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A tripartite organization of the urbilaterian brain: Developmental genetic evidence from Drosophila
Brain Research Bulletin 66 (2005) 491–494
A tripartite organization of the urbilaterian brain: Developmental genetic evidence from Drosophila Heinrich Reichert ∗ Institute of Zoology, Biozentrum/Pharmazentrum, University of Basel, Klingelbergstrasse 50, CH-4056 Basel, Switzerland Available online 5 March 2005
Abstract Developmental genetic studies suggest that the embryonic vertebrate brain has a tripartite ground plan consisting of a forebrain/midbrain, a hindbrain, and an intervening midbrain/hindbrain boundary region, which are characterized by the specific expression of the Otx, Hox and Pax-2/5/8 genes. Recent studies in Drosophila reveal similarities in the expression and function of these genes in patterning the embryonic brains of flies and vertebrates. Thus, in Drosophila, as is vertebrates, a Pax2/5/8 domain is located between an anterior otd/Otx2 region and a posterior Hox region of the embryonic brain. Moreover, in Drosophila, as in vertebrates, this Pax2/5/8 domain is located at the interface of the otd/Otx2 domain and a posterior unplugged/Gbx2 domain. Furthermore, in Drosophila, as in vertebrates, inactivation of otd/Otx2 or of unplugged/Gbx2 results in a comparable mispositioning or loss of orthologous gene expression domains in the embryonic brain. These developmental genetic similarities suggest that the tripartite ground plan, which characterizes the developing vertebrate brain, is also at the basis of the developing insect brain. This, in turn, implies that a tripartite organization of the embryonic brain may characterize all extant bilaterians, and thus may already have been established in the last common urbilaterian ancestor of all bilaterians. © 2005 Elsevier Inc. All rights reserved. Keywords: Tripartite organization; Urbilaterian brain; Drosophila; Brain development; Hox genes; Pax genes; Otx genes
1. Introduction Classical phylogenetic, neuroanatomical and embryological studies have led to the view of an independent evolutionary origin of protostome and deuterostome brains. Accordingly, bilaterians have been divided into two major groups with different CNS morphologies: the gastroneuralia, characterized by a ventral nerve cord, include protostomes such as arthropods, annelids and molluscs; and the notoneuralia, characterized by a dorsal nerve cord, include all chordates [15,3,10]. In contrast, more recent studies examining the expression patterns and functions of orthologous genes in embryonic nervous systems have revealed unexpected similarities in the embryonic brains of protostome insects and deuterostome vertebrates (for reviews see [2,1,5,7,13,18,12,17]). ∗
Tel.: +4161 2671612; fax: +4161 2671613. E-mail address: [email protected].
0361-9230/$ – see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.brainresbull.2004.11.028
Prominent examples for developmental control genes with similarities in expression and function in insects and vertebrates are the genes of the orthodenticle (otd/Otx) family and the genes of the homeotic (Hox) family. Thus, the otd/Otx genes are essential for proper development of the anterior brain. Mutations in these genes lead to severe brain phenotypes such as the absence of large neurogenic regions of the brains of both insects and vertebrates. Similarly, the Hox genes are important for patterning of the developing posterior brain and CNS. The comparable Hox gene mutant phenotypes that are observed in insects and vertebrates are indicative of comparable function of the homologous genes in embryonic development of the two corresponding brain types. Taken together with the results of comparative gene expression studies that have been carried out in other phyla, these results suggest that invertebrate brains and vertebrate brains are universally characterized by a rostral region specified by genes of the otd/Otx family and a caudal region specified by genes of the Hox family (Fig. 1).
