Replacement of posterior by anterior structures in the Drosophila wing caused by the mutation apterous-blot

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1 J. Embryo!, exp. Morph. Vol. 53, pp , Printed in Great Britain Company of Biologists Limited 1979 Replacement of posterior by anterior structures in the Drosophila wing caused by the mutation apterous-blot By J. ROBERT S. WHITTLE 1 From the School of Biological Sciences, The University of Sussex SUMMARY The recessive mutation apterous-blot in Drosophila melanogaster causes replacement of posterior wing structures by anterior ones, with variable penetrance and expressivity. Extreme transformations resemble mirror-image duplicate anterior wings as in the mutant engrailed. Anterior structures in the posterior wing only appear on the dorsal surface. Duplications solely of posterior structures are also seen. Clonal analysis shows that extra cell proliferation occurs in the posterior area but is complete by 108 h after egg deposition. Lineage analysis is consistent with a clonal perpetuation of the transformation. Genetic mosaics to test the cell-autonomy of apterous-blot show that it is not autonomously expressed in clones. The results of lineage analysis, the phenotypes of combinations of apterous-blot with other apterous alleles including a deletion for the locus and with various other homoeotic mutations, are together used to distinguish three alternative modes of action of this mutation. It is concluded that apterous-blot is unlikely to be a selector gene mutation but instead may cause the transformation by an event like transdetermination following a local failure in cell function in the wing disc. INTRODUCTION Analysis of genetic mosaics in Drosophila melanogaster reveals that determination events, in the form of cell lineage restrictions, occur during disc development, separating groups of cells (polyclones) with different assigned fates (Garcia-Bellido, Ripoll & Morata, 1973; Crick & Lawrence, 1975). One early restriction divides cells of the wing disc into two compartments, anterior and posterior (Garcia-Bellido et al., 1976). Garcia-Bellido (1975) has suggested that the distinctions between different polyclones are maintained throughout disc growth by the activity of particular genes (selector genes). Wings of the mutant engrailed show mirror-image symmetry about the anterior-posterior compartment border. This mutant has been ascribed a role in the maintenance of the difference between the posterior and anterior compartments in the wing and certain other discs (Lawrence & Morata, 1976) based upon its pattern of clonal growth, its cell autonomy in genetic mosaics with wild-type tissue, and 1 Author's address: School of Biological Sciences, The University of Sussex, Falmer, Brighton BNl 9QG, Sussex, U.K.

2 292 J. ROBERT S. WHITTLE its behaviour in combination with homoeotic mutations of the bithorax complex locus (Garcia-Bellido & Santamaria, 1972). This paper describes the analysis of another mutation, apterous-blot, which leads to the formation of wings phenotypically similar to those of engrailed. The conclusion is drawn that although apterous-blot causes transformation of posterior cells to anterior cells in the wing, yet it does so in a way unlike that expected from a failure in a selector gene. MATERIALS AND METHODS Flies were grown on normal yeast-agar-maize meal medium at 25 C. The recessive mutation apterous-blot (ap m ) was obtained from the stocks SM5/BI stw i8 ap tuf sp and from stw ap wt tuf. The linked mutations Bristle, straw, tufted and cinnabar were used for recombination analysis and to follow segregation of apterous-blot. Comparisons between genotypes were made, wherever possible, using segregants from the same mating. Wings were mounted in Berlese fluid for examination. Mitotic recombination was induced by irradiation with 1000 R from a 60 Co source delivering 2-5 KR/minute. Age at irradiation is expressed in hours after egg-laying (AEL). The cell markers straw (stw), pawn (pwn) and multiple wing hairs (mwh) were used to identify induced mosaics. All mutants are described in Lindsley & Grell (1968) except for pawn (Garcia- Bellido & Dapena, 1974). Minute-plus clones were induced using the Minute mutations M(2)c^&, M(3)h S37 or M(3)i 55. Comparisons of mean clone size were made by t tests after normalizing the data by log transformation. RESULTS Phenotypic description of apterous-blot wings The penetrance of this mutant is incomplete (for example, in a sample of 582 wings 38 % showed alterations in wing shape or bristle disposition visible at x 12 magnification), and expressivity is very variable. The most mild effect is blistering in the wing blade centred on the posterior crossvein. Some wings show clear mirror-image partial duplications only involving posterior structures (Fig. 1 A). Other wings have disruptions in the venation pattern involving veins 2-5 and the crossveins (Fig. 1B). These include extra veins and distal branching of veins 3 and 4, often concomitantly with the loss of parts of vein 5. Table 1 contains details of the extent of phenotypic variability in a sample of 50 wings showing alterations from the normal phenotype. The frequency of disruption of venation is highest for vein 5 and falls off for the veins in more anterior positions, but even so vein 3 in the anterior compartment is disrupted in 60 % of the sample. By comparison with wings of the mutant engrailed (Table 1) the disruptions in apterous-blot wings are less extreme for vein 4 but clearly involve veins 2 and 3, anterior compartment structures, which are unaffected by engrailed. Camera lucida drawings of the venation of 30 such wings were compared one to another to see whether each wing represented a combination of parts of two alternative

