Impairment of hippocampal long-term depression and defective spatial learning and memory in p35 mice

Cdk5 (cyclin-dependent kinase 5) activity is dependent upon association with one of two neuron-specific activators, p35 or p39. Genetic deletion of Cdk5 causes perinatal lethality with severe defects in corticogenesis and neuronal positioning. p35(-/-) mice are viable with milder histological abnormalities. Although substantial evidence implicates Cdk5 in synaptic plasticity, its role in learning and memory has not been evaluated using mutant mouse models. We report here that p35(-/-) mice have deficiencies in spatial learning and memory. Close examination of hippocampal circuitry revealed subtle histological defects in CA1 pyramidal cells. Furthermore, p35(-/-) mice exhibit impaired long-term depression and depotentiation of long-term potentiation in the Schaeffer collateral CA1 pathway. Moreover, the Cdk5-dependent phosphorylation state of protein phosphatase inhibitor-1 was increased in 4-week-old mice due to increased levels of p39, which co-localized with inhibitor-1 and Cdk5 in the cytoplasm. These results demonstrate that p35-dependent Cdk5 activity is important to learning and synaptic plasticity. Deletion of p35 may shift the substrate specificity of Cdk5 due to compensatory expression of p39.     

Evolution of clonality and polyploidy in a weevil system

The increased interest in asexual organisms calls for in-depth studies of asexual complexes that actively give rise to new clones. We present an extensive molecular study of the Otiorhynchus scaber (Coleoptera, Curculionidae) weevil system. Three forms have traditionally been recognized: diploid sexuals, triploid, and tetraploid parthenogens. All forms coexist in a small central area, but only the polyploid parthenogens have colonized marginal areas. Analyzing the phylogenetic relationship, based on three partial mitochondrial genes, of 95 individuals from 19 populations, we find that parthenogenesis and polyploidy have originated at least three times from different diploid lineages. We observe two major mitochondrial lineages, with over 2.5% sequence divergence between the most basal groups within them, and find that current distribution and phylogenetic relationships are weakly correlated. Quite unexpectedly, we also discover diploid clones that coexist with, and are morphologically indistinguishable from, the diploid sexual females. Our results support that these diploid clones are derived directly from the diploid sexuals. We also find that it is mainly an increase in ploidy level and not the benefits of asexual reproduction that confers to polyploid parthenogens the advantage over their diploid sexual relatives.     

Long‐term infection of adult mice with murine polyomavirus following stereotaxic inoculation into the brain

Murine polyomavirus is used in various models of persistent virus infection. This study was undertaken to assess the spatial and temporal patterns of MPyV infection in the brains of immunocompetent (BALB/c) and immunocompromised (KSN nude) mice. MPyV was stereotaxically microinfused into the brain parenchyma, and the kinetics of infection were examined by quantitative PCR. In BALB/c mice, the amount of viral DNA in the brain peaked at 4 days p.i. and then rapidly diminished. In contrast, MPyV DNA levels increased up to 4 days and then gradually decreased over the 30‐day observation period in the brain of KSN mice. In both mouse strains, viral DNA was readily detected around the sites of inoculation from 2 to 6 days p.i., and continued to be detected for up to 30 days p.i. In addition, MPyV infection did not lead to a drastic induction of innate immune response in the brains, nor did MPyV‐inoculated mice show any signs of disease. These results indicate that MPyV establishes an asymptomatic long‐term infection in the mouse brain.     

Damped long‐term host–parasite Red Queen coevolutionary dynamics: a reflection of dilution effects?

An increase in biological diversity leads to a greater stability of ecosystem properties. For host–parasite interactions, this is illustrated by the ‘dilution effect’: a negative correlation between host biodiversity and disease risk. We show that a similar mechanism might stabilise host–parasite dynamics at a lower level of diversity, i.e. at the level of genetic diversity within host species. A long‐term time shift experiment, based on a historical reconstruction of a Daphnia–parasite coevolution, reveals infectivity cycles with more stable amplitude in experienced than in naive hosts. Coevolutionary models incorporating an increase in host allelic diversity over time explain the detected asymmetry. The accumulation of resistance alleles creates an opportunity for the host to stabilise Red Queen dynamics. It leads to a larger arsenal enhancing the host performance in its coevolution with the parasite in which ‘it takes all the running both antagonists can do to keep in the same place’.     

A new hypothesis for the origin of pentaploid Holcus from diploid Holcus lanatus L. and tetraploid Holcus mollis L. in France

The Holcus complex in France consists of two species, Holcus lanatus L. (Yorkshire fog; 2n= 2x= 14) and Holcus mollis L. (Creeping soft-grass; 2n= 4x= 28) and an interspecific hybrid H.m.×H.l. (2n= 5x= 35), which is morphologically similar to Holcus mollis. A heterologous rDNA probe from wheat was used to detect the corresponding region in Holcus (s. l.) genomic DNA'fragments, for six to eight plants from 13 populations located south-west of Paris. A restriction enzyme map of the ribosomal RNA genes (rDNA) in Holcus (s. l.) was also constructed. The length polymorphism detected in the IGS region was used as a DNA fingerprint for the identification of different cytotypes and species of the Holcus complex and for the typing and delimitation of individuals in populations. In the light of the results we reconsider the assumption that the pentaploid hybrid H.m.×H.l. is purely clonal. New hypotheses concerning the origin of the pentaploid hybrid and its reproduction are proposed, and the consequences for genetic diversity in natural populations discussed.