Contractile properties of the articular capsule or ligament,in the primary spines of the sea-urchin Eucidaris tribuloides

1. The articular capsule or ligament, of the primary spines of the sea-urchin Eucidaris tribuloides behaves as a classical excitable tissue, responding with a shortening or contracture to a variety of stimuli including the cholinergic agonists acetylcholine (ACh), methacholine (MeACh), carbamylcholine (CCh) and nicotine.2. d-Tubocurarine failed to decrease the amplitude of the acetylcholine-induced contractures, while the contractures elicited by methacholine were blocked by atropine. Neostigmine, but not eserine, increased the amplitude of ACh-induced contractures, suggesting the presence of AChE in the preparation.3. The cholinergic agonists induced contracture of the ligament, but had quite different kinetics, the rate of rise of the contracture being fastest for ACh and decreasing in the following order: CCh, MeACh and nicotine.4. Tyramine and octopamine exert an inhibitory action on “catch” and a relaxing effect on the contracting ligament.5. The time courses of the contractures elicited in the same preparation were virtually identical to those of “catch”. Therefore, we propose that “catch” and contracture are only two different aspects of the same phenomenon; namely the contracture of the muscle fibers present in the ligament.     

Acetylcholine sensitivity of the spine-test articular capsule of the sea urchin Eucidaris tribuloides

1. The changes in the consistence of the spine-test articular capsule, or ligament, of the primary spines of Eucidaris tribuloides induced by acetylcholine (ACh) have been studied. Two complementary techniques were used: (a) "forced-vibration", which detects variations in the stiffness of the ligament along a single diametral plane; and (b) "forced-rotation" which records the spatial distribution of those changes. 2. ACh (1 microM to 1 mM) caused a rapid increase in the resistive force opposed by the ligament to passive stretching. Similar effects were elicited by several monoquaternary, N-substituted derivatives of trimethylammonium. 3. The opposite effect, i.e. softening, was induced by decamethonium, dimethylphenylpiperazine, and 2-ketoamyltrimethylammonium. 4. The involvement in these effects of ACh-binding groups with pharmacological properties similar to those of the "anionic sites" of nicotinic ACh receptors is suggested.     

Pharmacological sensitivity of the articular capsule of the primary spines of Eucidaris tribuloides

1. This paper describes the effects of several cholinergic agonists and antagonists, and of β-phenylethylamine (PEA) and some of its derivatives, on the articular capsule, or ligament, of the primary spines of Eucidaris tribuloides.2. Carbamylcholine (CCh), methacholine (MeACh), nicotine, and muscarine exert a stiffening effect similar to that of acetylcholine (ACh), although the time course of their actions varies widely.3. Atropine induced stiffening and blocked and responses to muscarine and MeACh. The responses to MeACh were blocked also by 4-diphenylacetoxy-N-methylpiperidine, suggesting the presence in the ligament of type M₃ muscarinic receptors, in addition to nicotinic ones. d-Tubocurarine induced stiffness of the ligament and failed to block the responses to ACh and nicotine.4. While ACh induced only a slight desensitization, CCh caused a long-lasting blockade of the stiffening effects of the cholinergic agonists. This shows that the receptors for ACh have a site or sites that recognize the ester moieties of these molecules.5. Eserine and neostigmine potentiate the responses to acetylcholine, indicating the presence of aeetyl-cholinesterase in the ligament.6. β-Phenylethy lamine, epinephrine, norepinephrine, and dopamine induce diphasic responses; usually a brief softening followed by a slow and irreversible stiffening of the ligament.7. In contrast to the above, tyramine and octopamine elicit a simple softening of ligaments which are stiff as a result of handling or by exposure to cholinergic agonists. However, tyramine and octopamine do not soften ligaments which become stiff as a result of exposure to adrenergic agonists.     

Cholinergic mechanisms in the neurocontrol of the cephalic aorta of the cephalopod Sepia officinalis L.

