Does not prevent the posterior localization of G-iα65A protein.

Low levels of bcd protein are present evenly throughout the embryo. gt expression is reduced in the anterior and posterior domains.

Male are fertile at 25^oC with only a partial reduction at 18^oC.

Interacts with RpII140^wimp maternal effect.

Neither hb or delocalized bcd transcripts are translated in the posterior as a result of nos activity: larvae have full complement of abdominal segments. If the embryos also contain bcd^Δ fewer abdominal segments are formed, if they contain bcd^hb1+hb2 five or six abdominal segments form.

In double mutants exu¹, fs(1)N¹² all abdominal segments are transformed to the eighth abdominal segment.

Male fertile at 25^oC, fertility reduced by 50% at 18^oC.

Eggs derived from homozygous females cellularise normally but become abnormal at gastrulation; the cephalic furrow is shifted forward and the anterior most cells form an inverted posterior end, forming a posterior midgut, proctodeal region and often Malpighian tubules. Anterior head structures of the cuticle are missing.

Embryos laid by exu¹ homozygous mothers have a uniform distribution of bcd RNA throughout the egg cytoplasm. A gradient becomes more pronounced as development proceeds due to posterior degradation of the RNA.

The thoracic and segmented head anlagen is enlarged while the posterior pattern is compressed. Low levels of bcd protein are distributed in a shallow gradient throughout the embryo. vas¹, exu¹ double mutant embryos exhibit a low and even level of bcd mRNA and protein along the anteroposterior axis: they have duplicated head and thorax at the posterior.

Embryos derived from homozygous females show a deletion of anterior head structures, but do not show defects in the thorax or anterior abdomen. The anterior structures that remain appear enlarged rather than reduced in size; when only the labrum is eliminated (29^oC) the antennal sense organ is normal in size, when both labrum and antenna are eliminated (18^oC) the maxillary structures are often increased in size. The thoracic region is expanded in size and there are an increased u of denticles in the prothorax. Occasional abdominal defects are restricted to the posterior abdominal region. More frequently, segmentation is normal, but the overall size of the hind end is reduced. The anterior phenotype is enhanced at 18^oC (compared to 29^oC). The temperature-critical period is during oogenesis. Embryos derived from bcd⁶/bcd⁷ females show some development of at least thoracic and frequently head structures when injected at the anterior tip with cytoplasm taken from the anterior tip of wild-type embryos. In contrast, cytoplasm taken from the anterior tip of embryos derived from exu¹/exu² females shows barely detectable rescuing activity when injected into the anterior tip of embryos derived from bcd⁶/bcd⁷ females. The phenotype of embryos derived from exu¹/exu² females is more or less unmodified if cytoplasm is removed from the anterior tip of the embryo. exu¹ does not alter the phenotype of embryos derived from bcd⁶/bcd⁷ females. However, in combination with bcd¹⁰/bcd⁹ further thoracic segments are deleted, leaving a somewhat enlarged but otherwise normally segmented abdomen. Mouth hooks or only prothoraces may be formed at both ends of the embryo in embryos derived from exu¹/exu¹; osk⁶/osk⁶ embryos.

Homozygous females produce embryos which have deletions of the most anterior head structures (labrum and chitinous mouth plates).

Analysis of germ-line clones indicates that the mutation is germ-line autonomous (Schupbach and Wieschaus, 1986). Causes shift in blastoderm fate map as indicated by ftz expression; thoracic stripes broadened and shifted anteriorly; abdominal stripes narrowed and compressed posteriorly (Mlodzik et al.). Eggs produced by exu mothers appear to have a more uniform distribution of bcd⁺ product, i.e., less concentration anteriorly (Fronhofer and Nusslein-Volhard, 1987).