Chytrid infections of Daphnia pulicaria: development, ecology, pathology and phylogeny of Polycaryum laeve

1. We combined ecological surveys, life table studies, microscopy and molecular sequencing to determine the development, ecology, pathology and phylogeny of Polycaryum laeve, an endoparasite of cladocerans. We report the first records of P. laeve from North America, where we have used a polymerase chain reaction primer and microscopic examination to confirm infections in 14 lakes. Infections are highly pathogenic and caused increased mortality, reduced growth, and reproductive castration in Daphnia pulicaria during life table studies. 2. Biweekly data from Allequash Lake (Wisconsin, U.S.A.) throughout 2003 indicated that fecundity and infection prevalence were inversely correlated. Infection prevalence was highest in late winter and early spring (up to 80%) and lowest during late summer. Epidemics were generally followed by sharp declines in host population density (up to 99%). 3. Within the haemocoel of its host, P. laeve forms thick‐walled sporangia, which occur systemically in later stages of infection. Fungal thalli associate closely with muscle fibres and connective tissue, leading to degeneration as the infection becomes advanced. Following death of the host, flagellated zoospores are released through an exit papilla on the sporangium. Based on the infection‐induced castration of the host and increases in infection prevalence with Daphnia size, we postulate that transmission is horizontal, but may be indirect through an additional host or free‐living stage. 4. Molecular and morphological data clearly indicate that P. laeve belongs in the fungal phylum Chytriodiomycota, order Blastocladiales. Based on ribosomal RNA gene sequences and morphological features, we transfer the genus Polycaryum from the Haplosporidia to the Chytridiomycota, and designate a lectotype and epitype for P. laeve. Considering the high prevalence of P. laeve infection within Daphnia populations, the frequency with which we detected infections among lakes, and the keystone importance of large‐bodied Daphnia in aquatic food webs, we suggest that P. laeve may exert a regulatory influence on Daphnia populations in lake ecosystems.     

Climate‐growth analysis for a Mexican dry forest tree shows strong impact of sea surface temperatures and predicts future growth declines

Tropical forests will experience relatively large changes in temperature and rainfall towards the end of this century. Little is known about how tropical trees will respond to these changes. We used tree rings to establish climate‐growth relations of a pioneer tree, Mimosa acantholoba, occurring in tropical dry secondary forests in southern Mexico. The role of large‐scale climatic drivers in determining interannual growth variation was studied by correlating growth to sea surface temperature anomalies (SSTA) of the Atlantic and Pacific Oceans, including the El Niño‐Southern Oscillation (ENSO). Annual growth varied eightfold over 1970–2007, and was correlated with wet season rainfall (r=0.75). Temperature, cloud cover and solar variation did not affect growth, although these climate variables correlated with growth due to their relations with rainfall. Strong positive correlations between growth and SSTA occurred in the North tropical Atlantic during the first half of the year, and in the Pacific during the second half of the year. The Pacific influence corresponded closely to ENSO‐like influences with negative effects of high SSTA in the eastern Pacific Niño3.4 region on growth due to decreases in rainfall. During El Niño years growth was reduced by 37%. We estimated how growth would be affected by the predicted trend of decreasing rainfall in Central America towards the end of this century. Using rainfall predictions of two sets of climate models, we estimated that growth at the end of this century will be reduced by 12% under a medium (A1B) and 21% under a high (A2) emission scenario. These results suggest that climate change may have repercussions for the carbon sequestration capacity of tropical dry forests in the region.     

Styloid crystals in Claoxylon (Euphorbiaceae) and allies (Claoxylinae) with notes on leaf anatomy

Claoxylon and Micrococca are the only Euphorbiaceae genera that have rough dried leaves (fresh ones are smooth) because of protruding styloid (needle-like) crystals more or less perpendicular to the leaf surface, which perforate the epidermis and cuticle. A broad leaf anatomical study of the subtribes Claoxylinae, the monogeneric Lobaniliinae, and Mercurialis of the Mercurialinae (95 of a possible 235 species in all six genera) showed that styloids are present in Claoxylon , Discoclaoxylon , Erythrococca , Lobanilia , and Micrococca , and lacking in Claoxylopsis and Mercurialis . Contrary to Claoxylon , the dried leaves of Discoclaoxylon , Erythrococca , Lobanilia , and Micrococca are not rough, because the styloids do not perforate the epidermis during drying and therefore herbarium leaves generally remain smooth. The presence of styloids supports a clade in a recent molecular phylogenetic study that unites subtribes Claoxylinae and Lobaniliinae as Claoxylinae s.l. Mercurialis (no styloids) is sister to all other taxa (with styloids) in the monophyletic Claoxylinae s.l. The styloids form a synapomorphy for a monophyletic part of the Claoxylinae ( Claoxylon , Discoclaoxylon , Erythrococca , Lobanilia , and Micrococca ). Other leaf anatomical notes are provided, together with an overview of the occurrence of styloids, stomata, and druse crystals for most species in the Claoxylinae. The indumentum ranges from (virtually) absent to a sparse or dense tomentum of straight and thick- or thin-walled unicellular hairs (most common), curly hairs, or two-armed hairs ('Malpighiaceous hairs'). The last two hair types probably form diagnostic characters for species groups within the genus Claoxylon .  © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society , 2008, 156 , 445–457.     

