Tarsal defects are not normally seen in legs of homozygous adults.

Mutant antennae show transformation of distal A3 and arista to tarsus.

ss^a/ss^D114.9 mutants have ectopic sex combs in t2 and an ectopic joint within t1.

Mutant flies have a reduced thermosensitivity at the low end, but are normal in their ability to detect temperature differences (3^o or 6^oC) in the range of their thermosensitivity. In a "heat-box" learning paradigm, mutant flies show avoidance behaviour during training and test phase performance is normal when using a 37^oC punishment temperature. When 30^oC is used as the punishment temperature, mutant flies show no avoidance behaviour, in contrast to wild type.

Hemizygotes exhibit incomplete tarsal segmentation. ss^a Ubx^bx-1 Ubx^Cbx-1 hemizygotes are characterised by reduction of the posterior part of the wing (Ubx^Cbx-1 phenotype) and by deterioration of tarsus segmentation of the second leg.

Antenna to leg transformation.

Shows mutant temperature gradient response. Flies are partially thermosensitive, and avoid temperatures above 35^oC. Behavior with respect to humidity is wild type.

Homozygotes and ss^a/ss^aP88 flies show transformation of the arista to distal mesothoracic tarsus. Expressivity of the phenotype is higher in ss^a/ss^aP88 flies than in ss^a homozygotes. The expressivity of the phenotype decreases with increasing temperature in ss^aB/ss^a flies. A high proportion of homozygous or ss^a/ss^aP88 flies produce an ectopic tarsal response to chemical stimulation at 25^oC. ss^a/ss^aP88 flies usually cannot fly.

The distal region of the antennal disc, the arista, is transformed to the corresponding leg region, the tarsus. The first 2-4 neurons appear in the homeotically transformed antennal disc at the pre-pupal stage, earlier than in wild-type. They are located at the centre of the disc (this region will develop into the tarsus). At 5 hours after puparium formation, the protrusion of the future tarsus is very obvious, and two axonal processes extend from the distal tip towards the antennal part of the disc. Until 10 hours after puparium formation, 2-4 additional neurons develop at the distal end of the tarsus. A cuticular, bag-like protrusion is formed around the homeotic tarsus as soon as it has reached the surface of the ventro-lateral head region. Antennal disc eversion is delayed by approximately 3 hours.

ss^a females lack aristae. They are able to distinguish between wild-type males that sing the courtship song, and males without wings that do not sing, and mate more readily with males that sing, suggesting that they can hear the courtship song. The homeotic legs of ss^a females are not required for the detection of the courtship song, since females whose homeotic legs have been removeed can still distinguish between singing and non-singing males.

Homozygous clones in the antenna show transformation of the distal antenna to leg. Homozygous clones also homozygous for Antp²⁵ or Scr² also show this transformation, indicating that the ss^a phenotype in the antenna is independent of Antp⁺ or Scr⁺ function.

The arista is transformed into a tarsus with mechanosensory bristles.

ss^a/ss^aP88 flies have an aristapedia phenotype; the arista and part of the third antennal segment are transformed into a well formed distal leg carrying 2 claws and 3 to 4 tarsal segments.

Antennae and aristae tarsuslike; mean number of tarsal segments = 4.0; incidence of claws = 97.1% (Garcia-Bellido, 1968); classified as an intermediate allele by Struhl. Third joint of antenna like parts of a tarsal row but with broad, flat, plate-like lobes below. Tarsal segments of legs display intermediate level of fusion (Struhl, 1982). Bristles like those of a medium to slight Minute. Frequent extra dorsocentral bristles. Transformed tissue is leg tissue in every attribute tested. Transformed tarsi elicit behavioral response similar to that of normal legs when exposed to sugar solutions (Deak, 1976). Regions of aristae converted into tarsi not affected by mutants affecting aristae (e.g., th and al) but are affected by those operating on tarsi (e.g., fj, d, app and ey) (Waddington, 1939; Braun, 1940; Mglinetz and Ivanov, 1975; Mglinetz, 1976). Dissociated cells from ss^a antennal discs aggregate with dissociated leg-disk cells but not with those from wild-type antennal discs (Garcia-Bellido, 1969). Antennal discs from ss^a larvae give rise to leg-like structures when transplanted into wild-type hosts as do both duplicated and regenerated antennal discs formed from eye-antenna-disk fragments (Gehring and Schubiger, 1975); when discs are pretreated with colchicine, the developing structures are more aristalike (Vogt, 1947). Homozygous clones of ss^a tissue produce antennal leg tissue when induced before (Roberts, 1964) but not after (Postlethwait and Girton, 1974) mid-third instar. Large clones conform to anterior and posterior compartments comparable to those induced in normal mesothoracic legs (Morata and Lawrence, 1979). No maternal effect; temperature independent.

Scr², hth^5E04, pb²⁷, put^p1, ss^a has visible | somatic clone phenotype

Dll^md23, ss^a/ss[+] has visible phenotype

Dll^md23/Dll[+], ss^a has visible phenotype

obk¹, ss^a has visible phenotype

Ubx^Cbx-1/Ubx^bx-1, ss^a has visible phenotype

Ubx^bx-3/Ubx^bxd-51j, ss^a has visible phenotype

Pc³, ss^a has visible phenotype

ss^a has arista phenotype, suppressible by Scer\GAL4^ptc-559.1/dan^EPgJ3-220

ss^a has tarsal segment | ectopic phenotype, suppressible by Scer\GAL4^ptc-559.1/dan^EPgJ3-220

ss^a is an enhancer of arista | somatic clone phenotype of ct^C145

ss^a is an enhancer of leg | ectopic | somatic clone phenotype of ct^C145

ss^a is an enhancer of phenotype of cg¹

Scr², hth^5E04, pb²⁷, put^p1, ss^a has leg | ectopic | somatic clone phenotype

Scr², hth^5E04, pb²⁷, put^p1, ss^a has labellum | somatic clone phenotype

Dll^md23, ss^a/ss[+] has tarsal segment phenotype

Dll^md23/Dll[+], ss^a has tarsal segment phenotype

obk¹, ss^a has leg | ectopic phenotype

obk¹, ss^a has antenna phenotype

Ubx^Cbx-1/Ubx^bx-1, ss^a has mesothoracic tarsal segment phenotype

Ubx^bx-3/Ubx^bxd-51j, ss^a has leg phenotype

Pc³, ss^a has sex comb | ectopic phenotype