/. Embryol. exp. Morph. Vol. 36, 1, pp. 109-125, 1976
Printed in Great Britain
109
Cell division patterns in the Drosophila head
disc: clones on the head cuticle
By ROBERT RANSOM 1
From the School of Biological Sciences, University of Sussex
SUMMARY
An account of the patterns of clones induced on the head cuticle of Drosophila melanogaster
is given. The system was studied using mitotic recombination, induced by X-rays at certain
developmental stages. In agreement with the findings of a computer model devised to simulate
growth in the head imaginal disc, cuticle clones are found to have a characteristic pattern,
sweeping round the central, eye-forming part of the disc. The similarity between model and
experiment suggests the validity of the model assumption. It is also shown that cuticle clones
are not smaller in the posterior of the head, when induced at the developmental stages studied:
this is in contrast to clones in the compound eye, where posterior clones are smaller than
anterior ones.
INTRODUCTION
Despite Becker's early work on the orientations of the clones induced by
somatic crossing in the eye region of the Drosophila head discs (Becker, 1957),
and subsequent studies by Baker (1967) on similar clones produced by positioneffect variegation, the shapes that clones induced on the surrounding head cuticle
would take have not been studied. In a recent publication (Ransom, 1975) I have
described a computer model which attempts to simulate clone growth in all
regions of the head disc. This model predicts the disposition and shapes of the
clones that should be seen on the head cuticle.
The model shows that the growth of the head disc, with its characteristic
forward-directed eye clones, can be mimicked by placing a simulated growing
constraint (perhaps analogous to the peripodial and basal membranes and surrounding tissues) around a two-dimensional sheet of dividing computer 'cells'
on a hexagonal lattice. The area in the centre of this computer-simulated 'head
disc', representing the presumptive eye part of the disc, gives clones as found by
the experimental results (Becker, 1957; Baker, 1967). The surrounding border
cells, taken to represent head chitin, show clones that sweep around the edge of
the eye part of the computer 'disc' (Fig. 1). The sweeping pattern of clones is
produced by the forward direction of the growing constraint; if the clone
orientations of the eye part of the disc were produced by forward-directed mitoses responding to, say, an anteriorly placed 'orienting signal', then it would be
1
Author's address: School of Biological Sciences, University of Sussex, Falmer, Brighton
BN1 9RH, Sussex, England.
110
ROBERT RANSOM
Fig. 1(A, B). Computer printouts of simulated head disc growth. 'Anterior' is to
the left of the page. Complete rules for the model are given in Ransom (1975): the
shapes in the outer perimeter, representing the boundary of the head disc, are the
clones produced after growth of the primordia shown boxed. Each number represents
the initial cell and the clone produced after the total primordium has grown to
about 700 cells. The inner perimeter shows the approximate boundary of the presumptive eye part of the head disc. (A) Growth from an eight-cell primordium.
Fig. 1 (B). Growth from a sixteen-cell primordium. Only eight cells and the clones
derived from them are drawn for clarity, the positions of the other cells in the primordium being marked O
expected that the same pattern of divisions as that seen in the eye would be carried on to the periphery of the disc (Fig. 2). If clones of the 'sweeping type'
could be demonstrated to exist in the real disc, it would lend weight to the
conclusion of the computer study: that is, that the characteristic clone orientations seen in the eye part of the head disc are produced by the combined effects of
Clones on Drosophila head cuticle
111
o 1
0
5
2
0
4
0
O 6 O
O 80 3
O
7
Fig. 1B. For legend see facing page.
the disc membrane and the constraints produced by surrounding tissues external
to this membrane.
The experiments reported here consist of the study of marked clones induced
on the head of Drosophila by mitotic recombination. Both clone orientations
and general data on clone sizes and distributions of clones induced on the head
cuticle up to the middle of the third larval instar are described and compared with the computer data. A further account of the clonal dynamics of
head-disc development with special regard to the induction of wild-type clones
in a Minute background is at present in preparation (W. K. Baker, personal
communication).
