Comparative genomic study of arachnid immune systems indicates loss of beta-1,3-glucanase-related proteins and the immune deficiency pathway.

Analyses of arthropod genomes have shown that the genes in the different innate humoral immune responses are conserved. These genes encode proteins that are involved in immune signalling pathways that recognize pathogens and activate immune responses. These immune responses include phagocytosis, encapsulation of the pathogen and production of effector molecules for pathogen elimination. So far, most studies have focused on insects leaving other major arthropod groups largely unexplored. Here, we annotate the immune-related genes of six arachnid genomes and present evidence for a conserved pattern of some immune genes, but also evolutionary changes in the arachnid immune system. Specifically, our results suggest that the family of recognition molecules of beta-1,3-glucanase-related proteins (ßGRPs) and the genes from the immune deficiency (IMD) signalling pathway have been lost in a common ancestor of arachnids. These findings are consistent with previous work suggesting that the humoral immune effector proteins are constitutively produced in arachnids in contrast to insects, where these have to be induced. Further functional studies are needed to verify this. We further show that the full haemolymph clotting cascade found in the horseshoe crab is retrieved in most arachnid genomes. Tetranychus lacks at least one major component, although it is possible that this cascade could still function through recruitment of a different protein. The gel-forming protein in horseshoe crabs, coagulogen, was not recovered in any of the arachnid genomes; however, it is possible that the arachnid clot consists of a related protein, spätzle, that is present in all of the genomes.

Micrognathozoa: a new class with complicated jaws like those of Rotifera and Gnathostomulida.

A new microscopic aschelminth-like animal, Limnognathia maerski nov. gen. et sp., is described from a cold spring at Disko Island, West Greenland, and assigned to Micrognathozoa nov. class. It has a complex of jaws in its pharynx, and the ultrastructure of the main jaws is similar to that of the jaws of advanced scleroperalian gnathostomulids. However, other jaw elements appear also to have characteristics of the trophi of Rotifera. Jaw-like structures are found in other protostome taxa as well-for instance, in proboscises of kalyptorhynch platyhelminths, in dorvilleid polychaetes and aplacophoran mollusks-but studies of their ultrastructure show that none of these jaws is homologous with jaws found in Gnathostomulida, Rotifera, and Micrognathozoa. The latter three groups have recently been joined into the monophylum Gnathifera Ahlrichs, 1995, an interpretation supported by the presence of jaw elements with cuticular rods with osmiophilic cores in all three groups. Such tubular structures are found in the fulcrum of all Rotifera and in several cuticular sclerites of both Gnathostomulida and Micrognathozoa. The gross morphology of the pharyngeal apparatus is similar in the three groups. It consists of a ventral pharyngeal bulb and a dorsal pharyngeal lumen. The absence of pharyngeal ciliation cannot be used as an autapomorphy in the ground pattern of the Gnathifera because the Micrognathozoa has the plesiomorphic alternative with a ciliated pharyngeal epithelium. The body of Limnognathia maerski nov. gen. et sp. consists of a head, thorax, and abdomen. The dorsal and lateral epidermis have plates formed by an intracellular matrix, as in Rotifera and Acanthocephala; however, the epidermis is not syncytial. The ventral epidermis lacks internal plates, but has a cuticular oral plate without ciliary structures. Two ventral rows of multiciliated cells form a locomotory organ. These ciliated cells resemble the ciliophores present in some interstitial annelids. An adhesive ciliated pad is located ventrally close to a caudal plate. As in many marine interstitial animals-e.g., gnathostomulids, gastrotrichs, and polychaetes-a special form of tactile bristles or sensoria is found on the body. Two pairs of protonephridia with unicellular terminal cells are found in the trunk; this unicellular condition may be the plesiomorphic condition in Bilateria. Only specimens with the female reproductive system have been found, indicating that all adult animals are parthenogenetic females. We suggest that 1) jaws of Gnathostomulida, Rotifera, and the new taxon, Micrognathozoa, are homologous structures; 2) Rotifera (including Acanthocephala) and the new group might be sister groups, while Gnathostomulida could be the sister-group to this assemblage; and 3) the similarities to certain gastrotrichs and interstitial polychaetes are convergent.

Self-injection of a dipteran parasitoid into a spider

A host invasion strategy hitherto unknown from other insect parasitoids was observed in the dipteran Acrocera orbicula (Fabricius) (Diptera: Acroceridae) parasitizing the wolf spider, Pardosa prativaga (L. Koch) (Araneida: Lycosidae). In laboratory experiments the free-living first instar acrocerid larvae attached themselves firmly to the spiders' integument by the mouthparts, cutting a tiny hole through the integument. No first instar larvae invaded the host. A week later the parasitoids molted, and a small, flexible, and glabrous second instar larva left each of the attached first instar exuviae and invaded the host through the attachment hole of the first instar larva. The novel host invasion pattern observed may reduce physical damage to the host in the initial phase of endoparasitism, enhancing parasitoid survival.

Evidence that receptors mediating central synaptic potentials extend beyond the postsynaptic density.

