Species of fly
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Drosophila metlerri, commonly known as the Sonoran Desert fly, is a fly in the genus "Drosophila". The species is found in North America and is most concentrated along the southern coast of California and in Mexico. "D. mettleri" are dependent on plant hosts, namely, the saguaro and cardon cacti. Thus, they are most prevalent in arid, desert conditions. It is able to detoxify chemicals found in the rotting liquid of cacti hosts, which allows it to use otherwise lethal soil as a nesting site.
"D. mettleri" was discovered in San Felipe, Baja California in October 1961 during an experiment where banana was used to attract several species of "Drosophila". Due to physical geographic barriers between Sonoran Desert flies, gene flow explains speciation.
It breeds in soil moistened by liquid excreted by rotting cacti and on necrotic patches. The liquid found in rotting cacti patches serves as a nutrient source for "D. mettleri" larvae, despite it containing chemicals toxic to the intestines of other "Dipterans". Several aspects of the fly species' courtship behavior, such as its courtship song and copulatory plugs, explain sexual selection methods in this fly.
Description.
Differences between "D. mettleri" and "D. nigrospiracula".
Of the Sonoran Desert fly species, "D. mettleri" behaves, breeds, and is distributed most similarly to "D. nigrospiracula". When "D. mettleri" are collected from cacti in prevalent geographic locations, they are isolated alongside "D. nigrospiracula". Both species exploit the toxic sap of their host plants. However, unlike "D. nigrospiracula," "D. mettleri" has a more efficient detoxification system that enables it to nest in areas not exploited by "D. nigrospiracula".
"D. mettleri" prefers to breed in soil moisturized in the sap of injured cacti, while "D. nigrospiracula" is found in the tissue of the cactus itself and is thus subjected to less concentrated toxin levels as compared to "D. mettleri". An up-regulation in the genes encoding for xenobiotic resistance are attributed to the specialized and highly efficient detoxification ability of "D. mettleri" in comparison to other species. This enables the species to use a breeding ground otherwise lethal to both larvae and adults.
Feeding differences.
"D. nigrospiracula" solely exploits cactus sap on the plant itself (i.e. in the tissues of the cacti). However, "D. mettleri" adults are located on both the cactus and in nearby soil, as the fly has the specialized ability to detoxify the highly concentrated sap found in surrounding soil. Females are more commonly located in their breeding ground (the soil), whereas males are situated in both areas.
Physical distinction.
Phenotypic differences and similarities may be used to distinguish between "D. mettleri" and "D. nigrospiracula". Similarities between these two species include large bodies, black carcasses, and thin, defined cheeks that stand in stark contrast to their elongated eyes. Differences between these two species that can be used to decipher between them include the following characteristics that D. "mettleri" has and "D. nigrospiracula" does not: toe extensions from the genital region, a frons pollinose that is angularly shaped like a "v", less maturated gonapophysics, and horns on the anterior only (no horns exist on the posterior region).
Distribution.
Distribution due to heat.
The species is most commonly found in North American deserts, specifically, in Mexico and Arizona. The species is also found on the southern coast of California, where they inhabit prickly pears. Of the four species of "Drosophila" found in the Sonoran Desert, "D. mettleri" have the lowest heat tolerance. "D. mettleri" are abundant through all seasons but are less prevalent during the summer period. Because the necrotic tissue of cacti in the arid environment of the desert characterizes intense heat conditions, the ability of "D. mettleri" to exploit the nearby soil of the cacti for breeding purposes provides a selective advantage wherein the pupa of this species have a higher survival rate than other "Drosophila" desert species. Other species of "Drosophila" are less successful in the heat of the Sonoran Desert in rearing young due to the intense conditions. Their inability to burrow into the soil has been shown to prove detrimental in increasing pupa survival, yet "D. mettleri" has an evolutionary advantage in their ability to exploit a niche environment for breeding. The hypothesis that the necrotic tissue of cacti serves as a similarly protective environment as the soil of the Sonoran Desert for other species of "Drosophila" has been disproven. While other "Drosophila" have higher heat tolerances due to their breeding grounds in the necrotic tissue of cacti that is higher in heat, "D. mettleri" have a lower thermal tolerance because there is no evolutionary advantage to such heat tolerance (ability to exploit the less heat-exposed area of cacti soil).
Effects of geographic barriers.
The geography of the area encompassed by the Sonoran Desert gives rise to genetic differentiation among desert "Drosophila" species who feed on necrotic rot. The Sonoran Desert incorporates the peninsular region of Baja California and mainland regions of California. After the peninsula was formed, the Gulf of California, which lies between the peninsula and the mainland, served as a barrier to dispersal opportunities in insects and land mammals. Another physical geographic barrier is the Gran Desierto de Alter, a large sand dune between the peninsula and the mainland.  These physical geographic barriers are the cause of gene flow among Sonoran Desert flies. Studies of allozymes in "D. mettleri" and "D. nigrospiracula" show differences in nuclear markers that would otherwise suggest differentiation among the regions of the Sonoran Desert. In "D. nigrospiracula", there were markers that showed differences within the species when they were located disparately among the peninsular and mainland regions of the desert. In "D. mettleri", however, there were no such nuclear marker differences among members of the species found on the peninsula versus those found on the mainland. These differences show that deviations among "D. mettleri" can be supported through the stepping stone hypothesis.
