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Morphometric analysis of thoracic muscles in wildtype and in bithorax Drosophila.

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THE ANATOMICAL RECORD 226:373-382 (1990)
Morphometric Analysis of Thoracic Muscles in
Wildtype and in Bithorax Drosophila
Department of Anatomy, UMDNJ-Robert Wood Johnson Medical School, Piscataway,
New Jersey 08854-5635 (M.D.E., S.H., B.P.); Department of Biology, Yale University,
New Haven, Connecticut 06511 (A.M.S., R.J.W.)
The tergotrochanteral (TTM) “jump” muscles in the second (T2)
and third (T3) thoracic segments of the fruit fly, Drosophila melanogaster, were
analyzed morphologically and morphometrically in wildtype (Canton-S)and bithorax mutants (abx b 2 pbx I DfT3RIP2). In the transformed T3 segments of mutant
flies, the TTMs were greatly increased in fiber number (330% of wildtype), length
(141%), and volume (460%), thus manifesting both hyperplasia and hypertrophy.
In contrast, TTMs in the “untransformed” T2 segments of mutant flies were both
hypoplastic and hypotrophic, in that significant decreases in fiber number (93% of
wildtype), length (go%), and volume (80%) were observed.
Two relationships emerged from analysis of the morphometric data: 1)Although
the fiber numbers and volumes of the transformed T3 TTMs in bithorax flies were
greatly increased, the total combined volumes of the TTMs in T2 + T3 remained
approximately the same in bithorax compared to wildtype flies. 2) The changes in
TTM volumes in bithorax flies compared to those in wildtype were proportional to
the relative changes in fiber numbers times the relative changes in muscle
lengths. These observations suggest that the genes of the bithorax complex influence the number and the length of tubular muscles fibers of the TTMs, but do not
significantly affect the mean cross-sectional areas of these fibers.
Fibrillar muscle fibers, which are not found at all in T3 segments in wildtype
flies, were observed in the transformed T3 segments of bithorax mutants in 11 of
18 cases (61%),but typically as wisps, not in complete muscles. We suggest that, in
the T3 segment of the bithorax flies, the relative differences between the massive
transformation of tubular TTMs vs. the minimal appearance of fibrillar muscles
may be related, in part, to the relative availability of muscle precursors.
The bithorax complex (BX-C) is a much-studied
group of homeotic genes in the fruit fly, Drosophila
melanogaster. If BX-C genes malfunction, posterior
body segments are transformed. In resultant mutant
flies, transformed posterior segments appear similar to
more anterior segments (Lewis, 1978, 1982; Ayala and
Kiger, 1984). A striking manifestation of BX-C transformation is that the mutant fly may have two pairs of
wings, one pair each projecting from the cuticle of the
second (T2) and the third (T3) thoracic segments (Fig.
11, in contrast to the wildtype fly, which has only one
pair of wings projecting from T2.
Most anatomical observations on BX-C phenotypes
have been made on epidermal structures (cuticle). In
this paper, we extend the anatomical observations of
four-winged BX-C mutants to a quantitative examination of muscles in T2 and T3.
The muscles of the thorax in the fly are principally of
two basic types (Miller, 1950): 1) tubular muscles,
which constitute most of the muscles of the thorax,
including the tergotrochanteral muscles (TTMs) which
are the “jump” muscles that provide the power to the
extension of the legs, and 2) fibrillar muscles, which
are found only in insects, and which are characteristic
of the indirect flight muscles (see, for instance, Tiegs,
1954; Pringle, 1957; Elder, 1975; Smith, 1984; for the
muscular anatomy of the thoracic segments in wildtype
flies, see Zalokar, 1947; and Miller, 1950; for the anatomy of the thoracic exoskeleton, see Ferris, 1950).