492
H. Reichert / Brain Research Bulletin 66 (2005) 491–494
Fig. 1. Conserved anteroposterior order of gene expression in embryonic brain development. Schematic of otd/Otx and Hox gene expression patterns in the developing CNS of Drosophila and mouse with the location of the DTB (Drosophila) and the MHB (mouse) indicated by arrow and vertical bar. Expression domains are color coded. Anterior is to the left. For Drosophila, gene expression corresponds to a stage 14 embryo; for the mouse, gene expression corresponds to a stage 9.5–12.5 embryo. The Drosophila brain is composed of an anterior supraesophageal ganglion and a posterior subesophageal ganglion. The supraesophageal ganglion is divided into the protocerebrum (B1), deutocerebrum (B2) and tritocerebrum (B3). The subesophageal ganglion is subdivided into the mandibular (S1), maxillary (S2) and labial (S3) neuromeres. The mouse brain is divided into a rostral region that comprises the telencephalon (T), diencephalon (D) and mesencephalon (M), and into a caudal region which has a metameric organization based on rhombomeres 1–8.
2. The tripartite brain of vertebrates In ascidian and vertebrate chordates, a third gene expression domain is located between the anterior Otx and the posterior Hox expression regions of the embryonic brain (reviewed in [7,18]). In embryonic vertebrate brain development, this domain is located between the presumptive mesencephalon and metencephalon and plays a central role in the development of the midbrain–hindbrain boundary (MHB) region or isthmus. The MHB region has an organizer function in the developing brain since tissue from the MHB region transplanted to the diencephalon or the rhombencephalon induces the cells surrounding it to develop mesencephalic fates in the diencephalon or cerebellar fates in the rhombencephalon. A number of transcription factors and signaling molecules are expressed in the presumptive MHB region, and these generate a complex genetic network, which establishes and maintains organizer features (reviewed in [8,14,21]). Among the transcription factors that play a central role in establishment and maintenance of the MHB region are those encoded by the Otx2, Gbx2, and Pax2/5/8 genes. During gastrulation and early neurulation, the homeobox genes Otx2 and gastrulation brain homeobox 2 (Gbx2) are expressed in mutually exclusive domains that lie anterior and posterior to the presumptive MHB. When the level of Otx gene expression is genetically reduced, the anterior margin of Gbx2 expression shifts to a more anterior position. Conversely, in Gbx mutant mutant brains, Otx2 expression is shifted to a more posterior position. These and other experiments indicate that Otx2 and Gbx2 control the induction and positioning of the MHB [20,16,9]. The genes encoding transcription factors of the Pax2/5/8 families delimit the intermediate expression domain at the interface of the Otx2 and Gbx2 expression domains that coincides with the presumptive MHB. Mutational
inactivation of either Pax2 or Pax5 alone or a combination of double mutants leads to partial or complete deletion of midbrain and cerebellum structures.
3. Evidence for a tripartite brain in Drosophila The central role of the MHB region in vertebrate brain development together with the conserved expression patterns of Pax2/5/8 genes in a corresponding embryonic brain region in urochordate ascidians have led to the proposal that a fundamental characteristic of the ancestral chordate brain was its tripartite organization characterized by Otx, Pax2/5/8 and Hox gene expressing regions [19]. To investigate whether protostomes also possess this type of tripartite organization of the brain, and to gain insight into the evolution of the bilaterian brain, a comparative analysis of expression and function of the orthologues that pattern the vertebrate MHB region has been carried out in the ecdysozoan Drosophila melanogaster. This investigation reveals striking similarities to the situation in vertebrates, suggesting that a tripartite ground plan is also found in the insect brain [6]. The Drosophila genome contains two genes, Pox neuro (Pox-n) and Drosophila Pax2 (DPax2), that are together considered to be orthologues of the Pax2/5/8 group [11] and one Gbx2 orthologous gene called unplugged (unpg) [4]. Remarkably, the Pax2/5/8 orthologues Pox-n and DPax2 are expressed in the embryonic fly brain in a transversal boundary region located posterior to the otd expression domain in the deutocerebral neuromere and anterior to the homeotic gene expression domain in the tritocerebral neuromere; this transversal domain is the detocerebral–tritocerebral boundary (DTB) region (Fig. 2). Furthermore, the Gbx2 orthologue unpg is expressed in the posterior deutocerebrum and its
H. Reichert / Brain Research Bulletin 66 (2005) 491–494
493
Fig. 2. A transversal domain of adjacent Pax2/5/8 orthologue expression at the DTB. Pox neuro (Poxn) and D-Pax2 are expressed in distinct domains at the DTB (deutocerebral/tritocerebral boundary) region in the embryonic brain. Laser confocal microscopy of P{lacZ}D-Pax2∆122 /+ stage 13/14 embryos, reconstructions of optical sections. Lateral views. Immunolabeling with anti HRP (red) to reveal neurons (left image). Triple-immunolabeling with anti-HRP (red), anti-Gal (green, yellow) and anti-Poxn (blue) to reveal neurons as well as Pax2 and Poxn expression (right image). Both images are the same optical section of the same preparation; arrow indicates the location of the DTB.
