Species of fly
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Drosophila innubila is a species of vinegar fly restricted to high-elevation woodlands in the mountains of the southern USA and Mexico, which it likely colonized during the last glacial period. "Drosophila innubila" is a kind of mushroom-breeding Drosophila, and member of the Drosophila quinaria species group. "Drosophila innubila" is best known for its association with a strain of male-killing "Wolbachia" bacteria. These bacteria are parasitic, as they drain resources from the host and cause half the infected female's eggs to abort. However "Wolbachia" may offer benefits to the fly's fitness in certain circumstances. The "D. innubila" genome was sequenced in 2019.
Symbiosis.
"Drosophila innubila" is stably infected by a strain of male-killing "Wolbachia" bacteria. The association between "Drosophila innubila" and "Wolbachia" can vary greatly within local populations. However, their relationship is very consistent across the overall "Drosophila innubila" species.
Male-killing by these "Wolbachia" results in the offspring of flies being entirely female, the biological sex with the higher reproductive output. Thus, this "Wolbachia" spreads in the population owing to the increased reproductive advantage of females it infects. However bacterial density varies over development. Bacterial density is lowest in the embryo and increases over the lifespan of the fly, reaching its highest densities in the ovaries of females. Bacterial density directly affects the efficiency of "Wolbachia" inheritance, as females with lower "Wolbachia" density also produce daughters with a low bacterial density. The forces driving differences in bacterial density may be epigenetic. If bacterial densities are low enough, females begin to produce males in spite of being infected with male-killing "Wolbachia". There is no evidence that a "Wolbachia" infection can be transmitted horizontally from an infected female fly to an uninfected female fly. Unlike extracellular symbiotic bacteria (e.g. "Spiroplasma"), "Wolbachia" live inside their host cells, which likely reduces its ability to move between hosts.
Infected sons are less likely to die from "Wolbachia" infection if their mother possessed a smaller bacterial density. It is speculated that the bacterial density of "Wolbachia" inside a host can vary, depending on the antibiotic activity of larval or adult food sources, possible host defense mechanisms, the age of the host, bacterial interactions within the host, as well as the environmental conditions that the fly experiences. Currently, there is no evidence for a mechanism in "Drosophila innubila" that suppresses or inhibits the male-killing effect of "Wolbachia."
Additionally, an unknown factor contributes to increased fitness benefits of "Wolbachia" infection. The presence of a "Wolbachia" infection can increase the fitness of a female fly by increasing her size and enhancing her fertility. Moreover, the "D. innubila" DNA nudivirus "DiNV" is a common viral infection amongst this species. It has been shown that certain "Wolbachia" can protect their hosts against viral infection, even leading to biocontrol strategies that use "Wolbachia" infection to suppress the spread of viral diseases. What role (if any) "Wolbachia" plays in defense against viruses is unclear. However, other studies that investigated the contribution of "Wolbachia" infection to the fitness of "Drosophila" species suggested that the bacteria can enhance survival of its host in the presence of oxidative stressors as well as prevent other pathogens from infecting the host by outcompeting them for host-derived resources like cholesterol. In addition, it is also suspected that "Wolbachia" may also be able to manipulate the expression of its host's DNA through utilization of host microRNA.
Immunity.
The genome of "D. innubila" was sequenced for a study in 2019, and boasts a very high quality of assembly, rivalling that of the classic genetic model "Drosophila melanogaster". This study highlighted the importance of the interaction between "D. innubila" and its viruses as implied by patterns of immune evolution in antiviral genes. Notably, natural selection on the immunity and antiviral pathways in "D. innubila" differ markedly from "D. melanogaster", implying divergent evolutionary pressures. The "D. innubila" DNA virus is similar to the "D. melanogaster" "Kallithea virus". As such, comparisons between "D. melanogaster" and "D. innubila" and their viruses promise to inform on the nature of host-virus interactions.
In some mushroom-feeding "Drosophila" species, such as "D. guttifera" and "D. neotestacea", the antimicrobial peptide gene "Diptericin B" has been pseudogenized. However this gene is maintained in "D. innubila," and is activated upon infection.
References.
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