A phenylalanine hydroxylase gene from the nematode C. elegans is expressed in the hypodermis.

We have identified an aromatic amino acid hydroxylase gene from the nematode C. elegans that likely encodes the worm phenylalanine hydroxylase (PheH). The predicted amino acid sequence is most similar to that of other PheH and TrpH proteins. Reporter gene fusions and staining with an antibody to mammalian PheH indicate the gene is expressed in hypodermal cells. A fusion protein expressed in bacteria can convert phenylalanine to tyrosine, and, to a lesser extent, tryptophan to 5-hydroxytryptophan. We hypothesize that the protein is necessary to produce additional tyrosine for protein cross-linking in the nematode cuticle.

Serotonin-deficient mutants and male mating behavior in the nematode Caenorhabditis elegans.

Defining a behavior that requires the function of specific neurons in the free-living nematode Caenorhabditis elegans can allow one to screen for mutations that disrupt the specification or function of those neurons. We identified serotonin-immunoreactive neurons required for tail curling or "turning" behavior exhibited by C. elegans males during mating. Males mutant in three different genes that reduce serotonin expression, cat-1, cat-4, and bas-1, exhibited defects in turning behavior similar to those of wild-type males in which these neurons were ablated. The turning defect of cat-4 males was rescued by exogenous serotonin, consistent with the idea that their behavioral defect is caused by a lack of serotonin. While the serotonin-deficient mutants we analyzed shared certain behavioral traits, they were blocked for serotonin synthesis at different steps. Analysis of these and additional serotonin-deficient mutants may help us understand how a neuron controls the expression of a serotonergic phenotype.

Multiple HOM-C gene interactions specify cell fates in the nematode central nervous system.

Intricate patterns of overlapping HOM-C gene expression along the A/P axis have been observed in many organisms; however, the significance of these patterns in establishing the ultimate fates of individual cells is not well understood. We have examined the expression of the Caenorhabditis elegans Antennapedia homolog mab-5 and its role in specifying cell fates in the posterior of the ventral nerve cord. We find that the pattern of fates specified by mab-5 not only depends on mab-5 expression but also on post-translational interactions with the neighboring HOM-C gene lin-39 and a second, inferred gene activity. Where mab-5 expression overlaps with lin-39 activity, they can interact in two different ways depending on the cell type: They can either effectively neutralize one another where they are both expressed or lin-39 can predominate over mab-5. As observed for Antennapedia in Drosophila, expression of mab-5 itself is repressed by the next most posterior HOM-C gene, egl-5. Thus, a surprising diversity in HOM-C regulatory mechanisms exists within a small set of cells even in a simple organism.

Mesenchyme of embryonic reproductive ducts directs process outgrowth of Retzius neurons in the medicinal leech.

In the two segments of the medicinal leech (Hirudo medicinalis) that contain the male (segment 5) and the female (segment 6) reproductive ducts, the paired Retzius (Rz) neurons are distinguished by several unique properties. For example, the muscles and glands of the body wall are the primary peripheral targets of Rz neurons in standard segments [Rz(X)], whereas the muscles and glands of the reproductive ducts are the primary peripheral targets of Rz neurons in the two reproductive segments [Rz(5,6)]. In this paper, we show that organogenesis and differentiation, which generate an epithelial tube surrounded by mesenchymal cells, occur in the embryonic reproductive ducts at approximately the time when Rz processes first contact these structures. The growth cones leading one branch of the posterior axon of Rz(5,6) contact the duct mesenchymal cells. Following initiation of this contact, these posterior growth cones enlarge and send out numerous filopodia. Secondarily, growth cones leading the anterior axon of each Rz(5,6) also modify their shapes and trajectories. When embryonic reproductive ducts were transplanted into posterior (nonreproductive) segments, the branch of the posterior Rz axon near the ectopic reproductive tissue produced enlarged growth cones and extended several secondary branches into the mesenchyme of the ectopic tissue. This result suggests that the reproductive mesenchyme is attractive to, and can modify the growth of, all Rz neurons. The behavior of Rz(5,6) growth cones suggests that the reproductive mesenchyme cells provide guidance cues that control the location in which Rz axons elaborate their peripheral arborization and form synapses, and that the mesenchyme may also stimulate the production of a densely branched arbor.

Segmental specialization of neuronal connectivity in the leech.