In dopamine (DA)-precontracted, ring-shaped muscle preparations of the cephalic aorta of Sepia officinalis L. acetylcholine (ACh) and several cholinomimetics showed vasodilaroty effects. The pD₂-values evaluated from the concentration-response curves reveal a vasodilatory potency range of: carbachol (pD₂ = 6.69) > ACh (pD₂ = 5.09) > muscarine (pD₂ = 4.09). The reduced action of nicotine results in a biphasic concentration-response curve that indicates a significantly lower intrinsic activity of this agonist. Under the ACh-antagonists applied, only tetraethylammonium (TEA) had a blocking effect (pA₂ = 5.46 TEA/ACh; pA₂ = 5.74 TEA/carbachol), whereas D-tubocurarine, α-bungaroxitin (α-BTX), atropine and pirenzepine enhanced or did not block the vasodilatory action of ACH and carbachol. The results suggest the presence of an “M-like” receptor in the cepahlic aorta of Sepia. The putative neurotransmitter FMRFamide which mimicked the vasodilatory effects of the cholinomimetics showed the highest potency (pD₂ = 8.24). Its action could not be blocked by TEA and seems to be mediated by another receptor mechanism.     

The distribution of acetylcholine sensitivity over uninnervated and innervated muscle fibers grown in cell culture

This paper describes the physiological and pharmacological parameters of the response of mature muscle fibers that develop from myoblasts in vitro to iontophoretically applied acetylcholine (ACh) and the distribution of ACh sensitivity over fibers innervated in vitro by spinal cord cells and uninnervated (control) fibers. Peaks of sensitivity were detected near nerve terminals on functionally innervated fibers, but the “extrasynaptic” chemosensitivity remained high. The distribution of chemosensitivity over uninnervated fibers is not uniform: peaks or “hot spots” were detected over most fibers. Autoradiography of cultures exposed to ¹²⁵I-α-bungarotoxin is consistent with the uneven distribution detected by iontophoresis. Sensitivity peaks were usually located in the immediate vicinity of obvious muscle nuclei and conversely the membrane near most nuclei was more sensitive than that over other regions along the same cell. The relation between innervation and distribution of ACh sensitivity is discussed.     

The molecular structure of the receptor-ionophore complex at the neuromuscular junction.

A general theory of the molecular structure of receptors for transmitters based only on protein has been presented elsewhere (Smythies, 1974a,b). The acetylcholine receptor at the neuromuscular junction is postulated in particular to be based on a Kusnetsov-Ghokov grid with four sequencestwo “primary” chains A-x-B-cys-A-x-B where A = arg or lys and B = glu or phosphoser and two “secondary” chains of sequence -gly-x-gly-pro-x-ile-cys-asp-x- forming a symmetrical receptor cup of rectangular form. The present paper extends the model to include the gate over the adjacent ionophore (or “ion conductance modulator”: ICM) and the linking mechanism from receptor to gate. These are postulated to consist of a second Kusnetsov-Ghokov grid generated by a third “primary” chain along the side that covers the orifice to the ion conducting channel. The action of ACh is postulated to be to displace an hydrated Ca⁺⁺ ion from the receptor cup and to disrupt the AB rungs in the receptor grid. The middle primary chain then slides 14 Å and the AB links reform. This replaces a bulky amino acid pair normally blocking the ion channel by a less bulky amino acid pair and so hydrated ions can be transmitted. It is further postulated that snake neurotoxins (ACh blockers) in a specified conformation bind mainly to the ionophore grid and prevent the sliding filament mechanism from opening; whereas the snake “cardiotoxins” (ACh agonists)—in a specified conformation—bind to the same sliding filament mechanism in its “open” ionophore gate and prevent it being closed: and histrionicotoxin binds to the same open “gate” but blocks it physically. The hypothesis may rigorously be tested by experiment as it makes detailed predictions on the X-ray structure of the snake neurotoxins and cardiotoxins.     

The influence of starvation and copper exposure on the composition of the dorsal carapace and distribution of trace metals in the shore crab Carcinus maenas (L.)