It may help to stress the definition of several terms used throughout this
article. Head disc refers to the area of the compound eye/antennal imaginal disc of
Drosophila producing the adult eye and chitinous regions of the head (including
112
ROBERT RANSOM
A
B
Fig. 2. Two different ideas of how the shapes of clones seen in the Drosophila eye in
the work of Becker (1957) and Baker (1967) might be related to the shapes of clones
on the surrounding head cuticle. Outer and inner perimeters represent boundaries of
the head disc, and eye part of the head disc, respectively. (A) The pattern based on
extrapolation from the eye clone data: all divisions are oriented in a forward direction, as seen in the eye clones. (B) The pattern suggested by the computer model.
Peripheral clones sweep around the perimeter of the head disc.
bristles). There are two head discs in each animal, each of which forms one
half of the total head, the two halves joining in the adult along the anterioposterior midline. The eye part of the head disc refers to the region of each disc
producing compound eye only.
EXPERIMENTAL DETAILS
The main Drosophila stock used was yellow white singed2 (y, 1-0-0; sn2,1-21-0;
w, 1-1-5). Using this mutant, both bristles and ommatidia can be marked, but
not hairs. In order to try to observe clones on the hairs, a second mutant strain
multiple wing hair was used (mwh, 3-0-0). Although Peyer & Hadorn (1965)
have described the expression of mwh on the head, it was found to be unreliable
for clone observations in the present study.
The experiments were done by mating y w sn2 females to Sevelen (wild-type)
males. This gave female offspring of the genotype y w sn2j + + +. During
development of the larvae, recombination can occur at mitosis - spontaneous
recombination occurs rarely (levels discussed below), but the frequency can be
increased with X-ray treatment; this also has the effect of setting the time at
which crossing over takes place. The exchange results in the production of two
homozygous cells from the heterozygotic parent, and the offspring of one of the
cells can be seen in the adult as a marked clone: recombination proximal to sn2
produces a clone of y w sn2 cells.
Accurately timed eggs were required for irradiation. The parents were allowed
to lay eggs over a set period, the egg laying being carried out on watch glasses
of freshly yeasted food in an egg-laying chamber. After a laying period of 12 h,
Clones on Drosophila head cuticle
113
Fig. 3. Standard chart of head regions as used in the present work.The abbreviations
for the bristle types are: IOC, inter-ocellar; OC, ocellar; F, frontal; OB, orbital;
FO, front orbital; VB, vibrissae; VT, vertical; PO, postorbital; SO, side occipital;
OT, occipital; PV, postvertical. E represents the compound eye. The left diagram represents the anterior, the right the posterior of the head. Bristles are marked • ,
points a, b, and c are common to anterior and posterior views.
the food was removed and the watch glasses were kept in moist containers until
the eggs were irradiated.
The irradiation parameters were 100 kV, 4 mA and 0-25 cm aluminium
filtration, giving a dose of 114-8 R/min. After irradiation, the eggs or larvae were
allowed to develop, were decapitated 2 days after emergence, and were then
stored in absolute alcohol for 1-2 days to remove red pigment. The heads were
squash mounted in groups of five on slides in DPX.
RESULTS AND ANALYSIS
A standard chart (Fig. 3) showing both the position and various types of
bristles was used to record the clone data obtained, and over 400 clones were
studied. The full numbers of postorbital and vibrissal bristles have not been
drawn for simplicity. Six time periods were analysed; 0-24, 24-36, 36-48,48-60,
60-72 and 72-84 h from oviposition respectively. The data were analysed in the
following ways.
EMB 36
114
ROBERT RANSOM
A
0-6-
0-4 -
0-2 -
I
12
I T
1 7
24
36
48
60
Time after oviposition (h)
1
72
B
Clones induced at
0-24 h
48-60 h
1 2-3 4-6 7-10 > 10
s
24-36 h
60-72 h
36-48 h
72-84 h
Fig. 4 A and B. For legend see facing page.