3 Wing transformation in Drosophila 293 B Fig. 1. Apterous-blot wings. (A) Duplication of posterior wing structures. (B) Mirrorimage duplicate veins, sensilla and bristles in the posterior margin. (C) Posterior compartment including dorsal vein network with many sensilla campaniformia. (D) Wing shape resembling the engrailed phenotype. The posterior margin includes dorsal triple row bristles. (E) Posterior margin of wing showing dorsal elements of triple row bristles opposite ventral cell hairs.

4 Table 1. Phenotypic variability of various parameters in wings of the genotypes apterous-blot, engrailed and the double mutant apterous-blot engrailed Genotype apterous-blot engrailed apterous-blot engrailed Number of wings Bristle number Veins severely reduced or disrupted (% of sample) MTR DR V2 V3 V4 V5 PXV* t J 100} 88 90J 100t * PXV = posterior cross vein, t Replaced by duplicate vein 3. % Absent. 840 ± ± ± ±15-3 5OO± ± ± ±5-l ± ±27-2 Marginal hair number between most distal DR bristles 96-5± ± ±13-7 o ww H Hr w

5 Wing transformation in Drosophila 295 vein patterns, but this could not be concluded from the drawings. All extra venation is dorsal, and extra sensilla campaniformia are formed, as many as 22 per wing (Fig. 1C). Very occasionally small outgrowths of wing-blade material project from the wing blade adjacent to the posterior crossvein, The shape of the wing is frequently altered towards a symmetrical structure about the long axis (Fig. 1D). The dorsal row of hairs on the posterior edge may be replaced by the two types of bristles normally found as the dorsal two rows of the anterior triple row (Fig. 1D) or by the dorsal element of the anterior double row bristles. When both double (DR) and triple (TR) row bristles appear in the posterior margin the double row bristles are always distal to the triple row elements. This aspect of the transformation is patchy and incomplete, the posterior edge often being composed of a mixture of bristles and double row hairs (Figs. IB, ID) more particularly amongst the DR bristles. The ventral double row hairs opposite these bristles are normal in their spacing; mean interhair distance was not significantly different between wild-type and in transformed apterous-blot wings (n = 100, t = 0-20). No single instance has been observed of a ventral bristle along the posterior margin. Counts were made of the number of median triple row bristles (MTR) and double row bristles (DR) separately in anterior and posterior locations in wings of genotype apterous-blot and of engrailed (Table 1). There are significantly fewer posteriorly located MTR bristles in apterous-blot wings than in engrailed wings (t 108 = 6-9, P < 1 %), and also significantly fewer posterior DR bristles in apterous-blot wings than in engrailed wings. However, since all those DR bristles in apterous-blot wings on the posterior margin are dorsal, the mean of 15-1 DR bristles is not different from half the DR mean bristle number in engrailed wings. The number of posterior marginal hairs between the most distal anterior DR bristle and the most distal posterior DR bristle was significantly larger in apterous-blot wings than in engrailed wings, reflecting the less complete nature of transformation of wing margin structures in the former (Table 1). Genetic analysis Analysis of the recombinants provided by females of the constitution Bl stw 48 ap m tuf sp/dp en bw showed that the wing phenotype described earlier behaved as a single mutation to the right of straw at 55-1 cms and to the left of tufted at 55-5 cms. Of 18 recombinants between Bristle (54-0 cms) and cinnabar (57-5 cms) nine showed recombination between Bristle and the factor; apterousblot is located at 55-2 cms. A number of combinations of apterous-blot with other mutations in chromosome two were constructed. The double heterozygote between apterous-blot and engrailed was wild-type whether the mutations were in cis or trans arrangement, as was the Fl between apterous-blot and tufted 1, or tufted 2, an extreme allele which is homozygous lethal (Whittle, unpublished). The heterozygotes

