Asian citrus psyllid stylet morphology and applicability to the model for inter-instar stylet replacement in the potato psyllid

In Hemiptera, presumptive stylets for each consecutive postembryonic instar are manufactured prior to ecdysis to replace the ecdysial stylets discarded with the exuviae. With the discovery that the bacterium “Candidatus” Liberibacter solanacearum accesses the tissues involved in the stylet replacement process of the potato psyllid, a hypothesis was formed that the bacterium could adhere to the stylets of freshly emerged instars and hence gain access to the host plant when feeding is resumed. Although unproven, it was imperative that a model for stylet replacement be built. Stylet morphology and the stylet replacement process of the Asian citrus psyllid (ACP), vector of “C.” L. asiaticus, causal pathogen of citrus greening disease, are comparable to the potato psyllid model system. Morphology consists of a basal terminus with its tab-shaped auricle, a base, shaft, and an apical terminus. Each of the four auricles act as a platform for the replacement apparatus, which is compacted into a tight aggregate of cells, the ‘end-cap’. As modeled, on apolysis of larval instar hypodermis, the aggregate ‘deconstructs’ and expands into a snail shell-shaped tube, the ‘atrium’, that houses the presumptive stylet as it is synthesized. Completed stylets then despool from the atrium and are fitted into their functional positions as the next instar emerges from its exuviae.     

Hunting‐mediated predator facilitation and superadditive mortality in a European ungulate

Predator‐prey theory predicts that in the presence of multiple types of predators using a common prey, predator facilitation may result as a consequence of contrasting prey defense mechanisms, where reducing the risk from one predator increases the risk from the other. While predator facilitation is well established in natural predator‐prey systems, little attention has been paid to situations where human hunters compete with natural predators for the same prey. Here, we investigate hunting‐mediated predator facilitation in a hunter‐predator‐prey system. We found that hunter avoidance by roe deer (Capreolus capreolus) exposed them to increase predation risk by Eurasian lynx (Lynx lynx). Lynx responded by increasing their activity and predation on deer, providing evidence that superadditive hunting mortality may be occurring through predator facilitation. Our results reveal a new pathway through which human hunters, in their role as top predators, may affect species interactions at lower trophic levels and thus drive ecosystem processes.     

mtDNA sequence diversity in Africa.

mtDNA sequences were determined from 241 individuals from nine ethnic groups in Africa. When they were compared with published data from other groups, it was found that the !Kung, Mbuti, and Biaka show on the order of 10 times more sequence differences between the three groups, as well as between those and the other groups (the Fulbe, Hausa, Tuareg, Songhai, Kanuri, Yoruba, Mandenka, Somali, Tukana, and Kikuyu), than these other groups do between one other. Furthermore, the pairwise sequence distributions, patterns of coalescence events, and numbers of variable positions relative to the mean sequence difference indicate that the former three groups have been of constant size over time, whereas the latter have expanded in size. We suggest that this reflects subsistence patterns in that the populations that have expanded in size are food producers whereas those that have not are hunters and gatherers.     

Habitatwahl nachtziehender Kleinv?gel bei Bodennebel

Zusammenfassung Nachtzieher brechen ihren Zug gewöhnlich während der Nacht oder vor Sonnenaufgang ab und landen meist in ihren artspezifischen Habitaten. Um Einblicke in das Verhalten von Nachtziehern während und nach der Landung zu erhalten, wurde die Habitatwahl von Kleinvogelarten während Bodennebel in verschiedenen Habitaten am Ufer des Neuenburgersees nach Fänglingen untersucht. Vögel, die den See überfliegen, treffen zuerst auf die Düne und den Schilfgürtel. Rotkehlchen, Amsel und Mönchsgrasmücke, die auf Gehölze angewiesen sind, wurden bei Bodennebel in höheren absoluten und relativen Zahlen auf der Düne und im Schifgürtel gefangen als ohne Bodennebel. Der typische Schilfbewohner Teichrohrsänger war dagegen bei Bodennebel noch stärker auf den Schilfgürtel konzentriert als ohne Bodennebel, während der Zilpzalp als überwiegender Tagzieher und eine Gruppe von gehölzbewohnenden Standvögeln keine Unterschiede in der Habitatwahl mit und ohne Bodennebel zeigten. Dies deutet darauf hin, daß die optische Erkennbarkeit der Habitate eine große Rolle bei der nächtlichen Habitatwahl spielt. Die tageszeitlichen Fangmuster und die Fettscores weisen darauf hin, daß die meisten Rotkehlchen, die im Schilfgürtel gefangen wurden, und viele, die auf der Düne auftraten, in der vorhergegangenen Nacht gezogen sind. Die meisten Rotkehlchen verließen den Schilfgürtel gleich nach Tagesanbruch mit und ohne Bodennebel, während Rotkehlchen auf der Düne damit bis zur Auflösung des Nebels zuwarteten.

