Developmental change in the steroid hormone signal for cell-autonomous, segment-specific programmed cell death of a motoneuron.

During metamorphosis of the hawkmoth, Manduca sexta, accessory planta retractor (APR) motoneurons undergo a segment-specific pattern of programmed cell death (PCD): e.g., APRs from abdominal segment six [APR(6)s] die at pupation in direct response to the prepupal rise in 20-hydroxyecdysone (20E), whereas APR(4)s survive through the pupal stage and die at eclosion (adult emergence). The hypothesis that the death of APR(4)s is triggered by the decline in 20E at eclosion was supported by findings that injection of 20E into developing pupae to delay the fall in 20E delayed APR(4) death. Furthermore, abdomen isolation to advance the fall in 20E caused precocious APR(4) death. In other experiments, APR(4)s were placed in primary cell culture 4 days before eclosion in medium containing 1 microg/ml 20E. A switch to hormone-free medium induced PCD in a significant proportion of APR(4)s, compared to APR(4)s that remained in 20E. Process fragmentation was a reliable early indicator of PCD. These results show that a decline in 20E triggers cell-autonomous PCD of APR(4)s, in contrast to the rise in 20E that triggers cell-autonomous PCD of APR(6)s. Thus, the PCD of homologous motoneurons in different body segments at different developmental times is triggered by different steroid hormone signals.     

Segment-specific muscle degeneration is triggered directly by a steroid hormone during insect metamorphosis

During metamorphosis of the hawkmoth, Manduca sexta, some larval muscles degenerate while others are respecified for new functions. In larvae, accessory planta retractor muscles (APRMs) are present in abdominal segments 1 to 6 (A1 to A6). APRMs serve as proleg retractors in A3 to A6 and body wall muscles in A1 and A2. At pupation, all APRMs degenerate except those in A2 and A3, which are respecified to circulate hemolymph in pupae. The motoneurons that innervate APRMs, the APRs, likewise undergo segment-specific programmed cell death (PCD), as a direct, cell-autonomous response to the prepupal peak of ecdysteroids. The segment-specific patterns of APR and APRM death differ. The present study tested the hypothesis that APRM death is a direct, cell-autonomous response to the prepupal peak of ecdysteroids. Prevention of the prepupal peak prevented APRM degeneration, and replacement of the peak by infusion of 20-hydroxyecdysone restored the correct segment-specific pattern of APRM degeneration. Surgical denervation of APRMs did not perturb their segment-specific degeneration at pupation, indicating that signals from APRs are not required for the muscles' segment-specific responses to ecdysteroids. The possibility that instructive signals originate from APRMs' epidermal attachment points was tested by treating the epidermis with a juvenile hormone analog to prevent pupal development. This manipulation likewise did not alter APRM fate. We conclude that both the muscles and motoneurons in this motor system respond directly and cell-autonomously to prepupal ecdysteroids to produce a segment-specific pattern of PCD that is matched to the functional requirements of the pupal body.     

Programmed cell death of an identified motoneuron in vitro: Temporal requirements for steroid exposure and protein synthesis

Ecdysteroid hormones trigger the programmed cell death (PCD) of a segmental subset of accessory planta retractor (APR) motoneurons at pupation in the moth, Manduca sexta. APRs from abdominal segment four [APR(4)s] survive through the pupal stage, whereas homologous APR(6)s die 24–48 h after pupal ecdysis (PE) (the shedding of the larval cuticle), in response to the prepupal peak of ecdysteroids. Following retrograde labeling with the vital fluorescent dye, DiI, the morphology of APR(4)s and APR(6)s in vivo was examined at PE and 24–48 h later. During this period, APR(4) somata remained large and ovoid while APR(6)s somata became shrunken and rounded. Similar phenotypes were observed when DiI-labeled APRs were cultured at PE and examined 24 h to 1 week later. During initial shrinkage and rounding of APR(6)s, the plasma membrane remained intact but DNA condensation occurred and mitochondrial activity was lost. The requirements for ecdysteroids and new protein synthesis for APR(6) death were tested by culturing cells with ecdysteroids and cycloheximide (CHX). When cultured at PE, the death of APR(6)s was independent of further exposure to ecdysteroids and could not be blocked by CHX. In contrast, APR(6)s cultured ∼24 h earlier required additional exposure to ecdysteroids to die and their death was inhibited by CHX. Thus, the final 24 h of larval life represents an important transition period in the commitment of APR(6)s to undergo PCD, and is of interest for pursuing underlying mechanisms of steroid-induced PCD. © 1998 John Wiley & Sons, Inc. J Neurobiol 35: 300–322, 1998     

