Homeotic gene expression in Drosophila

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Homeotic gene expression in Drosopl ila Michael S. Levine and Cathy J. Wedeen The genetic mechanisms that regulate complex developmental processes m higher eukaryotes are largely unknown An attractive model system for analys:ng this problem Involves the morphogenesls of the diverse anatomical segments of the frutO'ly, Drosophila melanogaster_ The deslrablhty of this system stems from the identification of genetic loci that appear to directly control the developmental commitment and differentiation of each body segment prtmordtum The larval and adult forms of Drosophda are composed of eight abdominal segments, three thoracic, and four to six head segments (Fig. 1A) The epidermal portions of each anatomical segment elaborate cutlcular patterns that are unique for a given segment In addition, there is some ewdence that several internal tissues also acquire a segment specific phenotype In particular, the ventral cord of the CNS IS composed of 11 repeating ventral ganglia 5 The epidermis of each of the three thoracic and eight abdominal segments is innervated by a corresponding ganghon of the ventral cord Recent histological studies have revealed subtle morphological differences between several of the embryonic ventral ganglia 6, more obvious differences can be discerned during subsequent larval development 7 Moreover, there is evidence that some of the mesodermally derived musculature also possesses segment specific properties s Thus, for Drosophila, it appears that several of the constituent tissues of a given segment acqmre morphological properties unique to that segment The various tissues of a particular adult segment arise from spatmlly associated embryonic progemtors 9-il. Homeotlc genes are thought to estabhsh the diverse developmental pathways that result in the acquisition of a d~stlnct adult segment phenotype by each of these embryonic segment p n m o r d m Mutation of a homeotlc gene sometimes results in the complete transformation of the epidermal tissues of one segment Into those of another segment Iz Ghysen and h~s associates have shown that certain homeotlc mutations also result in the segmental transformation of adult ventral neuromeres i3 Similarly, Lawrence has provided evidence that homeotlc genes also control segmentation of the larval musculature s Therefore, it appears that homeotlc transformations encompass the epidermal, neural and mesodermal tlS-

sues of the affected segment Proper morphogenesis of the different larval and adult body segments might require the autonomous expresston of homeotic genes in epidermal, neural and mesodermal tissues Over the past thirty years, E_ B Lewis has pioneered genetic and developmental studies of Drosophda homeotlc loci 14-16 Lewis has shown that many of the homeotlc loci that affect segment morphogenesis of the abdomen and postenor thorax are clustered within the same region of the Drosophda genome On the basis of this genetic linkage, Lewis proposed that these homeotlc loci occur wlthm a gene complex, designated the btthorax complex (BX-C) Lewis has demonstrated that the ehmlnatlon of a given BX-C function can result in reduced activities of nelghbonng btthorax genes This demonstratton of cts regulatory mteractlons* provides additional evidence that the bithorax genes comprise a complex A genetic map of the BX-C is shown in Fig 2A The chromosomal deficiency Df(3R) P9 deletes all known genes of the BX-C (Ref 15). Embryos homozygous for this allele die during the terminal stages of embryogenesls. In such embryos the epidermal and neural tissues of the metathorax and abdominal segments fall to acquire the diverse morphological features that normally characterize these segments 15 However, development of the head and anterior thoracic regions of BX-C mutant embryos appears normal The BX-C can be subdivided * A s an example, consider the Contrablthorax (Cbx) and Ultrabithorax (Ubx) mutations of the B X - C Lewis has proposed that Cbx alters a Ubx regulatory element Genetic e w d e n c e for this proposal involved the classical cts/trans complementatton test Cbx Ubx/++ heterozygotes are n o r m a l in a p p e a r a n c e In contrast, Ubx +/+ Cbx flies display the Cbx phenotype (which is a homeotlc transformation of meso- to metathorax) T h u s , the Cbx phenotype is observed only w h e n a wdd-type copy of the Ubx locus is in c~s 0 e on the same homologue) with the Cbx mutation