1. The influence of starvation and copper exposure on the composition of the carapace of the shore crab, Carcinus maenas has been investigated. The effects of the concentrations of selected trace metals in the carapace and midgut gland have also been examined.2. Differences in nutritional state did not affect the concentrations or relative proportions of the principle carapace components (calcium, chitin and protein), but starvation was associated with a reduction in carapace copper concentration. Copper concentration in the midgut gland increased in “starved” crabs, while midgut gland zinc and calcium concentrations remained unchanged.3. Starvation in combination with copper exposure (0.5 mg Cu/1) resulted in alterations in both carapace composition and trace metal loads. Carapace calcium concentrations were reduced significantly following copper exposure irrespective of the nutritional state of the crabs. However, the reduction was more marked in “fed” crabs than in “starved” animals.4. Copper-exposed crabs exhibited raised carapace copper concentrations while carapace zinc concentrations were depressed. Such changes were most pronounced in copper-exposed “starved” crabs.5. Calcium concentrations in the midgut glands of “starved” copper-exposed animals were significantly elevated compared with those in either control or “fed”, copper-exposed crabs.6. The findings are discussed with regard to their ecotoxicological significance.     

Acetylcholine and its metabolic enzymes in developing antennae of the moth, Manduca sexta.

Antennae of the moth, Manduca sexta, are thickly populated with sensory neurons, which send axons through antennal nerves to the brain. These neurons arise by cell divisions and differentiate synchronously during the 18 days of metamorphosis from pupa to adult. Biochemical studies support the hypothesis that antennal neurons use acetylcholine (ACh) as a neurotransmitter: (1) Antennae incubated with [¹⁴C]choline synthesize and store [¹⁴C]ACh; several other transmitter candidates do not accumulate detectably when appropriate radioactive precursors are supplied; (2) antennae and antennal nerves contain endogenous ACh; and (3) extracts of mature antennae contain choline acetyltransferase (ChAc) and acetylcholinesterase (AChE) with properties similar to those reported for the enzymes from other arthropods. Levels of ACh, ChAc, and AChE begin to increase in antennae soon after the sensory neurons are “born.” Levels rise exponentially for over a week as the neurons differentiate and then reach a plateau, at about the time the neurons reach morphological maturity, that is maintained into adulthood. In contrast, levels of carnitine acetyltransferase, cholinesterase, and soluble protein, presumably not confined to nervous tissue, change little during metamorphosis. Levels of ACh, ChAc, and AChE rise in an intracranial segment of antennal nerve at about the same time as in the antenna, indicating that axons can transport neurotransmitter machinery at an early stage in their development.     

The Role of Endogenous Calcitonin Gene-Related Peptide in the Neurotransmitter Quantal Size Increase in Mouse Neuromuscular Junctions

Changes in parameters of spontaneous acetylcholine (ACh) quantal secretion caused by prolonged high-frequency burst activity of neuromuscular junctions and possible involvement of endogenous calcitonin gene-related peptide (CGRP) and its receptors in these changes were studied. With this purpose, miniature endplate potentials (MEPPs) were recorded using standard microelectrode technique in isolated neuromuscular preparations of m. EDL–n. peroneus after a prolonged high-frequency nerve stimulation (30 Hz for 2 min). An increase in the MEPP amplitudes and time course was observed in the postactivation period that reached maximum 20–30 min after nerve stimulation and progressively faded in the following 30 min of recording. Inhibition of vesicular ACh transporter with vesamicol (1 μM) fully prevented this “wave” of the MEPP enhancement. This indicates the presynaptic origin of the MEPP amplitude increase, possibly mediated via intensification of synaptic vesicle loading with ACh and subsequent increase of the quantal size. Competitive antagonist of the CGRP receptor, truncated peptide isoform CGRP_8–37 (1 μM), had no effect on spontaneous secretion parameters by itself but was able to prevent the appearance of enhanced MEPPs in the postactivation period. This suggests the involvement of endogenous CGRP and its receptors in the observed MEPP enhancement after an intensive nerve stimulation. Ryanodine in high concentration (1 μM) that blocks ryanodine receptors and stored calcium release did not influence spontaneous ACh secretion but prevented the increase of the MEPP parameters in the postactivation period. Altogether, the data indicate that an intensive nerve stimulation, which activates neuromuscular junctions and muscle contractions, leads to a release of endogenous CGRP into synaptic cleft and this release strongly depends on the efflux of stored calcium. The released endogenous CGRP is able to exert an acute presynaptic effect on nerve terminals, which involves its specific receptor action and intracellular cascades leading to intensification of ACh loading into synaptic vesicles and an increase in the ACh quantal size.     