Fig. 3. Tripartite organization of the Drosophila and mouse brain based on expression patterns of orthologous genes. Schematic diagram of the expression of otd/Otx, unpg/Gbx2, Pax2/5/8, and Hox1 gene orthologues in the developing CNS of a stage 13/14 Drosophila embryo and a stage E10 mouse embryo. In both cases, a Pax2/5/8 expressing domain is located between an anterior otd/Otx expressing region and a posterior Hox expressing region in the embryonic brain, and a Pax2/5/8 expressing domain is positioned at the interface between the otd/Otx expression domain and a posteriorly abutting unplugged/Gbx2 expression domain. This otd/Otx-unpg/Gbx2 interface displays similar developmental genetic features in both Drosophila and mouse.
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H. Reichert / Brain Research Bulletin 66 (2005) 491–494
anterior-most expression in the brain coincides with the posterior border of otd expression. There are no cells found that co-express otd and unpg and thus, these domains exclude each other. Comparable to the situation in the vertebrates, in otd loss-of-function mutants unpg expression is shifted anteriorly. Null mutants for unpg, in contrast, reveal posteriorly shifted otd expression extending into the posterior deutocerebrum. Thus, in both Drosophila and vertebrates the otd/Otx2 and unpg/Gbx2 genes appear to negatively regulate each other at the interface of their expression domains in the DTB boundary zone located between anterior and posterior brain regions. In contrast to vertebrates, mutational inactivation of the Pox-n or DPax2 does not result in obvious brain defects in Drosophila [6].
4. An urbilaterian origin of the tripartite brain The similarities between insects and vertebrates at the level of gene expression and functional interactions of otd/Otx, unpg/Gbx2, Pax2/5/8 and Hox genes are striking and are likely to represent further examples of evolutionary conservation of brain patterning mechanisms. Moreover, these developmental genetic similarities indicate that the tripartite ground plan (anterior brain, boundary region, posterior brain), which characterizes the developing chordate brain, is also at the basis of the developing insect brain (Fig. 3). It is conceivable that the similarities of orthologous gene expression patterns and functional interactions in brain development evolved independently in insects and vertebrates. However, a more reasonable explanation is that an evolutionarily conserved genetic program underlies brain development in all bilaterians. This would imply that the generation of structural diversity in the embryonic brain is based on positional information that has been invented only once during evolution and is provided by genes like otd/Otx2, unpg/Gbx2, Pax2/5/8 and Hox, conferring on all bilaterians a common basic principle of brain development. If this is the case, one would predict that comparable orthologous gene expression and function should also characterize embryonic brain development in other invertebrate lineages such as the lophotrochozoans. Taken together, the results of comparative developmental genetic studies indicate that the tripartite ground plan that characterizes the developing chordate brain is also present in the developing insect brain. This suggests that a corresponding tripartite organization already existed in the brain of the last common urbilaterian ancestor of insects and chordates.
