The scuttle fly

Abstract

What is a scuttle fly? The scuttle fly, Megaselia scalaris, is a Dipteran fly in the family Phoridae named for the short, rapid bursts of running motion characteristic of adults. The species is alternatively referred to as the ‘humpbacked fly’ because of its hump-backed appearance (Figure 1Figure 1), or the ‘coffin fly’ because of its ability to dig deep into the ground to reach buried carrion and penetrate closed containers such as coffins. It is predominantly a warm climate species, but has been found in all biogeographic regions in North America, Asia, Africa, and Europe. Scuttle flies are present in habitats ranging from urban buildings to tropical rainforests, and thrive in unsanitary conditions such as trash containers, public restrooms, sewer pipes, and litter boxes.Figure 1A scuttle fly (photo by E. Spana).View Large Image | View Hi-Res Image | Download PowerPoint SlideM. scalaris is an omnivorous species, reportedly consuming a wider range of organic materials than any other insect. The larvae are capable of developing in living or decaying plant, animal, and fungal tissues, feces, milk, paint, and boot polish, and will consume almost anything that is, was, or came from a living organism. Larval development averages about seven days, with the subsequent puparial stage lasting approximately ten days; however, developmental time and percentage survival have been shown to decrease with increasing temperature from 21°C to 32°C. Scuttle fly adults grow to 2–3 mm in size, with females slightly larger than males. Females copulate soon after pupal emergence, and lay their eggs directly after copulation. Male fertility is purportedly limited to a short period after pupal emergence.M. scalaris is readily cultured in the laboratory, and its use as an experimental species in genetics, developmental biology, and drug and pollutant bioassay studies is increasing. Studies have compared the size, development, behavior, and genetics of M. scalaris to Drosophila in particular. Although M. scalaris eggs and adult males are both similar in size to those of Drosophila melanogaster, the larvae and pupae of M. scalaris are much larger than those of similarly reared D. melanogaster. The life cycle of M. scalaris is significantly longer than that of D. melanogaster, with a much slower rate of maturation. External temperatures influence the fecundity, sex ratio, and longevity of M. scalaris.Why are scuttle flies of interest? They are of medical and forensic importance because of their role in human myiasis and its colonization of human corpses. Myiasis is an invasion of dipterous fly larvae into the tissues or organs of vertebrates. In humans, myiasis can be an indicator of neglect in primary care facilities, and such cases have legal implications in the United States. M. scalaris larvae have been implicated in human myiasis in wounds, intestines, eyes, the respiratory system, and the urogenital system. Infestations of these flies are most common in the third world but have also been observed in the United States and Canada in schools and other facilities with variable sanitation. As a result, M. scalaris may cause disease both by spread of pathogens and by direct myiasis. Reports indicate that M. scalaris is capable of completing larval development, producing viable larvae, and giving rise to adults within the human intestine. The ability of M. scalaris to complete their life cycle on ripe bananas leads to human health concerns, where the vast consumption of affected fruit may potentially lead to human intestinal myiasis.Forensic entomology is often used to given an accurate estimation of the postmortem interval in homicide investigations. While blow flies (Calliphoridae) are typically the first flies to arrive at a carcass, their large size may delay their arrival at corpses in tightly enclosed spaces. The small size of scuttle flies allows them to colonize buried carrion or bodies in sealed rooms or coffins, often reaching decaying tissue that larger flies are unable to. Gravid females are strongly attracted by the olfactory cues that emanate from recently dead organisms, and can rapidly locate dead insects or animals in various environments. Although they are classified in a secondary forensic role because of their affinity to older decaying tissue, scuttle flies are frequently among the first insect colonizers of a corpse. Estimating the duration of development of M. scalaris since oviposition can be used to establish a postmortem interval. Because of their limited mobility and omnivorous diet, scuttle fly larvae ingest various compounds present in their environment that are incorporated into the sclerotized integument of the last larval instar. Thus, they have been used in bioassays to detect drugs in corpses that may have contributed to the cause or manner of death.Scuttle flies have also been used as an insect model for tolerance of anthropogenic pollutants. Although manganese and nickel have been shown to have a detrimental effect on some insect herbivores, the development and survival of M. scalaris were not affected by high concentrations of this metal in laboratory studies, suggesting that scuttle flies may be more tolerant of some pollutants than are other invertebrates. M. scalaris larvae exposed to high concentrations of hexavalent chromium, selenium, and methylmercury exhibited increased developmental times and levels of larval mortality in laboratory studies. Similar results in the natural environment would not only affect the ecology, population dynamics, and sustainability of M. scalaris populations, but potentially impact ecosystem stability through the loss of an important detritivore and abundant prey species.Finally, in a limited number of documented cases, scuttle flies have invaded living animals and plants to the extent of causing economic damage. For example, they are associated with cases of myiasis in cattle. M. scalaris also breed readily on ripe bananas and other fruits, various seeds, and ears of corn.Does the scuttle fly have a unique characteristic? Along with the unusual locomotory behavior of the adults, larvae of this species exhibit a unique behavior of swallowing air when exposed to a liquid environment. When placed in an aqueous environment, M. scalaris larvae quickly swallow air that produces bubbles in their gut. The resulting change in buoyancy allows the larvae to float, potentially enabling them to avoid drowning in natural environments. Because M. scalaris larvae are occasionally found in water-filled containers such as decaying coconuts and tree holes, this behavior may be a successful survival strategy for a species that occupies such a wide range of environmental conditions.Why might the scuttle fly serve as a genetic model system? Genetic interests in M. scalaris range from its unusual means of sex determination to its potential as an emerging model organism in ecology and evolutionary biology. Scuttle flies have been used to investigate the earliest stages of sex chromosome evolution, when homomorphic chromosomes having one or more sex determining factors evolved into heteromorphic pairs of sex chromosomes. Crossover suppression in the chromosomal region containing the sex-determining factor is thought to have initiated the differentiation of X and Y chromosomes, with the Y segment evolving independently from that of the X.M. scalaris has three pairs of homomorphic chromosomes, with the male sex determined by the presence of a male-determining factor, M, that can change locations among the chromosomes. In M. scalaris, the differentiation of new X and Y chromosomes can be initiated in independent lines by the transfer of M to another chromosome, whereby one copy of an original autosome becomes the new Y chromosome and the homologous chromosome becomes a new X chromosome. The new Y chromosome is then propagated and transmitted clonally to all male descendents in that lineage. Transposition of M among the three chromosomes has been observed at a frequency of 0.08–0.3%. Transposition of sex-determining factors has also been observed in other Diptera, including the midge (Chironomus tentans), the mosquito vector of Japanese encephalitis (Culex tritaeniorhynchus), and the housefly (Musca domestica).Recent work has generated genomic resources and biologically useful information for M. scalaris. We have found that the M. scalaris genome size is approximately 500 megabases, about four times larger than the D. melanogaster genome. Useful information on the scuttle fly genome was recently obtained from a preliminary low-coverage (0.05x) genome sequence survey, including types and relative abundances of repetitive element families, a nearly complete mitochondrial genome sequence (bearing identical arrangement of protein-coding genes and ribosomal subunit RNAs to other arthropods), and microsatellite sequences for population genetic analyses. Because published Dipteran genome sequences are currently limited to Drosophilids and mosquitoes, generating a complete scuttle fly genome sequence will allow for more comprehensive comparative and evolutionary genomic studies within this Order.