Modulation-Specific and Laminar-Dependent Effects of Acetylcholine on Visual Responses in the Rat Primary Visual Cortex

Acetylcholine (ACh) is secreted from cholinergic neurons in the basal forebrain to regions throughout the cerebral cortex, including the primary visual cortex (V1), and influences neuronal activities across all six layers via a form of diffuse extrasynaptic modulation termed volume transmission. To understand this effect in V1, we performed extracellular multi-point recordings of neuronal responses to drifting sinusoidal grating stimuli from the cortical layers of V1 in anesthetized rats and examined the modulatory effects of topically administered ACh. ACh facilitated or suppressed the visual responses of individual cells with a laminar bias: response suppression prevailed in layers 2/3, whereas response facilitation prevailed in layer 5. ACh effects on the stimulus contrast-response function showed that ACh changes the response gain upward or downward in facilitated or suppressed cells, respectively. Next, ACh effects on the signal-to-noise (S/N) ratio and the grating-phase information were tested. The grating-phase information was calculated as the F1/F0 ratio, which represents the amount of temporal response modulation at the fundamental frequency (F1) of a drifting grating relative to the mean evoked response (F0). In facilitated cells, ACh improved the S/N ratio, while in suppressed cells it enhanced the F1/F0 ratio without any concurrent reduction in the S/N ratio. These effects were predominantly observed in regular-spiking cells, but not in fast-spiking cells. Electrophysiological and histological findings suggest that ACh promotes the signaling of grating-phase information to higher-order areas by a suppressive effect on supragranular layers and enhances feedback signals with a high S/N ratio to subcortical areas by a facilitatory effect on infragranular layers. Thus, ACh distinctly and finely controls visual information processing in a manner that is specific for the modulation and cell type and is also laminar dependent.     

Cholinergic receptors and catecholamine secretion from adrenal chromaffin cells of the toad

1. The effects of cholinergic drugs on catecholamine (CA) secretion from adrenal chromaffin tissue of the toad were studied.2. CA secretion was induced by ACh or nicotine, but not by muscarine.3. Hexamethonium inhibited the CA release evoked by ACh or nicotine, while d-tubocurarine only affected the nicotinic response. Atropine did not prevent the secretory response.4. Muscarine abolished the secretion induced by the agonists, this effect being prevented by atropine or gallamine, but not by pirenzepine.5. In conclusion, CA secretion in the toad is stimulated by activation of nicotinic receptors. Inhibitory muscarinic receptors are present, most likely of type M₂, which may play a regulatory function.     

The allosteric site regulates the voltage sensitivity of muscarinic receptors

Muscarinic receptors (M-Rs) for acetylcholine (ACh) belong to the class A of G protein–coupled receptors. M-Rs are activated by orthosteric agonists that bind to a specific site buried in the M-R transmembrane helix bundle. In the active conformation, receptor function can be modulated either by allosteric modulators, which bind to the extracellular receptor surface or by the membrane potential via an unknown mechanism. Here, we compared the modulation of M₁-Rs and M₃-Rs induced by changes in voltage to their allosteric modulation by chemical compounds. We quantified changes in receptor signaling in single HEK 293 cells with a FRET biosensor for the G_q protein cycle. In the presence of ACh, M₁-R signaling was potentiated by voltage, similarly to positive allosteric modulation by benzyl quinolone carboxylic acid. Conversely, signaling of M₃-R was attenuated by voltage or the negative allosteric modulator gallamine. Because the orthosteric site is highly conserved among M-Rs, but allosteric sites vary, we constructed “allosteric site” M₃/M₁-R chimeras and analyzed their voltage dependencies. Exchanging the entire allosteric sites eliminated the voltage sensitivity of ACh responses for both receptors, but did not affect their modulation by allosteric compounds. Furthermore, a point mutation in M₃-Rs caused functional uncoupling of the allosteric and orthosteric sites and abolished voltage dependence. Molecular dynamics simulations of the receptor variants indicated a subtype-specific crosstalk between both sites, involving the conserved tyrosine lid structure of the orthosteric site. This molecular crosstalk leads to receptor subtype-specific voltage effects.     