Acknowledgement This work was supported by the Swiss NSF.
References [1] D. Arendt, K. N¨ubler-Jung, Comparison of early nerve cord development in insects and vertebrates, Development 126 (1999) 2309– 2325. [2] A.E. Bruce, M. Shankland, Expression of the head gene Lox22-Otx in the leech Helobdella and the origin of the bilaterian body plan, Dev. Biol. 201 (1998) 101–112. [3] R.C. Brusca, G.J. Brusca, Invertebrates, Sinauer Associates, Sunderland, MA, 1990. [4] C. Chiang, K.E. Young, P.A. Beachy, Control of Drosophila tracheal branching by the novel homeodomain gene unplugged, a regulatory target for genes of the bithorax complex, Development 121 (1995) 3901–3912. [5] F. Hirth, H. Reichert, Conserved genetic programs in insect and mammalian brain development, BioEssays 21 (1999) 677– 684. [6] F. Hirth, L. Kammermeier, E. Frei, U. Walldorf, M. Noll, H. Reichert, An urbilaterian origin of the tripartite brain: developmental insights from Drosophila, Development 130 (2003) 2365– 2372. [7] L.Z. Holland, N.D. Holland, Chordate origins of the vertebrate central nervous system, Curr. Opin. Neurobiol. 9 (1999) 596– 602. [8] A. Liu, A.L. Joyner, Early anterior/posterior patterning of the midbrain and cerebellum, Annu. Rev. Neurosci. 24 (2001) 869–896. [9] J.P. Martinez-Barbera, M. Signore, P. Pilo Boyl, E. Puelles, D. Acampora, R. Gogoi, F. Schubert, A. Lumsden, A. Simeone, Regionalisation of anterior neuroectoderm and its competence in responding to forebrain and midbrain inducing activities depend on mutual antagonism between OTX2 and GBX2, Development 128 (2001) 4789–4800. [10] C. Nielsen, Animal Evolution—Interrelationships of the Living Phyla, Oxford University Press, Oxford, 2001. [11] M. Noll, Evolution and role of Pax genes, Curr. Opin. Genet. Dev. 3 (1993) 595–605. [12] H. Reichert, Conserved genetic mechanisms for embryonic brain patterning, Int. J. Dev. Biol. 46 (2002) 81–87. [13] H. Reichert, A. Simeone, Conserved usage of gap and homeotic genes in patterning the CNS, Curr. Opin. Neurobiol. 9 (1999) 589–595. [14] M. Rhinn, M. Brand, The midbrain–hindbrain boundary organizer, Curr. Opin. Neurobiol. 11 (2001) 34–42. [15] R. Siewing, Lehrbuch der Zoologie, Gustav Fischer, Stuttgart, 1985. [16] A. Simeone, Positioning the isthmic organizer where Otx2 and Gbx2 meet, Trends Genet. 16 (2000) 237–240. [17] S.G. Sprecher, H. Reichert, The urbilaterian brain: developmental insights into the evolutionary origin of the brain in insects and vertebrates, Arthropod Struct. Funct. 32 (2003) 141–156. [18] H. Wada, N. Satoh, Patterning the protochordate neural tube, Curr. Opin. Neurobiol. 11 (2001) 16–21. [19] H. Wada, H. Saiga, N. Satoh, P.W. Holland, Tripartite organization of the ancestral chordate brain and the antiquity of placodes: insights from ascidian Pax-2/5/8, Hox and Otx genes, Development 125 (1998) 1113–1122. [20] K.M. Wassarman, M. Lewandoski, K. Campbell, A.L. Joyner, J.L.R. Rubenstein, S. Martinez, G.R. Martin, Specification of the anterior hindbrain and establishment of a normal mid/hindbrain organizer is dependent on Gbx2 gene function, Development 124 (1997) 2923–2934. [21] W. Wurst, L. Bally-Cuif, Neural plate patterning: upstream and downstream of the isthmic organizer, Nat. Rev. Neurosci. 2 (2001) 99–108.
