Biochemical Inhibition of Positional Signalling in the Avian Limb Bud

A region at the posterior margin of the developing avian limb bud, the zone of polarizing activity, appears to be responsible for signalling positional information along the limb antero-posterior axis. The mechanism of signalling is unknown and, unfortunately, no subcellular preparation from the polarizing region has shown polarizing activity in vivo. We have performed a series of experiments in which isolated polarizing regions were. treated with chemical agents or inhibitors prior to being grafted into anterior sites on host limb buds. A previous paper described the effects of some metabolic and biosynthetic inhibitors [5]. This paper describes the results of treatments with agents that primarily affect cell or cell surface integrity, or intracellular small molecules such as those involved in sulphydryl or cation balance. Chick and quail polarizing regions are compared, and a disaggregated cell assay is used to analyze inhibition. Drugs affecting cytostructure (colchicine, vinblastine, and cytochalasin B) inhibited the activity of polarizing regions, but did not affect the activity of treated cell suspensions, thus their action seemed dependent on retention of the agent by tissue. Inhibition observed with metabolic and biosynthetic inhibitors did not appear to involve drug retention. Agents interfering with cell surface integrity (meta-periodate, endoglycosidases, and concanavalin A) did not greatly interfere with polarizing activity at concentrations where they effectively eliminated cell spreading. Aldehyde fixatives or Triton X-100 abolished polarizing activity. Ouabain had little effect on positional signalling, but valinomycin abolished activity.     

Heterochrony and its role in sex determination of cryptically dioecious Consolea (Cactaceae) staminate flowers

Ovule development, megasporogenesis, and megagametogenesis were studied in six cryptically dioecious species of Consolea. All species showed uniform development typical for the Opuntioideae. Ovule development proceeds acropetally, but shows developmental asynchrony across floral morphs. At anthesis, female morph ovules are functional and available for fertilization, whereas staminate flower ovules are senescing and incapable of being fertilized. In occasional plants of some species, staminate flowers may reach anthesis with a few functional apical ovules capable of seed formation. Such plants are described as inconstant/leaky males. Ovule fertility differences across morphs are interpreted as resulting from heterochronic ovule development and senescence, although variation in embryo sac longevity cannot be ruled out. Significantly, ovule abortion follows a common pattern and timing in staminate flowers of both male morphs in all species. Thus, on the basis of this uniformity, a common origin for the cryptically dioecious breeding system in Consolea is hypothesized. Furthermore, staminate expression in Consolea appears to be controlled by a common, genetically determined heterochronic ovule developmental programme affecting the relative timing of ovule receptivity and flower opening. This is the first report of heterochrony as a mechanism of male sex determination.  © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society , 2008, 156 , 305–326.     

Sett density as an estimator of population density in the European badger Meles meles

• 1 Estimations of European badger population density in the UK are usually based on surveys of numbers of main setts. However, this approach cannot be used in low‐density areas, such as the Mediterranean region, where no main setts can be defined. Therefore, an alternative method is needed to estimate badger density over large areas.
• 2 We reviewed the existing published information to evaluate whether badger density is correlated to the density of all setts in an area (not only the main setts) and to ask whether badger density can be predicted from total sett number throughout the geographical range of the species.
• 3 In multiple regression analysis, badger density and the size of the study area explained 73% of the variance in sett density. Badger density had a significant positive effect on sett density, while the effect of study area size was not significant. Therefore, total sett density can be used to obtain an estimation of relative badger density in all habitats and regions throughout the badger's geographical range, allowing comparative research.

     

When dispersal fails: unexpected genetic separation in Gibraltar macaques (Macaca sylvanus)

Barbary macaques (Macaca sylvanus), now restricted in the wild to a few isolated forested areas of Morocco and Algeria, are present in a free‐ranging colony on Gibraltar. For many decades, the Gibraltar colony was exposed to multiple bottlenecks due to highly nonrandom removal of animals, followed by repeated introductions of animals from North Africa. Moreover, because of complete isolation, Gibraltar's several social groups of macaques provide an ideal system to study the genetic consequences of dispersal in cercopithecines in situ. Predictions of genetic consequences due to male‐biased dispersal in cercopithecines will be different for autosomal and maternally inherited genetic markers, such as the control region of the mitochondrial DNA. We used a panel of 14 highly polymorphic microsatellite loci and part of the hypervariable region I of the mitochondrial control region to estimate genetic structure between five social groups in Gibraltar. Surprisingly, for autosomal markers, both classical summary statistics and an individual‐based method using a Bayesian framework detected significant genetic structure between social groups in Gibraltar, despite much closer proximity than wild Algerian and Moroccan populations. Mitochondrial data support this finding, as a very substantial portion of the total genetic variation (70.2%) was found between social groups. Using two Bayesian approaches, we likewise identified not only a small number of male first‐generation immigrants (albeit less than expected for cercopithecines) but also unexpectedly a few females. We hypothesize that the culling of males that are more likely to disperse might slow down genetic homogenization among neighbouring groups, but may also and more perversely produce selection on certain behavioural traits. This may have important repercussions for conservation, as it could lead to evolutionary changes that are not due to inbreeding or genetic drift.     

Parallel polyploid speciation: distinct sympatric gene‐pools of recurrently derived allo‐octoploid Asplenium ferns

Although polyploidy is widespread, its significance to the generation of biodiversity remains unclear. Many polyploids have been derived recurrently. For a particular polyploid, gene‐flow between the products of independent origin is typical where they come into contact. Here, we use AFLP DNA‐fingerprinting and chloroplast DNA sequences to demonstrate parallel polyploid speciation within both of the ferns Asplenium cimmeriorum and A. gracillimum. Both of these taxa comprise at least two allopolyploids, recurrently derived from the same progenitor pair. Each of these allopolyploids remain genetically distinguishable even with extensive sympatry, and could therefore be considered distinct species. To our knowledge, parallel speciation on this scale amongst recurrent polyploids has not been previously reported. With their parallel origins, these ‘evolutionary replicates’ provide an unrivalled opportunity to investigate how the reproductive barriers and ecological differentiation necessary for speciation arise following polyploidy.     

Climate change effects on plant biomass alter dominance patterns and community evenness in an experimental old‐field ecosystem

Atmospheric and climatic change can alter plant biomass production and plant community composition. However, we know little about how climate change‐induced alterations in biomass production affect plant species composition. To better understand how climate change will alter both individual plant species and community biomass, we manipulated atmospheric [CO₂], air temperature, and precipitation in a constructed old‐field ecosystem. Specifically, we compared the responses of dominant and subdominant species to our climatic treatments, and explored how changes in plant dominance patterns alter community evenness over 2 years. Our study resulted in four major findings: (1) all treatments, elevated [CO₂], warming, and increased precipitation increased plant community biomass and the effects were additive rather than interactive, (2) plant species differed in their response to the treatments, resulting in shifts in the proportional biomass of individual species, which altered the plant community composition; however, the plant community response was largely driven by the positive precipitation response of Lespedeza, the most dominant species in the community, (3) precipitation explained most of the variation in plant community composition among treatments, and (4) changes in precipitation caused a shift in the dominant species proportional biomass that resulted in lower community evenness in the wet relative to dry treatments. Interestingly, compositional and evenness responses of the subdominant community to the treatments did not always follow the responses of the whole plant community. Our data suggest that changes in plant dominance patterns and community evenness are an important part of community responses to climatic change, and generally, that such compositional shifts can alter ecosystem biomass production and nutrient inputs.