emerged beetles have no differentiated oocytes in their single ovary. The eggs develop sequentially during a period of maturation feeding. The terminal oocyte is the only one ready to be laid at any one time. If conditions are unsuitable for oviposition, the oocyte is extruded from the ovariole into the hemocoele and nutrients are resorbed.

Fecundity of scarabaeine dung beetles is very low, but they produce relatively large eggs. The length of an average egg is about 33% of the adult female body, and the volume of the egg is about 2.5% of that of the female. Most species probably produce as few as 20 eggs/female/year in the field, because weather conditions or dung quality are rarely ideal for adult reproduction, and some species produce 5 or fewer. Competition for dung at times beetles are ovipositing is high. Adult investment in nesting behavior is high to enhance the success of the offspring that are produced. This is illustrated here in terms of adult male/female reproductive behavior and in female brood care.

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