The process of cell competition, whereby slowly dividing Minute cells are eliminated by faster-growing Minute+ cells in mosaic compartments of the Imaginal wing disc, is discussed. Evidence is presented suggesting that after completion of growth of the Imaginal discs, Minute+ cells no longer continue overgrowing and eliminating the Minute cells. The process of competition thus appears to be restricted to discs that are actively growing.

No cell competition can be detected in the histoblast cells that give rise to the adult abdomen. This observation, however, has been interpreted to be the result of an extremely long perdurance effect for the Minute+ product in these cells.

We have been studying patterns of growth in Drosophila Imaginal discs mosaic for populations of mutant cells with different growth rates. A phenomenon of cell competition has been described in these mosaic animals (Morata & Ripoll, 1975) such that the (normally non-Iethal) slower-growing cells are eliminated at the expense of the faster-growing ones. Cell competition only occurs within compartments, which are the basic units of development in Drosophila (Garcia-Bellido, Ripoll & Morata, 1976), and not between compartments (Simpson & Morata, 1981). It has also been shown that cell competition is a result of the differences in growth rate and that it depends upon local interactions between slow- and faster-growing cells (Simpson & Morata, 1981).

Compartments that are mosaic for cells of different growth rates are always of normal size and pattern. Therefore, although the overall amount of growth is controlled, the number of mitoses undergone by any particular cell is not important. The control of growth of the Imaginal discs is intrinsic to the discs themselves, that is intact discs in situ never grow to be larger than normal size, even when larval life is greatly prolonged and the hormonal and nutritive factors necessary for growth are still present (Simpson, Berreur & Berreur-Bonnenfant, 1980). These observations have led to the conclusion that the feedback interactions between growth and pattern size in Drosophila Imaginal discs probably results from the same type of local interactions that initially regulate the pattern itself. This conclusion is supported by two other lines of evidence: firstly that it is not possible to dissociate normal pattern and normal size (for discussion, see Simpson & Morata, 1980) and secondly, intercalary regeneration of in vivo cultured fragments of discs leads to stable pattern configurations and a cessation of growth (Bryant, 1978).

The experiments presented in this paper were designed to answer two questions. Firstly, does cell competition continue after a disc, mosaic for slow- and faster-growing cells, has completed its growth? Secondly, does cell competition occur in the histoblasts which form the adult abdomen in Drosophila?

Flies were grown on corn meal-yeast-agar medium, and were allowed to lay eggs in split bottles for 2 h periods. They were X-irradiated with a dose of 1000 R (Theta X-ray machine, Machlett tube, 1 mm Al filter, 40 KVP, 20 mA) in order to induce mitotic recombination (Becker, 1957). The relevant body parts were cooked in 10% KOH and mounted in Euparal for examination under the compound microscope. All slides were coded and scored blind.

Cross 1. Gynandromorphs were produced by means of the unstable ring-X chromosome, R(l)2wvC (Hinton, 1955). This chromosome is frequently lost during the early divisions in the egg, but remains stable throughout larval development. Ring-X-bearing females, also carrying multiple wing hairs (mwh), were mated to l(1)ts 504 sn3l(1)ts5697; M(3)i55/TM1 males. l(1)ts504 (Simpson & Schneiderman, 1975) and l(1)ts5697 (Arking, 1975), hereafter abbreviated to 504 and 5697, are sex-linked recessive lethals. These mutations have temperature-sensitive periods extending throughout the entire larval period. They both cause extensive cell death in the Imaginal discs at 30°C, the restrictive temperature. The mutations singed (sn3) and mwh were used as cell markers; they affect bristles or trichomes respectively, sn3 is on the X chromosome and served to reveal the male patches on the gynandromorphs, mwh is on the third chromosome and was used to mark clones induced by mitotic recombination. The use of a Minute mutation (M(3)i55) leads to the production of clones with an advantaged growth rate (Morata & Ripoll, 1975). TMI was used to balance (M(3)i55). See Lindsley & Grell (1968) for a description of mutations used. Animals were grown at 23°C, irradiated at 72 h after egg laying (AEL) and subjected to a heat pulse at 30°C between 72 and 120 h AEL. The time of eclosion of the flies was recorded, once each day at the same time. Gynandromorphs bearing (M(3)i55) were selected under the dissecting microscope. The female sn+ wings were examined for the presence of mwh M+ clones. Clones were drawn on to standard wing diagrams and their size estimated, taking into account the variable cell density in different parts of the wings (Garcia-Bellido & Merriam, 1971 d). Recombination in the left arm of chromosome 3 will give rise to mwh M+ clones and also mwh M(3)i55 clones in those cases where the mitotic exchange takes place between mwh and (M(3)i55). A bimodal distribution of clone sizes was observed by Morata & Ripoll (1975) after irradiation at any developmental stage. In the present experiment, clones smaller than 1000 cells were excluded; it was assumed that the remaining clones all belonged to the mwh M+ class.