Respecification of larval proleg motoneurons during metamorphosis of the tobacco hornworm, Manduca sexta: segmental dependence and hormonal regulation

The principal locomotory appendages of the Manduca sexta caterpillar, the prolegs, are present on the third through sixth abdominal segments (anal prolegs located on the terminal segment were not included in this study). Previous studies have characterized some of the proleg retractor muscles and their motoneurons. In the present study we identified additional proleg motoneurons and their putative homologs in the non-proleg-bearing segments. One of the motoneurons present in the proleg-bearing segments is absent in the non-proleg-bearing segments. At pupation the prolegs are lost, their muscles degenerate, and some of their motoneurons regress structurally. Subsequently, subsets of the proleg motoneurons and their homologs in other segments die in a segment-specific pattern. This is the first report of segment-specific motoneurons, and of segment-specific death of identified motoneurons, in Manduca. During adult development the surviving proleg motoneurons innervate the tergosternal muscle (TSM) and grow bilateral dendritic arbors. Dendritic growth is completed by about the 12th of the 18 days of adult development. Following adult emergence all but one of the respecified proleg motoneurons dies. The hormonal dependence of dendritic outgrowth was tested by isolating abdomens to eliminate the ecdysteroid-secreting glands in the thorax. Between the second and fifth days after pupation the motoneurons became progressively more competent to undergo dendritic outgrowth following abdomen isolation. The extent of dendritic outgrowth paralleled the degree of morphological development attained by isolated abdomens. It is concluded that ecdysteroids are required for motoneuron outgrowth, but our findings suggest that, unless an abdominal source of ecdysteroids exists in pupae, a relatively small exposure may be sufficient.     

Segment‐specific muscle degeneration is triggered directly by a steroid hormone during insect metamorphosis

During metamorphosis of the hawkmoth, Manduca sexta, some larval muscles degenerate while others are respecified for new functions. In larvae, accessory planta retractor muscles (APRMs) are present in abdominal segments 1 to 6 (A1 to A6). APRMs serve as proleg retractors in A3 to A6 and body wall muscles in A1 and A2. At pupation, all APRMs degenerate except those in A2 and A3, which are respecified to circulate hemolymph in pupae. The motoneurons that innervate APRMs, the APRs, likewise undergo segment‐specific programmed cell death (PCD), as a direct, cell‐autonomous response to the prepupal peak of ecdysteroids. The segment‐specific patterns of APR and APRM death differ. The present study tested the hypothesis that APRM death is a direct, cell‐autonomous response to the prepupal peak of ecdysteroids. Prevention of the prepupal peak prevented APRM degeneration, and replacement of the peak by infusion of 20‐hydroxyecdysone restored the correct segment‐specific pattern of APRM degeneration. Surgical denervation of APRMs did not perturb their segment‐specific degeneration at pupation, indicating that signals from APRs are not required for the muscles' segment‐specific responses to ecdysteroids. The possibility that instructive signals originate from APRMs' epidermal attachment points was tested by treating the epidermis with a juvenile hormone analog to prevent pupal development. This manipulation likewise did not alter APRM fate. We conclude that both the muscles and motoneurons in this motor system respond directly and cell‐autonomously to prepupal ecdysteroids to produce a segment‐specific pattern of PCD that is matched to the functional requirements of the pupal body. © 2004 Wiley Periodicals, Inc. J Neurobiol, 2005     

Role of caspases and mitochondria in the steroid-induced programmed cell death of a motoneuron during metamorphosis

Accessory planta retractor (APR) motoneurons of the hawk moth, Manduca sexta, undergo a segment-specific pattern of programmed cell death (PCD) 24 to 48 h after pupal ecdysis (PE). Cell culture experiments show that the PCD of APRs in abdominal segment 6 [APR(6)s] is a cell-autonomous response to the steroid hormone 20-hydroxyecdysone (20E) and involves mitochondrial demise and cell shrinkage. Twenty-four hours before PE, at stage W3-noon, APR(6)s require further 20E exposure and protein synthesis (as tested with cycloheximide) to undergo PCD, and death can be blocked by a broad-spectrum caspase inhibitor. By PE, death is 20E- and protein synthesis-independent and the caspase inhibitor blocks cell shrinkage but not loss of mitochondrial function. Thus, the commitment to mitochondrial demise precedes the commitment to execution events. The phenotype of necrotic cell death induced by a mitochondrial electron transfer inhibitor differs unambiguously from 20E-induced PCD. By inducing PCD pharmacologically, the readiness of APR(6)s to execute PCD was found to increase during the final larval instar. These data suggest that the 20E-induced PCD of APR(6)s includes a premitochondrial phase which includes 20E-induced synthetic events and apical caspase activity, a mitochondrial phase which culminates in loss of mitochondrial function, and a postmitochondrial phase during which effector caspases are activated and APR(6) is destroyed.     