A tripartite organization of the urbilaterian brain: Developmental genetic evidence from Drosophila Heinrich Reichert ∗ Institute of Zoology, Biozentrum/Pharmazentrum, University of Basel, Klingelbergstrasse 50, CH-4056 Basel, Switzerland Available online 5 March 2005
Abstract Developmental genetic studies suggest that the embryonic vertebrate brain has a tripartite ground plan consisting of a forebrain/midbrain, a hindbrain, and an intervening midbrain/hindbrain boundary region, which are characterized by the specific expression of the Otx, Hox and Pax-2/5/8 genes. Recent studies in Drosophila reveal similarities in the expression and function of these genes in patterning the embryonic brains of flies and vertebrates. Thus, in Drosophila, as is vertebrates, a Pax2/5/8 domain is located between an anterior otd/Otx2 region and a posterior Hox region of the embryonic brain. Moreover, in Drosophila, as in vertebrates, this Pax2/5/8 domain is located at the interface of the otd/Otx2 domain and a posterior unplugged/Gbx2 domain. Furthermore, in Drosophila, as in vertebrates, inactivation of otd/Otx2 or of unplugged/Gbx2 results in a comparable mispositioning or loss of orthologous gene expression domains in the embryonic brain. These developmental genetic similarities suggest that the tripartite ground plan, which characterizes the developing vertebrate brain, is also at the basis of the developing insect brain. This, in turn, implies that a tripartite organization of the embryonic brain may characterize all extant bilaterians, and thus may already have been established in the last common urbilaterian ancestor of all bilaterians. © 2005 Elsevier Inc. All rights reserved. Keywords: Tripartite organization; Urbilaterian brain; Drosophila; Brain development; Hox genes; Pax genes; Otx genes
1. Introduction Classical phylogenetic, neuroanatomical and embryological studies have led to the view of an independent evolutionary origin of protostome and deuterostome brains. Accordingly, bilaterians have been divided into two major groups with different CNS morphologies: the gastroneuralia, characterized by a ventral nerve cord, include protostomes such as arthropods, annelids and molluscs; and the notoneuralia, characterized by a dorsal nerve cord, include all chordates [15,3,10]. In contrast, more recent studies examining the expression patterns and functions of orthologous genes in embryonic nervous systems have revealed unexpected similarities in the embryonic brains of protostome insects and deuterostome vertebrates (for reviews see [2,1,5,7,13,18,12,17]). ∗
Tel.: +4161 2671612; fax: +4161 2671613. E-mail address: [email protected].
0361-9230/$ – see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.brainresbull.2004.11.028
Prominent examples for developmental control genes with similarities in expression and function in insects and vertebrates are the genes of the orthodenticle (otd/Otx) family and the genes of the homeotic (Hox) family. Thus, the otd/Otx genes are essential for proper development of the anterior brain. Mutations in these genes lead to severe brain phenotypes such as the absence of large neurogenic regions of the brains of both insects and vertebrates. Similarly, the Hox genes are important for patterning of the developing posterior brain and CNS. The comparable Hox gene mutant phenotypes that are observed in insects and vertebrates are indicative of comparable function of the homologous genes in embryonic development of the two corresponding brain types. Taken together with the results of comparative gene expression studies that have been carried out in other phyla, these results suggest that invertebrate brains and vertebrate brains are universally characterized by a rostral region specified by genes of the otd/Otx family and a caudal region specified by genes of the Hox family (Fig. 1).
