I think what Barbara is asking is how the, as yet unidentified, retrovirally derived gene maps to a robust phylogenetic tree. Does all the members of our best guess tree showing the placental mammals have this sequence? Are there clades or grades within the placental mammals defined by specific retrovirally derived genes?
I'd suggest that a better place for this discussion is the
Retroviral role in placental evolution thread I opened for this very topic, where we have still as yet to actually identify what the retroviral gene in question is supposed to be.
TTFN,
WK