Spatial variation of seedling distribution in an east Mediterranean pine woodland at the beginning of post-fire succession

Most of the area in pine woodlands is occupied by perennial seeders that regenerate from seeds in the first winter after the fire and by annuals. Control of the germination in the regenerating vegetation after wildfire is therefore a primary ecological component of the post-fire succession in this ecosystem. The aim of the study presented here was to determine the distribution of Pinus, Cistus and other plants seeds around burned Pinus halepensis trees, and to measure the conditions related to seed germination in the upper soil layers in the same locations. The study was carried out in a 50-year old planted Pinus halepensis woodland that was burned down by a wildfire in July 1995. The variation of seedbank density was determined by collecting samples under the canopies of burned trees and in a nearby open area. Pine seedbank density decreased and that of Cistus and annuals increased with increasing distance from the burned trunks. Most pine seeds were present in the ash layer while those of the other plants were in the soil. In situ germination experiments showed that seedling density decreased with distance from the burned trunks while the proportion of pines in the seedling population increased. This was a result of seedbank variation and germination inhibition by the high pH conditions caused by the ash. The establishment of sparse pine seedling under the dead tree canopies insured their rapid development without interference by other plants and played a key role in the regeneration and stability of the pine woodland community. The concomitant mass germination of the perennial seeders in the rest of the area prevented invasion by annuals.     

Limits to reproductive success of Sarracenia purpurea (Sarraceniaceae)

Plant biologists have an enduring interest in assessing components of plant fitness and determining limits to seed set. Consequently, the relative contributions of resource and pollinator availability have been documented for a large number of plant species. We experimentally examined the roles of resource and pollen availability on seed set by the northern pitcher plant Sarracenia purpurea. We were able to distinguish the relative contributions of carbon (photosynthate) and mineral nutrients (nitrogen) to reproductive success. We also determined potential pollinators of this species. The bees Bombus affinis and Augochlorella aurata and the fly Fletcherimyia fletcheri were the only floral visitors to S. purpurea that collected pollen. Supplemental pollination increased seed set by <10%, a much lower percentage than would be expected, given data from noncarnivorous, animal-pollinated taxa. Seed set was reduced by 14% in plants that could not capture prey and by another 23% in plants whose pitcher-shaped leaves were also prevented from photosynthesizing. We conclude that resources are more important than pollen availability in determining seed set by this pitcher plant and that reproductive output may be another "cost" of the carnivorous habit.     

Role of nesting resources in organising diverse bee communities in a Mediterranean landscape

Abstract.  1. The habitat components determining the structure of bee communities are well known when considering foraging resources; however, there is little data with respect to the role of nesting resources.
2. As a model system this study uses 21 diverse bee communities in a Mediterranean landscape comprising a variety of habitats regenerating after fire. The findings clearly demonstrate that a variety of nesting substrates and nest building materials have key roles in organising the composition of bee communities.
3. The availability of bare ground and potential nesting cavities were the two primary factors influencing the structure of the entire bee community, the composition of guilds, and also the relative abundance of the dominant species. Other nesting resources shown to be important include availability of steep and sloping ground, abundance of plant species providing pithy stems, and the occurrence of pre-existing burrows.
4. Nesting resource availability and guild structure varied markedly across habitats in different stages of post-fire regeneration; however, in all cases, nest sites and nesting resources were important determinants of bee community structure.     

Bimodal colour pattern of individual Pinus halepensis Mill. seeds: a new type of crypsis

Variation in seed traits is a well‐known phenomenon affecting plant ecology and evolution. Here we describe, for the first time, a bimodal colour pattern of individual seeds, proposing an adaptive explanation, using Pinus halepensis as a model. Pinus halepensis disperses its seeds either by wind on hot dry days, from regular cones, or after fires, mainly from serotinous cones. Post‐dispersal seeds are exposed to strong predation by passerine birds, making crypsis important for seed survival. Individual seeds from non‐serotinous cones have a bimodal colour pattern: one side is light brown and the other black, exposing only one colour when lying on the ground. Serotinous cones from most trees have seeds with similar bimodal colour patterns, whereas seeds from serotinous cones of some trees are light brown on both sides. The dark side provides the seed with better crypsis on dark soils, whereas the light‐brown side is better adapted to light‐coloured soils, and mainly to light‐grey ash‐covered soil, which is the natural post‐fire regeneration niche of P. halepensis. The relative reflection curves of the black and brown seed colours differ, and their calculated relative chromatic distance is 5: meaning that seed‐predating passerine birds see them differently, and probably prefer seeds that present a higher contrast against the soil background. We propose that such a bimodal colour pattern of individual seeds is probably an overlooked general phenomenon mainly linked to seed dispersal in post‐fire and other heterogeneous environments. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 271–278.     

