MESODERM-SPECIFIC B104 EXPRESSION IN THE DROSOPHILA EMBRYO IS MEDIATED BY INTERNAL CIS-ACTING ELEMENTS OF THE TRANSPOSON

The Drosophila melanogaster genome contains about 100 copies of the B1 04 transposable element, which is strongly expressed during embryogene sis. Here we show that B104 expression is restricted to the esophageal and amnioproctodeal regions of the embryo and to the developing mesod erm. Mesoderm-specific B104 expression requires the activity of the me soderm-determining factors twist and snail. Virtually the same express ion patterns were observed in Drosophila yakuba, a species that a sepa rated from D. melanogaster by some 15 million years of evolution. We s how that B104 expression is directed by internal sequences of the retr otransposon that are capable of acting as a cis-acting regulatory elem ent in front of a heterologous Drosophila promoter. Our findings sugge st that retrotransposon insertions can affect the expression patterns of endogenous genes by adding and distributing specific cis-acting con trol elements throughout the host genome. We therefore propose that tr ansposable elements in addition to reducing the fitness of their hosts may also provide a rich pool of cis-acting sequences that contribute to the long-term evolutionary potential of the population in a benefic ial manner.