mto two functional domains: the Ultrabtthorax or Ubx mutations and the infra-abdommal or tab senes of mutatlonsl7 18,45 Ubx expression is pnnclpally required for proper development of the metathorax and first abdominal segment Genes of the tab domain (i e abd-A and Abd-B) are involved m estabhshlng differentiation patterns for the second to through eighth abdommal segments Kaufman and his associates have demonstrated the occurrence of a second region of the Drosophila genome that contains multiple homeOtlC l o c i 19'20. By analogy to the BX-C, this second homeotlc gene cluster has been designated the Antennapedia gene complex (ANT-C) 19 In contrast to the BX-C, genes of the A N T - C are required for the establishment of diverse segmentation patterns in the anterior regions of the fly ANT-C mutations disrupt development of the posterior head segments as well as the pro- and meso-thorax A genetic map of the A N T - C IS shown In Fig, 2B. The Dfd, Scr, and Antp loci of the ANT-C appear to be required for proper estabhshment of the posterior-most head segment(s), prothorax, and mesothorax, respectively 2i,22 It has been proposed that A N T - C and BX-C gene products directly 'Instruct' embryonic segment primordla to acquire the morphological propertles that are unique for each of the different adult body segments 23. There are several critical predtctlons for this model Ftrst, a given homeotlc gene should be specifically expressed in the embryonic and larval segments that are known to be disrupted in mutants for the corresponding gene Second, this restricted expression of the homeotlc gene should be initiated dunng early embryonic stages of development_ It has been shown that the determmation of specific segment identities 1s initiated during, or just after, the cellulanzation of the syncttial blastoderm (which occurs approximately 2-3 h following fertihzatlon) 9,1° And third, expression of the gene should encompass each of the tissue-types that appear to acquire a unique segment-specific identity As previously discussed, it was anticipated that this might include epidermal, neural, as well as certain meso-

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dermal tissues The following studies have demonstrated that homeotlc gene expression roughly fulfils the criteria expected for regulatory cues that directly control development of the body segments Developmental and mosaic analyses of homeotlc gene functions have resulted in proposals regarding the temporal and spatial limits of homeotic gene expression during the D r o s o p h i l a life cycle For example, mutations that inactivate the U b x domain of the BX-C (see Fig 2A) most severely disrupt morphogenesls of the metathorax and first abdominal segment Is,45 (Fig 1B)_ More subtle morphological perturbations have been observed from the second to the eighth abdommal segments, while development of the anterior thorax and head appear to be unaffected 15 It was therefore suggested that expression of genes within the U b x domain would be largely confined to tissues of the metathorax and first abdominal segment and the corresponding segment prImordla is Inactivation of the A n t p locus of the A N T - C results in embryos that display homeotlc transformations of the meso- and meta-thoracic epidermis 21,24 (Fig 1C) Moreover, clones of homozygous A n t p - cells were induced in heterozygous embryos and larvae and allowed to differentiate into adult cuticular structures 24 Homozygous clones induced in the progenitors of the mesothoraclc leg were transformed into antennal structures whereas mosaic pro- and metat h o r a o c legs were less severely affected. Homozygous A n t p - clones induced m the head and abdominal segment progenitors had no detectable impact on the normal development of these structures On the basis of these and related studies, it has been proposed that the spatial limits of A n t p expression would be largely confined to the mesothoraclc segment and segment pnmordIum24 25 The molecular cloning of D r o s o p h i l a homeotIc genes has permitted a more direct assessment of the temporal and spatial hmlts of homeotlc gene expression 26-2s Homeotlc gene transcripts have been locahzed within D r o s o p h i l a tissue sections by m - s t t u hybridization 29-32 The method involves radlolabehng cloned D N A sequences that are derived from the coding regions of homeotmc genes and hybridizing these to serial tissue sections of D r o s o p h i l a embryos and larvae29 30 Tissues that accumulate

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Fig. 2 (A) GenetLc and molecular maps of the BX-C The relatzve map postt~ons of the genes of the B XC are indicated along the upper horizontal line ~5 tn 45 Genes tn the leftmost regton of the complex fall wLthtn the Ubx domain (see brackets above figure) Genes wtthzn the Ubx domain are necessary for segment tdentmes of 72 to A1 The nght-hand portLon of the map affects more postenor segments (A2 to AS), and ts deptcted as the abdommal-A (abd-A) and Abdommal-B (Abd-B) regLons of the mtraabdominal (tab) domatn Note that overlapping brackets loosely define the A - B boundary approxLmately wtthtn the zab-5 gene Overlapptng cloned genomtc D N A sequences that span the gene complex are mdLcated m ktlobase patrs along the lower horizontal hne Key abx, anteroblthorax, bx,