Some qualitative observations have already been
made of the musculature in four-winged BX-C mutants. Ferrus and Kankel (1981) found only tags of
muscles in the transformed third thoracic segment (T3)
of the bithorax mutant, bx? pbx I DfT3)P9. In no case did
they find muscles representative of T2 in T3. Schneiderman et al. (1987) repeated the observations of Ferrus and Kankel (1981) on muscular transformations in
T3, but used the bithorax mutant, (abx b 2 pbxl
DfT3R)P2) (Lewis, 1980a,b, 1982), which results more
reliably in a four-wing phenotype than does the mutant
studied by Ferrus and Kankel(l981). Schneiderman et
al. (1987) estimated that the TTMs in T2 of bithorax
Received August 2, 1988; accepted June 30, 1989.
Fig. 1. Photograph of a bithorax mutant (abx bn-?pbxIDfl3R)P.Z') fly (Drosophila melanogaster).
Fig. 2. Low-power electron micrograph of a cross-section through a
wildtype tergotrochanteral muscle, in the metathorax (third thoracic
segment). This muscle has eight tubular fibers, one of which has been
outlined. F, the outlined tubular fiber; My, one of the many myofibrils
included in each muscle fiber; N, centrally placed nucleus within a
tubular fiber; Tr,tracheole.
mutants were unchanged compared to wildtype, but
that in T3, the TTMs of mutants were greatly increased
in volume. Furthermore, Benson and King (1987),
studying the same mutant a s Schneiderman e t al.
(1987), found, unlike Ferrus and Kankel (1981))
strands of well-formed indirect flight muscles in the
transformed T3.
Our chief aims were 1)to extend the analysis of the
tubular muscles to the quantitative level, principally
by analyzing the effects of the genetic transformations
on i) the number of muscle fibers, ii) the lengths, and
iii) the volumes of the TTMs; 2) to determine, with
respect to the transformation of the T3 TTMs, whether
the increase in muscular volume reported by Schneiderman et al. (1987) was due to hypertrophy, to hyperplasia, or to some combination of the two; and 3) to
determine whether the transformation included fibrillar muscles, which in wildtype flies are found in T2,
but not in T3.
Our wildtype flies were of the Canton-S strain. The
parental strains of the bithorax flies were (abx bX3pbxl
T M l ) and (Df13R)P2 I T M l ) . Crosses were made with
flies that had been isolated as pupae, just prior to eclosion.
Flies were maintained in vials containing a mixture
of agar, yeast, white cornmeal, molasses, mold inhibitor, and water. Room temperature was thermostatically maintained between 23-26°C.
Flies selected for morphometry were late pupae on
the brink of eclosion (Bainbridge and Bownes, 1981,
stage P15 (i)).The sexes of all pupae were noted; some
pupae were weighed. All of our samples contained a n
equal number of males and females. The data were
analyzed taking sex into consideration. In order to
make the rearing conditions as equivalent a s possible,
we selected as many pupae as possible from the same
vial. We typically removed pupae only during the first
few days when mature pupae were observable in a
fresh vial. The bithorax flies that we chose for analysis
were those that appeared to be the most completely
transformed. On the other hand, the flies of the parental strains removed for analysis from the same vials as
the bithorax flies (“littermate” controls) appeared
grossly indistinguishable from wildtype.
Independent analysis of the two parental strains indicated that they did not differ from each other morphometrically. Furthermore, with only one exception
(Table 2), the parental strains did not differ significantly from wildtype on any measure.
Each pupa was dissected in ice-cold Ringer’s solution. The proboscis, legs, wings, and much of the abdomen were cut off, taking care not to pull, stretch, or
distort the head. A few small holes were pierced into
the dorsal thorax and in the eyes. Then the tissue was
immersed in ice-cold fixative (3% glutaraldehyde in
0.1M sodium cacodylate buffer (pH 7.4)) for 3-18 hr,
followed by three rinses in 0.1M sodium cacodylate
buffer containing 0.2M sucrose (pH 7.4). Postfixation
was in 1%osmium tetroxide in 0.1M sodium cacodylate
buffer (pH 7.4) for 2 h r at 4“C, followed by dehydration
throuph a n ethanolic series and embedment in EMbed812 (Electron Microscopy Sciences). Semithin sections
(0.8 bm) were examined by light microscopy (LM),
while ultrathin sections were examined by electron microscopy (EM) (Philips CM12).