Effects of Methylecgonidine on acetylcholine-induced bronchoconstriction and indicators of lung injury in guinea pigs

The fumarate salt of methylecgonidine (MEG; anhydroecgonine methylester), a pyrolysis product of cocaine, has previously been shown to antagonize contractions of guinea pig isolated trachea induced by acetylcholine (ACh) and other spasmogenics. We determined the effects of MEG fumarate on ACh-induced bronchoconstriction in vivo. Specific airway conductance (SGaw) was measured in guinea pigs receiving 30–300 mg/kg s.c. MEG fumarate and exposed one hour later to nebulized ACh (0.2–3.2%; by inhalation). MEG fumarate did not induce any changes in SGaw; neither did it antagonize dose-dependent decreases in SGaw induced by ACh. However, tremors, salivation, startle and increased numbers of fecal boli were observed after MEG administration. Thus, unlike antagonism of ACh-induced contractions of guinea pig isolated trachea observed in vitro, MEG fumarate does not antagonize ACh-induced bronchoconstriction in vivo, even at doses which induced changes in grossly-observable behavior. Inhalation of a condensation aerosol of MEG base induced lung damage as evidenced by the presence of blood and higher levels of protein and lactate dehydrogenase in the lung lavage fluid of MEG-treated animals than of control animals. Aerosols of MEG fumarate, on the other hand, did not induce lung damage when inhaled. These results extend previous observations that MEG base may contribute to detrimental pulmonary effects of crack smoking.     

Ultrastructure of salivary glands of Drosophila lebanonensis during normal development and after in vivo ecdysterone administration

Changes in the morphology of the salivary glands of Drosophila lebanonensis have been followed at both the light and electronmicroscopic level during a period of 30 hr before puparium formation and during puparium formation itself. Three striking differences were observed in comparison to other Drosophila species studied: (1) the secretion product of Drosophila lebanonensis has a different stainability to PAS reagent and uranyl acetate and no internal structures or “caps” can be observed; (2) the release of this secretion product is not restricted to a time period shortly before puparium formation but is a continuous process starting about 24 hr before puparium formation; and (3) the histolysis of these glands starts immediately after puparium formation, whereas in other Drosophila species this event starts 5 hr later.Puparium formation of Drosophila lebanonensis is controlled by the circadian oscillation. Injection of ecdysterone before the “gate” period results in changes in the cuticle as observed during normal development, but is not followed by the histolysis of the glands. Injection of ecdysterone after the “gate” is not followed by changes in the cuticle but histolysis is induced.     

Inhibition by Quinacrine of Depolarization-Induced Acetylcholine Release and Calcium Influx in Rat Brain Cortical Synaptosomes

The effects of quinacrine on depolarization-induced [³H]acetylcholine (ACh) release and ⁴⁵Ca²⁺ influx were examined in rat brain cortical synaptosomes. Quinacrine significantly reduced the stimulated release of [³H]ACh by high K⁺ and veratridine without affecting the spontaneous efflux from the preloaded synaptosomes. Quinacrine had no effect on ionophore A23187-induced release of [³H]ACh from the synaptosomes. Quinacrine (100 μM) markedly diminished the stimulated Ca²⁺ influx by veratridine and high K⁺ but not that by “Na⁺-free.” Trifluoperazine, a potent calmodulin antagonist, inhibited both Ca²⁺ influx and ACh release induced by the depolarizing agents. Inhibitory potencies of the two drugs on ACh release and Ca²⁺ influx were compared with the antagonism of calmodulin by two drugs, suggesting that the inhibition of depolarization-induced Ca²⁺ influx and ACh release by these drugs could not be explained by the antagonism of calmodulin.     