Clones on Drosophila head cuticle
115
Table 1. Data onyw sn2 clone sizes and frequencies obtained in these
experiments. All stocks were raised at 25 °C±1°
Age after oviposition at
irradiation (h)
0-24
24-36
36-48
48-60
60-72
72-84
Control,
unirradiated
stock
No. of flies
No. of clones
examined
Frequency of Average patch size in
clones per head
bristle no. + S.E.
391
354
456
153
220
133
23
74
124
63
119
63
006
0-21
0-27
0-41
0-54
0-47
61 + 31
5-0±2-8
30±20
2-0±l-5
l-6±l-6
11 ±0-4
406
13
003
30 + 4-2
(a) Frequency of clones with developmental stage at time of irradiation
The clone frequency of head bristles was found to differ markedly from the
data obtained when clones in the eye were considered. In the latter case, more
and smaller clones appeared later in development. The clones marked by
bristles reached a peak frequency at about 70 h of development, and then dropped
again at later times (see Fig. 4 A). This is because the clone size lessened the later
the clone was started, and so the chance of head bristles being included in the
clone was reduced because some areas of the head only contain a sparse population of bristles. Fig. 4 A also shows the 'background' level of spontaneous clones
generated in a control series without irradiating the larvae.
(b) Clone size
The asymmetry of the bristles on the head cuticle (see Fig. 3) made a study of
clone size using bristle markers rather inaccurate. For example, if a clone is
represented by two bristles, then a clone involving two 'close' bristles may be
much smaller than one involving only two widely spaced bristles. The spread
of final clone sizes in bristle numbers for clones induced at each developmental
stage is shown in Fig. 4B. For the purpose of analysis, arbitrary 'size classes'
were chosen to illustrate roughly logarithmic groupings. The size spread of
spontaneous clones is also included. Table 1 shows clone size and frequency data
in numeric form.
Fig. 4. Clone size distributions and clone frequencies on the head cuticle. (A) Frequency of y w sn2 clones on the head cuticle after X-irradiation at varying times of
development (O), and frequency of y w sn2 clones on unirradiated control flies
(dotted line): clones observed in the eye are marked ( • ) , (B) Clone size distributions
obtained, s = size class (classes were chosen logarithmically with log bristle
numbers in classes of 0-26, e.g. 0-0-26, 0-26-0-52 are the ranges for log bristle
numbers in the two smallest classes); p = proportion of all clones induced at the
time stage studied that are of each given class.
8-2
116
ROBERT RANSOM
(c) Clone orientation
Because the clone boundaries could not be observed at the cellular level due
to lack of adequate marker mutations, approximations had to be devised to
estimate clone orientations. Two estimates were used, the first using the following reasoning. If a clone has no distinct orientation, then a network of randomly
spaced bristles within the clone shows no clear 'long' axis - the distance between
furthest apart bristles on opposing sides of the clone will be approximately equal,
given that there are enough bristles in the clone to make this kind of analysis
possible. If the clone is oriented, then the distance between the furthest apart
bristles should indicate this orientation by showing a definite 'long axis' (Fig.
5 A, B). In the case of the head bristles, the spacing between bristles is clearly
not random, so to diminish errors, only clones with furthest bristles greater
than an arbitrary distance apart were analysed in this way. The arbitrary distance
chosen is shown, together with joined' furthest bristles' from the period 24-36 h
of development in Fig. 5C. The pattern of 'orientations' built up showed little
change at other stages, although the number of clones that could be treated in
this way grew less later in development, as the clones became smaller and
involved less bristles.
Several preferred orientations can be seen in Fig. 5 C, notably occipital bristles
to verticals, occipitals to side occipitals, orbitals tofrontals, postorbitals to
vibrissae, and eye to front orbitals and frontals. Although these results might be
subjectively interpreted to show clonal' sweeping' around the central eye region,
a more quantitative analysis is possible using the data on linkage between the
various bristle types in the same clone.
To study this, the number of times that every pair of regions was included in
the same clone was counted for each of the six progressive developmental stages
studied. In this way the relative 'linkage' of the various regions could be worked
out.
