The role of epiphytism in architecture and evolutionary constraint within mycorrhizal networks of tropical orchids

Characterizing the architecture of bipartite networks is increasingly used as a framework to study biotic interactions within their ecological context and to assess the extent to which evolutionary constraint shape them. Orchid mycorrhizal symbioses are particularly interesting as they are viewed as more beneficial for plants than for fungi, a situation expected to result in an asymmetry of biological constraint. This study addressed the architecture and phylogenetic constraint in these associations in tropical context. We identified a bipartite network including 73 orchid species and 95 taxonomic units of mycorrhizal fungi across the natural habitats of Reunion Island. Unlike some recent evidence for nestedness in mycorrhizal symbioses, we found a highly modular architecture that largely reflected an ecological barrier between epiphytic and terrestrial subnetworks. By testing for phylogenetic signal, the overall signal was stronger for both partners in the epiphytic subnetwork. Moreover, in the subnetwork of epiphytic angraecoid orchids, the signal in orchid phylogeny was stronger than the signal in fungal phylogeny. Epiphytic associations are therefore more conservative and may co‐evolve more than terrestrial ones. We suggest that such tighter phylogenetic specialization may have been driven by stressful life conditions in the epiphytic niches. In addition to paralleling recent insights into mycorrhizal networks, this study furthermore provides support for epiphytism as a major factor affecting ecological assemblage and evolutionary constraint in tropical mycorrhizal symbioses.     

Use of time in a decision‐making process by a parasitoid

1. Time perception is seldom studied in invertebrates, with the limited experimental evidence being insufficient to provide a comprehensive pattern of the capacity of invertebrates to measure time and use it in decision‐making processes. 2. In this study, it was hypothesized that insect parasitoids have evolved the capacity to measure time precisely and to use it to optimize foraging decisions related to host exploitation. To examine time perception in females of the gregarious egg parasitoid Trichogramma euproctidis, the present study used their ability to adjust their investment (number of eggs laid) in a host to the initial transit duration (interval between the first contact with the host and the following contact with the substrate). Females utilize this method to assess host egg size, as a large egg necessarily requires more time to evaluate than a small host. In this study, the initial transit duration for a given sized egg was artificially extended by suspending it. 3. For similar sized hosts, female T. euproctidis significantly increased both oviposition duration and progeny allocation following a longer initial transit duration. 4. These results demonstrate the intrinsic capacity of this parasitoid to measure time and to adjust their progeny investment accordingly. This is believed to be one of the few demonstrations of a retrospective measure of time in an invertebrate.     

Internal Nutrient Translocation in Chestnut Tree Stemwood: III. Dynamics Across an Age Series of Castanea sativa (Miller)

Nutrient translocation in chestnut tree stemwood was calculatedfrom the distribution of nutrient content throughout the tissuelife-span. The dynamics of internal translocation were followedduring the crop rotation by means of an age series of five coppicedstands (2–19 years). N, P, K, Ca and Mg contents in treerings were estimated from the concentrations along a verticaland radial gradient and from the ring volume obtained usingstem ring analysis.Real nutrient translocation was calculatedstepwise between successive stages in the age series;apparenttranslocation was computed on a complete tree rotation by comparingthe initial content just after the ring was formed with themineral content in the oldest stand. There was a marked translocationof N, P, K and Ca when the rings were physiologically-activetissues. Real translocation of N, P and K (but not Ca) increasedwith stand age, obviously in parallel with the enlarged stemwoodbiomass reaching 23.2 and 20.6 kg ha^-1for K and N in the lastyears of rotation, nearly 5 kg ha^-1for Mg and about 3 kg ha^-1forCa and P. Potassium was the most mobile element since translocationreached 60% of the total amount immobilized in the stemwoodat the end of the rotation, whereas values for N, P and Mg wereapproximately 25% and 10% for calcium. Total apparent translocationreached respectively 39.2 and 32.4 kg ha^-1for K, N, approximately12 and 7 kg ha^-1for Mg and Ca and only 4.4 kg ha^-1for P. Totalapparent translocation as a percentage of total wood immobilizationwas 114% for K, 83% for Mg, 63% for P, but only 39% for N and24% for calcium. Translocation; nutrient content; stemwood; tree ring; coppice; age series; dynamics; chestnut tree; Castanea sativa Miller     

Dynamic information and host acceptance by a tephritid fruit fly

Abstract. 1. Female apple maggot (Rhagoletis pomonella Walsh) flies held in field cages usually oviposited in an unparasitized (non-pheromone marked) fruit when it was encountered.
2. Oviposition in a previously parasitized (pheromone marked) fruit depended upon the time since the last oviposition (TSLO) and the percentage of infested fruit encountered during search for oviposition sites.
3. Previous theories of host acceptance suggest that the acceptance or rejection of a host should depend dichotomously on time since last oviposition and the fraction of marked hosts in the last five encounters. The experiments, however, show considerable variability and are thus not consistent with the theory.
4. A new theory for the experiments is introduced. This model involves physiological (egg complement) and informational state variables and leads to intuitive understanding of the experimental results. In particular, the model shows how the plasticity in oviposition site selection may arise from fitness maximizing behaviour. Alternative models are also discussed. All of the models stress the importance of physiological and informational states.     

