As I was working on my computer (in my office) this afternoon, a small critter was flying around my head. Based on my current location (in a building housing at least 3 Drosophila labs) and my previous whereabouts (our lab's fly room), I surmised that this was most likely a member of genus Drosophila. My suspicions were confirmed when, at 7:45pm (about 3 hours after I first noticed the pest), she (I think it was a she from the quick glance I got) landed on my computer monitor. Upon closer inspection (although not long enough to adequately surmise the sex) I noticed the fly was too lightly pigmented to be one of mine (rejecting the hitchhiker from the fly room hypothesis), but rather an escapee from one of the labs working with that cosmopolitan hexapod, Drosophila melanogaster.
Pictures comparing the two species in question can be found below the fold. And, yes, she took off from my monitor and is still flying around the office.
My species:
Drosophila melanogaster:
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That happens to me every now and then - we've got a melanogaster lab right across the hall from me. Fortunately, my own Drosophila are housefly-sized, so figuring out which is which ain't all that hard. And the 30 or so million years of evolution separating their lab critters from my field samples are enough to keep most of my primers from amplifying their samples.
Mike, the two species pictures above are separated by ~30my (or 50my if you go by the Tamura et al paper). The Drosophila/Sophophora split (which I think you're talking about) has to be more than 30 my. And, yeah, these dates are very approximate.
As for contamination from other species, I'd feel pretty safe spitting in my samples before running a PCR, knowing damn well I ain't gonna contaminate them. My big contamination worry comes from other strains within our lab. There probably are some primers, however, that will amplify throughout all Drosophilids, but they're rare. We're pretty well saturated across all synonymous and non-coding sits between our species and melanogaster, but that doesn't mean there aren't stretches of perfect identity for 20 nucleotides or so.