Evidence for fine-scale habitat specialisation in an invasive weed

Aims As an exotic species colonises a new continent, it must overcome enormous environmental variation in its introduced range. Local adaptation of introduced species has frequently been observed at the continent scale, particularly in response to latitudinal climatic variation. However, significant environmental heterogeneity can also exist at the landscape scale. A small number of studies have provided evidence that introduced species may also be capable of phenotypic and genetic differentiation at much smaller spatial scales. For example, previously we found US agricultural and non-agricultural populations of Sorghum halepense (Johnsongrass) to be phenotypically and genetically distinct. In this study, we investigated whether this phenotypic differentiation of agricultural and non-agricultural populations of S. halepense is the result of fine-scale local specialisation.Methods We surveyed a nationally collected S. halepense germplasm panel and also collected neighbouring agricultural and non-agricultural sub-populations of S. halepense at four sites throughout Western Virginia, USA, raising seedlings in common conditions mimicking both agricultural and non-agricultural habitats.Important findings At the national scale, we found evidence of habitat differentiation but not specialisation. However, at the local scale, we found evidence of specialisation in two of the four local populations to non-agricultural habitat, but no evidence of specialisation to agricultural habitat. These results show that local specialisation is a possible, but not guaranteed consequence of kilometre-scale habitat heterogeneity in invasive species. This finding contributes to a growing awareness of the importance of fine-scale local adaptation in the ecology and management of introduced and weedy species.     

The Internal Organization of Mycobacterial Partition Assembly: Does the DNA Wrap a Protein Core?

Before cell division in many bacteria, the ParBs spread on a large segment of DNA encompassing the origin-proximal parS site(s) to form the partition assembly that participates in chromosome segregation. Little is known about the structural organization of chromosomal partition assembly. We report solution X-ray and neutron scattering data characterizing the size parameters and internal organization of a nucleoprotein assembly formed by the mycobacterial chromosomal ParB and a 120-meric DNA containing a parS-encompassing region from the mycobacterial genome. The cross-sectional radii of gyration and linear mass density describing the rod-like ParB-DNA assembly were determined from solution scattering. A “DNA outside, protein inside” mode of partition assembly organization consistent with the neutron scattering hydrogen/deuterium contrast variation data is discussed. In this organization, the high scattering DNA is positioned towards the outer region of the partition assembly. The new results presented here provide a basis for understanding how ParBs organize the parS-proximal chromosome, thus setting the stage for further interactions with the DNA condensins, the origin tethering factors and the ParA.     

The Value of Artificial Stimuli in Behavioral Research: Making the Case for Egg Rejection Studies in Avian Brood Parasitism

Experimentation is at the heart of classical and modern behavioral ecology research. The manipulation of natural cues allows us to establish causation between aspects of the environment, both internal and external to organisms, and their effects on animals' behaviors. In recognition systems research, including the quest to understand the coevolution of sensory cues and decision rules underlying the rejection of foreign eggs by hosts of avian brood parasites, artificial stimuli have been used extensively, but not without controversy. In response to repeated criticism about the value of artificial stimuli, we describe four potential benefits of using them in egg recognition research, two each at the proximate and ultimate levels of analysis: (1) the standardization of stimuli for developmental studies and (2) the disassociation of correlated traits of egg phenotypes used for sensory discrimination, as well as (3) the estimation of the strength of selection on parasitic egg mimicry and (4) the establishment of the evolved limits of sensory and cognitive plasticity. We also highlight constraints of the artificial stimulus approach and provide a specific test of whether responses to artificial cues can accurately predict responses to natural cues. Artificial stimuli have a general value in ethological research beyond research in brood parasitism and may be especially critical in field studies involving the manipulation of a single parameter, where other, confounding variables are difficult or impossible to control experimentally or statistically.     