6 296 J. ROBERT S. WHITTLE ap hlt /ap 56t and ap blt /Df(2R)MS4 (a deficiency including the apterous locus) showed blistering but no transformation; the latter genotype has a Minute phenotype since the deficiency includes a Minute locus. The genotype ap m / M(2)c 33a has wings with slight upsets in venation near the posterior crossvein. The double homozygote apterous-blot engrailed was viable and the phenotype of wings from this genotype is described in Table 1, and may be contrasted with the phenotypes of each separate homozygote. By comparison with apterousblot, vein 3 in the double homozygote is significantly less often affected (X 2 = 12-8, P < 1 %) though this anterior structure is still disrupted in 19 % of wings. Vein 4 is always replaced by a mirror-image duplicate of vein 3, and the dorsal vestige of the vein in the normal location of vein 5 carries a sensillum. The mean number of posterior MTR bristles in the double homozygote is intermediate, but significantly different from either homozygote. The alula is no longer a separate discernible structure in the double homozygote though the large hairs characteristic of the alula are present on the margin intermingled with bristles typical of the distal costa region. About 1 % of the double homozygotes have antennae with mirror-image duplication from segment II. The more extreme allele apterous 561, which has no wings, drastically reduces haltere size, but there was no evidence of any transformation effect of apterousblot in the haltere, though the lack of distinctive morphological markers of the anterior compartments in the capitellum leaves some uncertainty on this point. The halteres of various genetic combinations with apterous-blot were examined for signs of transformations in the hope of identifying the compartment origin of the 'anterior' elements seen in the posterior wing margin. No differences in the dorsal metathoracic disc structures formed could be detected after the addition of apterous-blot either to the genotype bx 3 /Ubx lsq (37 halteres) or to the genotype pbx/ubx 130 (50 halteres). Neither did homozygosity for apterousblot change expressivity in the partially haltere-like dorsal mesothoracic disc derivatives formed by Contrabithorax heterozygotes. The penetrance and expressivity of apterous-blot in wings is severely reduced in combinations with the Minute mutations M(3)i 55 and M(3)h* 31, the abnormalities being reduced to local alterations in venation. Clonal analysis Table 2 shows the mean cell number in multiple wing hairs clones induced in homozygous apterous-blot genotypes at various ages. It is apparent that growth rates at these ages are not significantly different either between anterior and posterior compartments or between normal wings and wings which show the transformations of posterior structures. Regression analysis of posterior clone size on anterior clone size using pairs of such clones found in the same wing in this sample showed no significant differences in this relationship when normal and transformed wings were compared. This was true both for clones induced at 108 h AEL and 120 h AEL (19 pairs of clones in normal wings and 17 pairs

7 Wing transformation in Drosophila 297 Table 2. Clone size in phenotypically normal and transformed wings of apterous-blot Mean cell number in clone Age at induction of clone 108±8h AEL 120±4h AEL Anterior Posterior Normal Transformed Normal Transformed (1-21 ±119)* (0-51 ±0-44)* (1-30 ±1-12)* (0-72 ±0-63)* * Mean log clone size ± standard error (1-90 ±1-38)* (0-82 ±0-53)* (205 ±1-23)* (0-97 ±0-75)* of clones in wings with transformations at 108 h, t 36 = 0-10; at 120 h 11 and 12 clone pairs respectively and t 23 = 0-3). Thus the relative growth rates in the anterior and posterior compartments in phenotypically normal and in transformed apterous-blot wings were not different at this age. At 96 hours AEL, clone size in the anterior compartment was the same in normal and transformed wings (X = 30-6, In clone size = 2-93 ±1-11). Mean clone size in the posterior compartment of transformed wings (X = 136-8, In clone size = 4-62 ± 0-90) was not significantly larger than in the anterior compartment (t 25 = 1-0) but showed considerably greater variability. Partition of the posterior clones in wings showing transformation by their location with respect to the transformed structures yielded clearer information. Clones found within the areas with disrupted venation or close to bristles on the posterior margin were far larger (X = 191-3) than those elsewhere in. the transformed wings (X = 90-0, t 12 = 1-2). An examination was made of the structures included in clones induced at various ages during larval development in wings exhibiting transformations. No clone, of 137 found in the posterior region, embraced both transformed 'anterior' structures and also normal posterior structures. The criterion set was that the multiple wing hairs clone should clearly occupy an area including anterior margin bristles and posterior margin hairs. In the same sample of mosaics, no evidence was found for any clonal relationship between the cells contributing to the normal anterior structures of the wing and the transformed 'anterior' structures in the posterior wing. The suppression effect of Minute mutations upon the apterous-blot phenotype precluded the use of Minute-plus clones in the analysis of lineage relationships in transformed wings. Instead it was possible to test the cell-autonomy of the phenotypic suppression of the apterous-blot wing transformation using the same mosaics, since in the genotype stw ap m ; M(3)h S37 large multiple wing hairs clones

