Self-Generated Chemoattractant Gradients: Attractant Depletion Extends the Range and Robustness of Chemotaxis

Chemotaxis is fundamentally important, but the sources of gradients in vivo are rarely well understood. Here, we analyse self-generated chemotaxis, in which cells respond to gradients they have made themselves by breaking down globally available attractants, using both computational simulations and experiments. We show that chemoattractant degradation creates steep local gradients. This leads to surprising results, in particular the existence of a leading population of cells that moves highly directionally, while cells behind this group are undirected. This leading cell population is denser than those following, especially at high attractant concentrations. The local gradient moves with the leading cells as they interact with their surroundings, giving directed movement that is unusually robust and can operate over long distances. Even when gradients are applied from external sources, attractant breakdown greatly changes cells'' responses and increases robustness. We also consider alternative mechanisms for directional decision-making and show that they do not predict the features of population migration we observe experimentally. Our findings provide useful diagnostics to allow identification of self-generated gradients and suggest that self-generated chemotaxis is unexpectedly universal in biology and medicine.     

Does stream flow structure woody riparian vegetation in subtropical catchments?

The primary objective of this study was to test the relevance of hydrological classification and class differences to the characteristics of woody riparian vegetation in a subtropical landscape in Queensland, Australia. We followed classification procedures of the environmental flow framework ELOHA – Ecological Limits of Hydrologic Alteration. Riparian surveys at 44 sites distributed across five flow classes recorded 191 woody riparian species and 15, 500 individuals. There were differences among flow classes for riparian species richness, total abundance, and abundance of regenerating native trees and shrubs. There were also significant class differences in the occurrence of three common tree species, and 21 indicator species (mostly native taxa) further distinguished the vegetation characteristics of each flow class. We investigated the influence of key drivers of riparian vegetation structure (climate, depth to water table, stream‐specific power, substrate type, degree of hydrologic alteration, and land use) on riparian vegetation. Patterns were explained largely by climate, particularly annual rainfall and temperature. Strong covarying drivers (hydrology and climate) prevented us from isolating the independent influences of these drivers on riparian assemblage structure. The prevalence of species considered typically rheophytic in some flow classes implies a more substantial role for flow in these classes but needs further testing. No relationships were found between land use and riparian vegetation composition and structure. This study demonstrates the relevance of flow classification to the structure of riparian vegetation in a subtropical landscape, and the influence of covarying drivers on riparian patterns. Management of environmental flows to influence riparian vegetation assemblages would likely have most potential in sites dominated by rheophytic species where hydrological influences override other controls. In contrast, where vegetation assemblages are dominated by a diverse array of typical rainforest species, and other factors including broad‐scale climatic gradients and topographic variables have greater influence than hydrology, riparian vegetation is likely to be less responsive to environmental flow management.     

Epistatic partners of neurogenic genes modulate Drosophila olfactory behavior

The extent to which epistasis affects the genetic architecture of complex traits is difficult to quantify, and identifying variants in natural populations with epistatic interactions is challenging. Previous studies in Drosophila implicated extensive epistasis between variants in genes that affect neural connectivity and contribute to natural variation in olfactory response to benzaldehyde. In this study, we implemented a powerful screen to quantify the extent of epistasis as well as identify candidate interacting variants using 203 inbred wild‐derived lines with sequenced genomes of the Drosophila melanogaster Genetic Reference Panel (DGRP). We crossed the DGRP lines to P[GT1]‐element insertion mutants in Sema‐5c and neuralized (neur), two neurodevelopmental loci which affect olfactory behavior, and to their coisogenic wild‐type control. We observed significant variation in olfactory responses to benzaldehyde among F₁ genotypes and for the DGRP line by mutant genotype interactions for both loci, showing extensive nonadditive genetic variation. We performed genome‐wide association analyses to identify the candidate modifier loci. None of these polymorphisms were in or near the focal genes; therefore, epistasis is the cause of the nonadditive genetic variance. Candidate genes could be placed in interaction networks. Several candidate modifiers are associated with neural development. Analyses of mutants of candidate epistatic partners with neur (merry‐go‐round (mgr), prospero (pros), CG10098, Alhambra (Alh) and CG12535) and Sema‐5c (CG42540 and bruchpilot (brp)) showed aberrant olfactory responses compared with coisogenic controls. Thus, integrating genome‐wide analyses of natural variants with mutations at defined genomic locations in a common coisogenic background can unmask specific epistatic modifiers of behavioral phenotypes.     

