Genomic architecture of adaptive color pattern divergence and convergence in Heliconius butterflies.

Identifying the genetic changes driving adaptive variation in natural populations is key to understanding the origins of biodiversity. The mosaic of mimetic wing patterns in Heliconius butterflies makes an excellent system for exploring adaptive variation using next-generation sequencing. In this study, we use a combination of techniques to annotate the genomic interval modulating red color pattern variation, identify a narrow region responsible for adaptive divergence and convergence in Heliconius wing color patterns, and explore the evolutionary history of these adaptive alleles. We use whole genome resequencing from four hybrid zones between divergent color pattern races of Heliconius erato and two hybrid zones of the co-mimic Heliconius melpomene to examine genetic variation across 2.2 Mb of a partial reference sequence. In the intergenic region near optix, the gene previously shown to be responsible for the complex red pattern variation in Heliconius, population genetic analyses identify a shared 65-kb region of divergence that includes several sites perfectly associated with phenotype within each species. This region likely contains multiple cis-regulatory elements that control discrete expression domains of optix. The parallel signatures of genetic differentiation in H. erato and H. melpomene support a shared genetic architecture between the two distantly related co-mimics; however, phylogenetic analysis suggests mimetic patterns in each species evolved independently. Using a combination of next-generation sequencing analyses, we have refined our understanding of the genetic architecture of wing pattern variation in Heliconius and gained important insights into the evolution of novel adaptive phenotypes in natural populations.

Drought effects on litterfall, wood production and belowground carbon cycling in an Amazon forest: results of a throughfall reduction experiment.

The Amazon Basin experiences severe droughts that may become more common in the future. Little is known of the effects of such droughts on Amazon forest productivity and carbon allocation. We tested the prediction that severe drought decreases litterfall and wood production but potentially has multiple cancelling effects on belowground production within a 7-year partial throughfall exclusion experiment. We simulated an approximately 35-41% reduction in effective rainfall from 2000 through 2004 in a 1ha plot and compared forest response with a similar control plot. Wood production was the most sensitive component of above-ground net primary productivity (ANPP) to drought, declining by 13% the first year and up to 62% thereafter. Litterfall declined only in the third year of drought, with a maximum difference of 23% below the control plot. Soil CO2 efflux and its 14C signature showed no significant treatment response, suggesting similar amounts and sources of belowground production. ANPP was similar between plots in 2000 and declined to a low of 41% below the control plot during the subsequent treatment years, rebounding to only a 10% difference during the first post-treatment year. Live aboveground carbon declined by 32.5Mgha-1 through the effects of drought on ANPP and tree mortality. Results of this unreplicated, long-term, large-scale ecosystem manipulation experiment demonstrate that multi-year severe drought can substantially reduce Amazon forest carbon stocks.

Plasma vitellogenin in Morelet's crocodiles from contaminated habitats in northern Belize.

Vitellogenin induction has been widely used as a biomarker of endocrine disruption in wildlife, but few studies have investigated its use in wild reptiles living in contaminated habitats. This study examined vitellogenin induction in Morelet's crocodiles (Crocodylus moreletii) from wetlands in northern Belize contaminated with organochlorine (OC) pesticides. Vitellogenin was measured in 381 crocodile plasma samples using a vitellogenin ELISA previously developed for this species. Vitellogenin was detected in nine samples, all from adult females sampled during the breeding season. Males and juvenile females did not contain detectable levels of vitellogenin; however, many of these animals contained OC pesticides in their caudal scutes, confirming contaminant exposure. The lack of a vitellogenic response in these animals may be attributable to several factors related to the timing and magnitude of exposure to endocrine-disrupting chemicals and should not be interpreted as an absence of other contaminant-induced biological responses.

Mortality of large trees and lianas following experimental drought in an Amazon forest.