1. Every segmental ganglion of the leech Hirudo medicinalis contains two serotonergic Retzius cells. However, Retzius cells in the two segmental ganglia associated with reproductive function are morphologically distinct from Retzius cells elsewhere. This suggested that these Retzius cells might be physiologically distinct as well. 2. The degree of electrical coupling between Retzius cells distinguishes the reproductive Retzius cells; all Retzius cells are coupled in a non-rectifying manner, but reproductive Retzius cells are less strongly coupled. 3. Retzius cells in standard ganglia depolarize following swim motor pattern initiation or mechanosensory stimulation while Retzius cells in reproductive ganglia either do not respond or hyperpolarize. 4. In standard Retzius cells the depolarizing response caused by pressure mechanosensory neurons has fixed latency and one-to-one correspondence between the mechanosensory neuron action potentials and Retzius cell EPSPs. However, the latency is longer than for most known monosynaptic connections in the leech. 5. Raising the concentration of divalent cations in the bathing solution to increase thresholds abolishes the mechanosensory neuron-evoked EPSP in standard Retzius cells. This suggests that generation of action potentials in an interneuron is required for production of the EPSP, and therefore that the pathway from mechanosensory neuron to Retzius cell is polysynaptic. 6. P cells in reproductive segments have opposite effects on reproductive Retzius cells and standard Retzius cells in adjacent ganglia. Thus the difference in the pathway from P to Retzius is not localized specifically in the P cell, but elsewhere in the pathway, possibly in the type of receptor expressed by the Retzius cells.

Central synaptic inputs to identified leech neurons determined by peripheral targets.

Developing Retzius (Rz) neurons in different segments of the central nervous system of the medicinal leech have different peripheral targets: Rz cells in standard segments innervate the body wall, whereas Rz cells in the reproductive segments innervate reproductive tissue. Early removal of reproductive tissue primordia causes reproductive Rz cells to develop morphologically like their standard segmental homologs, suggesting that Rz cells depend on peripheral targets for signals that determine their central and peripheral morphology. Furthermore, after removal of reproductive tissue, reproductive Rz cells also receive synaptic inputs normally appropriate for standard Rz cells. These results suggest that the functional identity of these neurons is specified by the target they contact during embryogenesis.

Peripheral target choice by homologous neurons during embryogenesis of the medicinal leech. I. Segment-specific preferences of Retzius cells.

A pair of large serotonergic neurons, the Retzius (Rz) cells, is found in each segment of the leech nervous system. Most Rz cells innervate the body wall of their own segment as well as adjacent anterior and posterior segments. Rz cells in segments 5 and 6 [Rz (5,6)] instead innervate the reproductive tissue found only in those segments. Rz cells from adjacent segments [Rz (4,7)] provide the serotonergic innervation of the body wall of segments 5 and 6. During embryogenesis, the body wall and the reproductive tissue are apparently available to both Rz (5,6) and Rz (4,7), yet these neurons choose different targets. We asked how Rz (5,6) and Rz (4,7) choose their respective peripheral targets in the reproductive segments by ablating either the reproductive tissue or specific Rz cells. Ablation of the reproductive tissue caused Rz (5,6) to innervate body wall, although not as proficiently as did standard Rz cells, suggesting a preference of Rz (5,6) for reproductive tissue. Ablation of those Rz cells that would normally innervate the body wall of segments 5 and 6 did not cause Rz (5,6) to innervate body wall, ruling out competition for this target. When Rz (5,6) were ablated, Rz (4,7) innervated the body wall of segments 5 and 6 normally and did not innervate reproductive tissue. Thus, competition did not act in the choice of target by Rz (4,7) either. These results suggest that during normal development, Rz (5,6) and Rz (4,7) choose their targets independently of one another rather than competing for the available targets and that these cells have segment-specific target preferences.

Peripheral target choice by homologous neurons during embryogenesis of the medicinal leech. II. Innervation of ectopic reproductive tissue by nonreproductive Retzius cells.

Most Retzius (Rz) cells innervate the body wall of their own and adjacent segments, whereas Rz cells in segments 5 and 6 [Rz (5,6)] innervate the reproductive tissue, which is found only in those segments. Results from the preceding paper (Loer and Kristan, 1989a) showed that Rz (5,6) and standard Rz cells do not normally compete for their respective peripheral targets. These experiments did not, however, distinguish between 2 other possible mechanisms of target selection: intrinsic differences in target preference or differences in the timing of target contact. In order to separate these possibilities experimentally, we transplanted reproductive primordia to standard segments. We found that standard Rz cells were capable of densely innervating ectopic reproductive tissue, provided the target was transplanted at an appropriate time and location. Furthermore, after some processes of standard Rz cells contacted ectopic reproductive tissue, the rest of the cell's processes showed their growth in a way reminiscent of Rz (5,6) processes. These results strongly suggest that Rz (5,6) innervate reproductive tissue at least partly because their processes contact this target during a period that is optimal for them to associate with the target, or when the reproductive tissue is most attractive to Rz processes, or both.