The role of double-stranded break repair in the creation of phenotypic diversity at cereal VRN1 loci

Nonhomologous repair of double-stranded breaks, although fundamental to the maintenance of genomic integrity in all eukaryotes, has received little attention as to its evolutionary consequences in the generation and selection of phenotypic diversity. Here we document the role of illegitimate recombination in the creation of novel alleles in VRN1 orthologs selected to confer adaptation to annual cropping systems in barley and wheat.     

Recovery of human mesenchymal stem cells following dehydration and rehydration

As cell therapies advance from research laboratories to clinical application, there is the need to transport cells and tissues across long distances while maintaining cell viability and function. Currently cells are successfully stored and shipped under liquid nitrogen vapor. The ability to store these cells in the desiccated state at ambient temperature would provide tremendous economic and practical advantage. Human mesenchymal stem cells (hMSCs) have broad potential uses in tissue engineering and regeneration since they can differentiate along multiple lineages and support hematopoeisis. The current research applied recent technological advances in the dehydration and storage of human fibroblasts to hMSCs. Three conditions were tested: air-dried, air-dried and stored under vacuum (vacuum only), and incubated with 50 mM trehalose + 3% glycerol and then air-dried and stored under vacuum (vacuum + trehalose). Plates containing dehydrated hMSCs were shipped from San Diego to Baltimore overnight in separate FedEx cardboard boxes. The hMSCs were rehydrated with 3 ml of hMSC medium and were able to regain their spindle-shaped morphology and adhesive capability. In addition, they maintained high viability and proliferation capacity. Rehydrated and passaged cells continued to express the characteristic hMSC surface antigen panel. Additionally, cells showed constitutive levels of mRNA for a stromal factor and, when exposed to reagents known to induce differentiation, demonstrated upregulation of two tissue-specific messages indicative of differentiation potential for fat and bone. While our preliminary findings are encouraging, we still need to address consistency and duration of storage by considering factors such as cell water content, oxygen concentration, and the presence of free radicals.     

Connexin46 mutations in autosomal dominant congenital cataract

Loci for autosomal dominant "zonular pulverulent" cataract have been mapped to chromosomes 1q (CZP1) and 13q (CZP3). Here we report genetic refinement of the CZP3 locus and identify underlying mutations in the gene for gap-junction protein alpha-3 (GJA3), or connexin46 (Cx46). Linkage analysis gave a significantly positive two-point LOD score (Z) at marker D13S175 (maximum Z [Zmax]=>7.0; maximum recombination frequency [thetamax] =0). Haplotyping indicated that CZP3 probably lies in the genetic interval D13S1236-D13S175-D13S1316-cen-13pter, close to GJA3. Sequencing of a genomic clone isolated from the CZP3 candidate region identified an open reading frame coding for a protein of 435 amino acids (47,435 D) that shared approximately 88% homology with rat Cx46. Mutation analysis of GJA3 in two families with CZP3 detected distinct sequence changes that were not present in a panel of 105 normal, unrelated individuals. In family B, an A-->G transition resulted in an asparagine-to-serine substitution at codon 63 (N63S) and introduced a novel MwoI restriction site. In family E, insertion of a C at nucleotide 1137 (1137insC) introduced a novel BstXI site, causing a frameshift at codon 380. Restriction analysis confirmed that the novel MwoI and BstXI sites cosegregated with the disease in families B and E, respectively. This study identifies GJA3 as the sixth member of the connexin gene family to be implicated in human disease, and it highlights the physiological importance of gap-junction communication in the development of a transparent eye lens.