A 14,000-year oxygen isotope record from diatom silica in two alpine lakes on Mt. Kenya. (49/1413)

Oxygen isotopes are sensitive tracers of climate change in tropical regions. Abrupt shifts of up to 18 per mil in the oxygen isotope ratio of diatom silica have been found in a 14,000-year record from two alpine lakes on Mt. Kenya. Interpretation of tropical-montane isotope records is controversial, especially concerning the relative roles of precipitation and temperature. Here, we argue that Holocene variations in delta(18)O are better explained by lake moisture balance than by temperature-induced fractionation. Episodes of heavy convective precipitation dated approximately 11,100 to 8600, 6700 to 5600, 2900 to 1900, and <1300 years before the present were linked to enhanced soil erosion, neoglacial ice advances, and forest expansion on Mt. Kenya.  (+info)

Ecological studies of enzootic Venezuelan equine encephalitis in north-central Venezuela, 1997-1998. (50/1413)

From 1997-1998, we investigated the possible continuous circulation of epizootic Venezuelan equine encephalitis (VEE) virus suggested by a 1983 subtype IC interepizootic mosquito isolate made in Panaquire, Miranda State, Venezuela. The study area was originally covered by lowland tropical rainforest but has been converted into cacao plantations. Sentinel hamsters, small mammal trapping, mosquito collections, and human serosurveys were used to detect active or recent virus circulation. Six strains of subtype ID VEE virus were isolated from hamsters that displayed no apparent disease. Four other arboviruses belonging to group A (Togaviridae: Alphavirus), two Bunyamwera group (Bunyaviridae), and three Gamboa group (Bunyaviridae) arboviruses were also isolated from hamsters, as well as 8 unidentified viruses. Venezuelan equine encephalitis-specific antibodies were detected in 5 small mammal species: Proechimys guairae, Marmosa spp., and Didelphis marsupialis. Mosquito collections comprised of 38 different species, including 8 members of the subgenus Culex (Melanoconion), did not yield any virus isolates. Sera from 195 humans, either workers in the cacao plantation or nearby residents, were all negative for VEE virus antibodies. Sequences of 1,677 nucleotides from the P62 gene of 2 virus isolates indicated that they represent a subtype ID lineage that is distinct from all others characterized previously, and are unrelated to epizootic VEE emergence.  (+info)

Extreme morphological and ecological homoplasy in tropical salamanders. (51/1413)

Fossorial salamanders typically have elongate and attenuated heads and bodies, diminutive limbs, hands and feet, and extremely elongate tails. Batrachoseps from California, Lineatriton from eastern Mexico, and Oedipina from southern Mexico to Ecuador, all members of the family Plethodontidae, tribe Bolitoglossini, resemble one another in external morphology, which has evolved independently. Whereas Oedipina and Batrachoseps are elongate because there are more trunk vertebrae, a widespread homoplasy (parallelism) in salamanders, the genus Lineatriton is unique in having evolved convergently by an alternate "giraffe-neck" developmental program. Lineatriton has the same number of trunk vertebrae as related, nonelongated taxa, but individual trunk vertebrae are elongated. A robust phylogenetic hypothesis, based on sequences of three mtDNA genes, finds Lineatriton to be deeply nested within a clade characterized by generalized ecology and morphology. Lineatriton lineolus, the only currently recognized taxon in the genus, shows unanticipated genetic diversity. Surprisingly, geographically separated populations of L. lineolus are not monophyletic, but are sister taxa of different species of the morphologically generalized genus Pseudoeurycea. Lineatriton, long thought to be a unique monospecific lineage, is polyphyletic. Accordingly, the specialized morphology of Lineatriton displays homoplasy at two hierarchical levels: (i) with respect to other elongate lineages in the family (convergence), and (ii) within what is currently recognized as a single taxon (parallelism). These evolutionary events are of adaptive significance because to invade the lowland tropics salamanders must be either arboreal or fossorial; the repeated evolution of elongation and attenuation has led to multiple lowland invasions.  (+info)

Limitations to photosynthesis of lettuce grown under tropical conditions: alleviation by root-zone cooling. (52/1413)

Aerial parts of lettuce plants were grown under natural tropical fluctuating ambient temperatures, but with their roots exposed to two different root-zone temperatures (RZTs): a constant 20 degrees C-RZT and a fluctuating ambient (A-) RZT from 23-40 degrees C. Plants grown at A-RZT showed lower photosynthetic CO2 assimilation (A), stomatal conductance (gs), midday leaf relative water content (RWC), and chlorophyll fluorescence ratio Fv/Fm than 20 degrees C-RZT plants on both sunny and cloudy days. Substantial midday depression of A and g(s) occurred on both sunny and cloudy days in both RZT treatments, although Fv/Fm did not vary diurnally on cloudy days. Reciprocal temperature transfer experiments investigated the occurrence and possible causes of stomatal and non-stomatal limitations of photosynthesis. For both temperature transfers, light-saturated stomatal conductance (gs sat) and photosynthetic CO2 assimilation (A(sat)) were highly correlated with each other and with midday RWC, suggesting that A was limited by water stress-mediated stomatal closure. However, prolonged growth at A-RZT reduced light- and CO2-saturated photosynthetic O2 evolution (Pmax), indicating non-stomatal limitation of photosynthesis. Tight temporal coupling of leaf nitrogen content and P(max) during both temperature transfers suggested that decreased nutrient status caused this non-stomatal limitation of photosynthesis.  (+info)