Spiny east Mediterranean plant species flower later and in a drier season than non-spiny species

We compared the flowering phenology of spiny and non-spiny native species belonging to three families (Asteraceae, Fabaceae and Lamiaceae), which include the highest number of spiny species in the flora of Israel. We found that the peak of flowering (when the highest number of species flowered) was 4–8 weeks later for spiny species than for non-spiny species. The flowering peak of non-spiny species was in late March, while that of spiny species was at the beginning of May. The seasonal shift in flowering time from the main season, when most Mediterranean plants bloom, to the end of the flowering season, when fewer species bloom, might be the evolutionary result of a change in phenology reducing the competition for pollinators. Our results clearly indicate that spinescence of plants in the semi-arid east Mediterranean region is associated with a delayed flowering season at the beginning of the dry summer when most of the herbaceous vegetation is already dry. During this season, mammalian grazers consume any edible herbaceous vegetation, selecting for late flowering species that allocate more resources for anti-herbivore defenses than early flowering species. There is a well-known global geographical trend where the occurrence of spiny plants is higher in arid regions than in humid ones. In parallel to the global trend, we show a seasonal one, that non-spiny plants grow and flower in the spring, which is the main flowering season in the Mediterranean basin, while spiny plants flower later, in the hot and dry summer. Under the current trend of global warming, there are prospects of future increase in the dominance of spiny species in the Mediterranean region.     

A sheep in wolf's clothing: do carrion and dung odours of flowers not only attract pollinators but also deter herbivores?

Carrion and dung odours of various flowers have traditionally been considered an adaptation for attracting the flies and beetles that pollinate them. While we accept the role of such odours in pollinator attraction, we propose that they may also have another, overlooked, anti‐herbivore defensive function. We suggest that such odours may deter mammalian herbivores, especially during the critical period of flowering. Carrion odour is a good predictor for two potential dangers to mammalian herbivores: (1) pathogenic microbes, (2) proximity of carnivores. Similarly, dung odour predicts faeces‐contaminated habitats that present high risks of parasitism. These are two new types of repulsive olfactory aposematic mimicry by plants: (1) olfactory feigning of carcass (thanatosis), a well‐known behavioural defensive strategy in animals, (2) olfactory mimicry of faeces, which also has a defensive visual parallel in animals.     

The story of cell fusion: big lessons from little worms

The ability of two or more cells to unite to form a new syncytial cell has been utilized in metazoans throughout evolution to form many complex organs, such as muscles, bones and placentae. This requires migration, recognition and adhesion between cells together with fusion of their plasma membranes and rearrangement of their cytoplasmic contents. Until recently, understanding of the mechanisms of cell fusion was restricted to fusion between enveloped viruses and their target cells. The identification of new factors that take part in developmental cell fusion in C. elegans opens the way to understanding how cells fuse and what the functions of this process are. In this review, we describe current knowledge on the mechanisms and putative roles of developmental cell fusion in C. elegans and how cell fusion is regulated, together with other intercellular processes to promote organogenesis.     

Dimerization in vivo and inhibition of the nonreceptor form of protein tyrosine phosphatase epsilon

cyt-PTP epsilon is a naturally occurring nonreceptor form of the receptor-type protein tyrosine phosphatase (PTP) epsilon. As such, cyt-PTP epsilon enables analysis of phosphatase regulation in the absence of extracellular domains, which participate in dimerization and inactivation of the receptor-type phosphatases receptor-type protein tyrosine phosphatase alpha (RPTPalpha) and CD45. Using immunoprecipitation and gel filtration, we show that cyt-PTP epsilon forms dimers and higher-order associations in vivo, the first such demonstration among nonreceptor phosphatases. Although cyt-PTP epsilon readily dimerizes in the absence of exogenous stabilization, dimerization is increased by oxidative stress. Epidermal growth factor receptor stimulation can affect cyt-PTP epsilon dimerization and tyrosine phosphorylation in either direction, suggesting that cell surface receptors can relay extracellular signals to cyt-PTP epsilon, which lacks extracellular domains of its own. The inactive, membrane-distal (D2) phosphatase domain of cyt-PTP epsilon is a major contributor to intermolecular binding and strongly interacts in a homotypic manner; the presence of D2 and the interactions that it mediates inhibit cyt-PTP epsilon activity. Intermolecular binding is inhibited by the extreme C and N termini of D2. cyt-PTP epsilon lacking these regions constitutively dimerizes, and its activities in vitro towards para-nitrophenylphosphate and in vivo towards the Kv2.1 potassium channel are markedly reduced. We conclude that physiological signals can regulate dimerization and phosphorylation of cyt-PTP epsilon in the absence of direct interaction between the PTP and extracellular molecules. Furthermore, dimerization can be mediated by the D2 domain and does not strictly require the presence of PTP extracellular domains.