Assembly of an MCP receptor, CheW, and kinase CheA complex in the bacterial chemotaxis signal transduction pathway.

We examined the binding interactions of the methylation-dependent chemotaxis receptors Tsr and Tar with the chemotaxis-specific protein kinase CheA and the coupling factor CheW. Receptor directly bound CheW, but receptor-CheA binding was dependent upon the presence of CheW. These observations in combination with our previous identification of a CheW-CheA complex suggest that CheW physically links the kinase to the receptor. The ternary complex of receptor, CheW, and CheA is both kinetically and thermodynamically stable at physiological concentrations. Stability is not significantly altered by changes associated with attractant or repellent binding to the receptor. Such binding greatly modulates the kinase activity of CheA. Our results demonstrate that modulation of the kinase activity does not require association-dissociation of the ternary complex. This suggests that the receptor signal is transduced through conformational changes in the ternary complex rather than through changes in the association of the kinase CheA with receptor and/or CheW.     

Do microsporidia function as “biological weapon” for Harmonia axyridis under natural conditions?

Invasive alien species, such as the multicoloured Asian ladybird Harmonia axyridis, are often regarded as major drivers of biodiversity loss. Therefore understanding which characteristics or mechanisms contribute to their invasive success is important. Here the role of symbiotic microsporidia in the hemolymph of H. axyridis was investigated in the context of intraguild predation between wild‐caught H. axyridis and the native ladybird species Coccinella septempunctata. The microsporidia were recently discussed to contribute to the unpalatability of Harmonia for other coccinellids during intraguild predation and to function as “biological weapons”. In the present study, visual detection of microsporidia in hemolymph samples revealed that 73.5 % of H. axyridis were infected. Intraguild predation experiments between larvae of the two species showed a significant competitive advantage for H. axyridis, even against larger larvae of C. septempunctata. Adult C. septempunctata always killed and fed on H. axyridis larvae. However only 11.4 % (4 of 47) of C. septempunctata that fed on infected H. axyridis died within 4 months. In contrast to previous studies this suggests that microsporidia or harmonine, the chemical defense compound of H. axyridis, do not lead to death of C. septempunctata preying on larvae of H. axyridis. Instead our results support the idea that competitive advantage during intraguild predation greatly facilitates the success of H. axyridis and that this may help this highly invasive species to outcompete native species. The impact of microsporidia on Harmonia itself as well as on interspecific interactions require further studies.     

Fine structure of the spermatophore and spermatozoa in inseminated females of Pergamasus mites (Acari: Gamasida: Pergamasidae)

This study describes the spermatophore of Pergamasus mites after transfer into the female endogynial sac and modifications of the structure of the female-penetrating spermatozoa. The spermatophore is saccular and contains largely unmodified spermatozoa and variously structured secretions. The spermatozoa that leave the spermatophore reach the inner (proximal) end of the vaginal duct where they presumably escape from the genital tract to continue their route through the hemocoel into the ovarian tissue where fertilization occurs. During this transit, the structure of such spermatozoa changes considerably, in particular when in contact with the ovary. These alterations include modifications of the cell periphery and of certain inclusion bodies. Furthermore, the spermatozoa form protrusions that fit into corresponding invaginations of the somatic tissue of the ovary.     

Fine structure of the genital system in the females of Pergamasus mites (Acari: Gamasida: Pergamasidae)

The female reproductive system in Pergamasus mites consists of an unpaired vagina, vaginal duct, uterus, and ovary. Additionally, there are paired vaginal glands, as well as unpaired ventral and paired lateromedial glandular complexes. The vagina and vaginal duct are cuticle‐lined. In the dorsal wall of the vagina, this lining forms the endogynium which possesses a “sac” and two conspicuous “spherules” and is armed with “stipula” and other cuticular protrusions. The endogynium functions as a spermatheca, being a storing site for the spermatophore. The spherule procuticle is perforated by microvilli of underlying cells that are structurally very unusual. The lining of the vaginal duct forms numerous cuticular fibers directed toward the vagina. There is an external layer of muscles, supposedly functioning as a sphincter. The uterus is an organ in which the fertilized egg is stored for some time and starts embryonic development. Its wall is composed of glandular epithelial cells. The ovary consists of inner and outer parts. The former part is formed by a nutritive syncytium, whereas the latter contains growing oocytes. Two groups of glands connect with the genital tract. Paired vaginal glands are composed of glandular and secretion‐storing parts and open into the vagina. Paired lateromedial and unpaired ventral glandular complexes empty into the genital tract between the vaginal duct and uterus. The structure of the female genital system is discussed in terms of its function and phylogeny. J. Morphol. 240:195–223, 1999. © 1999 Wiley‐Liss, Inc.     

Heat stress reduces the contribution of diazotrophs to coral holobiont nitrogen cycling

Efficient nutrient cycling in the coral-algal symbiosis requires constant but limited nitrogen availability. Coral-associated diazotrophs, i.e., prokaryotes capable of fixing dinitrogen, may thus support productivity in a stable coral-algal symbiosis but could contribute to its breakdown when overstimulated. However, the effects of environmental conditions on diazotroph communities and their interaction with other members of the coral holobiont remain poorly understood. Here we assessed the effects of heat stress on diazotroph diversity and their contribution to holobiont nutrient cycling in the reef-building coral Stylophora pistillata from the central Red Sea. In a stable symbiotic state, we found that nitrogen fixation by coral-associated diazotrophs constitutes a source of nitrogen to the algal symbionts. Heat stress caused an increase in nitrogen fixation concomitant with a change in diazotroph communities. Yet, this additional fixed nitrogen was not assimilated by the coral tissue or the algal symbionts. We conclude that although diazotrophs may support coral holobiont functioning under low nitrogen availability, altered nutrient cycling during heat stress abates the dependence of the coral host and its algal symbionts on diazotroph-derived nitrogen. Consequently, the role of nitrogen fixation in the coral holobiont is strongly dependent on its nutritional status and varies dynamically with environmental conditions.Subject terms: Microbial ecology, Climate-change ecology     

Carbohydrate composition of mucus from scleractinian corals from the central Red Sea

Coral Reefs - Coral mucus is continuously released by most corals and acts as an important protective barrier and as a substrate for host-associated microbial communities due to its complex...