Morphological changes in leech Retzius neurons after target contact during embryogenesis.

Segmental variation in identified neurons may provide an opportunity to examine extrinsic influences on neuronal phenotype, since segmentally homologous neurons must contain much the same intrinsic information, having arisen from very similar or identical precursors. Two large serotonergic Retzius (Rz) cells are found in each segmental ganglion of the leech Hirudo medicinalis. While most Rz cells innervate the body wall in their own segment and, by way of axons in the interganglionic connectives, the body wall of adjacent segments, the Rz cells in ganglia 5 and 6 [Rz(5,6)] lack interganglionic axons and innervate only the reproductive tissue (Glover and Mason, 1986). Here we describe and quantify the development of differences between Rz(5,6) and other Rz cells in peripheral innervation, neuropilar arborization, and soma size. We filled individual Rz cells with Lucifer yellow or HRP in adults and in staged embryos. During the first 72 hr of outgrowth of Rz cell processes, the morphology of Rz(5,6) was indistinguishable from that of other Rz cells. Only after the processes of Rz(5,6) reached the reproductive tissue did they begin to differ from their segmental homologs. This temporal correlation suggests that these morphological differences arise because of some interaction between Rz(5,6) and their target tissue.

Segment-specific morphogenesis of leech Retzius neurons requires particular peripheral targets.

In most segments of the leech, a pair of Retzius (Rz) cells innervate the body wall musculature and skin; however, in the segments specialized for reproduction (midbody segments 5 and 6), these neurons innervate the reproductive tissue instead. Whereas all Rz cells have the same morphology early in embryogenesis, those in the reproductive segments [Rz(5,6)] become considerably different from their segmental homologs. Unlike standard Rz cells, Rz(5,6) do not have axons in the interganglionic connectives or in the body wall (Glover and Mason, 1986). Rz(5,6) also have significantly smaller somata and fewer branches in the ganglionic neuropil than do standard Rz cells (Jellies et al., 1987). Since these differences between Rz cells do not become apparent until after Rz(5,6) processes appear to contact the reproductive tissue primordia, interactions between Rz(5,6) processes and the reproductive tissue may determine the segmental specializations of these neurons. We have tested this possibility by ablating the reproductive tissue primordia early in embryogenesis and subsequently examining Rz(5,6) morphology. In the absence of reproductive tissue, Rz(5,6) became more like standard Rz cells: they retained axons in the interganglionic connectives, they projected into the body wall, and the density of their arborization within the neuropil increased. These results indicate that the development of some segmental specializations of Rz(5,6) involves an interaction with their unique target tissue.

Development of segmental differences in the pressure mechanosensory neurons of the leech Haementeria ghilianii.

Using a monoclonal antibody specific for the pressure mechanosensory neurons (P cells) of the leech Haementeria ghilianii, we have examined the segmental differences between P cells in the adult nerve cord, as well as the development of these differences during embryogenesis. The standard segmental ganglion contains two pairs of P cells of about the same size and staining intensity. The sex ganglia appear to be missing the P cells that normally innervate ventral skin, and ganglia 20 and 21 have much smaller ventral P cells than most segments. The pattern of P cells in the head and tail ganglia also differs slightly from that of the standard ganglia. During embryogenesis, when the neurons are first stained by the antibody, there are two pairs of P cells of equal size in each segmental ganglion. Obvious segmental differences arise subsequently, modifying an initially identical set of cells.

Neuronal cell death in grasshopper embryos: variable patterns in different species, clutches, and clones.

Previous studies showed that cell death plays an important role in adjusting the segment-specific number of ganglionic neurones during grasshopper embryogenesis (Bate, Goodman & Spitzer, 1979; Goodman & Bate, 1981). In every segment, the single midline precursor 3 (MP3) divides once to produce two progeny. In some segments, one or both of these two progeny die; there is a general pattern of cell death of the MP3 progeny across the thoracic and abdominal segments. In the present study we examined the pattern of cell survival versus death of the MP3 progeny in 472 embryos from four different species, from the genetically related offspring within different clutches of the same species and from the genetically identical offspring within isogenic clones of the same species. We find variability in the pattern of cell survival versus death amongst embryos of the same species, clutch and clone, suggesting a significant epigenetic influence on this pattern. However, our results also show significant differences in the pattern of cell death between different genera and species, and between different clones and clutches within a single species, suggesting a genetic influence on this pattern as well.