btthorax, Cbx, Contrab~thorax,Ubx, Ultrablthorax, bxd, blthoraxo~d,pbx, postb~thorax, ~ab, mfraabdominal ('B)Geneac and molecular maps of the A N T - C The relative posttzons of genes wtthln the A N T - C are mdtcated on the genetzc map (upper hne) The Deformed (Dfd), Sex Combs Reduced (Scr), and AntennapedLa (Antp) locL are requLred for morphogenests of the posterior-most head segments, prothorax, and mesothorax, respectively Some of the A N T - C genes have been correlated wah poszttons on the molecular map (lower hne) The molecular scale zs measured tn kdobase pairs Key pb, probosopedm, zen, zerknulI, ftz, fUshl tarazu

R N A s specified by the gene are identified by autoradlography The method is quite sensitive and has been estimated to allow detection of as few as 10 to 100 R N A molecules in a cell of average dimension 29 Such m - s t t u localization studies are d e p e n d e n t upon the availability of D N A clones that derive from homeotlc loci Bender, Splerer and Hogness pioneered techniques that resulted in the Isolation of molecular clones for BX-C (Fig 2A) 26 More recently, Kaufman and Gehrlng's research groups have independently isolated cloned chromosomal D N A sequences that span most of the A N T - C (Fig 2B) 27,28 The initial t n - s t t u localization studies involved the use of cloned U b x coding sequences and a c D N A derived from a transcription product of the A n t p locus a° 31 On the basis of the in-sttu hybrldl-

zation studies, as well as Northern analyses, initiation of A n t p and U b x expression appears to occur during cellulanzatlon of the syncltial blastoderm (Ref 31 and A k a m , M , unpublished observations) By mld-embryomc periods of development, the principal, but not exclusive, focus of both U b x and A n t p expression corresponds to the ventral cord of the CNS 3°,31 In particular, A n t p transcripts largely accumulate in the ganglion cells of the mesothorax (Fig 3A), whereas U b x transcripts are principally detected in the metathoracic and first abdominal ganglia (Fig 3B) As previously discussed, development of the epidermal portions of the mesothorax has been demonstrated to be most severely disrupted in A n t p mutations Ubx mutations most dramatically alter proper morphogenesis of the metathorax and first

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Fig. 3. (Facing page) Locahzanon of Antp and Ubx transcripts tn mid-embryos and third mstar larvae Tissue secnons of mid-stage embryos were hybridized to an Antp cDNA and to a Ubx genomtc DNA fragment (A) Antp transcripts are prmopally observed m the pro- and mesothoracw pornons of the ventral ganghon, weak labelhng ts also detected posteriorly along the ventral cord through A 7 (B) Ubx transcripts are most abundant tn the metathoractc ganghon and the first abdominal ganghon Lower levels o f Ubx transcripts are observed along the ventral cord postertor to the most intense hybrtdtzaaon signals (C) Antp transcripts are locahzed in a horizontal secnon of a wmg tmagmal disc (mesothoracw origin) from a third mstar larva (see arrow) (D) A darkfield photomicrograph of (C) (E, F) Bright and dark field photomwrographs of Ubx tranvcrtpts tn the haltere tmagtnal disc (metathoractc origin) ofathzrdmstarlarva (Figs 3a andc al, and

d and e 3" are reprinted by kind permission of the E M B O Journal 3b ~s reprinted from Ref 33 )

abdominal segment Thus, there is a close correlation between the embryonic ventral ganglia that accumulate high steady-state levels of transcripts specified by Ubx and Antp, and the larval and adult segments that display the most severe developmental defects by mutation of these loci_ Additional features of Ubx and Antp expression are remarkably similar For example, during the terminal stages of third-lnstar larval development, Antp and Ubx transcripts also accumulate in some of the hypodermal and mesodermal tissues of the segments that express these genes In particular, Ubx transcripts have been shown to accumulate in the haltere disk of the larval metathorax (Fig_ 3E and F) 3° Subsequently, during meta-