Crosses 2, 3 and 4 were performed as follows.
formula
These led to the production of the following females: f36a/ +; pwn/ +; mwh/ + (genotype A),f36a/T(1;2) Bld/pwn; mwh/ + (genotype B) and ywaf36a/M(l)oSp (genotype C). In these flies forked (f36a) and pawn (pwn - see Garcia-Bellido & Dapena, 1974), were used to mark clones of bristles resulting from mitotic recombination. A description of the genotypes of these clones is given in the text where relevant. Embryos were irradiated 3 h after the middle of the egglaying period and some at 72 h AEL. Tergites 2 to 6 inclusive were scored for the presence of clones.

Cell competition after the completion of growth in the Imaginal discs

In the gynandromorphs resulting from cross 1, the female discs have a wildtype phenotype, whereas the XO male discs are mutant for the marker sn3 and the two temperature-sensitive cell-lethal mutations. This means that during the 48 h heat pulse, cell death occurs only in the male discs. The lesions are then repaired by the process of regeneration which causes pupariation to be delayed (Russell, 1974; Simpson & Schneiderman, 1975). Pupariation is retarded for a variable number of days depending upon the amount of male mutant tissue (Simpson et al. 1980). Since the female discs are unaffected by the heat pulse, they therefore undergo metamorphosis a considerable time after they have presumably completed growth. In the present experiment the emergence of the Minute gynandromorphs varied from day 13 to day 18 AEL. Earlier observations have shown that the delay in eclosion of the imagos is a consequence of a similar delay in pupariation; little or no variation in the length of the pupal period is seen (Simpson & Schneiderman, 1975).

Exclusively female wings from these flies were scored for the presence of large overgrowing mwh M+ clones (see Materials and Methods). Clones from flies emerging from day 13 to day 15 (early eclosing, EE) have been pooled and also those from flies emerging from day 16 to day 18 (late eclosing, LE). Forty-one clones were obtained from the EE series and 26 from the LE series. There is no detectable difference in clone size between the two series (EE: 3800 ±600; LE: 4100 ± 800). It will be remembered that the larvae were all irradiated at the same stage, prior to the heat pulse and subsequent cell death in the male discs.

During the growth of mosaic M/M+ discs, the M+ cells normally divide extensively and eliminate the M cells quite rapidly (Morata & Ripoil, 1975; Simpson, 1979). M+ clones induced early in the larval stage often fill an entire developmental compartment (Simpson & Morata, 1981). In the present experiment, if, after the disc has reached its full size, the M+ cells were to continue to divide and eliminate the M cells at a similar rate, then one would expect most of the clones in the LE series to fill an entire compartment. In fact only one of them did. The results suggest that after the completion of growth, the M+ cells stop dividing and do not continue to eliminate the M cells. However, a very slow continuous overgrowth might not have been detected. This conclusion is of course based on the assumption that in the late pupariating gynandromorphs, the female discs complete their growth long before pupariation intervenes, and are not retarded due to the presence of the regenerating male discs. It is not proven that this assumption is valid, but preliminary unpublished observations (Berreur & Simpson) suggest that it is.

Analysis of Minute clones in the tergites

Description of growth

Clonal analyses performed by previous workers have established that after an initial one or two divisions during embryogenesis (Wieschaus & Gehring, 1976; Lawrence, Green & Johnston, 1978) no further cell divisions take place in the histoblasts until pupariation (Garcia-Bellido & Merriam, 1971 b;,Guerra, Postlethwait & Schneiderman, 1973). This pattern of growth has been confirmed by histological observations which have also shown that during the larval period the cells increase in volume about 57-fold but do not become polytene or polyploid (Madhavan & Schneiderman, 1977). After pupariation, histoblast growth can be divided into two phases (Roseland & Schneiderman, 1979). During the first phase, from 3 to 15 h after pupariation, the histoblasts undergo about four rapid cell divisions in a manner resembling cleavage. During this time, they remain in their original area, that is in two groups of cells, continuous with the larval epithelium, situated anteriorly and posteriorly in each hemitergite. During the second phase, from 15 to 36 h after pupariation, the histoblast nests enlarge, the cells continue division and migrate over the larval polytenal cells which they replace.

