New Sclerocactus (Cactaceae) from Nevada

Described is Sclerocactus blainei Welsh & Thorne, sp. nov. from Nye County, Nevada.        

Variation in Lewsia kelloggii (Portulacaceae) with description of a new species endemic to Idaho

Lewisia kelloggii has been understood as a rare plant with a disjunct range in California and Idaho. Examination of herbarium specimens and analysis of isozymes in 6 Idaho and 7 California populations revealed consistent differences between plants of the 2 states. Fixed differences in alleles at 2 loci (AAT2 and PGI1) distinguished Idaho from California plants. Genetic identities based on isozymes between Idaho and California populations averaged 0.58, lower than the average for congeneric plant species. Idaho plants were smaller than most California plants, but California plants were variable. The most consistent morphological difference between Idaho and California specimens was the difference in the number of glands on the margins of bracts and sepals. Idaho plants had 0 (-5) pink glands on each margin of these organs, all on teeth near the tips. In California plants these organs had 12-25 glands on each margin, the distal ones elevated on teeth and the proximal ones sessile. We recognize the Idaho plants as a new species, L. sacajaweana , and retain the name L. kelloggii for the California populations.     

The mixture method of clustering applied to three-way data

Clustering or classifying individuals into groups such that there is relative homogeneity within the groups and heterogeneity between the groups is a problem which has been considered for many years. Most available clustering techniques are applicable only to a two-way data set, where one of the modes is to be partitioned into groups on the basis of the other mode. Suppose, however, that the data set is three-way. Then what is needed is a multivariate technique which will cluster one of the modes on the basis of both of the other modes simultaneously. It is shown that by appropriate specification of the underlying model, the mixture maximum likelihood approach to clustering can be applied in the context of a three-way table. It is illustrated using a soybean data set which consists of multiattribute measurements on a number of genotypes each grown in several environments. Although the problem is set in the framework of clustering genotypes, the technique is applicable to other types of three-way data sets.     

Selective development of CD4+ T cells in transgenic mice expressing a class II MHC-restricted antigen receptor

T lymphocytes are predisposed to recognition of foreign protein fragments bound to cell-surface molecules encoded by the major histocompatibility complex (MHC). There is now compelling evidence that this specificity is a consequence of a selection process operating on developing T lymphocytes in the thymus. As a result of this positive selection, thymocytes that express antigen receptors with a threshold affinity for self MHC-encoded glycoproteins preferentially emigrate from the thymus and seed peripheral lymphoid organs. The specificity for both foreign antigen and MHC molecules is imparted by the alpha and beta chains of the T-cell antigen receptor (TCR). Two other T-cell surface proteins, CD4 and CD8, which bind non-polymorphic regions of class II and class I MHC molecules respectively, are also involved in these recognition events and play an integral role in thymic selection. In order to elucidate the developmental pathways of class II MHC-restricted T cells in relation to these essential accessory molecules, we have produced TCR-transgenic mice expressing a receptor specific for a fragment of pigeon cytochrome c and the Ek (class II MHC) molecule. The transgenic TCR is expressed on virtually all T cells in mice expressing Ek. The thymuses of these mice contain an abnormally high percentage of mature CD4+CD8- cells. In addition, the peripheral T-cell population is almost exclusively CD4+, demonstrating that the MHC specificity of the TCR determines the phenotype of T cells during selection in the thymus.     

Analysis of specificity for antigen, Mls, and allogenic MHC by transfer of T-cell receptor alpha- and beta-chain genes

The majority of peripheral T lymphocytes bear cell-surface antigen receptors comprised of a disulphide-linked alpha beta dimer. In an immune response, this receptor endows T cells with specificities for foreign antigenic protein fragments bound to cell surface glycoproteins encoded in the major histocompatibility complex (MHC). At a high frequency (greater than 1%), the same population of T lymphocytes responds to allogeneic MHC glycoproteins, or to differences at other genetic loci termed Mls, in conjunction with MHC. The alpha beta-antigen receptor has been implicated in alloreactivity and Mls reactivity. In fact, many monoclonal T-cell lines recognize a foreign protein fragment bound to self-MHC molecules and, in addition, recognize allogeneic MHC glycoproteins, an Mls-encoded determinant, or both. For at least one T-cell clone, a monoclonal antibody directed against the alpha beta antigen receptor has been shown to block activation induced by either antigen-bound self-MHC or by allogeneic MHC. However, it remains to be demonstrated directly that a single alpha beta receptor can mediate antigen specificity, alloreactivity and Mls reactivity, a prerequisite to understanding the structural basis of these high-frequency cross-reactivities. To address this issue we have performed transfers of receptor chain genes from a multiple-reactive T-cell clone into an unrelated host T lymphocyte. We now demonstrate definitively that the genes encoding a single alpha beta-receptor chain pair can transfer the recognition of self-MHC molecules complexed with fragments of antigen, allogeneic MHC molecules, and an Mls-encoded determinant (presumably in conjunction with MHC). In this case the transfer of antigen specificity and alloreactivity requires a specific alpha beta-receptor chain combination, whereas Mls reactivity can be transferred with the beta-chain gene alone into a recipient expressing a randomly selected alpha-chain.     

Plants of Arizona: a new species of Astragalus from the Kaibab Plateau

Astragalus atwoodii Welsh and Thorne is named and described on the basis of materials collected on the Kaibab Plateau in Coconino County, Arizona.     

The ageing systemic milieu negatively regulates neurogenesis and cognitive function

In the central nervous system, ageing results in a precipitous decline in adult neural stem/progenitor cells and neurogenesis, with concomitant impairments in cognitive functions. Interestingly, such impairments can be ameliorated through systemic perturbations such as exercise. Here, using heterochronic parabiosis we show that blood-borne factors present in the systemic milieu can inhibit or promote adult neurogenesis in an age-dependent fashion in mice. Accordingly, exposing a young mouse to an old systemic environment or to plasma from old mice decreased synaptic plasticity, and impaired contextual fear conditioning and spatial learning and memory. We identify chemokines--including CCL11 (also known as eotaxin)--the plasma levels of which correlate with reduced neurogenesis in heterochronic parabionts and aged mice, and the levels of which are increased in the plasma and cerebrospinal fluid of healthy ageing humans. Lastly, increasing peripheral CCL11 chemokine levels in vivo in young mice decreased adult neurogenesis and impaired learning and memory. Together our data indicate that the decline in neurogenesis and cognitive impairments observed during ageing can be in part attributed to changes in blood-borne factors.