I have separated the head into 12 regions for study, and, excluding the eye,
these can be recognized by the bristle groups that they contain. The groups are
indicated in the legend to Fig. 3. If the shapes of the clones on the head are not
random, it would be expected that certain of the bristle-group regions are more
likely to be present in the same clone than are other combinations. The bristlegroup linkage data was derived as follows. A certain number of clones, n, was
found involving any specific bristle group A after induction at any set time of
larval development. These clones either involved A bristles only, or A bristles
in combination with bristles of other types. The proportion of A clones involving
bristles of a second type B was added to the proportion of B clones that involved
A bristles (total B clones = m). To average this figure, the added proportions
were halved. Calling n' the total number of clones involving both A and B
bristles, group linkage between A and B at set time = ^(n'/n+m/n').
Tables 2-5 indicate the proportion of clones found that involved every
Clones on Drosophila head cuticle
111
Fig. 5. (A) Drawing a line between the two 'furthest apart' bristles in a randomly
growing clone gives no clear orientation direction: there are several possible clone
orientations. (B) The approproximate orientation direction is clearly shown by a
line between' furthest apart' bristles in this clone. (C) Clone orientation (measured by
link-up between two furthest apart bristles of clones) seen in clones induced 24—36 h
after oviposition. The line in the centre shows the ' cut-off distance' below which
clone orientations were not scored.
118
ROBERT RANSOM
Table 2. Proportions of clones involving each pair of bristle types induced 0-24 h
after oviposition. Abbreviations of bristle types are defined in Fig. 3. Figures in
parentheses refer to the number of clones induced at this stage that included the
indicated marker region
E
F
FO
VB
OB
OC
IOC
VT
PV
OT
PO
SO
E
(5)
F
(3)
FO
(4)
10
0-33
0-75
0-25
0-5
—
—
0-2
—
—
—
0-2
10
0-25
—
0-5
0-66
0-66
0-2
—
0-25
—
0-6
0-33
10
—
0-33
—
—
0-2
—
—
—
VB
(4)
0-2
—
—
10
—
—
—
—
—
0-25
OB
(6)
OC
(3)
0-6
10
0-5
—
10
0-66
0-66
0-2
10
0-25
—
0-66
—
—
0-33
10
10
0-2
10
0-25
—
IOC
(3)
0-66
—
—
0-33
10
10
0-2
10
0-25
—
VT
(5)
(0
OT
(4)
PO
(8)
SO
(0)
0-2
0-33
0-25
—
0-17
0-33
0-33
10
—
—
0-375
0-33
—
—
017
0-33
0-33
—
10
0-25
—
0-33
—
—
017
0-33
0-33
—
10
10
—
—
—
0-5
—
—
—
0-6
—
—
10
—
—
—
—
—
—
—
—
—
—
PV
Table 3. Proportions of clones involving each pair of bristle types
induced 24-36 h after oviposition
E
F
FO
VB
OB
OC
IOC
VT
PV
OT
PO
SO
E
(18)
F
(17)
FO
(20)
VB
(26)
OB
(18)
OC
(9)
IOC
(8)
VT
(20)
PV
(5)
OT
(14)
PO
(19)
SO
(9)
10
018
0-60
015
0-44
—
—
—
—
—
—
—
016
10
0-25
—
0-50
0-22
0-25
010
0-20
—
—
—
0-66
0-29
10
—
0-5
0-22
0-25
—
0-20
—
—
—
0-22
—
—
10
—
—
—
—
—
0-7
0-21
—
0-44
0-52
0-46
—
10
0-33
0-25
0-20
0-20
007
—
—
012
010
—
016
10
0-875
—
0-20
007
—
—
012
010
—
011
0-77
10
—
0-20
—
—
—
012
—
—
0-22
—
—
10
0-20
0-5
0-31
0-44
006
010
—
005
011
0125
005
10
0-7
—
—
—
—
003
005
011
—
0-35
0-20
10
0-26
0-44
—
—
015
—
—
—
0-30
—
0-36
10
0-77
—
—
—
—
—
—
0-20
—
0-38
0-36
10
pairing of regions for the stages up to 60 h (after this time clones rarely crossed
between regions, making the analysis impracticable). Table 6 shows the linkage