Severe drought episodes such as those associated with El Niño Southern Oscillation (ENSO) events influence large areas of tropical forest and may become more frequent in the future. One of the most important forest responses to severe drought is tree mortality, which alters forest structure, composition, carbon content, and flammability, and which varies widely. This study tests the hypothesis that tree mortality increases abruptly during drought episodes when plant-available soil water (PAW) declines below a critical minimum threshold. It also examines the effect of tree size, plant life form (palm, liana, tree) and potential canopy position (understory, midcanopy, overstory) on drought-induced plant mortality. A severe, four-year drought episode was simulated by excluding 60% of incoming throughfall during each wet season using plastic panels installed in the understory of a 1-ha forest treatment plot, while a 1-ha control plot received normal rainfall. After 3.2 years, the treatment resulted in a 38% increase in mortality rates across all stems >2 cm dbh. Mortality rates increased 4.5-fold among large trees (>30 cm dbh) and twofold among medium trees (10-30 cm dbh) in response to the treatment, whereas the smallest stems were less responsive. Recruitment rates did not compensate for the elevated mortality of larger-diameter stems in the treatment plot. Overall, lianas proved more susceptible to drought-induced mortality than trees or palms, and potential overstory tree species were more vulnerable than midcanopy and understory species. Large stems contributed to 90% of the pretreatment live aboveground biomass in both plots. Large-tree mortality resulting from the treatment generated 3.4 times more dead biomass than the control plot. The dramatic mortality response suggests significant, adverse impacts on the global carbon cycle if climatic changes follow current trends.

Effects of partial throughfall exclusion on the phenology of Coussarea racemosa (Rubiaceae) in an east-central Amazon rainforest.

Severe droughts may alter the reproductive phenology of tropical tree species, but our understanding of these effects has been hampered by confounded variation in drought, light and other factors during natural drought events. We used a large-scale experimental reduction of throughfall in an eastern-central Amazon forest to study the phenological response to drought of an abundant subcanopy tree, Coussarea racemosa. We hypothesized that drought would alter the production and the timing of reproduction, as well as the number of viable fruits. The study system comprised two 1-ha plots in the Tapajos National Forest, Para, Brazil: a dry plot where 50% of incoming precipitation (80% throughfall) was diverted from the soil during the six-month wet season beginning in January 2000, and a wet plot that received natural rainfall inputs. Fruit production of C. racemosa was quantified every 15 days using 100 litter traps (0.5 m(2)) in each plot. The production of new leaves and flowers was recorded monthly for C. racemosa individuals. Soil water, pre-dawn leaf water potential and solar radiation were measured to help interpret phenological patterns. Over the approximately 3.5-year period (April 2000 through December 2003), total fruit production remained similar between plots, declining by 12%. In 2003, production was four times higher in both plots than in previous years. In the dry plot, fruit fall shifted 40 and 60 days later into the dry season in 2002 and 2003, respectively. Total fruit fall dry mass production was variable across the study period. Foliage and flower production coincided with peak irradiance early in the dry season until delays in flowering appeared in the dry plot in 2002 and 2003. Plant water stress, through its influence on leaf developmental processes and, perhaps, inhibition of photosynthesis, appears to have altered both the timing of fruit fall and the quality and number of seeds produced.

Drought stress and carbon uptake in an Amazon forest measured with spaceborne imaging spectroscopy.

Amazônia contains vast stores of carbon in high-diversity ecosystems, yet this region undergoes major changes in precipitation affecting land use, carbon dynamics, and climate. The extent and structural complexity of Amazon forests impedes ground studies of ecosystem functions such as net primary production (NPP), water cycling, and carbon sequestration. Traditional modeling and remote-sensing approaches are not well suited to tropical forest studies, because (i) biophysical mechanisms determining drought effects on canopy water and carbon dynamics are poorly known, and (ii) remote-sensing metrics of canopy greenness may be insensitive to small changes in leaf area accompanying drought. New spaceborne imaging spectroscopy may detect drought stress in tropical forests, helping to monitor forest physiology and constrain carbon models. We combined a forest drought experiment in Amazônia with spaceborne imaging spectrometer measurements of this area. With field data on rainfall, soil water, and leaf and canopy responses, we tested whether spaceborne hyperspectral observations quantify differences in canopy water and NPP resulting from drought stress. We found that hyperspectral metrics of canopy water content and light-use efficiency are highly sensitive to drought. Using these observations, forest NPP was estimated with greater sensitivity to drought conditions than with traditional combinations of modeling, remote-sensing, and field measurements. Spaceborne imaging spectroscopy will increase the accuracy of ecological studies in humid tropical forests.