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blot. These analyses indicated the mites carried both P and Drosophila ribo-
somal DNA sequences. Mites isolated from M colonies (which lack P ) lacked the
P sequences.
For P. regalis to have transferred P elements from D. willistoni to D. mela-
nogaster , several conditions had to occur in the proper sequence ( Houck
1993 ). Females of D. melanogaster and D. willistoni had to deposit their eggs
in proximity and mites had to feed sequentially on one and then the other,
in the correct order. The recipient egg had to be < 3 hours old, the germ line
of the recipient embryo had to incorporate a complete copy of the exogenous
P , the transformed individual had to survive to adulthood, and the adult had to
reproduce.
A second potential mechanism for horizontal transfer of P involves inter-
specific crosses. Crosses between the sibling species Drosophila simulans and
D. mauritiana produce sterile males, but fertile females. When F 1 females are
backcrossed to males of either species, a few fertile males are produced. To
determine whether interspecific transmission of P might occur, the two species
were crossed and the hybrid progeny were evaluated by in situ hybridization of
larval salivary glands and Southern blot ( Montchamp-Moreau et al. 1991 ). The
results indicated that the P element is able to pass from one species to another
when the postmating sterility barrier is incomplete. Hybridization, although
rare, occurs between some Drosophila species.
P elements have been found in other Diptera, including Opomizydae and
Trixoscelididae ( Anxolabehere and Periquet 1987 ). Inactive P elements were found
in the sheep blowfly Lucilia cuprina (Calliphoridae) and the house fly Musca
domestica (Muscidae) ( Lee et al. 1999 ). The P elements in M. domestica differed
from those in D. melanogaster by having two introns in exon 2 (as does the P from
L. cuprina ). The lack of a functional exon 3 in the house fly P likely is the basis for
the element's inactivity. P elements found in families other than the Drosophilidae
suggests that P elements are not limited to this lineage ( Lee et al. 1999 ).
9.7 P Vectors and Germ-Line Transformation
9.7.1 Protocols
After P elements were cloned ( Rubin et al. 1982 ), they were genetically engi-
neered to serve as vectors to insert exogenous DNA into the germ line of
D. melanogaster ( Spradling and Rubin 1982 ). Many different vectors with differ-
ent characteristics have been produced subsequently ( Fujioka et al. 2000 ). The
following example outlines the procedures involved in P -mediated transforma-
tion of Drosophila ( Figure 9.2 ):
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