>In article <eesnyder.697915657 at beagle> eesnyder at boulder.Colorado.EDU (Eric E. Snyder) writes:
>>Just as _Drosophila_ P-elements appeared to have
>>arisen _de novo_ in the 1920s...
> P elements are thought to have been transfered laterally
to _D. melanogaster_ from a species in the _D. Willistoni_ group. The
vector is thought to be the mite _Proctolaelaps regalis_. P elements
are abundant in willistoni group flies, but absent in species closely
related to melanogaster. See Houck, et.al., 1991, Possible Horizontal
Transfer of _Drosophila_ Genes by the Mite _Proctolelaps regalis_,
Science 253, 1125-1128
The author's present several lines of indirect evidence.
1.) High sequence similarity in _melanogaster_
2.) Abundance in _willistoni_ group flies (It would be interesting
to see how variable they are in/among those species)
3.) Absense in species closely related to _melanogaster_
4.) Overlapping ranges where mites interact with both flies.
They also present some diect evidence. The mite pierce
the flies and take in fluids. The authors show that mites that
live on strains carrying P, can obtain it. And since the mite
pierces the fly, it could transfer the element (it's mobile after
all) to it's host.
The paper makes a much better case for it than I do here --
a good read.