of possible cross-homologies_ McGinnls et al 34,35 and Scott et al 36 have independently demonstrated the occurrence of a similar - 200 bp protein coding sequence within several ANTC and BX-C loci This shared sequence has been designated the homeo box A total of at least seven borneo box copies are known to occur within the A N T - C and BX-C (McGinhiS, W., unpublished observations) Each of these h o m e o box copies is associated with a gene involved In the developmental process of segmentation H o m e o box homologies appear to be extremely well conserved amongst these seven loci In fact, the h o m e o box contained within the Antp locus has been used as a hybridization probe for the direct isolation of the Dfd and abd-A genes (Fig 2) from a Drosophda genomlc D N A hbrary 34 More recently, cloned genomic DNA sequences that contain the Abd-B locus of the BX-C have also been isolated on the basis of h o m e o box cross-hybridization (McGinnls, W , Bender, W and Levine, M , unpubhshed observations) The cloned Dfd, abd-A and Abd-B genomic D N A sequences have been used as ln-sttu hybridization probes for the localization of the transcripts they specify in Drosophila tissue sections, (Ref. 34 and Harding, K. and Levme, M , unpublished observations) The regions of Dfd, abd-A and Abd-B expression largely correspond to the embryonic segments and segment p n m o r d l a that are most altered by mutation of these loci More-

morphosis, the haltere lmaglnal disk will differentiate into much of the dorsal cutlcular structures of the adult m e t a t h o r a x (including a pmr of 'vestigial wings' or halteres) Similarly, Antp transcripts are present in the wing disk of the larval mesothorax (Fig 3C and D) 3I The wing lmaglnal disks give rise to the dorsal cuticle of the adult mesothorax, including a pair of wings_ The wing and haltere imaglnal disks arise from the epidermis of the embryonic meso- and metathorax, respectively Thus, Antp and Ubx are both expressed m several distinct tissues of a larval segment In summary, the results of previous genetic analyses as well as the more recent m-sttu localization studies are consistent with the proposal that Antp and Ubx products have direct regulatory roles in the establishment of developmental pathways that result in the f o r m a h o n of s p e c i f i c adult body segments Lewis proposed that the homeotlc loci of the BX-C arose from a common ancestral gene by duphcation and divergence 15 It is also possible that genes of the A N T - C arose from such a c o m m o n progenitor Indeed, the striking slmdaritles in the spatial and temporal limits of Antp and Ubx expression tend to support this view A more critical prediction of this model ts that the different genes contained within the A N T - C and BXC share direct D N A sequence homology The recent avadablhty of molecular clones derived from homeotlc loci permitted systematic assessment

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Fig. 5. Locahzanon of Antp transcripts m BX-C embryos In the lower left ts a schematic representation of the cuttcular pattern o f a BX-C (Df(3R)Pq homozygote) embryo Note that 7"2 through A 7 display a mesothoraczc phenotype A sagu~al section through a D f OR) P9 homozygote at the terminal stages o f embryogenests was hybridized to an Antp cDNA probe The Antp transcripts are locahzed to the T2 through A T poslttons of the ventral cord as demonstrated by the bright (A) and dark field (B) photomicrographs of this secUon (Repnnted by permxsslon from Nature, Ref 44

over, the patterns of Dfd, abd-A and Abd-B expression are slmdar to those obtained for Ubx and Antp in that the embryonic ventral cord contains the most abundant levels of Dfd, abd-A and Abd-B transcripts. In mid-staged embryos, the p n n o p a l sites of Dfd, abd-A and Abd-B expression correspond to the subesophogeal ganghon, the second through seventh abdominal ganglia and the postenor-most abdominal ganglion, respectively (Ref 34 and Harding, K and Levine, M , in preparation) In addition to its occurrence amongst Drosophila homeotic loci, the homeo box has been shown to be evolutionanly conserved and present in a variety of metazoans 35 H o m e o box containing chromosomal D N A sequences have been isolated from Xenopus 37, mice 38 and humans 39 In each case, the vertebrate homeo boxes possess over 70% putative peptide homology with Drosophda homeo boxes (Fig. 4) It is possible that vertebrate homeo boxes function in a manner homologous to those in Drosophda and are involved in the specification of embryonic somltes However, there is currently no evidence to support, or disprove, this proposal. It has been postulated that the homeo box specifies a DNA-bindmg protein domain 4°,41 Weak structural homologies have been shown to be conserved between the homeo box protein domain and known D N A binding proteins. Consistent with this notion is the recent demonstration that Ubx proteins largely accumulate in the nuclei of cells that express the