Each of the relatives of "D. mettleri" that are closest phylogenetically do not inhabit the Sonoran Desert region; therefore, the four species of "Drosophila" that do inhabit the Sonoran Desert region experienced parallel evolution instead of speciation within the desert. The species endemic to the Sonoran region, other than "D. mettleri", are "D. nigrospiracula", "D. mojavensis", and "D. melanogaster".
Habitat.
Host plants.
The main host plants include the saguaro ("Carnegiea gigantea"), the cardon ("Pachycereus pringlei"), and the senita ("Lophocereus schottii") cacti.
"D. mettleri" is most commonly found in the saguaro and cardon cacti, in part for these cacti's larger size and cooler internal environments. In addition, these species of cacti release more exudant material into the soil; this enriched soil serves as a nesting site. The necrotic tissues of these cacti may also be used as feeding and nesting sites, yet the exploitation of the rot patches in these cacti is less common in "D. mettleri" in comparison to other desert "Drosophila" species.
Selection of host plant by rot patches.
A characteristic pertinent to each of the Sonoran Desert "Drosophila" species is host cactus selection. Each of these flies chooses to exploit a different host plant for its resources (breeding, food). The selection of host cactus is important to the amount of resources available to each of these species and plays a role in the growth and development of the species.
Other factors leading to host selection.
Each fly species exploits necrotic rot patches on the cactus for nutrients. The largest necrotic patch sizes rank in order from highest to lowest: cardon, saguaro, organpipe, and senita. The size and arm spans of the cacti did not influence the size of the necrotic patches. Instead of rot patch size being regulated by spatial factors, seasonal differences in the growth of the host plants determined how large and how abundant necrotic patches were. Further, temporal factors determined the amount of time a necrotic patch lasted; during the spring, patches are least abundant and during the summer, patches are most abundant. While all other species of the four Sonoran Desert Drosophila showed a correlation between the resource richness of their hosts’ necrotic patches and population size, no such distinct correlation was found for "D. mettleri". "D. mettleri" exploits both necrotic patches and soil near rotting cacti; further, this species can thrive on a variety of hosts, leading to more dispersal of these flies under a given spatial region. The overall development periods and availability of host cacti strongly affects the growth of both larvae and the reproductive success of adults. For larvae, optimal host conditions entail long-lived necrotic plots as this gives the larvae enough time to grow and develop into larger, more fit adults. For adults, adequate conditions include hosts that grow more regularly because adults will have a regular site to lay eggs.
Defense mechanism.
The host cacti contain toxic chemicals and compounds that otherwise prevent flies from feeding on cacti. "D. metlerri" contains genes that enable it to detoxify these toxic compounds and thus feed off of the cacti as a source of nutrients. These genes are linked to the evolution of xenobiotic resistance.
Life history.
The unique ability of "D. mettleri" to detoxify the high concentration of alkaloids present in the soil surrounding cacti with rot spots enables adult females to lay their eggs and sustain their larvae in the same nesting area. Other species of desert "Drosophila" that inhabit the same geographic region lack this ability and thus reduce the amount of competition females face in finding nesting sites. One species ("D. majavensis") is composed of adults who cannot withstand the high alkaloid concentration in cacti soil unlike the larva that is only slightly sensitive. While this may encourage larval deposits to form in the soil, the inability of adult females to survive in these soil conditions prevents them from nesting in the soil, thus decreasing the amount of competition "D. mettleri" faces in finding appropriate nesting sites.
Food resources.
Rot material.
Like other desert species of "Drosophila", "D. mettleri" relies on the exudant juices of cacti as a feeding substrate. The juice found in the necrotic tissue of the cacti is also used as a feeding substrate when levels of exudates in the soil become low or scarce.
Yeast and bacteria.
The area of the cactus often exploited by "D. mettleri" (when exudants are not extracted from soil) are "rot pockets". These pockets form when the cactus is injured; aggregates of bacteria and yeast conglomerate on the surface near the injury site and begin to form a pocket-like indent. The fermentation of the yeast and bacteria in this pocket serve as a "signal" to "D. mettleri" so that the site can be exploited for food.
Communities of yeast on host species of "D. mettleri" have been isolated and serve to decompose the host cacti. During mating, both sexes of the fly gift each other yeast and bacteria. This mixture is then placed on breeding sites, later exposing larvae to natural florae necessary in increasing their fitness.
Mating.
Courtship behavior.
The courtship behaviors of desert "Drosophila" of the Eremophila complex are similar. "D. mettleri", belonging to this complex, begin courting as soon as they have their first intrasexual contact. Once the male brushes against a female, courting begins and the male begins to follow the female. The male initiates in courtship song through vibration of his wings. The male will choose to either follow a side of the female or follow behind her. Depending on the spot the male decides to occupy will determine the vibrational frequency of the male's wings. The farther the male is to the female, the shorter the wing pulses are. When the female stops moving, vibrational frequency increases. During this courtship song, the male may choose to lick the genitals of the female. If the female chooses to accept a male during his courting song, the female's wings will expand and the male will terminate his song. The flies will begin to copulate soon after. In comparison to other species in the Eremophila complex, males of "D. mettleri" are more likely to force copulation with females.
Courtship song.
The courting song of "D. mettleri" is characterized by distinct vibrational pulses and frequencies. The more mobile a female is, the more irregular the vibrational pulses of the male will be. To initiate the courting song, a male will extend his wings seventy-eighty degrees and release a few short pulses. After the initiation process, more regular pulses are released by males.
Male-female controls.
In "D. mettleri", after the male inserts his sperm (i.e. ejaculates) into his mate, swelling of the vaginal wall occurs. This may serve as a