Influence of interganglionic interactions on steroid-mediated dendritic reorganization and death of proleg motor neurons during metamorphosis in Manduca sexta

In Manduca sexta, the larval abdominal prolegs and their muscles degenerate at pupation. The proleg motor neurons undergo a period of dendritic regression, after which a specific subset of them dies. The surviving motor neurons undergo dendritic outgrowth during pupal-adult development, and most die after adult emergence. All of these events are regulated hormonally by ecdysteroids and juvenile hormone, but interactions of the motor neurons with other cells may potentially contribute as well. To investigate the possible influence of interganglionic neural interactions, we chronically isolated individual abdominal ganglia by severing the adjacent rostral and caudal connectives in the larval stage. Subsequent metamorphic changes in proleg motor neurons were examined in the isolated ganglia and ganglia adjacent to the isolated ganglia. Two abnormalities were observed: (1) some imprecision in the timing of motor neuron death, both at pupation and after adult emergence, and (2) the growth of ectopic neurites outside the neuropil boundaries during pupal-adult development (in ganglia with or without neuromas caused by connective transections). Other aspects of proleg motor neuron metamorphosis, including the segment-specific death of motor neurons at pupation, were the same as that in intact and sham-operated insects. Thus, interganglionic interactions appear to play a relatively minor role in the steroid-mediated metamorphic transformation of proleg motor neurons. © 1994 John Wiley & Sons, Inc.     

Segment-specific retention of a larval neuromuscular system and its role in a new, rhythmic, pupal motor pattern in Manduca sexta

At pupation in Manduca sexta, accessory planta retractor muscles and their motoneurons degenerate in segment-specific patterns. Accessory planta retractor muscles in abdominal segments 2 and 3 survive in reduced form through the pupal stage and degenerate after adult emergence. Electromyographic and electrophysiological recordings show that these accessory planta retractor muscles participate in a new, rhythmic `pupal motor pattern' in which all four muscles contract synchronously at ∼4 s intervals for extended bouts. Accessory planta retractor muscle contractions are driven by synaptic activation of accessory planta retractor motoneurons and are often accompanied by rhythmic activity in intersegmental muscles and spiracular closer muscles. The pupal motor pattern is influenced by descending neural input although isolated abdominal ganglia can produce a pupal motor pattern-like rhythm. The robust pupal motor pattern first seen after pupal ecdysis weakens during the second half of pupal life. Anemometric recordings indicate that the intersegmental muscle and spiracular closer muscle component of the pupal motor pattern produces ventilation. Accessory planta retractor muscle contractions lift the flexible abdominal floor, to which the developing wings and legs adhere tightly. We hypothesize that, by a bellows-like action, the accessory planta retractor muscle contractions circulate hemolymph in the appendages. Morphometric analysis shows that dendritic regression is similar in accessory planta retractor motoneurons with different pupal fates, and that accessory planta retractor motoneurons begin to participate in the pupal motor pattern while their dendrites are regressed. Accepted: 29 March 1998     

Programmed cell death in the embryonic central nervous system of Drosophila melanogaster

Although programmed cell death (PCD) plays a crucial role throughout Drosophila CNS development, its pattern and incidence remain largely uninvestigated. We provide here a detailed analysis of the occurrence of PCD in the embryonic ventral nerve cord (VNC). We traced the spatio-temporal pattern of PCD and compared the appearance of, and total cell numbers in, thoracic and abdominal neuromeres of wild-type and PCD-deficient H99 mutant embryos. Furthermore, we have examined the clonal origin and fate of superfluous cells in H99 mutants by DiI labeling almost all neuroblasts, with special attention to segment-specific differences within the individually identified neuroblast lineages. Our data reveal that although PCD-deficient mutants appear morphologically well-structured, there is significant hyperplasia in the VNC. The majority of neuroblast lineages comprise superfluous cells, and a specific set of these lineages shows segment-specific characteristics. The superfluous cells can be specified as neurons with extended wild-type-like or abnormal axonal projections, but not as glia. The lineage data also provide indications towards the identities of neuroblasts that normally die in the late embryo and of those that become postembryonic and resume proliferation in the larva. Using cell-specific markers we were able to precisely identify some of the progeny cells, including the GW neuron, the U motoneurons and one of the RP motoneurons, all of which undergo segment-specific cell death. The data obtained in this analysis form the basis for further investigations into the mechanisms involved in the regulation of PCD and its role in segmental patterning in the embryonic CNS.     