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H. Reichert / Brain Research Bulletin 66 (2005) 491–494
Fig. 1. Conserved anteroposterior order of gene expression in embryonic brain development. Schematic of otd/Otx and Hox gene expression patterns in the developing CNS of Drosophila and mouse with the location of the DTB (Drosophila) and the MHB (mouse) indicated by arrow and vertical bar. Expression domains are color coded. Anterior is to the left. For Drosophila, gene expression corresponds to a stage 14 embryo; for the mouse, gene expression corresponds to a stage 9.5–12.5 embryo. The Drosophila brain is composed of an anterior supraesophageal ganglion and a posterior subesophageal ganglion. The supraesophageal ganglion is divided into the protocerebrum (B1), deutocerebrum (B2) and tritocerebrum (B3). The subesophageal ganglion is subdivided into the mandibular (S1), maxillary (S2) and labial (S3) neuromeres. The mouse brain is divided into a rostral region that comprises the telencephalon (T), diencephalon (D) and mesencephalon (M), and into a caudal region which has a metameric organization based on rhombomeres 1–8.
2. The tripartite brain of vertebrates In ascidian and vertebrate chordates, a third gene expression domain is located between the anterior Otx and the posterior Hox expression regions of the embryonic brain (reviewed in [7,18]). In embryonic vertebrate brain development, this domain is located between the presumptive mesencephalon and metencephalon and plays a central role in the development of the midbrain–hindbrain boundary (MHB) region or isthmus. The MHB region has an organizer function in the developing brain since tissue from the MHB region transplanted to the diencephalon or the rhombencephalon induces the cells surrounding it to develop mesencephalic fates in the diencephalon or cerebellar fates in the rhombencephalon. A number of transcription factors and signaling molecules are expressed in the presumptive MHB region, and these generate a complex genetic network, which establishes and maintains organizer features (reviewed in [8,14,21]). Among the transcription factors that play a central role in establishment and maintenance of the MHB region are those encoded by the Otx2, Gbx2, and Pax2/5/8 genes. During gastrulation and early neurulation, the homeobox genes Otx2 and gastrulation brain homeobox 2 (Gbx2) are expressed in mutually exclusive domains that lie anterior and posterior to the presumptive MHB. When the level of Otx gene expression is genetically reduced, the anterior margin of Gbx2 expression shifts to a more anterior position. Conversely, in Gbx mutant mutant brains, Otx2 expression is shifted to a more posterior position. These and other experiments indicate that Otx2 and Gbx2 control the induction and positioning of the MHB [20,16,9]. The genes encoding transcription factors of the Pax2/5/8 families delimit the intermediate expression domain at the interface of the Otx2 and Gbx2 expression domains that coincides with the presumptive MHB. Mutational
inactivation of either Pax2 or Pax5 alone or a combination of double mutants leads to partial or complete deletion of midbrain and cerebellum structures.
3. Evidence for a tripartite brain in Drosophila The central role of the MHB region in vertebrate brain development together with the conserved expression patterns of Pax2/5/8 genes in a corresponding embryonic brain region in urochordate ascidians have led to the proposal that a fundamental characteristic of the ancestral chordate brain was its tripartite organization characterized by Otx, Pax2/5/8 and Hox gene expressing regions [19]. To investigate whether protostomes also possess this type of tripartite organization of the brain, and to gain insight into the evolution of the bilaterian brain, a comparative analysis of expression and function of the orthologues that pattern the vertebrate MHB region has been carried out in the ecdysozoan Drosophila melanogaster. This investigation reveals striking similarities to the situation in vertebrates, suggesting that a tripartite ground plan is also found in the insect brain [6]. The Drosophila genome contains two genes, Pox neuro (Pox-n) and Drosophila Pax2 (DPax2), that are together considered to be orthologues of the Pax2/5/8 group [11] and one Gbx2 orthologous gene called unplugged (unpg) [4]. Remarkably, the Pax2/5/8 orthologues Pox-n and DPax2 are expressed in the embryonic fly brain in a transversal boundary region located posterior to the otd expression domain in the deutocerebral neuromere and anterior to the homeotic gene expression domain in the tritocerebral neuromere; this transversal domain is the detocerebral–tritocerebral boundary (DTB) region (Fig. 2). Furthermore, the Gbx2 orthologue unpg is expressed in the posterior deutocerebrum and its
H. Reichert / Brain Research Bulletin 66 (2005) 491–494
493
Fig. 2. A transversal domain of adjacent Pax2/5/8 orthologue expression at the DTB. Pox neuro (Poxn) and D-Pax2 are expressed in distinct domains at the DTB (deutocerebral/tritocerebral boundary) region in the embryonic brain. Laser confocal microscopy of P{lacZ}D-Pax2∆122 /+ stage 13/14 embryos, reconstructions of optical sections. Lateral views. Immunolabeling with anti HRP (red) to reveal neurons (left image). Triple-immunolabeling with anti-HRP (red), anti-Gal (green, yellow) and anti-Poxn (blue) to reveal neurons as well as Pax2 and Poxn expression (right image). Both images are the same optical section of the same preparation; arrow indicates the location of the DTB.
