The search for new species brings scientists to places as near as their lab bench and as distant as the Antarctic. They may be looking for traces of species long extinct or sightings of contemporary rarities. But one new study has found evidence for the existence of an unknown organism by looking at the DNA of another species.
Drosophila quinaria, a type of fruit fly, is well known to scientists. But in the course of studying its DNA, and the prevalence of a parasitic bacterium, researchers found evidence that, sometime in the past, D. quinaria hybridized with another fruit fly species unknown to science.
This evidence comes from a comparison of nuclear and mitonchondrial DNA. Here’s where I need to back up and explain some background genetics:
Mitochondria are the parts of the cell that produce, in an intricate series of chemical reactions, the energy needed for all our biological functions. They are fascinating structures for many reasons, but particularly because they contain their own DNA (usually abbreviated as mtDNA). What you probably think of as the human genome—two copies of each chromosome (apart from the sex chromosomes), one inherited from from each parent—lives in a part of the cell called the nucleus. mtDNA comes in a single copy and is only inherited from the mother, largely because egg cells are so much bigger than sperm and thus contribute all of the non-nuclear component of the cell.
Mitochondrial DNA has another interesting property: it doesn’t recombine. That is, if two mutations arise in the same mitochondrial genome, they’ll stay together in their descendents, while two mutations in the same nuclear genome won’t necessarily both end up in the offspring. This makes it easier to trace the evolutionary history of a mitochondrial genome than of an entire nuclear genome.
Back to the fruit flies: researchers found two main types of mtDNA in Drosophila quinaria. This is not particularly unusual. What was unexpected was just how different the two mtDNA types were. Not only were they as different as one might expect when looking at two different, though related, species, they were just as different from all of D. quinaria‘s closest relatives (data were unavailable for a handful of rare species). Despite this huge gulf, all the flies, regardless of mtDNA type, had similar nuclear DNA, indicating that they are indeed members of the same species.
What could be the cause of this pattern? It’s not unheard of for hybridization—ancient or recent—to allow a species to pick up the mitochondrial genome of another. But the fact that the mystery mtDNA was not particularly closely related to that of any known fruit flies suggests that the species D. quinaria hybridized with is either undiscovered or no longer existent.
Strange, no? It’s about to get stranger. What allowed the mtDNA of another species to creep into D. quinaria‘s genome and stay there? It may well have been a versatile bacterium called Wolbachia. This bug lives inside the cells of many insects and their ilk, with variable and sometimes sinister effects (more on which another day). Since it lives inside cells, but not in the nucleus, it is transmitted across generations just like mitochondria. Some, but not all, populations of D. quinaria host Wolbachia, and one of the populations in this study actually has a mix of infected and uninfected flies. And lo and behold, whether an individual fly has Wolbachia predicts almost perfectly which mtDNA group it belongs to (the single exception could mean that the occasional fly is “cured” of its infection). Wolbachia sometimes helps protect its host from viral infections, so maybe D. quinaria acquired it by hybridization (not at all unusual among flies), along with some mtDNA, and kept it because of these effects.
This fascinating study is probably not grounds for describing a new species, but if the unknown Drosophila is not extinct, we’ll be able to recognize it, and we’ll have an exciting new system for studying hybridization, the effects of Wolbachia, and possibly the genetics of speciation.
DYER, K., BURKE, C., & JAENIKE, J. (2011). Wolbachia-mediated persistence of mtDNA from a potentially extinct species Molecular Ecology DOI: 10.1111/j.1365-294X.2011.05128.x
Read Full Post »