Effect of climatic factors and population density on varicella zoster virus epidemiology within a tropical country. (53/1413)

Blood samples were collected from healthy subjects, aged 9 months-29 years in urban and rural communities from 4 distinct regions in Thailand, to determine the seroprevalence rate of varicella-zoster virus (VZV) antibody and its relationship with demographic, climatic, and socioeconomic factors. The overall seroprevalence rate was 52.8% and increased from 15.5% in the 9-month to 4-year-old group to 75.9% in the 20-29 year-olds. The age-adjusted seroprevalence was significantly higher in the cooler than in the warmer regions. In the warmer regions only, the age-specific seroprevalence was significantly higher in the urban population than in the rural population. In Thailand, climate is the main determinant of VZV seroprevalence. The delayed onset of natural immunity is more marked in warmer climate areas. Population density is a secondary determinant; in the warmer areas, the pattern of adolescent and adult susceptibility was greater in rural than in urban areas.  (+info)

Remotely sensed biological production in the equatorial Pacific. (54/1413)

A combination of ship, buoy, and satellite observations in the tropical Pacific during the period from 1992 to 2000 provides a basin-scale perspective on the net effects of El Nino and La Nina on biogeochemical cycles. New biological production during the 1997-99 El Nino/La Nina period varied by more than a factor of 2. The resulting interannual changes in global carbon sequestration associated with the El Nino/La Nina cycle contributed to the largest known natural perturbation of the global carbon cycle over these time scales.  (+info)

Assessment of genetic variability of resistance to gastrointestinal nematode parasites in Creole goats in the humid tropics. (55/1413)

Goat production is widespread in the tropics. Goats are very susceptible to gastrointestinal nematode infection, but there is less evidence of their genetic resistance. Genetic resistance of Creole goats to gastrointestinal nematodes has been studied at Guadeloupe in the French West Indies since 1995. The objective of this research was to investigate genetic variation for resistance to gastrointestinal nematode infection, in order to introduce this trait into breeding schemes. Genetic variability was assessed within a Creole experimental flock. Forty-nine sire groups were characterized at weaning and 55 during fattening after weaning. Kids were naturally infected, mainly by Haemonchus contortus and Trichostrongylus colubriformis. Fecal egg counts were determined once at weaning and every 6 and 7 wk after drenching during fattening. Blood samples were collected every 7 wk during fattening for determination of packed cell volume. Live weights were recorded at weaning and at the beginning and middle of every infection period during fattening. Genetic parameters were estimated using the REML for multivariate animal models. The heritability estimate for transformed fecal egg count was 0.37+/-0.06 at weaning. During fattening, it increased from 0.14+/-0.05 at 4 mo to 0.33+/-0.06 at 10 mo. Heritabilities of packed cell volume ranged from 0.10 to 0.33. At weaning, maternal heritability of fecal egg count reached 0.26 and direct heritability 0.20. After 6 mo of age, maternal effects were found to be unimportant for fecal egg count and packed cell volume. Live weights presented significant genetic variability. Genetic relationships between fecal egg counts and live weight in infected pastures were never significant. Genetic correlations between packed cell volume and live weight decreased from 0.47 to 0.10 from weaning to 10 mo of age. These results demonstrated the feasibility of breeding for improved resistance to nematodes in Creole kids.  (+info)

Colletotrichum gloeosporioides pelB is an important virulence factor in avocado fruit-fungus interaction. (56/1413)

Colletotrichum gloeosporioides is an important pathogen of tropical and subtropical fruits. The C. gloeosporioides pelB gene was disrupted in the fungus via homologous recombination. Three independent isolates, GD-14, GD-23, and GD-29, did not produce or secrete pectate lyase B (PLB) and exhibited 25% lower pectate lyase (PL) and pectin lyase (PNL) activities and 15% higher polygalacturonase (PG) activity than the wild type. The PLB mutants exhibited no growth reduction on glucose, Na polypectate, or pectin as the sole carbon source at pH 3.8 or 6.0, except for a 15% reduction on pectin at pH 6.0. When pelB mutants were inoculated onto avocado fruits, however, a 36 to 45% reduction in estimated decay diameter was observed compared with the two controls, the wild type and undisrupted transformed isolate. In addition, these pelB mutants induced a significantly higher host phenylalanine ammonia lyase activity as well as the antifungal diene, which is indicative of higher host resistance. These results suggest that PLB is an important factor in the attack of C. gloeosporioides on avocado fruit, probably as a result of its virulence factor and role in the induction of host defense mechanisms.  (+info)