Fig. 3. Tripartite organization of the Drosophila and mouse brain based on expression patterns of orthologous genes. Schematic diagram of the expression of otd/Otx, unpg/Gbx2, Pax2/5/8, and Hox1 gene orthologues in the developing CNS of a stage 13/14 Drosophila embryo and a stage E10 mouse embryo. In both cases, a Pax2/5/8 expressing domain is located between an anterior otd/Otx expressing region and a posterior Hox expressing region in the embryonic brain, and a Pax2/5/8 expressing domain is positioned at the interface between the otd/Otx expression domain and a posteriorly abutting unplugged/Gbx2 expression domain. This otd/Otx-unpg/Gbx2 interface displays similar developmental genetic features in both Drosophila and mouse.
494
H. Reichert / Brain Research Bulletin 66 (2005) 491–494
anterior-most expression in the brain coincides with the posterior border of otd expression. There are no cells found that co-express otd and unpg and thus, these domains exclude each other. Comparable to the situation in the vertebrates, in otd loss-of-function mutants unpg expression is shifted anteriorly. Null mutants for unpg, in contrast, reveal posteriorly shifted otd expression extending into the posterior deutocerebrum. Thus, in both Drosophila and vertebrates the otd/Otx2 and unpg/Gbx2 genes appear to negatively regulate each other at the interface of their expression domains in the DTB boundary zone located between anterior and posterior brain regions. In contrast to vertebrates, mutational inactivation of the Pox-n or DPax2 does not result in obvious brain defects in Drosophila [6].
4. An urbilaterian origin of the tripartite brain The similarities between insects and vertebrates at the level of gene expression and functional interactions of otd/Otx, unpg/Gbx2, Pax2/5/8 and Hox genes are striking and are likely to represent further examples of evolutionary conservation of brain patterning mechanisms. Moreover, these developmental genetic similarities indicate that the tripartite ground plan (anterior brain, boundary region, posterior brain), which characterizes the developing chordate brain, is also at the basis of the developing insect brain (Fig. 3). It is conceivable that the similarities of orthologous gene expression patterns and functional interactions in brain development evolved independently in insects and vertebrates. However, a more reasonable explanation is that an evolutionarily conserved genetic program underlies brain development in all bilaterians. This would imply that the generation of structural diversity in the embryonic brain is based on positional information that has been invented only once during evolution and is provided by genes like otd/Otx2, unpg/Gbx2, Pax2/5/8 and Hox, conferring on all bilaterians a common basic principle of brain development. If this is the case, one would predict that comparable orthologous gene expression and function should also characterize embryonic brain development in other invertebrate lineages such as the lophotrochozoans. Taken together, the results of comparative developmental genetic studies indicate that the tripartite ground plan that characterizes the developing chordate brain is also present in the developing insect brain. This suggests that a corresponding tripartite organization already existed in the brain of the last common urbilaterian ancestor of insects and chordates.
Acknowledgement This work was supported by the Swiss NSF.