Post-juvenile dispersal of Hazel Grouse Bonasa bonasia in an expanding population of the southeastern French Alps

We studied the post-juvenile dispersal of 18 radiotagged juvenile Hazel Grouse Bonasa bonasia (14 males, four females) in an expanding population in the southeastern French Alps between 1998 and 2001. The mean dispersal distances between the capture sites of juveniles in September–October and the centre of the home range in the following spring was 4 km for males (range 0.1–24.9 km) and 2 km for females (range 0.2–5.6 km). The distances recorded for two long-dispersing males (15 and 24.9 km) are greater than those reported to date for Hazel Grouse. Using our radiotracking data, we interpret the pattern of range expansion that has been occurring since the 1950s around our study area. Barriers to dispersal included rocky ground and other alpine habitats above 2000 m and over 1 km wide, but Hazel Grouse did cross open agricultural land at lower elevation. Two patterns of dispersal movements were recognized in juveniles: erratic movements that led to settlement on or near the natal site, and direct movements to a new range relatively far from the natal area. We discuss the adaptive consequences of these different behaviour patterns.     

Modifications to the photosynthetic apparatus of higher plants in response to changes in the light environment

A brief review of the photosynthetic apparatus of higher plants is given, followed by a consideration of the modifications induced in this apparatus by changes in light intensity and light quality. Possible strategies by which plants may optimize photosynthetic activity by both long- and short-term modifications of their photosynthetic apparatus in response to changing light regimes are discussed.     

SEM Characterization of an Extracellular Matrix Around Somatic Proembryos in Roots of Cichorium

Roots of in vitro plantlets of a hybrid Cichorium intybus L.x C. endivia L. placed for 10 d in an agitated liquid inductionmedium in darkness at 35 °C give somatic embryos of varioussizes which disrupt the epidermis. Proembryos can be observedinside the root; they show a fibrillar network linking the surfacecells. The network is observed in agitated and static liquidmedium; it is not well developed on solid medium. It is notremoved by lipid solvents and pectinase but disappears partlywith protease. Ether-methanol (1:1 v/v) for 45 min and Tris-HC1buffer for 3 h at 30 °C destroy it. As in animal cells suchexternal proteic networks are constituents of an extracellularmatrix linked to the cytoskeleton, we tested microtubule destabilizationby colchicine and cold treatments which removed the network;this effect was reversed by DMSO and high temperature. Cichorium, extracellular matrix, extracellular proteins, somatic embryogenesis     

Pollen flow, population composition, and the adaptive significance of distyly in Linum tenuifolium L. (Linaceae)

The function of distyly as a device promoting inter-morph pollination was proposed by Darwin over a century ago, though only recently have experimental studies been undertaken to test this hypothesis. These studies are unrealistic since they assume regular spatial distribution of morphs within a population and fail to consider intra-flower pollination in assessing distyly's effects. Pollen flow in two populations of Linum tenuifolium L. was investigated using a fluorescent-dye marker technique. One population was dimorphic and self-incompatible; the other was monomorphic and self-compatible. In both populations pollen dispersal was leptokurtic with over 75% of grains detected within 5 m from the source. Intra-flower pollinations comprised a large proportion of individual stigma loads (42% for the dimorphic form; 49% for the monomorphic form). Total stigmatic loads for the dimorphic population showed equal proportions of pin and thrum pollen on both long-style and short-style stigmas, though variation in individual loads was marked. Values ranging from 34.2 to 61.5% legitimate pollen were recorded. The irregular distribution of morphs observed at the dimorphic site will reduce the efficiency of heterostyly as a device promoting intermorph pollination. The reduction in intra-flower pollination suggests adaptive significance of this floral dimorphism in increasing the efficiency of pollen dispersal to the individual.     

Effects of Drought on Partitioning of Nitrogen in Two Wheat Varieties Differing in Drought-tolerance

A model was constructed to describe the translocation and partitioningof nitrogen on the seventh day after anthesis for well-wateredand droughted plants of two wheat varieties (Triticum aestivumL. cv. Warigal and Condor). The glasshouse-grown plants weredetillered so that a simplified model could be derived for themain stem. A 9-d drought treatment was imposed just after anthesisand this coincided with the period of endosperm cell divisionin the grains. Warigal, which had a higher grain yield thanCondor under drought, absorbed up to 15-times more nitrogenand translocated 1.5-fold more nitrogen to the shoot via thexylem. In both varieties, nitrogen redistributed from vegetativeorgans accounted for more than 60 per cent in control and 70per cent in droughted plants of the nitrogen needed for eargrowth. The net loss of nitrogen increased by 4-3 per cent inthe leaves, but decreased by 60 per cent in the stem under drought.Stem and roots appeared to play an important role in the nitrogeneconomy of droughted plants: less nitrogen was translocateddirectly to the grains from the senescing leaves and 40–60per cent more nitrogen was translocated to the roots. Nearlyall the nitrogen reaching the roots in the phloem was reloadedinto the xylem stream and translocated back to the shoot. Thetransfer of nitrogen through the stem was reduced under droughtand this resulted in a constant C:N ratio of the grains whichmay be important in the regulation of endosperm cell division. Triticum aestivum L., wheat, drought, nitrogen, senescence, translocation     

Factors involved in resistance to blight (Phytophthora infestans (Mont.) de Bary) in potatoes and assessment of resistance using detached leaves

In leaves of potatoes inoculated with Phytophthora infestans early in the growing season, more but smaller lesions with later onset of sporulation and fewer sporangia developed than on those inoculated later. None of these factors was uniform on detached leaves of all clones, so could not be used separately to forecast field resistance satisfactorily; when considered together, a reasonable prediction was feasible. With some clones, tests of detached leaflets gave a better indication of behaviour in the field than laboratory tests of whole plants.