Clone size and frequency

Clones were induced by irradiation at blastoderm (3 h AEL) and at 72 h AEL, in animals of genotypes A: f36a/ +; pwn/+; mwh/ +, B: f36a/T1; 2)Bld/pwn; mwh/ + and C: ywaf36a/ M(I)oSP. Clones of f36a and pwn bristles of animals of genotypes A served as controls. The genetic constitution of females of genotype

B and of the clones that were induced in them is depicted in Fig. 1. These females carried two doses of M(2)c+ and M(1)Bld+ and therefore developed at the rate for wild-type animals. Mitotic recombination in chromosome 1 resulted in f36a clones bearing only one dose of M(2)c+ and therefore mutant for that M due to the haplo-insufficiency of the M loci. Similarly recombination in 2R led to pwn M(1)Bld clones (mwh clones were ignored as mwh does not mark bristles). The results are given only for f36aM(2)c clones since pwn did not appear to be a reliable marker for abdominal bristles.* Recombination in chromosome 1 of females of genotype C led to the production of ywaf36aM+ clones in a M(1)oSp background (y clones resulting from recombination distal to f33a were not scored).

Fig. 1

The genetic constitution of Minute+ females (B) f36a/T(r,2) Bld/pwn\ mwh/ +, and the genotype of clones produced by mitotic recombination. Only one of the two chromatids per chromosome is shown in the mother cell. The thin solid line represents the X chromosome and the solid bar chromosome 2. Irradiation leads to the production of f36aM(2)c clones after recombination in the X chromosome and pwn M(1)Bld clones after recombination in chromosome 2.

Fig. 1

The genetic constitution of Minute+ females (B) f36a/T(r,2) Bld/pwn\ mwh/ +, and the genotype of clones produced by mitotic recombination. Only one of the two chromatids per chromosome is shown in the mother cell. The thin solid line represents the X chromosome and the solid bar chromosome 2. Irradiation leads to the production of f36aM(2)c clones after recombination in the X chromosome and pwn M(1)Bld clones after recombination in chromosome 2.

The average sizes of clones obtained are presented in Table 1. In the control cross (animals of genotype A) clones induced at 3 h AEL are larger than those induced at 72 h AEL. This confirms the observations of Wiechaus & Gehring (1976) and Lawrence et al. (1978) that some cell division occurs in embryogenesis.

Table 1

Average size, in number of bristles, of f36a clones induced in flies of genotypes A: f36a/+; pwn/+; mwh/ +, B: f36a/T(l; 2)Bld/pwn; mwh/ + and C: ywaf36a/M(1)oSp

Average size, in number of bristles, of f36a clones induced in flies of genotypes A: f36a/+; pwn/+; mwh/ +, B: f36a/T(l; 2)Bld/pwn; mwh/ + and C: ywaf36a/M(1)oSp
Average size, in number of bristles, of f36a clones induced in flies of genotypes A: f36a/+; pwn/+; mwh/ +, B: f36a/T(l; 2)Bld/pwn; mwh/ + and C: ywaf36a/M(1)oSp

It can be seen that whereas M(2)c clones induced by irradiation at 72 h AEL do not differ from the controls, those induced at 3 h AEL are considerably smaller than the controls (P < 0·001). Therefore only clones induced at 3 h expressed the M phenotype of slow growth. The distribution of clone sizes is given in Fig. 2. The lower mean clone size for M(2)c clones induced at blastoderm is due to a decrease in the number of large clones: only 7% of the clones contained more than five bristles, compared with 36% of the control clones (P < 0·01) and 30% of the M+ clones in animals of genotype C (P < 0·01).

Fig. 2

Distribution of the different sizes of f3Sa clones produced by irradiation at 3 h AEL of flies of genotype A: f36a/ +; pwn/ +; mwh/ +, B: f36a/T(1; 2)Bld/pwn-, mwh/ + and C: ywaf360/M(1)oSp.

Fig. 2

Distribution of the different sizes of f3Sa clones produced by irradiation at 3 h AEL of flies of genotype A: f36a/ +; pwn/ +; mwh/ +, B: f36a/T(1; 2)Bld/pwn-, mwh/ + and C: ywaf360/M(1)oSp.