For measuring the volumes of the TTMs in T2 and
T3, every 20th horizontal section through the fly was
photographed (LM), and the cross-sectional area of the
muscle of interest was determined by using a Bioquant
morphometric system. The width of the endomysium,
as well a s extramuscular spaces, appeared almost
negligible in the muscle sections on which the crosssectional area of the muscles were measured, for the
wildtype, parental, and bithorax mutant flies alike.
Accordingly, the total volumes of the muscles were calculated without correcting for the slight contributions
of extramuscular tissues or spaces.
Muscle length was reconstructed by counting and
adding together the number of cross sections on which
the muscle could be seen, having calculated that the
distance between each section was 16 pm (20 x 0.8
Fm). Virtually all observed muscle fibers extended for
the entire length of the muscles they comprised. Individual muscle fibers were counted from the LM cross
sections. No corrections were made for shrinkage. To
test the reliability of the method, the volumes of the
muscles from several flies were independently deter-
Fig. 3. Higher-power electron micrograph of a cross-section through
a tubular fiber from a tergotrochanteral muscle in the mesothorax
(second thoracic segment) of a wildtype fly. Long arrow points to
glycogen granules, G ; arrowheads indicate border of muscle fiber;
Mi, mitochondrion; My, myofibril; N, nucleus.
Fig. 4. Light micrograph of a horizontal section (0.8 pm) through the
thorax of a late pupa, just before eclosion, of a wildtype female fly
(Canto&). The upper, large outlined muscles are the tergotrochanteral muscles in the mesothorax (T2 TTMs). The lower, small outlined
muscles (indicated by arrows) are the tergotrochanteral muscles of
the metathorax (T3 TTMs). The heavy broken line indicates the posterior boundary ofT2. At this horizontal level, most ofthe exoskeleton
of T3 lies ventral to the section. C, cardia; DLM, dorsal longitudinal
muscle; DVM, dorsoventral muscle.
Fig. 5. Light micrograph of a horizontal section (0.8 pm) through the
thorax of a late pupa, just before eclosion, of a female bithorax fly (ubx
br-7 pbxiDfT3RIP2). This section is cut in a more dorsal plane than the
section in Figure 4. The upper, large outlined muscles are the tergotrochanteral muscles in the mesothorax (T2 TTMs). The lower, small
outlined muscles (indicated by arrows) are the tergotrochanteral muscles in the transformed metathorax (T3 TTMs). The heavy broken line
indicates the boundary, a t this level, between T2 and T3. DLM, dorsal
longitudinal muscle; DVM, dorsoventral muscle.
TABLE 1. Wildtype (Canton-S)tergotrochanteral
muscles (TTMs)
Fiber Length Volume Fiber Length Volume
No. (pm) (10-6 pm3) No. (pm) (10-6 pm3)
(n = 6)
(_n= 6 )
mined by two different investigators. The measurements were typically within a fraction of a percent of
each other.
Because no systematic right-left differences were observed in any of the data, measurements from paired
muscles of each fly were pooled and analyzed as total
values for a pair of muscles/fly by analysis of variance.
Differences between group means were assessed by a
Newman-Keuls test (Winer, 1971).
In the second thoracic (T2) and third thoracic (T3)
segments of wildtype (Canton-S) and bithorax (abx b 2
pbxlDfT3R)P2) mutant flies (Fig. l ) , we examined the
fiber numbers (number of individual muscle fibersl
muscle), overall lengths, and total volumes of the tergotrochanteral muscles (TTMs) (Figs. 2, 3). The morphometric observations were made on horizontally
sectioned wildtype (Fig. 4) and bithorax (Fig. 5) flies.
Similar observations were made on the bithorax parental strains [(DfT3R)P2ITMl) and (abx b$ pbxlTMl)]
(Tables 1, 2).