gene 42,46 It is therefore possible that Drosophda homeotic genes specify developmental processes by the direct trans modulation of target genes These target genes, or 'realisator loci' might function to impose specific morphogenetic properties upon cells that express particular homeotic genes 23 A central problem in the speoflcation of discrete body segments by Drosophila homeotic loci is how particular homeotlc genes come to be expressed only m certain regions of developing embryos. Mutations in homeotic genes can result in the proper morphogenesis of a particular anatomical structure in the wrong position along the body axis of the fly As described below, at least some of the homeotlc mutant phenotypes observed are associated with the accumulanon of a functional homeotic gene product m embryonic progenitors that normally do not express the gene. The restriction of Dfd, Antp, Ubx, abd-A and Abd-B to discrete positions of the embryonic ventral cord appears to involve luerarchical interactions of the genes contained within the A N T - C and BX-C (Ref 25) Embryos that lack all known genes of the BX-C display an epidermal and neural transformation of the metathorax and first seven abdominal segments into the homologous tissues of the mesothorax Is It has been proposed that this transformation results from a posterior extension of the normally mesothoracic realm of Antp expression 25-43 Consistent with this suggestion ~s the demonstration that Antp R N A s accumulate in the metathoracic and first seven abdominal ganglia of

B X - C embryos (l e Df(3R)P9 homozygotes, see Fig. 5) ~ On the basis of these and other findings 2s it has been proposed that Antp transcripts might be prevented from reaching high steady-state levels in posterior thoracic and abdominal gangha of wild-type embryos by the BX-C products which are also present in these cells In particular, BX-C products might interfere with the accumulation of Antp R N A s in the metathoraclc and abdominal gangha 44 The basis for possible cross-regulatory interactions amongst Drosophda homeotic loci is not known. It ~s intriguing to speculate that these interactions are mediated by the different homeo box protein domains encoded by the vanous ANT-C and BX-C loci For example, it is possible that Antp protein products autoregulate the Antp locus and sustain its expression in the embryonic and larval mesothoraclc ganglion. The Antp homeo box protern domain rmght be responsible for such tram" autoregulatlon of Antp expression In embryonic ventral ganglia posterior to the mesothorax, autoregulatlon of Antp might be hindered by the competitive binding of BX-C homeo box containing proteins to the Antp promotor. H o m e o box protein domains encoded by BX-C should have an appreciable affinity for the Antp promotor since the different A N T - C and BX-C homeo boxes share such extensive sequence homology. Thus, Antp might be expressed at relatively low levels in posterior ventral ganglia since BX-C products largely block Antp promotor binding sJtes for interaction with Antp proteins

T I N S - J u n e 1985 T h e r e 1S c u r r e n t l y n o direct e v i d e n c e to s u p p o r t this m o d e l . H o w e v e r , t h e availability o f c l o n e d h o m e o t i c D N A sequences and current progress tow a r d s t h e isolation of h o m e o U c p r o teins 42,46 s h o u l d p e r m i t a s s e s s m e n t o f this a n d o t h e r p r o p o s a l s r e g a r d i n g t h e spatial l o c a h z a t l o n o f h o m e o t i c g e n e p r o d u c t s to discrete r e g i o n s o f d e v e l o p lng e m b r y o s