found between eye and surrounding head cuticle markers. Clones of the type
predicted by the computer model would minimize the linkage between certain
bristle groups, for instance, occipital and postorbital bristles should only occur
together in the same clone in rare cases, because clones involving these markers
should be swept upwards or downwards, and would only meet in the largest
clones. Orbitals and ocelli/inter-ocelli should also not be seen together in small
Clones on Drosophila head cuticle
119
Table 4. Proportions of clones involving each pair of bristle types
induced 36-48 h after oviposition
E
E
F
F O VB O B O C IOC V T PV O T P O SO
(12) (22) (21) (24) (20) (8) (6) (25) (7) (9) (30) (6)
1 0 0 1 0 0-60 0 1 0 0-20 _ _ _
_
_ _ _
F
FO
VB
OB
OC
IOC
VT
PV
OT
PO
SO
006
0-40
005
012
—
—
—
—
—
—
—
10
012
013 1 0
—
—
0-53 018
014
—
—
—
—
—
—
—
0125 —
—
—
—
—
—
0-60
—
0-20
10
—
—
10
—
—
—
—
—
—
016
—
0125
—
0142 —
—
—
006
_
—
—
10
0-6
009
—
0125
—
—
—
—
—
—
—
005
—
—
—
—
0-42 0-28
0-2
0-2
10
005 1 0
0-5
016 016 1 0
0125 0125 0125
—
0-39 007
0-5
—
—
_
—
_
006
—
—
006
014
0-2
005
016
10
—
0-33
—
_
0-20
—
—
—
0-52
0-32
—
10
0-33
—
_
—
—
—
—
014
—
0-25
007
10
Table 5. Proportions of clones involving each pair of bristle types
induced 48-60 h after oviposition
F
(9)
FO
(6)
10
—
10
0175 —
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
10
—
10
—
0-33
—
—
E
(1)
E
F
FO
VB
OB
OC
IOC
VT
PV
OT
PO
SO
—
—
—
—
VB
(20)
OB
(3)
—
—
10
—
—
—
—
—
—
—
—
—
—
0175 —
—
—
—
10
—
10
—
0-5
—
—
—
0-5
—
—
—
—
—
—
OC
(4)
IOC
(4)
VT
(9)
PV
(2)
OT
(9)
PO
(3)
so
—
—
—
—
0-5
10
—
0-5
—
—
—
—
—
—
—
—
—
10
0-5
—
0-33
0-2
—
—
—
—
0-25
0-25
011
10
—
—
0-2
—
—
—
—
—
—
—
—
10
—
—
—
—
—
—
—
—
011
—
—
10
—
—
—
—
—
—
—
011
0-5
—
—
10
(5)
clones, because the clones should be swept downwards at the dorsal anterior of
the disc. Clones involving the ocelli should either go forward to include the
frontals or back to the postverticals. Similarly, clones involving the postverticals should include inter-ocelli or occipitals, but not orbitals or verticals.
Consideration of Fig. 6 shows the validity of these predictions. As the distance
between bristle groups varies, a corrective method has been used to estimate the
relative linkages between the groups: this method is described in the legend to
Fig. 6. It can be seen that the 'strongest' linkages (verticals to occipitals; occipitals to postverticals; ocellars to frontals; orbitals to frontals) are those predicted as agreeing with the hypothesis that cuticle clones 'sweep' around the
central eye part of the head disc. An anomalous situation is seen with linkage
120
ROBERT RANSOM
oc
Q- ^
1156
OB
1-249
Fig. 6. Bristle group linkage seen in clones on the dorsal half of the adult head.
Expected frequent linkages if the ' sweeping hypothesis' is correct are shown as solid
lines, other linkages as dotted lines. If clone growth is totally random then the
proportions of clones of two bristle groups that appear in the same clone should
be a function of the distance between the two bristle groups. For example, if the
average proportion p of clones of types A and B that include each other = 0-25, and
the distance d between groups A and B is 10 units, then assuming random growth
the average proportion of clones of types C and D that include each other should
= 0-125 if the distance C to D is 20 units.
