Mutations at the spineless locus display three different phenotypes: (1) The spineless phenotype is characterized by the reduction in size of all bristles; (2) the aristapedia phenotype corresponds to the transformation of distal
antennal segments, specifically the arista and the distal portion of the third antennal segment, into distal mesothoracic
leg segments, i.e., tarsal segments; and (3) leg-segment
fusion manifested as fusion of tarsal segments on all eight
legs. Homeotic tissue does not conform to developmental compartment boundaries; therefore, ss does not qualify as a
selector gene (Struhl). Expression of these phenotypes varies
among alleles. Some alleles show only the spineless phenotype. Aristapedia alleles, symbolized ssa, vary in expression; weak alleles show a swelling of the third antennal joint
and rudimentary tarsal transformation of the base of the
arista; as expression becomes more extreme, more tarsal joints
are formed until in the most extreme alleles four tarsal segments and terminal claws are formed. Tarsal fusions are
characteristic of extreme aristapedia alleles.
Variegates for spineless character but completely
mutant for aristapedia. Male sterile. RK2A.
Bristles only a little larger than hairs; dorsocentrals least reduced; postscutellars erect. No effect on legs
or aristae. Growth of bristles slows during development [Lees
and Waddington, 1943, Proc. Roy. Soc. (London), Ser. B
131: 87-110]. Dominant to aristapedia phenotype of all ssa
From Bridges and Brehme, 1944, Carnegie Inst. Washington Publ.
No. 552: 179.
Antennae and aristae tarsuslike; mean number of tarsal segments = 4.0; incidence of claws = 97.1% (Garcia-Bellido, 1968, Genetics 59: 487-99); 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). 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, Nature 260: 252-54).
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, Growth, Suppl. 1, pp. 37-44; Braun, 1940,
Genetics 25: 143-49; Mglinetz and Ivanov, 1975, Genetika
11(#11): 27-33; 1976, Genetika 12(#12): 87-94]. Dissociated
cells from ssa antennal disks aggregate with dissociated leg-disk cells but not with those from wild-type antennal disks
(Garcia-Bellido). Antennal disks from ssa larvae give rise to
leg-like structures when transplanted into wild-type hosts as
do both duplicated and regenerated antennal disks formed from
eye-antenna-disk fragments (Gehring and Schubiger, 1975, J.
Embryol. Exp. Morph. 33: 459-69); when disks are pretreated
with colchicine, the developing structures are more aristalike
(Vogt, 1947, Experientia 3: 156-59). Homozygous clones of
ssa tissue produce antennal leg tissue when induced before
(Roberts, 1964, Genetics 49: 593-98) but not after
(Postlethwait and Girton, 1974, Genetics 76: 767-74) mid-third instar. Large clones conform to anterior and posterior
compartments comparable to those induced in normal mesothoracic legs [Morata and Lawrence, 1979, Dev. Biol. 70: 355-71
(fig.)]. No maternal effect; temperature independent.
Cold sensitive; phenotype normal in flies raised at
29; fully penetrant at 17 [mean number of tarsal segments =
2.8; incidence of tarsal claws = 1.3% as recorded by Garcia-Bellido (1968, Genetics 59: 487-99) probably in flies raised
at 25]. Phenocritical stage in first half of third instar
[Grigliatti and Suzuki, 1971, Proc. Nat. Acad. Sci. USA
68: 1307-11 (fig.)]. Shifts from 17 to 29 early in third
instar restrict transformation to base of arista; distal
extent of transformation increases as shift is effected later
in development (Grigliatti and Suzuki). Opposite response to
temperature for antennal transformation but not tarsal fusion
reported by Mglinetz (1977, Genetika 13: 70-75). Developmental compartments demonstrated in antennal legs [Morata and
Lawrence, 1979, Dev. Biol. 70: 355-71 (fig.)].
ssaB: spineless-aristapedia of Bridges
Bristles of female like a slight Minute, especially
postscutellars. At 25, aristae inconspicuously thickened at
base, plumed or threadlike for rest of extent. At 18
penetrance is 7% and expression weak; at 14, ssaB enhanced and
resembles ssa; al causes reduction of ssaB arista at 25 but
has no effect on transformed arista at 14 (Villee, 1943, J.
Exp. Zool. 93: 75-98). Legs frequently have lumps at
second joint of tarsi; more pronounced in male and result in doubling of sex combs, which are
strung along first and second fused joints. Eyes a little
flattened. Except at low temperatures, all characters slight
and may overlap wild type. ssaB/ss1 has slight Minute phenotype but wild-type legs and aristae. ssaB/ssaSp like ssaSp
with large tarsal aristae. RK2. Struhl (1982, Genetics
102: 737-49) and Posakony.
Homozygous viable and fertile. Displays enlargement
of base of arista. Increased number of sex-comb teeth on basitarsus of first leg in males (12-38 with mean of 24 teeth per
*ssasnB (A. Hannah Alava)
Description incomplete. Extreme fusion of tarsal
segments; basitarsus of male first leg has 20-50 sex-comb
teeth (mean = 40); sex-comb teeth not uncommon on second leg.
Reported to be homozygous lethal and lethal in combination with Df(3R)bxd100 (Bownes, Bournias-Vardiabasis, and
Spare, 1979, Mol. Gen. Genet. 174: 67-74); however both of
these combinations viable in the hands of
Homozygote is wild type. ssiso53/ssa,
ssiso53/ssa63c, and ssiso53/ssa53e have thickened proximal
segments of aristae, like ssaB. RK3.