Phylogeny Friday -- 11 April 2008

Drosophila Are Not Fruit Flies Edition

As I have mentioned before, Drosophila are not fruit flies. Tephritids are fruit flies. Drosophila feed on rotting fruit, while true fruit flies feed on fresh fruit. That makes true fruit flies agricultural pests. Drosophila, on the other hand, are connoisseurs of the finer things in life -- wine, beer, cheese, and the like.

In addition to rotting fruit, Drosophila also feed on mushrooms and crabs. Yes, crabs. Well, they don't actually feed on the crabs, just like they don't actually feed on the fruit. The flies and their larva are more interested in the yeast that ferment fruit or the microbes found on crabs.

Drosophilids that live on crabs can be found in two locations: the Caribbean and Christmas Island. One species of Caribbean Drosophila, D. endobranchia, was last observed in the mid-1960s. It's placement in phylogeny of Drosophila was not well resolved, and it was thought that it may be extinct. A new paper in PLoS ONE reports the rediscovery of that species, as well as the evolutionary relationship of the crabby flies (doi:10.1371/journal.pone.0001942.g001)

The geographic location of D. endobranchia and its host, the black crab, as well as the life history of these flies, is summarized in the following figure.

(A) The black crab (Gecarcinus ruricola, black morph). (B) The male holotype of D. endobranchia collected by H.L. Carson in 1966, now in the collections of the National Museum of Natural History, Washington, D.C. (C) Grand Cayman; Numbers refer to sites were crab flies were found. Scale bar 5 km. Image courtesy of NASA. (D) Male fly courting a female fly under the watchful eye of their host (a yellow morph black crab). (E) First instar fly larvae are found in the nephric pads (yellow arrow). The larvae feed on microorganisms, which cleanse the urine (exuded from the green gland; red arrow) of nitrogenous waste compounds. (F) Second instar is spent inside the gill chambers.

The article itself (from which the preceding image was taken) is written in a colloquial style not common in scientific publications. Here's a taste, in which the authors describe their collection protocol:

The flies strike a peculiar sight in real life. They essentially hardly move at all, are extremely reluctant in leaving their host crabs and are hard pushed to take flight. Although the flies are sluggish, the crabs on which they reside are anything but. Chasing after crabs through a pitch-black jungle (growing on a razor sharp labyrinthine limestone ground), while trying to aspirate flies from their carapaces is not trivial. Obtaining large amounts of flies in this way is simply a nightmare. The scarcity of the flies, and the nocturnal and shy nature of their hosts made it a daunting task to figure out the biology of these odd flies.

It really captures the joy (and struggle) of field work. The article goes on to describe various observation made by the researchers based on their collections. But this is Phylogeny Friday, so we're going to skip right to the evolutionary relationships of flies that live on crabs.

The other two species of crab-living flies were pretty well placed in the Drosophila phylogeny. The Christmas Island fly, Lissocephala poweilli belongs to a primative genus closely related to Drosophila. The other Caribbean fly, D. carcinophila falls in the repleta group, which probably means nothing to most people reading this blog. For some context, the repleta group is more closely related to picture-wing Drosophila and the cactus-feeding Drosophila than it is to D. melanogaster.

Stensmyr et al. sequenced five genes from D. endobranchia in order to resolve its phylogenetic position in the Drosophila genus. Here are the trees they constructed using three of the genes (the position of D. endobranchia is indicated by the black arrow):

D. endobranchia falls within the virilis-repleta radiation, which also contains the other Caribbean species, D. carcinophila. However, D. endobranchia does not appear to be in the repleta group, but it is part of a closely allied group (the canalinea group). That means the two species of Caribbean Drosophila that inhabit crabs are closely related, but their similar life-histories are not the result of shared ancestry. Rather, they evolved into crab-flies independently, perhaps because something about Neotropical repleta flies predisposes them to host-shift onto crabs.

Green MM. 2002. It really is not a fruit fly. Genetics 162: 1-3 [link]

Kimura MT. 1980. Evolution of Food Preferences in Fungus-Feeding Drosophila: An Ecological Study. Evolution 34: 1009-1018. [link]

Stensmyr MC, Stieber R, Hansson BS, Vosshall LB. 2008. The Cayman Crab Fly Revisited -- Phylogeny and Biology of Drosophila endobranchia. PLoS ONE 3: e1942. doi:10.1371/journal.pone.0001942