8 298 J. ROBERT S. WHITTLE produced by mitotic recombination lacked the Minute mutation. In particular, posterior compartment clones both dorsal and ventral which touched the posterior border were inspected to see whether they formed 'anterior' bristles on the posterior margin or distorted wing shape towards that characteristic for apterous-blot homozygotes. None of the 119 clones found (Table 3, column 5) showed any alteration in posterior wing structures even though many clones were in excess of several thousand cells; the average size of these clones is given in Table 3. Column 7 of Table 3 shows the number of clones touching the anterior-posterior border over a distance of more than 20 cells. Every clone respected this border. The crucial question in investigating the basis of the apterous-blot mutation was whether it showed cell-autonomous behaviour, whether the phenotype of marked clones homozygous for apterous-blot growing in wing discs otherwise heterozygous for the mutation would resemble that of the homozygous mutation. The genotypes stw ap hlt pwn/m(2)c 33ai and pwn/m(2)c 33& were irradiated as larvae and a search was made for pawn clones. Clones were only recovered in the controls (for example, at 72 h AEL five clones in the 30 wings in the controls and none in 79 wings of the experimental group). This apparent cell-lethality of the clones of genotype stw ap ut pwn was confirmed by their absence from the tergites of the same sample. The failure to recover straw pawn clones in stw ap m pwn/'m(2)c 33a flies may be attributable to the combination of the two cell-marker mutations, and so an alternative approach was adopted. The genotypes stw ap^/pwn and It stw/pwn were irradiated during larval life to induce clones. Pawn is clearly detectable both in bristles and trichomes (Garcia-Bellido & Dapena, 1974), and the expectation was that many of the mitotic recombination events producing pawn clones would also give a straw twin-spot. Examination of the abdomens of the two genotypes above after irradiation between 72 and 96 h AEL showed that approximately half of the pawn clones were accompanied by a straw twin-spot clone (Table 4). Straw has the drawback that it can only be unambiguously classified in bristles in the wing, but each pawn clone was examined to see firstly whether it was associated with any local disruptions in wing organization. Secondly, where pawn clones were found dorsally in the posterior margin, a search was made for adjacent twin straw clones showing transformations including triple row or double row bristles. Table 4 indicates the numbers of pawn clones recovered and their mean size. No disturbances were associated with any of the pawn clones found in the posterior compartment nor was any of the pawn clones on the posterior margin associated with a clone including straw marginal bristles. An anterior dorsal pawn clone in the genotype stw ap h]t /pwn included dorsal triple row bristles and an adjacent straw twin-spot, demonstrating that it was possible to recognize such twin-spots were they produced.

9 ng transforma tion in Drcjsophite Table 3. Multiple wing hairs Minute-plus clones in wings of genotype stw ap blt ; M(3)h S37 /mwh Age at induction of clone (h AEL) 48 72* 96* Total wings examined Clones per wing D Clones A ^ i \ V D Clones touching posterior margin V N Clones touching anteriorposterior border Clone size loge clone size ±S.E. 6-2 ± ± ±2-6 60±21 20 ±1-9 * Includes clones induced in ap hlt ; i M(3)i 55 /mwh. Table 4. Numbers and clone sizes o/pawn clones found in the wings of the genotypes stw ap at various stages blfc /pwn and It stw/pwn after X-irradiation Age at induction of clone (hael) Anterior Posterior stw ap wt /pwn A Clone frequency Clone size Anterior Posterior Tergites pawn clones K) // stw/pwn A Clone frequency Clone size pawn/straw twin-spots clones per abdomen 7 27 pawn clones pawn/straw twin-spots clones per abdomen

