Pc+ may be considered a negative regulator of the
bithorax complex (BXC) and the Antennapedia complex (ANTC),
with a decreasing gradient of activity from anterior to posterior. When homozygous or hemizygous, Pc mutants are late
embryonic lethals. Embryos with at least one dose of the BXC
show incomplete head development and caudad transformations,
the thoracic and first seven abdominal segments being partially transformed into the eighth abdominal segment (Lewis,
1978; Denell, 1982; Haynie, 1983, Dev. Biol. 100: 399-411;
Denell and Frederick, 1983, Dev. Biol. 97: 34-47). This
homeotic effect in homozygotes is enhanced by increasing the
dosage of the BXC. Transformations involve brain and ventral
nerve cord as well as epidermis (Jimenez and Campos-Ortega,
1981). Pc+ alleles in the mother weaken the homeotic effect
(Denell, 1982; Lawrence, Johnson, and Struhl, 1983, Cell
35: 27-34). Pc2/Pc2 or Pc3/Pc3 clones induced in leg and
eye-antennal tissue during larval development also show similar posteriorly-directed transformations (Struhl, 1981; Duncan
and Lewis, 1982).
Pc/+ flies carrying at least one dose of the BXC show caudad
transformations, i.e. partial conversion of wings into halteres and of anterior abdominal segments into more posterior
ones. Some Pc heterozygotes show phenotypes characteristic of
ANTC mutants, i.e. partial conversion of antennae into legs
and of second and third legs into first legs (with sex combs
in males) (Hannah-Alava, 1958; Duncan, 1982). The frequency
of wing transformations varies directly with the BXC dosage,
but does not seem to be changed by variation in ANTC dosage
(Duncan and Lewis, 1982; Botas et al., 1982). The number of
abdominal transformations, however, varies inversely with the
doses of the BXC while it increases as the doses of the ANTC
are increased (Duncan, 1982; Duncan and Lewis, 1982).
Other changes observed in Pc/+ flies include a transformation of ventral to dorsal wing (Tiong; Sato et al., 1983),
elevated, divergent, or crinkled wings, terminal gaps in the
L4 wing vein, bent humeral or notopleural bristles, and defective sternopleural bristles, all abnormalities being less
extreme in males than in females (sometimes absent in males).
When doubly heterozygous with AntpYu and AntpB, Pc enhances
Antp. The expression of all Pc mutant heterozygotes (including deficiencies for the locus) is enhanced by the second
chromosome dominant, E(Pc) (Sato et al., 1983, 1984).
Pc3/Pc3/Dp(1;3;4)7 flies (carrying a Pc+ duplication) show
stronger leg and wing transformations than E(Pc)/+;Pc3/+ flies
(Duncan and Lewis, 1982; Sato et al., 1983).