The relative importance of dispersal and local processes in structuring phytoplankton communities in a set of highly interconnected ponds

1. The recognition that both local and regional processes act together in shaping local communities makes determining their relative roles in natural communities central to understanding patterns in community structure. 2. We investigated the relative influence of these processes on the phytoplankton communities of a highly interconnected pond system. We sampled the phytoplankton communities of 28 ponds concurrently with 20 local environmental variables. 3. We found that phytoplankton community variation, in terms of both phytoplankton community composition (PCC) and diversity, was only significantly explained by local environmental variables. These were mainly associated with the contrasting clear‐water and turbid ecological states of the shallow ponds studied. Clear‐water conditions favoured only a few taxa, resulting in a significantly lower taxon diversity and richness under these conditions. 4. The failure to explain variation in PCC by a dispersal model based on the water flow between ponds points at very effective species sorting. This is attributed to the high population turn‐over rates and sensitivity to environmental conditions of phytoplankton communities. Some evidence was found, however, that dispersal influences local communities through mass effects between neighbouring ponds. 5. Overall, our results emphasize both the strong selection pressure that components of the food web exert on phytoplankton communities and the high potential of these communities to respond to such environmental change, thereby effectively opposing the homogenizing effects of continuous dispersal.     

Phloem Unloading in Stem Parts Parasitized by Cuscuta: The Release of 14C and K+ to the Free Space at 0°C and 25°C

By washing out ¹⁴C-solutes or K⁺ from the free space of stem segments of Vicia faba L. and Urtica dioica L., efflux from stem tissues was measured. At 25°C the efflux of ¹⁴C or K⁺ from non-parasitized stem parts was found to be comparable with the pattern known for parenchyma cells. The efflux pattern from parasitized stem parts kept at 0°C was also a normal one, but at 25°C a more complicated pattern was found. After addition of metabolic inhibitors, however, the efflux pattern appeared to be comparable with the pattern found in 0°C experiments. Also, after washing stem segments for several hours in water at 0°C, Phloem unloading was shown to start unimpeded after transfer to 25°C. Stem parts parasitized by Cuscuta can be regarded as a model system for studying the fundamental processes underlying movement out of sieve tubes in strong sink regions.     

LOCATION AND ULTRASTRUCTURE OF PRIMORDIAL GERM CELLS (PGCs) IN AMBYSTOMA MEXICANUM

The location and ultrastructure of the primordial germ cells (PGCs) were studied in Ambystoma mexicanum larvae of stages 23 to 47.
PGCs were found in the spaces between the endodermal cell mass and the lateral plate mesoderm at stages 23 to 35. Some of the PGCs at stage 35, and most of them at stages 40 and 42, were located near the Wolffian duct. At stages 46 and 47 all the PGCs were situated in the genital ridges. Cilia, which have hitherto never been reported in PGCs, were occasionally seen in PGCs of Ambystoma from stage 23 till stage 46.
No "germinal plasm" was found in the PGCs prior to stage 40. Specific structures or "nuage material", corresponding to the germinal granules or their derivatives in Xenopus , were first recognized in the vicinity of the nucleus at stage 40. Between stages 40 and 46, the amount of "nuage material" markedly increased. It was finally localized mainly in "intermitochondrial spaces". A possible transfer of material from the nucleus to the cytoplasm or vice versa through nuclear pores was first noticed at stage 40, the material concerned being quite similar in ultrastructure to the "nuage material".     

Effect of Low Temperature upon Lipid and Fatty Acid Composition of Roots and Leaves of Winter Rape Plants

When winter rape plants were transferred from favourable temperature conditions (25/20°C day/night temperature) to 5°C, the frost resistance of the leaves was increased whereas the frost tolerance of the roots remained unaffected. This permitted an analysis of the changes in lipid and fatty acid composition both as related to functioning of the plant at low temperature alone (roots) and as related to adaptation to freezing and functioning at low temperature (leaves). — Transfer of the plants to 5°C lead to an increase in the level of linolenic acid in roots and leaves. This increase was most evident in the phosphatidyl choline and ethanolamine fractions of the leaves, and in the neutral lipids and in an unidentified phospholipid from the roots. It was concluded that upon transfer of the plants to 5°C a general and non-specific increase in linolenic acid level contributed to functioning of the rape plants at low temperature; and that parallel but minor increases in linolenic acid level of digalactosyl diglyceride, phosphatidyl inositol and the unknown phospholipid in roots and leaves could only contribute to low-temperature functioning in specific membrane enzyme locations. Combined adaptation of the leaves to freezing tolerance and low-temperature functioning was correlated with a higher level of phosphatidyl choline and ethanolamine, predominantly esterified with linolenic acid.