Physiological recordings and computer simulations of unitary inhibitory postsynaptic potentials in the Mauthner cell of the goldfish central nervous system have been used to estimate the expected size of the postsynaptic receptor matrix at individual junctions. Simultaneous pre- and postsynaptic recordings were used to determine the kinetic parameters of the quantal responses under normal conditions and in the presence of strychnine, a competitive antagonist of glycine, which is the putative transmitter at these synapses. Calculations indicate that if the postsynaptic density, which has a radius of 0.1 micron, were to accommodate the population of channels estimated to be opened during a quantal response, the glycine binding site density in that region would be unrealistically high. Computer simulation of the quantal responses included transmitter diffusion, transmitter-receptor interactions, and channel activation under conditions including both normal and lowered binding site densities, the latter corresponding to the experimental data obtained with strychnine. The data indicate that the synaptic receptors involved in generating unitary responses are widely distributed to include regions located outside the junctional area, which directly faces the presynaptic release sites. We further suggest that the receptor matrix is surrounded by a restricted diffusional space; this geometrical organization may underlie the finding that response rise times are relatively independent of receptor binding site densities.

Localization of active sites along the myelinated goldfish Mauthner axon: morphological and pharmacological evidence for saltatory conduction.

Injections of Lucifer Yellow (LY) and horseradish peroxidase (HRP) were made within the myelin sheath of the goldfish Mauthner axon to determine the domains of individual oligodendrocytes. Long segments of the myelin sheath were stained with both markers. The lengths and locations of these sheath segments were analyzed in whole mount preparations (LY) or in reconstructions of serial vibratome sections (HRP). The termination sites of individual myelin sheaths, relative to gross anatomical landmarks of the brain, were consistent within and between all fish studied. In particular, the average locations of the termination sites were separated by 2.2 to 2.6 mm and corresponded to the brain regions where active site foci have been previously localized electrophysiologically. Individual sheath segments generally spanned the entire distance between adjacent active sites. The node-internode-node structure of the Mauthner axon that is suggested by these findings was further tested by ejecting tetrodotoxin (TTX) at various discrete rostral-caudal locations just outside the fiber. Large all-or-nothing components of the antidromic action potential were rapidly blocked (within seconds) only when the TTX ejections were made within a few hundred micrometers of the active site foci. The amplitudes of these blocked components are also consistent with predictions based upon previous electrophysiological analyses which demonstrated an active site spacing of 2.2 to 2.6 mm, a space constant of 5.0 mm, and a safety factor of 6 for impulse propagation. It is concluded from these morphological, pharmacological, and electrophysiological observations that the Mauthner axon possesses nodes separated by 2.2 to 2.6 mm and that a single oligodendrocyte spans an internodal region. Although nodal ultrastructure remains to be described, these results rule out the possibility that each of the short (approximately 50 micron), closely spaced (average separation = 155 micron) axon collaterals is a site of action current production.

Measurement of myelin sheath resistances: implications for axonal conduction and pathophysiology.

As commonly understood, the myelin sheath of axons insulates the internodal axolemma and essentially restricts transmembrane currents to nodal regions. However, recordings obtained from within the myelin sheath showed that its apparent resistance to current generated by action potentials is similar in magnitude to that of the internodal axolemma. This suggests that the sheath does not appreciably limit transmembrane current flow, presumably because there is a longitudinal shunt under the myelin and through the paranodal region. Thus, in some demyelinating diseases and other axonopathies, the safety factor for impulse conduction may be lowered by a loosening or a reduction in the number of paranodal axoglial junctions.

Electrotonic and dye coupling in hippocampal CA1 pyramidal cells in vitro.

The presence of electrotonic and dye coupling in region CA1 of the guinea-pig hippocampus was investigated in the in vitro hippocampal slice preparation. No electronic coupling potentials were observed in simultaneous recordings from 101 pairs of pyramidal cells. Also, no electrotonically-coupled short latency depolarizations were observed in more than 75 pyramidal cells in response to antidromic activation of the pyramidal cell population, either in normal bathing medium or in medium with lowered Ca2+ concentration and added Mn2+. When the fluorescent dye Lucifer Yellow was injected into pyramidal cell somas, spread of the dye to other cells (dye coupling) was often observed. Injection of Lucifer Yellow into the dendrites of these neurons resulted in many fewer cases of dye coupling. The failure to find electrophysiological evidence of electrotonic coupling among CA1 pyramidal cells suggests that such coupling is not a functionally important feature of this area of the CNS. The lack of electrophysiological evidence of coupling combined with the observation that the site of Lucifer Yellow injection influences the extent of dye coupling further suggests that at least part of the observed dye coupling may be artifactual. Electrotonic coupling may exist in a small percentage of hippocampal pyramidal cells. However, it is not clear that this small amount of coupling is either necessary or sufficient for the synchronization of neural activity as has been hypothesized to occur during epileptogenesis.

Mauthner axon diameter and impulse conduction velocity decrease with growth of goldfish.

Conduction velocities of antidromically evoked impulses along the goldfish Mauthner axon were found to be inversely correlated with body length. To test the hypothesis that such a relation is accompanied by a reduction in axonal diameter with increasing fish size, Mauthner axon diameters were measured. A parabolic relationship with respect to body length was obtained, axonal diameter being maximal in 9.5 cm fish. To our knowledge, this is the first report of a decrease in conduction velocity and axonal size during growth of an organism were functioning of the cell is maintained.