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En México, las comunidades de helmintos en anfibios han recibido poca atención. Durante 2005 y 2006, un total de 52 individuos del sapo marino (Rhinella marina) y 54 del sapo de la costa del golfo (Incilius valliceps) fueron recolectados en las lagunas de Yalahau (Yucatán, México) para estudiar dichas comunidades. Para comparar entre sus comunidades, se produjeron curvas de rarefacción y extrapolación basadas en el tamaño de la muestra y se estimó la diversidad usando los números de Hill. Se calcularon los 3 primeros números que se asocian a los estimadores de riqueza y dominancia de especies y se construyeron las curvas de rarefacción y extrapolación respectivas. Los resultados mostraron que con el tamaño de la muestra obtenido se registró la mayor parte de las especies de parásitos en ambos hospederos. Una mayor diversidad de helmintos y número de especies se registraron en R. marina frente a I. valliceps. Sin embargo, la extrapolación mostró que al alcanzar los 50 individuos no hay diferencias estadísticas entre ambas comunidades. Las comunidades de estos anfibios en esta región del trópico se ajustan al patrón descrito en latitudes neárticas. Igualmente, el hábitat que ocupan es un factor importante en la estructuración de sus comunidades helmínticas. ABSTRACT Helminth communities in amphibians in Mexico have received little attention. During 2005 and 2006, we collected a total of 52 individuals of the marine toad (Rhinella marina) and 54 of the southern Gulf Coast toad (Incilius valliceps) in the Lagunas de Yalahau (Yucatán, Mexico), in order to study their helminth communities. We produced rarefaction and extrapolation sample-size-based and coverage-based curves to provide asymptotic diversity estimators based on Hill numbers to compare the communities. We calculated the first 3 Hill numbers, which are associated with estimators of species richness and species dominance. In general, all results showed that the sample size was large enough to register most of t

We characterize the diversity of nectar-living yeasts of a tropical host plant community at different hierarchical sampling levels, measure the associations between yeasts and nectariferous plants, and measure the effect of yeasts on nectar traits. Using a series of hierarchically nested sampling units, we extracted nectar from an assemblage of host plants that were representative of the diversity of life forms, flower shapes, and pollinator types in the tropical area of Yucatan, Mexico. Yeasts were isolated from single nectar samples; their DNA was identified, the yeast cell density was estimated, and the sugar composition and concentration of nectar were quantified using HPLC. In contrast to previous studies from temperate regions, the diversity of nectar-living yeasts in the plant community was characterized by a relatively high number of equally common species with low dominance. Analyses predict highly diverse nectar yeast communities in a relatively narrow range of tropical vegetation, suggesting that the diversity of yeasts will increase as the number of sampling units increases at the level of the species, genera, and botanical families of the hosts. Significant associations between specific yeast species and host plants were also detected; the interaction between yeasts and host plants impacted the effect of yeast cell density on nectar sugars. This study provides an overall picture of the diversity of nectar-living yeasts in tropical host plants and suggests that the key factor that affects the community-wide patterns of nectar traits is not nectar chemistry, but rather the type of yeasts interacting with host plants.

† Background and Aims Variation in the composition of floral nectar reflects intrinsic plant characteristics as well as the action of extrinsic factors. Micro-organisms, particularly yeasts, represent one extrinsic factor that inhabit the nectar of animal-pollinated flowers worldwide. In this study a ‘microbial imprint hypothesis’ is formulated and tested, in which it is proposed that natural community-wide variation in nectar sugar composition will partly depend on the presence of yeasts in flowers. † Methods Occurrence and density of yeasts were studied microscopically in single-flower nectar samples of 22 animal-pollinated species from coastal xeric and sub-humid tropical habitats of the Yucata´n Peninsula, Mexico. Nectar sugar concentration and composition were concurrently determined on the same samples using high-per- formance liquid chromatography (HPLC) methods. † Key Results Microscopical examination of nectar samples revealed the presence of yeasts in nearly all plant species (21 out of 22 species) and in about half of the samples examined (51.8 % of total, all species combined). Plant species and individuals differed significantly in nectar sugar concentration and composition, and also in the incidence of nectar yeasts. After statistically controlling for differences between plant species and individuals, nectar yeasts still accounted for a significant fraction of community-wide variance in all nectar sugar parameters considered. Significant yeast × species interactions on sugar parameters revealed that plant species differed in the nectar sugar correlates of variation in yeast incidence. † Conclusions The results support the hypothesis that nectar yeasts impose a detectable imprint on community- wide variation in nectar sugar composition and concentration. Since nectar sugar features influence pollinator attraction and plant reproduction, future nectar studies should control for yeast presence and examine the extent to which microbial signatures on necta

Factors that contribute to variation in nectar sugar composition, nectar concentration and volume have been a central concern in studies of pollinator assemblages in angiosperms. In an effort to better under- stand the mechanisms underlying variation in nectar traits, we designed a series of experiments with flowering Helleborus foetidus individuals under natural and glasshouse conditions, to identify intraplant variation in nectar traits which depend on both intrinsic (sexual phases of individual flowers) and exter- nal (pollinator visits and plant growth conditions) factors. The results showed that nectar volume, sugar composition and concentration in Helleborus foetidus varied between floral sexual phases, environmental growing conditions, and levels of flower exposure to pollinator visits. Processes of mate-limitation in male reproductive success or pollen-limitation in female success, as well as flower protogyny and holocrine secretion of nectaries may be involved in nectar variability between floral phases. By comparing different environments we observed that nectar volume and concentration at the nectary and flower level were plastic traits sensitive to external conditions, emphasizing responsiveness to environmental changes and a consequent plasticity in nectar traits such as sugar concentration and volume. Nectar sugar composi- tion did not respond to different growing conditions, suggesting that this is an intrinsic characteristic of this species, but pollinator exposure produced significant changes in the nectar of single nectaries, particularly in the sucrose–fructose balance. Future research on nectar ecology and nectar chemistry will need to consider that nectar traits exhibit different kinds of variation at the intraplant level and under different environmental conditions.