10 300 J. ROBERT S. WHITTLE DISCUSSION The wings of penetrant homozygotes for apterous-blot show replacement of some posterior structures by anterior structures; extra dorsal venation, sensilla campaniformia and margin bristles are all characteristic of the anterior compartment. There is a symmetry about the long axis of the wing in shape, disposition of the extra dorsal veins and of margin bristles particularly at the junction of veins with the margin. Although the ventral posterior region of these wings is frequently larger in area, it otherwise retains the characteristics of the posterior compartment, invariably forming hairs and not bristles at the ventral posterior margin, and no duplicate to vein 2 is seen. Waddington (1939) described this mutant as showing posterior duplications (cf. Fig. 1A) but the extra anterior structures have not been reported before. The more extreme alterations in apterous-blot wings are very similar to those of the mutation engrailed (Garcia-Bellido & Santamaria, 1972) but are distinguished from the latter in that there are no ventral bristles on the posterior margin, anterior compartment structures (for example vein 3) are also disrupted, and some wings display duplications within the posterior compartment whilst having normal anterior structures. The fact that the double mutant ap hlt en has a wing phenotype very similar to engrailed argues that both mutations interfere with a similar event or process in wing development. Apterous-blot flies do not, however, have duplicated sex combs in males. Three alternative explanations are considered here as possibilities for the origin of this transformation phenotype, and they can be distinguished by the experiments described. Alternative 1. The mutation is a partial failure in a gene specifically concerned in the maintenance of posterior compartment determination during cell proliferation and therefore having a role similar to that ascribed to engrailed as a compartment-specific selector gene (Garcia-Bellido & Santamaria, 1972; Lawrence & Morata, 1976). It would be distinguished from engrailed in that its function is restricted to the dorsal posterior polyclone. This explanation, although the most interesting, is unlikely in the light of the following observations. Firstly the mutant does not act cell-autonomously in clones, an essential criterion for a gene involved in the maintenance of a specific determined state through cell proliferation. The formation of large Minute-plus clones of apterous-blot in the posterior compartment as early as 48 h AEL does not relieve the phenotypic suppression by the Minute. Although twin clones are expected to be common following mitotic recombination in the genotype stw ap hlt /pwn, no single transformation, for example upsets in venation or size of the posterior compartment, let alone any posterior edge transformed clone including straw bristles was found accompanying any pawn clone. The most critical evidence is the absence of transformed straw twin clones adjacent to pawn clones in the dorsal posterior margin. However, apterous-blot wings show widespread

11 Wing transformation in Drosophila 301 disruptions of veins and increase in cell number and frequently show transformations to anterior structures in the posterior area yet have an uninterrupted posterior margin of hairs, and so absence of any disruption or transformation adjacent to 'interstitial' pawn clones in the wing blade is also evidence against cell-autonomy of apterous-blot. The allele ap hlt is intermediate in the allelic series at this locus. The dominant allele ap^& shows wing-scalloping and cell-death (Fristrom, 1969). The allele tf/? 56f behaves as recessive to ap hlt and yet shows no homoeotic effects. The mutant combined with a deficiency for the apterous locus (ap hlt /Df(2R)MS4) shows no transformation effects, thus not behaving as a hypomorphic selector gene mutant (see Garcia-Bellido, 1977). It is also difficult on this hypothesis to explain why a selector gene failure should occasionally lead to posterior duplications (Fig. 1 A). Finally, the effect of the mutation is not restricted to the posterior compartment but abnormalities frequently include anterior structures (Fig. ID). Alternative 2. The second possibility is that the mutation ap hlt disrupts integrity of cell-cell contacts near the anterior-posterior compartment border so that some cells of the anterior polyclone become separated and enclosed by cells of the posterior polyclone. These 'trapped' cells would be supposed to maintain their state of determination, behaving as do mosaics of posterior compartment clones homozygous for engrailed in wild-type wings. Locally and clonally they would lead to a 'patchy' appearance of anterior structures in the posterior compartment, possibly causing local increases in wing size near them (Lawrence & Morata, 1976). This explanation clearly predicts that anterior structures in the posterior wing will show close lineage relationships to structures in the anterior compartment, whereas anterior and posterior areas are normally established as separate lineages (polyclones) in early embryogenesis (Garcia-Bellido, et al. 1973). No clones were found which behaved in this manner. If this mutant functioned in the same way in the haltere it would have been expected that some ap hlt ; pbx/ubx 1ZQ halteres would show unusual areas of haltere tissue in the homoeotic posterior wing tissue formed by postbithorax. Furthermore, halteres of ap hlt ; bx/ubx 130 might show 'invasion' of posterior haltere tissue by anterior wing structures, yet neither were seen. However, the results of Adler (1978) on the behaviour of cultured fragments of bithorax haltere discs, the tendency of haltere and wing disc cells to separate, and the low expressivity of apterous-blot in the mesothoracic wings of these flies leave considerable doubt about how definitive these experiments are. Byrant (1975) demonstrated that during intercalary regeneration cells could change their compartment-determined state, and it has recently been shown that when cells cross the anterior/posterior border in situ they clearly do redetermine to the state of the cells around them on that side of the compartment border (Simpson, Szabad & Nothiger, 1979) making unlikely the hypothesis considered as alternative 2. It is interesting to note that these authors also 2O EMB 53