The M+ clones induced in M(1)oSp flies at 3 h AEL display an average size similar to that of the controls. Therefore, such wild-type cells undergo the same number of divisions whether they are in an M or M+ animal.

Spontaneous clones

The frequency of spontaneous clones has been found to vary considerably for different stocks (Brown & Welshons, 1955; Weaver, 1960). Table 2 shows that for the three crosses used here the frequency is quite different even for the same marker and the same chromosome arm. The enhancing effect of the M mutations on the frequency of mitotic recombination has been well documented (Stern, 1936; Kaplan, 1953; Ferrus, 1975). The size of such clones however remains fairly constant (Table 1), the majority being composed of one or two bristles as reported by earlier authors (Brown & Welshons, 1955; Weaver, 1960; Baker, Carpenter & Ripoll, 1978; Kennison & Ripoll, personal communication). Examination of Table 2 shows that the clone frequencies in the unirradiated series áre higher than in those irradiated at 3 h AEL. It was not possible therefore to correct the clone sizes obtained after irradiation at 3 h by subtraction of spontaneous clones as has been done by earlier authors. J. Kennison and P. Ripoll (personal communication) report that, for wing discs, a lower frequency of small clones is observed after irradiation than in nonirradiated wings.

Table 2

Frequency of f3(5a clones per abdomen in flies of genotypes A: f36a/ +; pwn/ +; mwh/ +,B: f36a/T(l; 2)Bld/pwn; mwh/ +, and C: ywaf36a/M(l)oSp

Frequency of f3(5a clones per abdomen in flies of genotypes A: f36a/ +; pwn/ +; mwh/ +,B: f36a/T(l; 2)Bld/pwn; mwh/ +, and C: ywaf36a/M(l)oSp
Frequency of f3(5a clones per abdomen in flies of genotypes A: f36a/ +; pwn/ +; mwh/ +,B: f36a/T(l; 2)Bld/pwn; mwh/ +, and C: ywaf36a/M(l)oSp

Perdurance in the histoblasts

Two of the observations made in the experimental series require explanation. First, why are the M+ clones, induced in M animals at blastoderm, no larger than control clones, whereas M clones induced in M+ animals at blastoderm are smaller than control clones? Second, why are all clones induced at 72 h AEL the same size? I interpret these results to be the consequence of a long perdurance effect, as follows.

The increase in cell volume occurring during larval development probably allows the accumulation of reserves enabling the initial, rapid, cleavage-like divisions to take place at a time when the animal stops feeding. If this is the case then it may have special consequences for clonal analysis, since it may result in an extremely long perdurance effect (Garcia-Bellido & Merriam, 1971c) for mutations such as the Minutes. Perdurance is the amount of time that is necessary, after the recombination event leading to the production of an M cell, to reduce the amount of wild-type M+ products synthesized by the cell. It has been concluded, from a comparison of clone sizes, that in the wing disc perdurance lasts about three cell divisions in the case of M(1)Bld and M(2)c (Simpson & Morata, 1981). In the case of the histoblasts, this perdurance effect may well involve many more cell divisions. If the irradiation is performed during the larval period the recombination event only takes place at the first division after pupanation. The initial cleavage divisions may not require synthesis of new products for division, and it is possible that a further three divisions after the four cleavage divisions may be required to effectively reduce the amount of M+ product in the cell. As it has been calculated (Roseland & Schneiderman, 1979; Madhavan & Madhavan, 1980) that only about 8 doublings are required to make the adult abdominal epidermis, this would mean that the cells have virtually no time to express the M phenotype of slow growth and may explain why Morata & Ripoll (1975) observed only very small differences in clone sizes between M+ clones in M flies and M clones in M+ flies after larval irradiation. On the other hand, the developmental delay observed for M animals occurs almost entirely during the larval phase; very little delay is observed in the pupal period (Dunn & Mossige, 1937; Brehme, 1939; Steiner, 1976). Since M flies are similar in size to wild-type and bear the same number of bristles (Brehme, 1939) this must mean that the histoblasts in M flies divide at a similar rate to those of M+ flies. This may mean that sufficient M+ products are accumulated during larval life to tide the histoblasts over the entire pupal development. In the case of M larvae the extended larval period would allow the synthesis of similar amounts of M+ products to those in wild-type larvae. If this were true, perdurance may well cover the whole of the pupal stage. It could be the reason, therefore, why all clones observed here were of a similar size, irrespective of their M constitution.