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Habitat selection of night-migrating passerines during ground fog

Night migrants generally terminate migration during the night or before sunrise and usually land in their species-specific habitats. In order to get insights into their behaviour during and after landing, habitat selection of small passerines was examined during ground fog in a sequence of habitats at the shores of Lake Neuchâtel. On days with ground fog,Erithacus rubecula,Turdus merula andSylvia atricapilla, depending on wood and bushes, were caught in higher relative and absolute numbers on the dune and in the reed-beds which they meet first when flying across the lake, than on days without ground fog.Acrocephalus scirpaceus, a typical reed species, however, was more concentrated in the reed-belt during ground fog than without fog. A day-migrant and a group of sedentary species showed no significant differences. This indicates that visual cues are important when selecting habitats during nocturnal landing. The diurnal distribution of captures and the fat scores ofErithacus rubecula indicate that most individuals caught in the reed-bed and many on the dune had been migrating the previous night. During the first day-light hours, Robins in the reed-bed disperse, probably to more appropriate habitats, irrespective of fog, while Robins that have landed on the dune wait until fog has dissolved.

     

Chemical composition of Casparian strips isolated from Clivia miniata Reg. roots: evidence for lignin

Endodermal cell walls and xylem vessels were isolated enzymatically from Clivia miniata Reg. roots. Transmission-electron-microscopic investigation of cross-sections of intact C. miniata roots and scanning-electron-microscopic investigation of isolated endodermal cell walls indicated that the root endodermis of C. miniata is essentially in its primary state of development. Isolated Casparian strips and xylem vessels were subjected to two different degradation methods usually applied to prove the existence of lignin, namely, cupric oxide oxidation and thioacidolysis. The reaction products obtained were typical aromatic derivatives of the natural lignin precursors coniferyl and sinapyl alcohols, and, in traces, of p-coumaryl alcohol, indicating the occurrence of lignin in the polymers from both Casparian strips and xylem vessels. The qualitative chemical compositions of the polymers from the two sources were similar, whereas the quantitative compositions were different, indicating that the molecular structure of the lignin polymer in the Casparian strips was different from that in the xylem vessels. Thus, for the first time, direct chemical evidence has been obtained that Casparian strips of C. miniata roots contain lignin as a major cell wall polymer.The author is indebted to Prof. Dr. G. Krohne (Zentrale Abteilung für Elektronenmikroskopie, Universität Würzburg, Germany) and to Prof. Dr. R. Guggenheim (Labor für Rasterelektronenmikroskopie, Universität Basel, Schweiz) for offering the opportunity for transmission-electron-microscopic and low-temperature scanning-electron-microscopic investigations, respectively. Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.     

Two heads are better than one: crystal structure of the insect derived double domain Kazal inhibitor rhodniin in complex with thrombin.

Rhodniin is a highly specific inhibitor of thrombin isolated from the assassin bug Rhodnius prolixus. The 2.6 Angstrum crystal structure of the non-covalent complex between recombinant rhodniin and bovine alpha-thrombin reveals that the two Kazal-type domains of rhodniin bind to different sites of thrombin. The amino-terminal domain binds in a substrate-like manner to the narrow active-site cleft of thrombin; the imidazole group of the P1 His residue extends into the S1 pocket to form favourable hydrogen/ionic bonds with Asp189 at its bottom, and additionally with Glu192 at its entrance. The carboxy-terminal domain, whose distorted reactive-site loop cannot adopt the canonical conformation, docks to the fibrinogen recognition exosite via extensive electrostatic interactions. The rather acidic polypeptide linking the two domains is displaced from the thrombin surface, with none of its residues involved in direct salt bridges with thrombin. The tight (Ki = 2 x 10(-13) M) binding of rhodniin to thrombin is the result of the sum of steric and charge complementarity of the amino-terminal domain towards the active-site cleft, and of the electrostatic interactions between the carboxy-terminal domain and the exosite.     

Cyclin D1 is dispensable for G1 control in retinoblastoma gene-deficient cells independently of cdk4 activity.

To elucidate the regulator-versus-target relationship in the cyclin D1/cdk4/retinoblastoma protein (pRB) pathway, we examined fibroblasts from RB-1 gene-deficient and RB-1 wild-type littermate mouse embryos (ME) and in human tumor cell lines that differed in the status of the RB-1 gene. The RB+/+ and RB-/- ME fibroblasts expressed similar protein levels of D-type cyclins, cdk4, and cdk6, showed analogous spectra and abundance of cellular proteins complexed with cdk4 and/or cyclins D1 and D2, and exhibited comparable associated kinase activities. Of the two human cell lines established from the same sarcoma biopsy, the RB-positive SKUT1B cells contained cdk4 that was mainly associated with D-type cyclins, contrary to a predominant cdk4-p16INK4 complex in the RB-deficient SKUT1A cells. Antibody-mediated neutralization of cyclin D1 arrested the RB-positive ME and SKUT1B cells in G1, whereas this cyclin appeared dispensable in the RB-deficient ME and SKUT1A cells. Lack of requirement for cyclin D1 therefore correlated with absence of functional pRB, regardless of whether active cyclin D1/cdk4 holoenzyme was present in the cells under study. Consistent with a potential role of cyclin D/cdk4 in phosphorylation of pRB, monoclonal anti-cyclin D1 antibodies supporting the associated kinase activity failed to significantly affect proliferation of RB-positive cells, whereas the antibody DCS-6, unable to coprecipitate cdk4, efficiently inhibited G1 progression and prevented pRB phosphorylation in vivo. These data provide evidence for an upstream control function of cyclin D1/cdk4, and a downstream role for pRB, in the order of events regulating transition through late G1 phase of the mammalian cell division cycle.