Antagonistic roles for Ultrabithorax and Antennapedia in regulating segment-specific apoptosis of differentiated motoneurons in the Drosophila embryonic central nervous system

The generation of morphological diversity among segmental units of the nervous system is crucial for correct matching of neurons with their targets and for formation of functional neuromuscular networks. However, the mechanisms leading to segment diversity remain largely unknown. We report here that the Hox genes Ultrabithorax (Ubx) and Antennapedia (Antp) regulate segment-specific survival of differentiated motoneurons in the ventral nerve cord of Drosophila embryos. We show that Ubx is required to activate segment-specific apoptosis in these cells, and that their survival depends on Antp. Expression of the Ubx protein is strongly upregulated in the motoneurons shortly before they undergo apoptosis, and our results indicate that this late upregulation is required to activate reaper-dependent cell death. We further demonstrate that Ubx executes this role by counteracting the function of Antp in promoting cell survival. Thus, two Hox genes contribute to segment patterning and diversity in the embryonic CNS by carrying out opposing roles in the survival of specific differentiated motoneurons.     

Segmentally distributed metamorphic changes in neural circuits controlling abdominal bending in the hawkmoth Manduca sexta

During the metamorphosis of Manduca sexta the larval nervous system is reorganized to allow the generation of behaviors that are specific to the pupal and adult stages. In some instances, metamorphic changes in neurons that persist from the larval stage are segment-specific and lead to expression of segment-specific behavior in later stages. At the larval-pupal transition, the larval abdominal bending behavior, which is distributed throughout the abdomen, changes to the pupal gin trap behavior which is restricted to three abdominal segments. This study suggests that the neural circuit that underlies larval bending undergoes segment specific modifications to produce the segmentally restricted gin trap behavior. We show, however, that non-gin trap segments go through a developmental change similar to that seen in gin trap segments. Pupal-specific motor patterns are produced by stimulation of sensory neurons in abdominal segments that do not have gin traps and cannot produce the gin trap behavior. In particular, sensory stimulation in non-gin trap pupal segments evokes a motor response that is faster than the larval response and that displays the triphasic contralateral-ipsilateral-contralateral activity pattern that is typical of the pupal gin trap behavior. Despite the alteration of reflex activity in all segments, developmental changes in sensory neuron morphology are restricted to those segments that form gin traps. In non-gin trap segments, persistent sensory neurons do not expand their terminal arbors, as do sensory neurons in gin trap segments, yet are capable of eliciting gin trap-like motor responses. Accepted: 10 January 1997     

Segment-specific generation of Drosophila Capability neuropeptide neurons by multi-faceted Hox cues

In the Drosophila ventral nerve cord, the three pairs of Capability neuropeptide-expressing Va neurons are exclusively found in the second, third and fourth abdominal segments (A2–A4). To address the underlying mechanisms behind such segment-specific cell specification, we followed the developmental specification of these neurons. We find that Va neurons are initially generated in all ventral nerve cord segments and progress along a common differentiation path. However, their terminal differentiation only manifests itself in A2–A4, due to two distinct mechanisms: segment-specific programmed cell death (PCD) in posterior segments, and differentiation to an alternative identity in segments anterior to A2. Genetic analyses reveal that the Hox homeotic genes are involved in the segment-specific appearance of Va neurons. In posterior segments, the Hox gene Abdominal-B exerts a pro-apoptotic role on Va neurons, which involves the function of several RHG genes. Strikingly, this role of Abd-B is completely opposite to its role in the segment-specific apoptosis of other classes of neuropeptide neurons, the dMP2 and MP1 neurons, where Abd-B acts in an anti-apoptotic manner. In segments A2–A4 we find that abdominal A is important for the terminal differentiation of Va cell fate. In the A1 segment, Ultrabithorax acts to specify an alternate Va neuron fate. In contrast, in thoracic segments, Antennapedia suppresses the Va cell fate. Thus, Hox genes act in a multi-faceted manner to control the segment-specific appearance of the Va neuropeptide neurons in the ventral nerve cord.     

