Under siege: virus control in plant meristems and progeny

In the arms race between plants and viruses, two frontiers have been utilized for decades to combat viral infections in agriculture. First, many pathogenic viruses are excluded from plant meristems, which allows the regeneration of virus-free plant material by tissue culture. Second, vertical transmission of viruses to the host progeny is often inefficient, thereby reducing the danger of viral transmission through seeds. Numerous reports point to the existence of tightly linked meristematic and transgenerational antiviral barriers that remain poorly understood. In this review, we summarize the current understanding of the molecular mechanisms that exclude viruses from plant stem cells and progeny. We also discuss the evidence connecting viral invasion of meristematic cells and the ability of plants to recover from acute infections. Research spanning decades performed on a variety of virus/host combinations has made clear that, beside morphological barriers, RNA interference (RNAi) plays a crucial role in preventing—or allowing—meristem invasion and vertical transmission. How a virus interacts with plant RNAi pathways in the meristem has profound effects on its symptomatology, persistence, replication rates, and, ultimately, entry into the host progeny.

We review what is known about the biological mechanisms regulating virus exclusion from—or invasion of—plant host meristems and progeny, including possible consequences and implications of these phenomena.     

Spider diversity in a tropical habitat gradient in Chiapas, Mexico

This paper presents an assessment of spider diversity in a complex landscape of southern Mexico. Eighteen different habitats were identified, measured and mapped across this fragmented landscape. Habitat types were characterized by measuring various features, including number of plant forms, tree cover and litter depth. Each month from February to April (dry season) and from June to August 2002 (wet season), spiders were sampled on each habitat by using pitfall traps and direct collection. Correlations between spider diversity and habitat characteristics were carried out to explore the relative contribution of each habitat variable as related to changes in spider composition and richness. In total, 115 spider species were recorded in 18 habitat types, and the mean number and density of species per habitat were 21 (± 3, standard error of mean) and 57 (± 9), respectively. The species recorded represent 41% of the fauna recorded in the Mexican state of Chiapas and 4% of the fauna recorded in Mexico. Relatively pristine habitats (e.g. deciduous forest) contained an important proportion of spider diversity in this fragmented landscape. Epigean spider diversity was significantly correlated with tree cover and with the diversity of plant forms during the rainy season. No correlation was detected between soil spider diversity and the habitat variables measured for any season. The results of this work suggest that in highly fragmented tropical landscapes, some habitat types (e.g. coffee plantations, hedgerows) might play an important role for the persistence of spider populations. The prevalence of relatively stable conditions in some of these habitats can allow spiders species (e.g. Nephila clavipes ) to overcome adverse conditions such as a decrease in humidity and dramatic changes in temperature and wind exposure, allowing them to recolonize when favourable conditions return.     

Mitochondrial DNA under siege in avian phylogeography

Mitochondrial DNA (mtDNA) has been the workhorse of research in phylogeography for almost two decades. However, concerns with basing evolutionary interpretations on mtDNA results alone have been voiced since the inception of such studies. Recently, some authors have suggested that the potential problems with mtDNA are so great that inferences about population structure and species limits are unwarranted unless corroborated by other evidence, usually in the form of nuclear gene data. Here we review the relative merits of mitochondrial and nuclear phylogeographical studies, using birds as an exemplar class of organisms. A review of population demographic and genetic theory indicates that mitochondrial and nuclear phylogeographical results ought to concur for both geographically unstructured populations and for populations that have long histories of isolation. However, a relatively common occurrence will be shallow, but geographically structured mtDNA trees--without nuclear gene corroboration--for populations with relatively shorter periods of isolation. This is expected because of the longer coalescence times of nuclear genes (approximately four times that of mtDNA); such cases do not contradict the mtDNA inference of recent isolation and evolutionary divergence. Rather, the nuclear markers are more lagging indicators of changes in population structure. A review of the recent literature on birds reveals the existence of relatively few cases in which nuclear markers contradict mitochondrial markers in a fashion not consistent with coalescent theory. Preliminary information from nuclear genes suggests that mtDNA patterns will prove to be robust indicators of patterns of population history and species limits. At equilibrium, mitochondrial loci are generally a more sensitive indicator of population structure than are nuclear loci, and mitochondrial estimates of F(ST)-like statistics are generally expected to exceed nuclear ones. Hence, invoking behavioural or ecological explanations of such differences is not parsimonious. Nuclear genes will prove important for quantitative estimates of the depths of haplotype trees, rates of population growth and values of gene flow.     

Keepers of maize in Chiapas,Mexico

This study concerns the knowledge and selection of biological diversity of maize (Zea mays) within its center of domestication in Mesoamerica. Maize farmers in central Chiapas of Southern México keep local varieties (landraces) belonging to six races and four race mixtures. Fifteen local varieties are recognized. In spite of widespread adoption of a modern, high yielding variety, maize farmers continue to select local varieties for specific soils and because of agronomic and use criteria. Farmers maintain maize varieties primarily through seed selection. Spatial and temporal separation do not seem sufficient to maintain varieties. The management of improved varieties can lead from a uniform population to a heterogeneous one as hybridization with local maize populations occurs.     

Maternal serum alpha-fetoprotein screening activities of state health agencies: a survey.

Maternal serum alpha-fetoprotein (MSAFP) screening has been demonstrated to be cost-effective on a population basis and is becoming standard practice. The American Society of Human Genetics has twice published policy statements to define the essential elements of a quality screening program. The present study reviewed the impact of these policy statements on state public-health agencies with respect to regulation or provision of MSAFP screening in their jurisdictions. With a few exceptions, states have not elected to play a major role in provision or regulation of this test. There is need to address issues of funding, standards, and data collection in a collaborative effort, if policy statements on genetic services are to be translated into effective state population screening.     

The thymus under siege: Lmo2 induces precancerous stem cells in a mouse model of T-ALL

Comment on: McCormack MP, et al. Science 2010; 327:879-83.     

Mayan Voices for Human Rights: Displaced Catholics in Highland Chiapas

Mayan Voices for Human Rights: Displaced Catholics in Highland Chiapas. Christine Kovic. Austin: University of Texas Press, 2005. 238 pp.     

Histories and Stories from Chiapas: Border Identities in Southern Mexico.

Histories and Stories from Chiapas: Border Identities in Southern Mexico. R. Aída Hernández Castillo. Texas: University of Texas Press, 2001. 317 pp.