TABLE 2. Percentage comparisons of wildtype with
bithorax mutants and parental strains for
tergotrochanteral muscles (TTMs) (all values for
wildtype defined as = 100%)
No. Length Volume No. Length Volume
Yabx bx? pbxlDfl3RiP2). All the values in this row of the table differ
significantly from wildtype (P <0.05).
'(abx bx? pbxITM1) and (Dfl3R)P2ITMl). The only value in this row
significantly different from wildtype is that for T2 TTM fiber number
In summary, the T2 TTMs of our bithorax flies had
only 93% as many fibers, 90% of the length, and only
80% a s much volume as in the wildtype flies. Such
changes are not due to possible differences in overall
body size between bithorax and wildtype flies, because
our measurements of body weights of wildtype and
bithorax flies revealed no statistically significant differences. In fact, the bithorax flies tended to be heavier
(Table 3).
TergotrochanteralMuscles (TTMs) in T3
Analysis of variance of data for the T3 TTMs (see
Tables 1 and 2) revealed that, with respect to fiber
number, males had more fibers than the females
(P<0.005), there were statistically significant effects of
sex x genotype (P<0.005), and the genotypes differed
(P<O.OOl). With respect both to muscle lengths and
volumes, there were no significant sex differences or
sex x genotype interactions, but the genotypes differed
More specifically for the T3 TTMs, in bithorax flies,
Tergotrochanteral Muscles (TTMs) in T2
the mean number of fibers (26.0)-more than three
Analysis of variance with respect to fiber number times the mean number of T3 TTM fibers in wildtype
revealed no difference between the sexes for the T2 flies (7.9) or in flies of the parental strains (8.51-was
TTMs, nor were there any significant sex x genotype almost identical to the mean number of fibers in their
interactions. However, there was a statistically signif- T2 TTMs. The mean length of the T3 TTMs in bithorax
icant (P<0.05) effect of genotypes for the T2 TTMs, in flies (382 pm) was significantly (P<0.05) longer than
that the mean number of fibers counted in bithorax that in wildtype flies (272 pm), or than in flies of the
flies (26.1), a s well as the mean number seen in flies of parental strains (288 pm), which did not differ signifthe parental strains (26.41, were significantly less icantly from wildtype. The mean volume of the T3
(P<0.05) than the mean number of fibers counted in TTMs in bithorax flies (1.49 x 106pm3) was more than
four times greater than in wildtype flies (0.32 x 106
wildtype flies (28.2).
Analysis of variance of data for muscle lengths and pm3) or in flies of the parental strains (0.33 x 106
volumes revealed that for the T2 TTMs, the females pm3). Thus, in bithorax flies, the volumes of the T3
had longer and larger muscles than the males TTMs were 40% of volumes of the T2 TTMs, compared
(P<O.Ol). This was expected because females are larger to 7% in wildtype and parental strains, a relative inthan males, and, most likely, muscle length is directly crease of almost six-fold.
An analysis of variance of the [T2 TTMs + T3 TTMs]
influenced by the size of the thoracic cavity. We also
found that the genotypes differed (P<0.005), in that muscle volume/f ly revealed that there was apparently
the mean length of the T2 TTM muscles in bithorax no effect of genotype, suggesting that this volume
flies (476 pm) was significantly shorter (P<0.05) than might be conserved in wildtype versus bithorax flies,
in the wildtype flies (529 pm) or in flies of the parental even though the total number of [T2 TTMs + T3
strains (523 pm). With respect to volume, the mean TTMsl muscle fibers in bithorax flies was almost twice
volume was significantly (P<O.Ol) smaller in bithorax that in wildtype flies. That is, in bithorax flies, the
flies (3.61 x 106 pm3) than in wildtype (4.52 x 106 TTMs in T2 + T3 are markedly increased in total numpm3) or in flies of the parental strains (4.58 x 106 ber of muscle fibers, but not increased in total volume
(Fig. 6). This constancy in the total volume of the TTMs
Mean Volumes of Tergot rochanteral Muscles In Second (T2)
and Third ( T 3 ) Thoracic Segments
. .