References [1] D. Arendt, K. N¨ubler-Jung, Comparison of early nerve cord development in insects and vertebrates, Development 126 (1999) 2309– 2325. [2] A.E. Bruce, M. Shankland, Expression of the head gene Lox22-Otx in the leech Helobdella and the origin of the bilaterian body plan, Dev. Biol. 201 (1998) 101–112. [3] R.C. Brusca, G.J. Brusca, Invertebrates, Sinauer Associates, Sunderland, MA, 1990. [4] C. Chiang, K.E. Young, P.A. Beachy, Control of Drosophila tracheal branching by the novel homeodomain gene unplugged, a regulatory target for genes of the bithorax complex, Development 121 (1995) 3901–3912. [5] F. Hirth, H. Reichert, Conserved genetic programs in insect and mammalian brain development, BioEssays 21 (1999) 677– 684. [6] F. Hirth, L. Kammermeier, E. Frei, U. Walldorf, M. Noll, H. Reichert, An urbilaterian origin of the tripartite brain: developmental insights from Drosophila, Development 130 (2003) 2365– 2372. [7] L.Z. Holland, N.D. Holland, Chordate origins of the vertebrate central nervous system, Curr. Opin. Neurobiol. 9 (1999) 596– 602. [8] A. Liu, A.L. Joyner, Early anterior/posterior patterning of the midbrain and cerebellum, Annu. Rev. Neurosci. 24 (2001) 869–896. [9] J.P. Martinez-Barbera, M. Signore, P. Pilo Boyl, E. Puelles, D. Acampora, R. Gogoi, F. Schubert, A. Lumsden, A. Simeone, Regionalisation of anterior neuroectoderm and its competence in responding to forebrain and midbrain inducing activities depend on mutual antagonism between OTX2 and GBX2, Development 128 (2001) 4789–4800. [10] C. Nielsen, Animal Evolution—Interrelationships of the Living Phyla, Oxford University Press, Oxford, 2001. [11] M. Noll, Evolution and role of Pax genes, Curr. Opin. Genet. Dev. 3 (1993) 595–605. [12] H. Reichert, Conserved genetic mechanisms for embryonic brain patterning, Int. J. Dev. Biol. 46 (2002) 81–87. [13] H. Reichert, A. Simeone, Conserved usage of gap and homeotic genes in patterning the CNS, Curr. Opin. Neurobiol. 9 (1999) 589–595. [14] M. Rhinn, M. Brand, The midbrain–hindbrain boundary organizer, Curr. Opin. Neurobiol. 11 (2001) 34–42. [15] R. Siewing, Lehrbuch der Zoologie, Gustav Fischer, Stuttgart, 1985. [16] A. Simeone, Positioning the isthmic organizer where Otx2 and Gbx2 meet, Trends Genet. 16 (2000) 237–240. [17] S.G. Sprecher, H. Reichert, The urbilaterian brain: developmental insights into the evolutionary origin of the brain in insects and vertebrates, Arthropod Struct. Funct. 32 (2003) 141–156. [18] H. Wada, N. Satoh, Patterning the protochordate neural tube, Curr. Opin. Neurobiol. 11 (2001) 16–21. [19] H. Wada, H. Saiga, N. Satoh, P.W. Holland, Tripartite organization of the ancestral chordate brain and the antiquity of placodes: insights from ascidian Pax-2/5/8, Hox and Otx genes, Development 125 (1998) 1113–1122. [20] K.M. Wassarman, M. Lewandoski, K. Campbell, A.L. Joyner, J.L.R. Rubenstein, S. Martinez, G.R. Martin, Specification of the anterior hindbrain and establishment of a normal mid/hindbrain organizer is dependent on Gbx2 gene function, Development 124 (1997) 2923–2934. [21] W. Wurst, L. Bally-Cuif, Neural plate patterning: upstream and downstream of the isthmic organizer, Nat. Rev. Neurosci. 2 (2001) 99–108.