Substrate specificity and gene expression of two Penicillium chrysogenum α-l-arabinofuranosidases (AFQ1 and AFS1) belonging to glycoside hydrolase families 51 and 54

We previously isolated two α-l-arabinofuranosidases (ABFs), termed AFQ1 and AFS1, from the culture filtrate of Penicillium chrysogenum 31B. afq1 and afs1 complementary DNAs encoding AFQ1 and AFS1 were isolated by in vitro cloning. The deduced amino acid sequences of AFQ1 and AFS1 are highly similar to those of Penicillium purpurogenum ABF 2 and ABF 1, respectively, which belong to glycoside hydrolase (GH) families 51 and 54, respectively. Pfam analysis revealed an “Alpha-L-AF_C” domain in AFQ1 and “ArabFuran-catal” and “AbfB” domains in AFS1. Semi-quantitative RT-PCR analysis indicated that the afq1 gene was constitutively expressed in P. chrysogenum 31B at a low level, although the expression was slightly induced with arabinose, arabinitol, arabinan, and arabinoxylan. In contrast, expression of the afs1 gene was strongly expressed by the above four carbohydrates and less strongly induced by galactan. Recombinant enzymes (rAFQ1 and rAFS1) expressed in Escherichia coli were active against both p-nitrophenyl α-l-arabinofuranoside and polysaccharides with different specificities. ¹H-NMR analysis revealed that rAFS1 degraded arabinofuranosyl side chains that were both singly and doubly linked to the backbones of arabinoxylan and l-arabinan. On the other hand, rAFQ1 preferentially released arabinose linked to C-3 of single-substituted xylose or arabinose residues in the two polysaccharides.     

Function of Interfacial Prolines at the Transmitter-binding Sites of the Neuromuscular Acetylcholine Receptor

The neuromuscular acetylcholine (ACh) receptor has two conserved prolines in loop D of the complementary subunit at each of its two transmitter-binding sites (α-ϵ and α-δ). We used single-channel electrophysiology to estimate the energy changes caused by mutations of these prolines with regard to unliganded gating (ΔG₀) and the affinity change for ACh that increases the open channel probability (ΔG_B). The effects of mutations of ProD2 (ϵPro-121/δPro-123) were greater than those of its neighbor (ϵPro-120/δPro-122) and were greater at α-ϵ versus α-δ. The main consequence of the congenital myasthenic syndrome mutation ϵProD2-L was to impair the establishment of a high affinity for ACh and thus make ΔG_B less favorable. At both binding sites, most ProD2 mutations decreased constitutive activity (increased ΔG₀). LRYHQG and RL substitutions reduced substantially the net binding energy (made ΔG_B^ACh less favorable) by ≥2 kcal/mol at α-ϵ and α-δ, respectively. Mutant cycle analyses were used to estimate energy coupling between the two ProD2 residues and between each ProD2 and glycine residues (αGly-147 and αGly-153) on the primary (α subunit) side of each binding pocket. The distant binding site prolines interact weakly. ProD2 interacts strongly with αGly-147 but only at α-ϵ and only when ACh is present. The results suggest that in the low to-high affinity change there is a concerted inter-subunit strain in the backbones at ϵProD2 and αGly-147. It is possible to engineer receptors having a single functional binding site by using a α-ϵ or α-δ ProD2-R knock-out mutation. In adult-type ACh receptors, the energy from the affinity change for ACh is approximately the same at the two binding sites (approximately −5 kcal/mol).     