Developmental attenuation of the pre-ecdysis motor pattern in the tobacco hornworm,Manduca sexta

Summary At the culmination of each molt, the larval tobacco hornworm exhibits a pre-ecdysis behavior prior to shedding its old cuticle at ecdysis. Both pre-ecdysis and ecdysis behaviors are triggered by the peptide, eclosion hormone (EH). Pre-ecdysis behavior consists of rhythmic abdominal compressions that loosen the old larval cuticle. This behavior is robust at larval molts, but at the larval-pupal molt the only comparable behavior consists of rhythmic dorso-ventral flexions of the anterior body. These flexions appear to be an attenuated version of the larval pre-ecdysis behavior because (1) they show the same EH dependence, and (2) the motor patterns recorded from EH treated, deafferented larval and pupal preparations are similar except that the pupal pattern is much weaker. Both patterns are characterized by rhythmic, synaptically-driven bursts of action potentials in motoneurons MN-2 and MN-3, which occur synchronously in all segments. However, the synaptic drive to the motoneurons and their resultant levels of activity are reduced during the pupal pre-ecdysis motor pattern, especially in posterior abdominal segments. Although the dendritic arbors of both motoneurons regress somewhat during the larval-pupal transformation, this does not appear to be the primary source of diminished synaptic drive because regression is greatest in the segments in which synaptic inputs remain the strongest. The developmental weakening of the pre-ecdysis motor pattern thus may be due to changes at the interneuronal level.Abbreviations A2, A3... abdominal segments 2, 3, etc. - ALE anterior lateral external muscle - day L3 third day of the 5th larval instar - day P0 the day of pupal ecdysis - DN _a anterior branch of the dorsal nerve - EH eclosion hormone - HPLC high performance liquid chromatography - TP tergopleural muscle     

Possible interactions of a steroid hormone and neural inputs in controlling the death of an identified neuron in the moth Manduca sexta

The emergence of the adult Manduca sexta moth is followed by the loss of almost half of this insect's abdominal motoneurons and interneurons (Truman, 1983). This programmed cell death completes the transformation of the nervous system of the caterpillar into that of the moth. The death of these neurons has been previously shown to be a response to an endocrine signal: the decline in ecdysteroids that occurs at the end of metamorphosis (Truman and Schwartz, 1984). Our current research is focussed on the regulation of the fate of a pair of identified motoneurons, the MN-12 cells, in the third abdominal ganglion. Isolation of this ganglion from anterior parts of the nervous system can prevent the death of these cells at the time when they would normally die in response to the decline in ecdysteroids. Transection of the ventral nerve cord at various levels revealed that the source of this regulatory "death signal" is the fused pterothoracic ganglion and that it is transmitted via the interganglionic connectives. We hypothesize that the factors mediating this effect may act in concert with the ecdysteroid decline to specify the exact time of death for individual neurons.     

Evidence for an endogenous neurocidin in the Manduca sexta ventral nerve cord

Half of the neurons in the abdominal nervous system of the moth Manduca sexta die after adult eclosion. Two physiological signals regulate post-eclosion neuronal death in adult moths. The first is endocrine: a decline in blood ecdysteroids is necessary for the death of neurons in the segmental ganglia. The second signal, which is highly specific for a pair of motoneurons found at the posterior midline in each of the three unfused abdominal ganglia, originates in the nervous system. It is transmitted from the fused pterothoracic ganglion to abdominal ganglion A3 via the intersegmental connectives. To characterize the signal of neural origin, we have developed an in vitro bioassay for neuron-killing factors (“neurocidins”). Aqueous extracts of pterothoracic ganglia were prepared and applied to cultured ventral nerve cords. These extracts exhibited concentration-dependent effectiveness in killing motoneurons. The active component of the extract was heat-stable and protease-sensitive. Size fractionation studies suggested that the active component has a molecular mass between 10 and 30 kD. This is the first report of an endogenous neuron-killing protein from an insect nervous system. © 1995 Wiley-Liss, Inc.     

