Drosophila nigrospiracula is a fly species indigenous to the Sonoran Desert, spanning Arizona, Baja California, and part of Sonora, Mexico. D. nigrospiracula share the Sonoran Desert with three other species of Drosophila: D. pachea, D. mettleri, and D. mojavensis. This fly breeds on the decomposing tissues of two species of cacti that are also endemic to the region: cardón (Pachycereus pringlei) and saguaro (Carnegiea gigantea).
Taxonomy
D. nigrospiracula is part of the D. anceps species complex. This species is categorized as part of the D. mulleri subgroup, one of the five subgroups within the Drosophila repleta group of the Drosophila genus. The four Drosophila fly species inhabiting the Sonoran Desert are not particularly closely related to each other but are all, however, under the repleta group. Each species came to inhabit this region independently, and they experience different resource availability. As such, they adapted independently to the environment of the Sonoran Desert and came to use different cactus species as hosts.
Habitat
D. nigrospiracula are only found in the Sonoran Desert and specifically on cardón (Pachycereus pringlei) and saguaro (Carnegiea gigantean) cacti. Cardón and saguaro are tall, columnar cacti with similar morphology, chemical composition, and low alkaloid composition (their main allelochemical). Saguaro grow on the main part of the desert, while the cardón species grows only on the Baja peninsula. D. nigrospiracula is restricted to these two forms of cacti because they emit the alkaloids gigantine and carnegine, which D. nigrospiracula can metabolize. These allelochemicals are usually harmful to an insect without the proper allelochemical detoxification enzymes – namely the cytochrome p450 system. In general, formation of plant decay can come from injury or freezing of the cactus. Then, exudate flows down the plant, creating a place for D. nigrospiracula to congregate.
Mechanism of metabolism of host plant
The cytochrome p450 system is found in nearly all life forms and is responsible for processing pheromones and steroids and also metabolizing insecticides, drugs, mutagens, and carcinogens. The p450 system allows Drosophila to metabolize and detoxify the secondary compounds that the cacti use as a function of herbivore defense in order to viably use the plant as a host.
Home range
Typical population sizes range from 361 to 3,999 flies. These flies commute to other saguaro cacti at rates three times higher than any other Drosophila species; they live in short-lived yet widespread habitats, requiring constant relocation to look for resources. D. nigrospiracula travel up to 2 km per day to gain resources between the low density cacti.
Life history
D. nigrospiracula eggs form in follicles that consist of 15 nurse cells, and a given female's paired ovaries consist of hundreds of follicles. The chromosomes of these flies are polyploid and are actively transcribed to RNA, which is then passed to oocytes to help with embryogenesis. In the final stages of oogenesis, nurse cells dispose of their nuclear components into growing oocytes, providing them resources for embryonic development.
Role of phosphorus in egg formation
D. nigrospiracula exhibit sexual dimorphism through differing ratios of phosphorus concentration in adult males and females. Females have about 3 times as much phosphorus in their gonads as males. This inequality is due to the importance of phosphorus in the synthesis of nucleic acids during oogenesis. The cacti on which D. nigrospiracula breed and feed have low concentrations of phosphorus, but research has shown that the higher concentration in females comes from the phosphorus transferred from the male to the female during copulation. The phosphorus from the male ejaculate is incorporated into the female ovaries and traceable in RNA and DNA. Inside the female, phosphorus is then used to synthesize nucleic acids necessary in egg production. Reduced phosphorus levels in mature female diets slowed oogenesis.
Mating
Larger males tended to be more successful in mating than smaller males, as they win the male-male competition for females. Females approach males to mate at non-resource-based territories controlled by a male.
D. nigrospiracula has a sex ratio that favors females, as females can mate multiple times in one morning. Additionally, females also are ready to mate 3 days after birth, but males do not reach sexual maturation until they are 9 to 10 days old. These two factors greatly increase the operational sex ratio in favor of females.