The figure pxd should therefore remain constant for all pairings if random
growth occurs. In the above example, pd = 2-5. pd figures were calculated for the
pairs of bristle groups shown in the figure, and the obtained values are given, d values
were obtained in arbitrary micrometer eyepiece scale units and/? values were averaged from tables 2-5. Each p value is therefore the average for the developmental
period 0-60 h after oviposition.
111
Clones on Drosophila head cuticle
Table 6. Proportions of clones of certain bristle types bordering the eye that cross
the eye/head cuticle border at the developmental ages studied. The proportion
decreases both with increasing age, and posterior position of the cuticle marker
group studied
Age (h)
FO
OB
VB
VT
0-24
24-36
36-48
48-60
60-72
72-84
0-6
0-7
0-6
0-2
01
0-6
0-4
0-2
—
—
0-2
0-2
01
—
—
0-2
PO
Fig. 7. Outlines of examples of clones seen after irradiation between 24 and 48 h after
oviposition. Boundaries are drawn only between marker bristles in each clone and
are therefore not very accurate. Stippled clones are included for clarity.
between ocellar bristles and orbital bristles; this may be due to inclusion of
both these groups in a number of large clones induced early in development, as
linkage for ocellar and orbital bristles is high only when clones are induced in
the period 0-24 h. Other pairings of bristle groups that would not be expected
with 'sweeping' (post orbitals to postverticals; orbitals to front orbitals) have
the 'weakest' linkages as seen in Fig. 6).
Table 6 indicates that clones cross from eye to head chitin most frequently at
the anterior of the eye. This agrees with the findings of the computer model (see
122
ROBERT RANSOM
Fig. 1A, clone 4; Fig. IB, clone 5). Examination of the alternative modes of
growth of the head disc (Fig. 2) shows that the model predicts maximal crossing
of the eye/chitin border at this point, whilst general forward-directed growth of
the type seen in the eye part of the disc (Fig. 2B) would be expected to produce
clones enveloping chitin and eye in more posterior areas.
(d) Individual clone shapes
The shapes of individual clones also suggest the validity of cuticle clones being
oriented around the outside of the eye. The boundaries of the clones of Fig. 7
only show the clones as marked by the bristles they contain, and not by their
cellular borders. Similar orientations to those abstracted in Fig. 5 are favoured,
and the clones should be compared with their computer-generated counterparts
in Fig. 1.
DISCUSSION
The data support the hypothesis that clone shapes like those predicted by the
computer model are prevalent on the head cuticle. This conclusion is based on
the following evidence: (i) the general orientation of clones as shown by
'furthest bristle' analysis (Fig. 5), (ii) the most favoured bristle group pairings
in the dorsal part of the head are those predicted by the computer model (Fig. 6),
(iii) individual clones show similar shapes and orientations as model clones.
Clones at the rear of the head rarely cross from cuticle to compound eye (see
Table 6). This is perhaps due to the properties of growth in the eye region of the
disc. Becker (1966) has noted that clones induced in the rear of the presumptive
eye appear smaller in the adult than those induced in the anterior. This suggests
that less cell divisions and/or a slower mitotic rate are acting in the posterior of
the eye part of the disc. Partly because no evidence existed that this was the case
in the cuticular regions of the head disc as well, this factor was not taken into
account in the computer model.
As it could be argued that cell-division differentials of this kind might hinder
crossing of the eye/head border at the posterior of the eye by limiting clone
sizes and thus the likelihood of clones being large enough to cross the border,
a rough analysis was carried out to see if clones induced in the posterior head
cuticle were smaller than those in the anterior. To do this, the head was arbitrarily subdivided into two halves, the anterior half being marked by bristle types
FO, F, OB, VB, OC, and IOC (see legend to Fig. 3); the posterior by the remaining bristle types. The average number of bristles present in clones involving each
of the halves at each of the six irradiation times was then counted (the 'halves'
were chosen so that approximately the same number of bristles were present in
each half). If there is a similar difference in clone size between front and rear
bristles to that seen in anterior and posterior regions of the eye, it would be
expected to be shown in a statistical analysis of the clone sizes in the two halves.