N = 12
(abx bX3 pbx I Df (313)~ 2 )
Fig. 6. Histograms ofthe total volumes ofthe tergotrochanteral muscles ("Ms)
in T2 and T3 of flies of wildtype (Canton-S), bithorax
mutants (abx bn3 pbxlDff3R)P2j and of the parental strains (abx bx?
pbxlTM1) and (Dfl3RjP2ITMlj of the bithorax mutants. The areas of
the histograms filled with parallel lines indicate the mean total volumes of the T2 WMs; the dotted areas of the histograms indicate the
mean total volumes of the T3 "Ms; the capped lines in the histograms indicate estimated standard deviations.
TABLE 3. Weights just prior to eclosion (mg) (Stage
P15 (i) of Bainbridge and Bownes, 1981)
fibers making up the muscles were constant, so that
changing the total number of fibers by a certain percentage would be equivalent to changing the cross-sectional area of the whole muscle by the same percentage.
If one assumes such a cylindrical model for the measured muscles, then i t is possible to calculate a n approximate mean cross-sectional area (as well a s a n approximate mean diameter) for the T2 and the T3 TTMs.
(af -- Vtn x L, where af is the approximate "mean
cross-sectional area" of a muscle fiber, n = number of
fibers, L = length of the muscle, V = volume of the
muscle. This "mean cross-sectional area" is a computed, rather than a measurable, quantity, because the
individual fibers were not actually cylindrical, but fusiform. Also, the cross-sectional perimeters were distorted from true circles due to packing and other factors. This calculation does not include a correction for
the proportion of the total muscle cross-sectional area
due to extracellular space.)
For the T2 TTMs, the mean calculated crosssectional fiber area was 303 pm2 in the wildtype and
291 pm2 in the bithorax flies. (The corresponding calculated diameters were 19.6 and 19.2 pm.) For the T3
TTMs, the mean calculated cross-sectional fiber area
was 149 pm2 in wildtype and 150 pm2 in bithorax flies.
(n = 6)
(n = 7)
'(abx bn3 pbxlDff3R jP2).
in T2 + T3 cannot reflect any limiting of space for
insertion on the cuticle or within the thoracic cavity,
because the total volume enclosed by the T2 + T3 exoskeleton is clearly markedly increased in bithorax
mutants compared to wildtype flies.
Another relationship emerged by comparing percentage changes in fiber number, length, and volume in the
T2 and in the T3 TTMs of bithorax mutants vs. wildtype. In both these muscles, the observed changes in
volume were proportional to the changes in fiber number times the changes in overall length. If these muscles were true cylinders, this would be exactly the relationship expected if the cross-sectional areas of the
(The corresponding calculated diameters were both
13.8 Fm). In view of the marked differences in the sizes
of the muscles, especially between the T3 TTMs in
wildtype vs. bithorax flies (the mean volume in the
bithorax flies was 460% that of the wildtype), the similarity of these parameters is truly striking, suggesting
that the genes of the BX-C which produced our mutants affected the number and length of tubular muscles fibers of the TTMs but not the mean cross-sectional
areas of these fibers. Note that the mean calculated
cross-sectional areas of the fibers of the TTMs in T3
were much smaller (about 50%) than those in T2, in
both wildtype and bithorax flies. In other words, in
spite of massive transformation in the bithorax flies,
the individual T3 TTM fibers in some sense retained
their identity as T3 fibers (though their number and
lengths were significantly increased). That is, the T3
TTMs were incompletely transformed; they were phenotypically T2 in fiber number but remained phenotypically T3 in mean fiber diameter.
Another way of describing this transformation is
that the T3 TTMs in the bithorax flies were increased
both as a result of hyperplasia in the number of the
muscle fibers, and as a result of hypertrophy, manifested as a n increase in fiber length. While the increased fiber length may have been a n obligatory effect
of the markedly increased distance between the sites of
origin and insertion that resulted from the increased
size of the transformed T3 cuticle, the increased fiber
number most likely resulted from a genetic change induced in the transformed tubular T3 TTMs.