Selected references 1 Ferns, G F (1950) m Biology of Drosophila (Demerec, M , ed ), pp 368-419, Wdey, NY 2 North, G (19831 Nature (London) 303, 134136 3 Lewis, E B (1982) Embryomc Development, Part A Genenc Aspects, pp 269-288, Alan R Llss, lnc, NY 4 Morata, G and Kerndge, S (1982) Nature (London) 300, 191-192 5 Poulson, D F (19501 in Bmlogy of Drosophda (Demerec, M , ed ), pp 168-274, Wiley, NY 6 JJmenez, F and Campos-Ortega, J A (19811 Wilhelm Roux'Arch Dev Bml 190, 370-373 7 Kankel, D R , Ferrus, A , Garen, S H , Harte, P J and Lews, P E (19801 m The Genetics and Biology ofDrosophda, Vol 2, (Ashburne, M and Wright, T S , eds), pp 295-363, Academm Press, NY 8 Lawrence, P A and Johnson, P (1984) Cell 36, 775-782 9 Wmschaus, E and Gehnng, W J (19761 Dev Bml 50, 249-263 10 Lawrence, P and Morata, G (19771 Dev Btol 56, 40-51 1l Lohs-Schardln, M , Cremer, C and Nusslem-Volhard, C (19791 Dev Bml 73, 239255 12 Ouweueel, W H (1976) Adv Genet 18, 179 13 Teugels, E and Ghysen, A (1983) Nature (London) 304, 440--442 14 Lew]s, E B (19631 Am Zool 3, 33-56 15 Lewis, E B (19781 Nature (London) 2/6, 565-570 16 Duncan, I and Lewis, E B (1982) m Development Order Its Ongm and Regulation, pp 533-554, Alan R Llss. Inc, NY 17 Morata, G , Botas, J , Kerndge, S and Struhl, G (1983)J Embryol Exp Morphol 78,319-341 18 Lawrence P A and Morata, G (19831 Cell 35, 595-601 19 Kaufman, T C , Lewis, R and Waklmoto, B (1980) Genencs 94. 115-133 20 Denell, R E , Hummels, K R , Wakamoto, B T andKaufman, T C (1981)Dev Bml 81, 43--50 21 Waklmoto, B T and Kaufman, T C (19811 Dev Bml 81, 51--64 22 Hazelngg, T and Kaufman, T C (19831 Genetics 105, 581~300 23 Garcla-Belhdo, A (1977) Am Zool 17, 613-629 24 Struhl. G (1980) Nature 292, 635-638 25 Struhl, G (19821 Proc NmlAead Scl USA 79. 7380-7384 26 Bender. W , Akam, M , Karch, F , Beachy, P A . Pfeffer. M , Spmrer, P , Lewis, E B and Hogness, D S (19831 Science 221, 23

245 27 Garber, R L , Kurolwa, A and Gehnng, W J (1983) EMBOJ 2, 2027 28 Scott, M P , Wemer, A J , Hazelngg, T I , Pohsky, B A , Plrotta, V , Scalenhe, F and Kaufman, T C (1983) Cell 35, 763 29 Hafen, E , Levme, M , Garber, R L and Gehnng, W J (1983) EMBO J 2, 617-623 30 Akam, M (1983) EMBO J 2, 2075-2084 31 Lewne, M , Hafen, E , Garber, R L and Gehnng, W J (1983) EMBO J 2, 20372O46 32 Hafen, E , KuroLwa, A and Gehnng, W J (1984) Cell 37,833--841 33 Scott, M P (1984) Trends NeuroSct 7,221223 34 McGmms, W , Levme, M S , Hafen, E , Kurolwa, A and Gehnng, W J (1984) Nature 308, 428-433 35 McGmms, W , Garber, R L , Wlrz, J , Kurolwa, A and Gehnng, W J (1984) Cell 37, 403-408 36 Scott, M P and Wemer, A J (1984) Proc Natl Acad Scl USA 81, 4115-4119 37 Carrasco, E , McGmms, W , Gehrmg, W J

and DeRobertts, E M (1984) Cell 37, 409 38 McGmnls, W , Hart, C , Gehnng, W J and Ruddle, F H (1984) Cell 38, 675-680 39 Levme, M , Rubm, G M and Tqan,, R (1984) Cell 38, 667-673 40 Laughon, A and Scott, M P (1984) Nature 310, 25-31 41 Shepherd, J C W , MeGlnnts, W , Carrasco, A E , DeRobertls, M and Gehnng, W J (1984) Nature (London) 310, 70--71 42 White, R and Wdcox, M (1984) Cell 00, 000-000 43 Duncan, I (1981) Genencs 100, 520 44 Hafen, E , Levme, M and Gehnng, W J (1984) Nature (London) 307,287-289 45 Sanchez-Herrero, E , Vernos, I , Marco, R and Morata, G (1985) Nature 313, 108--113 46 Beachy, P A , Helfand, S L and Hogness, D S (1985) Nature 313,545-551 Michael S Levme and Cathy J Wedeen are at the Fmrchdd Center, Department of Biological Sciences, Columbm Umversity, New York. N Y 10027, USA

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Oro$oolffla rnelanogaster J. A.

