Drosophila simulans
Drosophila simulans is a species of fly closely related to "D. melanogaster", belonging to the same "melanogaster" species subgroup. Its closest relatives are "D. mauritiana" and "D. sechellia". Taxonomy. This species was discovered by the fly geneticist Alfred Sturtevant in 1919, when he noticed that the flies used in Thomas Hunt Morgan's laboratory at the Columbia University were actually two distinct species: "D. melanogaster" and "D. simulans". Males differ in the external genitalia, while trained observers can separate females using colour characteristics. "D. melanogaster" females crossed to "D. simulans" males produce sterile F1 females and no F1  males. The reciprocal cross produces sterile F1 males and no female progeny. "Drosophila simulans" was found later to be closely related to two island endemics, "D. sechellia" and "D. mauritiana". "D. simulans" will mate with these sister species to form fertile females and sterile males, a fact that has made "D. simulans" an important model organism for research into speciation. "D. simulans" are monomorphic in their pheromone profiles where both males and females largely produce the cuticular hydrocarbon pheromone 7-tricosene (7-T). The ability of males within the "D. melanogaster" subgroup to discriminate between conspecific and heterospecific females is  due in part to the differential valence of the cuticular hydrocarbon 7,11-heptacosadiene (7,11-HD), which is produced by "D. melanogaster" and "D. sechellia" females. Perfuming a "D. simulans" female with 7,11-HD is sufficient to suppress "D. simulans" male courtship. Studies have provided evidences that paternal leakage is an integral part of the inheritance of this species. "Wolbachia" infections give insight into how certain species of "Drosophila" are related. Through the analysis of cytoplasmic incompatibility and similar mitochondrial DNA, it has been shown that "D. simulans" and "D. mauritiana" are more closely related to each other than to "D.  sechellia." Cytoplasmic incompatibility causes egg and sperm cells to fail in creating viable offspring, a common feature in "Wolbachia"-infected "D. simulans" and "D. mauritiana" individuals. "Drosophila sechellia" has significantly distinct mitochondrial DNA, further emphasizing the evolutionary differences between the three species. Relationship with "Wolbachia". Infections of "Wolbachia", a commonly infectious strain of bacteria observed in many insects such as "Trichogramma" and "Muscidifurax uniraptor" wasps, are transmitted between generations of "Drosophila simulans." "Wolbachia" is inherited through maternal heredity. The infection is maintained through a process involving cytoplasmic incompatibility (CI) in which "Wolbachia" hinders uninfected individuals from producing offspring. "Wolbachia" has formed  a symbiotic relationship with "D. simulans". "Wolbachia" infects the cytoplasm of a cell; once infected, a female fly will pass the infection to all resulting offspring through the cytoplasm of her eggs. Two separate "Wolbachia" infection events have occurred in the ancestors of "D. simulans," suggesting the evolutionary advantage of "Wolbachia" infections to "D. simulans." Effects of "Wolbachia" infection. "Wolbachia" infections have significantly decreased virus-induced mortality in "D. simulans." While the mechanism for the decreased virus-induced mortality is still unknown, "Wolbachia" provides antiviral properties, potentially perpetuated by outcompeting the virus. Furthermore, different strains of "Wolbachia" have varying levels  of antiviral properties; for example, some strains can protect against DCV (Drosophila C virus) while other strains cannot. Benefits of "Wolbachia" studies. "Drosophila simulans" has also played an important role in sequencing the genomes for certain Wolbachia strains. "D. simulans" eggs were infected with the wRi "Wolbachia" strain in order to better understand how "Wolbachia" recombines. Further studies can help understand how "Wolbachia" strains coexist with "D. simulans" individuals. Studying "Wolbachia" strains and their mechanisms of infection can provide insight into the complex phylogenetic relationships of arthropods.