Fibrillar Muscles
In the T2 of both wildtype and bithorax flies, the
conspicuous indirect flight muscles, such a s the dorsal
longitudinal muscles (DLMs) and the dorsoventral
muscles (DVMs), are composed of fibrillar fibers (Fig.
7). We found no fibrillar muscle fibers in T3 of wildtype
flies, but of the 18 bithorax flies in which we surveyed
T3 for fibrillar muscle, we found some fibrillar fibers in
11 flies (61%), typically in small wisps of muscle (Fig.
However, we found no consistently transformed
fibrillar muscle in T3 that clearly resembled the T2
DLMs and DVMs, in marked contrast to our observations of the tubular T3 TTMs, which were invariably
present and markedly enlarged in the bithorax flies.
With respect to our studies of fibrillar muscle in the
transformed metathorax, our observations fell between
those of Ferrus and Kankel(l981) and those of Benson
and King (1987), with the former observing much less,
and the latter somewhat more complete transformations of normally metathoracic (T3) structures toward
the mesothoracic (T2). Ferrus and Kankel (1981) examined a different genotype, (bx? pbxlDfl;3)P9), which
would be expected to produce less completely transformed phenotypes than the triple mutation that we
examined (abx b 9 pbxlDfl;3R)P2). Benson and King
(19871, however, did examine the same genotype that
we studied. They obtained light microscopic and scanning electron micrographs of transformed DVM fibrillar muscle fibers in T3 that appear quite similar to
those found in T2 of wildtype.
Fig. 7. Electron micrograph of a cross-section through a fibrillar fiber from a dorsoventral indirect flight muscle in the mesothorax (T2)
of a wildtype fly. Arrow indicates glycogen granules, G; Mi, mitochondrion; My, myofibril.
Do BX-C Gene Effects Respect
Parasegmental Boundaries ?
In fruit flies, the BX-C genes are crucial for the organization of the second and third thoracic (T2, T3) and
first eight abdominal segments (Al-A8) (Lewis, 1978,
1982). It has been suggested that each gene is predominantly responsible for specifying the identity of a portion of a segment or parasegment (Lewis, 1978; Martinez-Arias and Lawrence, 1985; Lawrence, 1988). The
cellular localizations of BX-C gene products provide
indirect molecular biological support for this model
(e.g., Beachy e t al., 1985; Canal and Ferrus, 1987;
Hooper, 1986; Wedeen e t al., 1986; White and Wilcox,
However, anatomical evidence that the cuticle is always transformed within segmental or parasegmental
boundaries is somewhat weak (see, e.g., Cole and
Palka, 1982). In addition, Peifer and Bender (19861,
who studied a variety of bithorax mutants, observed a
decrease in the size of the mesonotum (anterior T2 cuticle) of their most extremely transformed bx mutants.
According to a standard model of BX-C function (Morata et al., 19861, bx mutants should affect sequences
which code for, or regulate the production of, product(s)
38 1
model. We extended the observations on the bithorax
mutants to the musculature, in which we found a reduction in TTM volume, length, and fiber number in
T2-that is, anterior to the regions affected by the mutated genes according to the parasegmental model of
gene action.
Does Cuticular Development Determine Musculature?
Does Muscular Development Determine Cuticle?
Fig. 8. Light micrograph of a horizontal section (0.8 pm) through the
thorax of a late pupa, just before eclosion, of a male bithorax fly (abx
b*7 pbxlDff3RIPZ). The thick broken line indicates the boundary between the second thoracic (T2) and third thoracic (T3) segments. The
thin broken line outlines a cross-section through a fibrillar muscle
fiber, F. The solid line outlines the transformed metathoracic tergotrochanteral muscle (T3 TTM). A portion of a mesothoracic dorsoventral muscle (DVM) is visible. Note that in wildtype flies, fibrillar
muscles are not found in T3. The T3 fibrillar muscle (F)in this section
occupies a position somewhat similar to that of a DVM in the T2 of
wildtype flies. This T3 fibrillar muscle (F) extended virtually
throughout the dorsoventral extent of the T3 cavity, about 430 pm in
With respect to the possible developmental interactions of cuticle and muscle, Ferrus and Kankel (1981)
concluded that the epidermis does not control muscular
development, because they failed to find in their bithorax mutants any muscles representative of T2 in T3.