A motoneuron spared from steroid-activated developmental death by removal of descending neural inputs exhibits stable electrophysiological properties and morphology

Neurons die during the development of nervous systems. The death of specific, idenified motoneuros during metamorphosis of the tobacco hornworm, Manduca sexta, provides an accessible model system in which to study the regulation of postembryonic neuronal death. Hormones and descending neural inputs have been shown toinfluence the survival of abdominal motoneurons during the first few days of adult life in this insect. Motoneurons prevented from undergoing the normal process of developmental degeneration by removal of neural inputs were examined at the physiological and structural levels using several cell imaging techniques. Although these neurons lost their muscle targets and experienced the endocrine cue that normally triggers death, they showed no overt electrophysiological or morphological signs of degeneration. Thus, by appropriate intervention, the MN-12 motoneuron can be spared from developmental neuronal death and remain as a functioning supernumerary element in the mature nervous system. © 1995 John Wiley & Sons, Inc.     

Increase of 30K protein in identified motoneurons by hemolymph results in inhibition of programmed cell death in silkworm, Bombyx mori (Lepidoptera, Bombycidae)

This study demonstrates that a 30K protein was gradually synthesized in primary-cultured motoneurons from the accessory planta retractor (APR) of the 6th abdominal ganglion (APR6) in silkworm ventral ganglia through stimulation of hemolymph. An increase in 30K protein synthesis resulted in an inhibition of programmed cell death (PCD) of APR6 motoneurons. The 30K protein was gradually synthesized from the 30Kc6 gene of identified APR6s in day-6 4th instars to day-9 5th instar larvae, but synthesis of the 30K protein ceased in isolated APR6s of day-1 pupa, which normally begin to undergo PCD. When pupal APR6s were treated with larval hemolymph, however, the 30K protein was synthesized suggesting the existence of an anti-PCD factor in the larval hemolymph. An increase of 30K protein within the APR6s was confirmed by antiserum made against the recombinant 30K protein that originated from the APR 30Kc6 gene. Larval APR6, in which PCD was induced with 20-hydroxyecdysone (20E) added to the primary culture, exhibited some PCD characteristics of shrinkage of cell bodies, axonal fragmentation and loss of mitochondrial function. These results provide new insights on the survival or PCD of insect motoneurons through stimulation of hemolymph.     

Roles of Hox genes in the patterning of the central nervous system of Drosophila

One of the key aspects of functional nervous systems is the restriction of particular neural subtypes to specific regions, which permits the establishment of differential segment-specific neuromuscular networks. Although Hox genes play a major role in shaping the anterior-posterior body axis during animal development, our understanding of how they act in individual cells to determine particular traits at precise developmental stages is rudimentary. We have used the abdominal leucokinergic neurons (ABLKs) to address this issue. These neurons are generated during both embryonic and postembryonic neurogenesis by the same progenitor neuroblast, and are designated embryonic and postembryonic ABLKs, respectively. We report that the genes of the Bithorax-Complex, Ultrabithorax (Ubx) and abdominal-A (abd-A) are redundantly required to specify the embryonic ABLKs. Moreover, the segment-specific pattern of the postembryonic ABLKs, which are restricted to the most anterior abdominal segments, is controlled by the absence of Abdominal-B (Abd-B), which we found was able to repress the expression of the neuropeptide leucokinin. We discuss this and other examples of how Hox genes generate diversity within the central nervous system of Drosophila.Keyword: development, Hox genes, central nervous system, Drosophila, cell fate specification     

Balancing sex chromosome expression and satisfying the sexes

One of the key aspects of functional nervous systems is the restriction of particular neural subtypes to specific regions, which permits the establishment of differential segment-specific neuromuscular networks. Although Hox genes play a major role in shaping the anterior-posterior body axis during animal development, our understanding of how they act in individual cells to determine particular traits at precise developmental stages is rudimentary. We have used the abdominal leucokinergic neurons (ABLKs) to address this issue. These neurons are generated during both embryonic and postembryonic neurogenesis by the same progenitor neuroblast, and are designated embryonic and postembryonic ABLKs, respectively. We report that the genes of the Bithorax-Complex, Ultrabithorax (Ubx) and abdominal-A (abd-A) are redundantly required to specify the embryonic ABLKs. Moreover, the segment-specific pattern of the postembryonic ABLKs, which are restricted to the most anterior abdominal segments, is controlled by the absence of Abdominal-B (Abd-B), which we found was able to repress the expression of the neuropeptide leucokinin. We discuss this and other examples of how Hox genes generate diversity within the central nervous system of Drosophila.