Pattern of nucleotide substitution at major histocompatibility complex class I loci reveals overdominant selection

The major histocompatibility complex (MHC) loci are known to be highly polymorphic in humans, mice and certain other mammals, with heterozygosity as high as 80-90% (ref. 1). Four different hypotheses have been proposed to explain this high degree of polymorphism: (1) a high mutation rate, (2) gene conversion or interlocus genetic exchange, (3) over dominant selection and (4) frequency-dependent selection. In an attempt to establish which of these hypotheses is correct, we examined the pattern of nucleotide substitution between polymorphic alleles in the region of the antigen recognition site (ARS) and other regions of human and mouse class I MHC genes. The results indicate that in ARS the rate of nonsynonymous (amino acid altering) substitution is significantly higher than that of synonymous substitution in both humans and mice, whereas in other regions the reverse is true. This observation, together with a theoretical study and other considerations, supports the hypothesis of overdominant selection (heterozygote advantage).     

Stimulation and recording from single cells in whole animal preparations ofAplysia

Résumé Nous avons introduit des microélectrodes dans les neurones des ganglions pédieux et pleural de l'Aplysie suspendue d'une manière qui permettait des mouvements limités. Les stimulations physiologiques agissant sur des neurones pédieux sont plus simples que celles qui excitent les neurones pleuraux. La stimulation intracellulaire montre que certains neurones pédieux opèrent comme des «motor-neurones», tandis que les neurones pleuraux (la cellule géante de gauche (LGC) comprise) présentent un comportement complexe.     

Experiments on the respiration of the trigger fish (Balistes capriscus)

Zusammenfassung Druck- und Bewegungsregistrierungen haben gezeigt, dass die Kiemenventilation beim plectognathen FischBalistes capriscus primär denselben doppelten Pumpmechanismus benützt, wie andere Knochenfische. Messungen des Ventilationsvolumens wurden durch Zufuhr von Wasser unter konstantem Druck zur Mundöffnung durch einen Gummischlauch, der an der steifen Haut des Fisches festgekblet wurde, vorgenommen. Es wurden Sauerstoffausnützungen von 90% beobachtet.     

Energy source of flagellar type III secretion

Bacterial flagella contain a specialized secretion apparatus that functions to deliver the protein subunits that form the filament and other structures to outside the membrane. This apparatus is related to the injectisome used by many gram-negative pathogens and symbionts to transfer effector proteins into host cells; in both systems this export mechanism is termed 'type III' secretion. The flagellar secretion apparatus comprises a membrane-embedded complex of about five proteins, and soluble factors, which include export-dedicated chaperones and an ATPase, FliI, that was thought to provide the energy for export. Here we show that flagellar secretion in Salmonella enterica requires the proton motive force (PMF) and does not require ATP hydrolysis by FliI. The export of several flagellar export substrates was prevented by treatment with the protonophore CCCP, with no accompanying decrease in cellular ATP levels. Weak swarming motility and rare flagella were observed in a mutant deleted for FliI and for the non-flagellar type-III secretion ATPases InvJ and SsaN. These findings show that the flagellar secretion apparatus functions as a proton-driven protein exporter and that ATP hydrolysis is not essential for type III secretion.     

Tissue ascorbic acid and a liver dehydroascorbatase in guinea-pigs.

The extraction, partial purification and assay of a dehydroascorbatase from guinea-pig liver is described. There was no evidence that changes in dehydroascorbatase activity could account for the modified tissue ascorbic acid concentrations associated with aging or with the ingestion of fluoride, flavonoids or anthocyanin material.     

Histochemistry of some trout respiratory muscles.

A histochemical study has been made of the main cranial muscles which produce ventilation movements of the rainbow trout. It is shown that a greater proportion of red(aerobic) fibres is present in those muscles known to be active during shallow ventilation than those which become active at greater ventilation volumes. An ordered recruitment of red, pink and white fibres within these muscles is also likely.