They also concluded that muscles do not play a dominant role in producing cuticular folding. The transformed T3 in their bithorax mutants appeared to form
a relatively normal notum (the dorsal thoracic cuticle
found in T2 in wildtype), although the underlying musculature was largely absent. That normal cuticular development can occur in the absence of muscle was confirmed by Costello and Wyman (1985).
The question of the dependence of muscular determination on cuticular form was also taken up by
Lawrence and Johnston (1984, 1986). Lawrence (1982)
had demonstrated that, unlike the cuticle, the muscles
of the thoracic segments are not subdivided into anterior and posterior compartments. By observing the location of a male-specific muscle in mosaics, Lawrence
and Johnston (1984) were able to demonstrate that
muscle can develop independently of cuticular segmental identity. Subsequently, Hooper (1986), in studying
the development of musculature in a variety of BX-C
mutants of the Ultrabithorax group, found that epidermal transformations occurred in different segments
than the muscular transformations, with the transformations of cuticle occurring about 1.5 segments anterior to the corresponding transformations in muscles.
Our results, too, support the conclusion that muscular transformations can occur independently of cuticular changes.
Anatomical evidence suggests that BX-C gene functions affect the morphology of cuticle (e.g., Costello
and Wyman, 1985; Ferrus and Kankel, 1981; Lewis,
1978, 1982; Morata and Kerridge, 1980; Morata et al.,
necessary for the correct differentiation of the cuticle of 1986; Peifer and Bender, 1986), the nervous system
anterior T3; abx should affect primarily posterior T2 (e.g., Benson and King, 1987; Burt and Palka, 1982;
and anterior T3; and pbx should primarily affect pos- Green, 1981; Ghysen e t al., 1983, 1985; Jimenez and
terior T3. Thus, Peifer and Bender’s (1986) observation Campos-Ortega, 1981; Schneiderman et al., 1987;
of effects of bx on anterior T2 cuticle indicates a n ex- Strausfeld and Singh, 1980; Teugels and Ghysen, 1983,
tension of bx effects beyond the hypothesized paraseg- 1985; Thomas and Wyman, 19841, and muscles (e.g.,
mental domain of action of that gene. Also, Peifer and Hooper, 1986; Lawrence and Johnston, 1984, 19861,
Bender (1986) noted t h a t the differences which they though the interactions among the various tissues and
observed in the halteres (which project from T3) and the segmental distributions of effects are not undernotal structures affected by abx or bx followed no clear stood. Although no set of homeotic mutations has been
compartmental boundaries. They concluded that “the as much studied genetically and biochemically a s that
effects of neither abx or bx coincide well with a single of the BX-C we are still far from understanding how
parasegment or compartment.” They suggested that the BX-C genes control adult form. Our data, by proBX-C mutations may affect cell-by-cell regulation, viding quantitative analysis of phenotypes of major
which could result in effects coinciding with neither BX-C mutations, contribute toward understanding in
detail how alterations of functioning in three BX-C
segments nor parasegments.
Our data support those of Peifer and Bender (1986) genes may influence adult morphology.
A major finding of this study was that, in the bithoin questioning the applicability of the parasegmental
rax mutant which we examined, the major transformation of a tubular muscle (TTM in T3) occurred primarily as a hyperplastic transformation, which, in part,
appeared to be compensated for by a corresponding hypoplastic change in the anterior neighboring segment
(TTM in T2).
This work was partially supported by a grant to
M.D.E. from the Foundation of the University of Medicine and Dentistry of New Jersey (UMDNJ).
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muscle, morphometric, bithorax, thoracica, analysis, drosophila, wildtype
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