The results of the study carried out are summarized in Table 7, and it is shown
Clones on Drosophila head cuticle
123
Table 7. Differences in average clone sizes and frequencies in subdivided posterior
and anterior regions of the head cuticle at the developmental stages studied
No. of clones
Average size
A
1
Age(h)
0-24
24-36
36-48
48-60
60-72
72-84
Anterior
Posterior
Anterior
Posterior
Sig.
14
67
98
53
80
47
14
46
70
38
41
17
4-66
4-50
2-58
1-70
1-21
104
5-82
5-86
2-76
1-87
200
105
0-25
0-62
0-96
0-80
0-92
0-90
The number of bristles included in both the anterior region (see text for bristle types included)
and the posterior region is about 40 each. The anterior region is smaller than the posterior,
and the bristles are more evenly spread: this may explain the gradual decrease in clone proportion seen in the posterior region with increasing age of irradiation, although clone sizes are
about the same in both cases. The final column in the table shows that the differences in mean
clone sizes between anterior and posterior regions are not significant (Mann-Whitney U-test
for comparing non-normally distributed means, Mann & Whitney, 1947). The figures given
are the probabilities that any two samples with the same distribution will exceed the differences seen in the two samples shown. It is therefore concluded that the two samples observed
have the same distribution, that clone size in anterior and posterior regions is similar, and
that no difference in mitotic activity like that suggested to occur in the eye occurs in the
presumptive head cuticle regions of the head disc.
that the variation in clone sizes seen between anterior and posterior at the
various developmental stages is in agreement with the hypothesis that both
samples have the same distribution.
There is an alternative explanation for non-crossing of clones between the
posterior cuticular regions of the head and the compound eye. Sprey & Oldenhave (1974) have suggested that certain parts of the head of Drosophila may be
formed by the peripodial membrane and not by the disc epithelium proper.
These authors conclude that the post-orbital bristles are formed from the peripodial membrane. This may mean that instead of forward-directed cell division
pushing presumptive posterior cuticle clones into the eye region, the clones
would be pushed away over the top of the underlying ommatidial region (see
Fig. 8).
The finding that clone shapes seen on the adult head of Drosophilafitwith the
predictions of a computer model using simple cell-division rules and the
presence of mechanical constraints surrounding the disc as parameters appears
to validate the assumptions of the model. In another publication (Ransom,
1976) a further test of the model has been carried out: it has been shown that if
all external constraints are removed from the growing computer 'cells', then
clones like those induced in the leg disc of Drosophila are generated - sectorshaped clones that give rise to parallel-sided clones stretching down the side of
the leg in the adult.
124
ROBERT RANSOM
B
Fig. 8. A possible explanation of separation of posterior head clones from posterior
eye clones during head disc development. (A) View of mature eye/antennal disc
complex. A antennal disc, E (dotted region) eye part of head disc, PO presumptive
postorbital region in overlying peripodial membrane. (B) Side view of disc cut along
the line ap. Postorbital and eye presumptive regions may be separated during development as shown, pm peripodial membrane, a, p, d, v, anterior, posterior, dorsal,
ventral.
The single difference between growth in the concentric discs, leg, wing and
haltere, which seem to grow in a radial fashion, and the more asymmetric
growth of the head disc appears to be the constraint system imposed on head
disc growth.
This work was done at the Hubrecht Laboratory, Utrecht, and I would like to thank the
staff members, particularly Dr W. J. Ouweneel, for their help during my stay. The Royal
Society generously supported me whilst I was in the Netherlands. Professor J. H. Sang and
Dr J. R. S. Whittle kindly read and commented on this manuscript.
Clones on Drosophila head cuticle
125
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{Received 12 December 1975; revised 2 April 1976)