Acetylcholine sensitivity of innervated and noninnervated Xenopus muscle cells in culture

The chemosensitivity of Xenopus muscle cells grown in culture to iontophoretically applied acetylcholine (ACh) in the presence or absence of neurons was examined. Muscle cells grown without nerve cells are sensitive to ACh over their entire surface (2.4 mV/pC) with occasional spots of high chemosensitivity (“hot spots”). In cultures containing neural tube cells, the ACh sensitivity of muscle cells increased by approximately 50% regardless of the presence of nerve contacts or functional synapses. A similar increase in the ACh sensitivity was observed in muscle cells cultured in medium conditioned by neural tube cells. The ACh sensitivity of the extrajunctional region in functionally innervated muscle cells was not different from that of noninnervated cells growing in the same cultures. However, the chemosensitivity at the junctional region was about fivefold higher than that of the extrajunctional area. This increase in junctional chemosensitivity may well account for the increase in miniature endplate potential amplitude which has previously been reported to occur during nerve-induced ACh receptor accumulation.     

A new conception of the shoot of higher plants

According to the classical model, the “shoot” consists only of the categories “caulome” (“stem” sensu lato) and “phyllome” (“leaf” sensu lato), (and “root” in cases of “adventitious” root formation). If lateral shoots are present, their position is axillary. Consequently, caulome as well as phyllome are inserted on the caulome and only on the caulome. This classical model of the shoot has two disadvantages of great consequence: (1) Intermediate organs cannot be accepted as such, but have to be interpreted (i.e. categorized) as either caulome or phyllome (or root) by distortion of the actual similarity. (2) Certain positional changes of organs cannot be accepted as such, but have to be “explained” by congenital fusion. The new conception of the shoot will have the advantages of the classical model but not its disadvantages. Hence, the shoot may consist of the following parts: (main and lateral) shoot, caulome, phyllome, root, emergence, and structures intermediate between (i.e. partially homologous to) any of the preceding. Thus, the five categories of the classical model, namely “shoot”, “caulome”, “phyllome”, “root” and “emergence” are no longer mutually exclusive; they may merge into each other due to an actual or potential continuum. Intermediate organs are therefore accepted as such; for example, an organ may be characterized as an intermediate form between a caulome and a phyllome. Besides intermediate forms, all changes in position are accepted as such. Hence, the following positional relations are possible: caulome and phyllome may be inserted on the caulome, caulome and phyllome may be inserted on the phyllome; roots may be inserted on caulome or phyllome; intermediate forms may be inserted on the caulome, phyllome, or other intermediate forms. Consequences of the new conception for morphological research are pointed out, especially for homologization, evolutionary considerations, and the direction in which research progresses.     

Nature of the ribosomal RNA transcribed from the X and Y chromosomes of Drosophila melanogaster

In Drosophila melanogaster there is one nucleolar organizer (NO) on each X and Y chromosome. Experiments were carried out to compare the ribosomal RNAs derived from the two nucleolar organizers. ³²PO₄-labelled ribosomal RNA was isolated from two strains of D. melanogaster, one containing only the X chromosome NO, the other containing only the Y chromosome NO. 28 S and 18 S RNA from the two strains were subjected to a variety of “fingerprinting” and sequencing procedures. Fingerprints of 28 S RNA were very different from those of 18 S RNA. Fingerprints of “X” and “Y” 28 S RNA were indistinguishable from each other, as also were fingerprints of “X” and “Y” 18 S RNA. In combined “T₁ plus pancreatic” RNAase fingerprints several distinctive products were characterized and quantitated. Identical products were obtained from X and Y RNA, and the molar yields of the products were indistinguishable. Together these findings imply that the rRNA sequences encoded by the X and Y NOs are closely similar and probably identical to each other.Two further findings were of interest in “T₁ plus pancreatic” RNAase fingerprints: (1) in 28 S (as well as in 18 S) fingerprints several distinctive products were recovered in approximately unimolar yields. This indicates that 28 S RNA does not consist of two identical half molecules, though it does consist of two non-identical half molecules together with a “5.8 S” fragment. (2) Several methylated components in Drosophila rRNA also occur in rRNA from HeLa cells and yeast. This suggests that certain features of rRNA structure involving methylated nucleotides may be highly conserved in eukaryotic evolution.