Chromosomal region 68D/E is required for various aspects of Drosophila
gut development; within this region maps the Brachyury homolog T-rela
ted gene (Trg), DNA of which rescues the hindgut defects of deficiency
68D/E, From a screen of 13,000 mutagenized chromosomes we identified
six non-complementing alleles that are lethal over deficiencies of 68D
/E and show a hindgut phenotype. These mutations constitute an allelic
series and are all rescued to viability by a Trg transgene. We have n
amed the mutant alleles and the genetic locus they define brachyentero
n (byn); phenotypic characterization of the strongest alleles allows d
etermination of the role of byn in embryogenesis. byn expression is ac
tivated by tailless, but byn does not regulate itself, byn expression
in the hindgut and anal pad primordia is required for the regulation o
f genes encoding transcription factors (evert-skipped, engrailed, caud
al, AbdominalB and orthopedia) and cell signaling molecules (wingless
and decapentaplegic). In byn mutant embryos, the defective program of
gene activity in these primordia is followed by apoptosis (initiated b
y reaper expression and completed by macrophage engulfment), resulting
in severely reduced hindgut and anal pads. Although byn is not expres
sed in the midgut or the Malpighian tubules, it is required for the fo
rmation of midgut constrictions and for the elongation of the Malpighi
an tubules.