Campos-Ortega

Cells o f the neurogen,c ectoderm o f insects m u s t decide between two alternative fates, the n e u r o g e m c a n d the e p i d e r m o g e m c In the last f e w years several genes h a v e been identified m D r o s o p h i l a which are revolved in controlling this decision The analys~s o f m u t a u o n s m these genes has led to hypotheses a b o u t the c o m m i t m e n t o f the n e u r o g e m c e c t o d e r m a n d o f the neural progenitors. E a r l y n e u r o g e n e s i s c o m p n s e s t h e proc e s s e s t h a t l e a d to t h e f o r m a t i o n of a n e u r a l p r i m o r d l u m reside the e m b r y o . In m o s t a n i m a l s early n e u r o g e n e s l s p r o c e e d s t h r o u g h t h e s a m e stages, w h i c h i n c l u d e . (1) t h e c o m m i t m e n t of p a r t o f t h e e c t o d e r m a l g e r m layer as n e u r o g e n l c e c t o d e r m , f r o m w h i c h (2) n e u r a l cell p r o g e n i t o r s will o n g i n a t e , a n d (3) t h e m o r p h o g e n e t i c m o v e m e n t s performed by these progenitors m o r d e r to r e a c h t h e definitive location of the neural primordtum The study o f e a r l y n e u r o g e n e s i s ts c o n c e r n e d with t h e m e c h a n i s m s t h a t lead to the origin o f cell diversity in t h e d e v e l o p lng a n i m a l , for e c t o d e r m a l cells g o i n g to give n s e to t h e C N S h a v e in g e n e r a l to d e c i d e b e t w e e n the n e u r o g e m c a n d a n u m b e r o f o t h e r possible fates. T h e p r o g e n i t o r cells o f the insect C N S a r e large cells called n e u r o blasts 1. E a r l y in d e v e l o p m e n t t h e y m o v e f r o m the n e u r o g e n l c r e g i o n o f t h e e c t o d e r m , f r o m w h i c h they are d e r i v e d , to d e e p e r levels of the e m b r y o 1-'6 In t h e n e u r o g e n t c ectod e r m p r e s u m p t i v e n e u r o b l a s t s are i n t e r m i n g l e d with p r o g e n i t o r ceils of t h e e p i d e r m i s 4's- T h u s , in the n e u r o g e n i c r e g i o n o f insects n e t g h b o u r i n g

cells h a v e to c h o o s e b e t w e e n two d i f f e r e n t fates, t e b e t w e e n m a k i n g e p i d e r m i s a n d neurons_ H o w IS this d e c i s i o n m a d e ? A p p a r e n t l y , several different neurogemc mechanisms have e v o l v e d in the a n i m a l k i n g d o m In v e r t e b r a t e s , for e x a m p l e , t h e n e u r o genlc e c t o d e r m , the n e u r a l plate, is clearly s e p a r a t e f r o m the e p l d e r m o g e n t c r e g i o n s o f the e m b r y o a n d i n v a g l n a t e s as a w h o l e to f o r m t h e p n m o r d i u m o f the C N S F u r t h e r m o r e it is k n o w n for s e v e r a l s p e c m s that the commitment of a parhcular region of t h e e c t o d e r m a l g e r m l a y e r to f o r m t h e n e u r a l p l a t e is largely d e p e n d e n t o n inductive influences from the mesod e r m a l l a y e r (see, for e x a m p l e , ref 7) O n t h e o t h e r h a n d , in m s e c t s cells f r o m the n e u r o g e m c e c t o d e r m segregate individually a n d m o v e into t h e embryo, where they subsequently g r o u p to f o r m t h e n e u r a l p r i m o r d m m , whilst t h e r e m a i n i n g cells of the n e u r o g e n l c r e g i o n give rise to s u b s t a n tial p o r t i o n s of t h e e p i d e r m i s . T h e inductive influences from the mesod e r m do n o t s e e m to be i m p o r t a n t in i n s e c t s since n e u r o g e n e s l s m a y o c c u r in a b s e n c e o f m e s o d e r m s N e v e r t h e less, in principle n e u r o g e n e s l s in verte-

(~ 1985,ElsevmrScmncePubhshersB V Amslerdam 0378- 5912/85/$0200