12 302 J. ROBERT S. WHITTLE report the appearance of bristles characteristic of the anterior compartment on the posterior margin of wings that during larval life had been subjected either to surgical operations in situ or to heat-shock when carrying temperaturesensitive cell-lethal mutations. Alternative 3. The mutant causes transdetermination of some cells of the posterior polyclone so that they become anterior. Such a type of mutation has in fact been anticipated by Garcia-Bellido (1977) as likely to be found within homoeotic mutations as a general class. Again one expects such transdetermined cells to behave as do mosaics of engrailed in the posterior compartment, but there is no reason to suppose that the mutant itself need behave cellautonomously in the initial lesion it produces. The duplication of posterior structures (Fig. 1A) might reflect either the varying location or extent of the initial cell lesion in the disc. The position of ap mt in the allelic series as an allele of intermediate effect is now more understandable, the dominant allele #/? Xa causing some cell death and the recessive extreme allele ap 1 not supporting the growth or differentiation of cells in the wing blade at all. The atypically large size of clones induced at 96 h AEL in transformed areas of these wings may reflect cell loss and subsequent proliferation during replacement, and this may provide the opportunity for this 'transdetermination' from posterior to anterior to occur. This analysis makes clear again the value of investigating clonal parameters of a mutant before assigning a role to a gene (Garcia-Bellido, 1975). The intriguing aspect remaining to this mutant is the restriction of all the transformations to cells of the dorsal surface of the wing, and this is being further investigated. I thank the Science Research Council for funding this research, the British Council for travel support to a discussion meeting and Mrs Corryn Willsone for excellent technical support. REFERENCES ADLER, P. N. (1978). Mutants of the bithorax complex and determinative states in the thorax of Drosophila melanogaster. Devi Biol. 65, BRYANT, P. J. (1975). Pattern formation in the imaginal wing disc of Drosophila melanogaster: fate map, regeneration and duplication. /. exp. Zool. 193, CRICK, F. H. C. & LAWRENCE, P. A. (1975). Compartments and polyclones in insect development. Science, N.Y. 189, FRISTROM, D. (1969). Cellular degeneration in the production of some mutant phenotypes in Drosophila melanogaster. Molec. Gen. Genet. 103, GARCIA-BELLIDO, A. (1975). Genetic control of wing disc development in Drosophila. In 'Cell Patterning', Ciba Foundation Symposium, vol. 29, pp GARCIA-BELLIDO, A. (1977). Homoeotic and atavic mutations in insects. Amer. Zool. 17, GARCIA-BELLIDO, A. & DAPENA, J. (1974). Induction, detection and characterization of cell differentiation mutants in Drosophila. Molec. gen. Genet. 128, GARCIA-BELLIDO, A. & SANTAMARIA, P. (1972). Developmental analysis of the wing disc in the mutant engrailed of Drosophila. Genetics 72,

13 Wing transformation in Drosophilia 303 GARCIA-BELLIDO, A., RIPOLL, P. & MORATA, G. (1973). Developmental compartmentalisation in the wing disc of Drosophila. Nature New Biology 245, GARCIA-BELLIDO, A., RIPOLL, P. & MORATA, G. (1976). Developmental segregations in the dorsal mesothoracic disc of Drosophila. Devi Biol. 48, LAWRENCE, P. A. & MORATA, G. (1976). Compartments in the wing of Drosophila: A study of the engrailed gene. Devi Biol. 50, LINDSLEY, D. L. & GRELL, EL. (1968). Genetic variations of Drosophila melanogaster. Carnegie Just. Wash. Publ. no SIMPSON, P., SZABAD, J. & NOTHIGER, R. (1979). Regeneration and compartments in Drosophila. J. Embryol. exp. Morph. 49, WADDINGTON, C. H. (1939). Preliminary notes on the development of the wings in normal and mutant strains of Drosophila. Proc. natn. Acad. Sci., U.S.A. 25, (Received 22 February 1979, revised 9 May 1979)

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