Turbidity Structures the Controls of Ecosystem Metabolism and Associated Metabolic Process Domains Along a 75-km Segment of a Semiarid Stream

Ecosystems - Stream ecosystem metabolism contributes to global carbon cycling, yet predicting metabolism across ecosystems remains elusive. Even within stream segments, spatial variation in...     

mtDNA and language support a common origin of Micronesians and Polynesians in Island Southeast Asia

The origins and relationships among Micronesians, Polynesians, and Melanesians were investigated. Five different mtDNA region V length polymorphisms from 873 individuals representing 24 Oceanic and Asian populations were analyzed. The frequency cline of a common deletion and the distributions of a rare expanded length polymorphism support the origin of both Micronesians and Polynesians in Island Southeast Asia. Genetic, linguistic, and geographic distances were compared to assess the relative importance of isolation and gene flow during the prehistory of 19 Austronesian-speaking populations subdivided into five potential spheres of interaction. We observed significant correlations (P < 0.05) between genetic and linguistic distances in four of five comparisons. These data indicate extensive gene flow throughout much of Micronesia, but substantial isolation in other Pacific regions. Although recent advancements in our understanding of intentional voyaging within Remote Oceania have challenged the existence of the “myth of the primitive isolate,” we caution against the adoption of panmictic alternatives. Am J Phys Anthropol 105:109–119, 1998. © 1998 Wiley-Liss, Inc.     

Macroalgal spore dysfunction: ocean acidification delays and weakens adhesion

Early life stages of marine organisms are predicted to be vulnerable to ocean acidification. For macroalgae, reproduction and population persistence rely on spores to settle, adhere and continue the algal life cycle, yet the effect of ocean acidification on this critical life stage has been largely overlooked. We explicitly tested the biomechanical impact of reduced pH on early spore adhesion. We developed a shear flume to examine the effect of reduced pH on spore attachment time and strength in two intertidal rhodophyte macroalgae, one calcified (Corallina vancouveriensis) and one noncalcified (Polyostea robusta). Reduced pH delayed spore attachment of both species by 40%–52% and weakened attachment strength in C. vancouveriensis, causing spores to dislodge at lower flow‐induced shear forces, but had no effect on the attachment strength of P. robusta. Results are consistent with our prediction that reduced pH disrupts proper curing and gel formation of spore adhesives (anionic polysaccharides and glycoproteins) via protonation and cation displacement, although experimental verification is needed. Our results demonstrate that ocean acidification negatively, and differentially, impacts spore adhesion in two macroalgae. If results hold in field conditions, reduced ocean pH has the potential to impact macroalgal communities via spore dysfunction, regardless of the physiological tolerance of mature thalli.     

Expression of HvHMA2 in tobacco modifies Zn–Fe–Cd homeostasis

HvHMA2 is a plasma membrane P_1B-ATPase from barley that functions in Zn/Cd root-to-shoot transport. To assess the usefulness of HvHMA2 for modifying the metal content in aerial plant parts, it was expressed in tobacco under the CaMV35S promoter. Transformation with HvHMA2 did not produce one unique pattern of Zn and Cd accumulation; instead it depended on external metal supply. Thus Zn and Cd root-to-shoot translocation was facilitated, but not at all applied Zn/Cd concentrations. Metal uptake was restricted in HvHMA2-transformed plants and the level in the shoot was not enhanced. It was shown that HvHMA2 localizes to the plasma membrane of tobacco cells, and overloads the apoplast with Zn, which could explain the overall decrease in metal uptake observed. Despite the lower levels in the shoot, HvHMA2 transformants showed increased Zn sensitivity. Moreover, introduction of HvHMA2 into tobacco interfered with Fe metabolism and Fe accumulation was modified in HvHMA2-transformants in a Zn- and Cd-concentration dependent manner. The results indicate that ectopic expression of the export protein HvHMA2 in tobacco interferes with tobacco metal Zn–Cd–Fe cross-homeostasis, inducing internal mechanisms regulating metal uptake and tolerance.