When animals are irradiated prior to the larval stage, for example at blastoderm, then the recombination event will occur before the larval non-mitotic period of growth. If the above reasoning is correct, and if a genetically M cell amasses the M+ products necessary for division at a rate slower than that for wild-type cells, then an M clone induced at blastoderm in an M+ animal would have insufficient time to synthesize the required amount of M+ product and should therefore express a slower rate of cell division during the pupal period.

The converse, that of an M+ clone in an M fly, would not necessarily result in extra growth of the M+ clone, for even if the M+ cells accumulate more reserves than is usual during the prolonged M larval stage, it is unlikely that they would then divide faster than is normal.

No cell competition can be detected in the tergites

Examination of Table 2 shows that there is no decrease in the frequency of M(2)c clones induced at 3 h AEL in flies of genotype 3. Therefore, although these cells are dividing at a slow M rate, there is apparently no loss of clones. Loss of a small number of cells, however, would go undetected. The main reason for the lack of cell competition in the abdomen may be that there is no overgrowth of M+ cells (as shown in M flies of genotype C). It has been demonstrated that the loss of M cells through cell competition is associated with extra growth of neighbouring M+ cells (Simpson & Morata, 1981). This result is consistent with the observation that very little regulation is possible in the abdomen (Santamaria & Garcia-Bellido, 1972; Roseland & Schneiderman, 1979), since the duration of the pupal period varies little.

It can be concluded from the foregoing that no cell competition can be detected in the tergites. However, the pattern of growth of the histoblasts is quite different from that occurring in the Imaginal discs, and this result may be due to an extremely long perdurance for the M+ products.

Previous studies have shown that while M+ cells divide rapidly and eliminate M cells with which they are in contact they do not cross the boundaries between compartments to invade M territories of neighbouring compartments (Garcia-Bellido et al. 1976). Although M+ clones may grow to be very large their growth is in no way comparable to an invasive, cancerous type of growth; mosaic M/M+ animals develop quite normally. Results presented here suggest also that M+ cells stop dividing in mosaic M/M+ wing discs that have reached full size. Therefore, whatever the nature of the signal telling the cells to stop dividing, M+ cells that are still in contact with M cells remain sensitive to it.

It is probable that the factors leading to a cessation of growth are not external but are intrinsic to the Imaginal discs themselves (Simpson et al. 1980; Bryant, 1978). In a fly in which pupariation is retarded because of lesions in only some of the Imaginal discs, the other intact discs have an extended period of time for growth before the onset of metamorphosis. They do not, however, give organs or appendages that are any larger than the usual size (Simpson et al. 1980). The hormonal conditions necessary for growth are fulfilled none the less, since growth and regeneration proceed in the damaged discs. Experimental evidence for the production of giant adult insects by prolonged feeding due to juvenile-hormone treatment is not convincing. In all cases growth of the larval body decreases and eventually stops with each supernumerary larval instar. Less than 1% of the large larvae metamorphose, and when they do the resulting adults are abnormal (Sláma, Romanuk & Sorm, 1974). The one reported instance of an additional moult in the Diptera was not followed by adult differentiation (Zdarek & Slama, 1973). The so-called giant mutant in Drosophila owes its slightly enlarged body size to an increased cell size and not to an additional number of cells (unpublished observations). In cases in which enlarged Imaginal discs have been described, these discs either differentiate abnormal or duplicated patterns (Bryant & Schubiger, 1971; Martin, Martin & Shearn, 1977) or else prove incapable of differentiation (Gateff & Schneiderman, 1969; Stewart, Murphy & Fristrom, 1972; Garen, Kauvar & Lepesant, 1977). Fragmentation of Imaginal discs usually leads to a regenerating and a duplicating fragment, and after regeneration or duplication growth stops, leaving normal-sized patterns (Bryant, 1978).

I thank Renée Dupré for her expert technical assistance, R. Arking for the mutation l(V)ts5697 and Peter Lawrence for comments on the manuscript. This work was supported by the Centre National de la Recherche Scientifique.

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*

Frequently, many pwn bristles were found clustered closely together and many pwn clones were associated with empty bristle sockets. Either pwn is not a reliable marker for abdominal bristles, or else the pwn chromosome used here had accumulated a deleterious mutation affecting bristle differentiation.