Plant size, latitude, and phylogeny explain within-population variability in herbivory

Interactions between plants and herbivores are central in most ecosystems, but their strength is highly variable. The amount of variability within a system is thought to influence most aspects of plant-herbivore biology, from ecological stability to plant defense evolution. Our understanding of what influences variability, however, is limited by sparse data. We collected standardized surveys of herbivory for 503 plant species at 790 sites across 116° of latitude. With these data, we show that within-population variability in herbivory increases with latitude, decreases with plant size, and is phylogenetically structured. Differences in the magnitude of variability are thus central to how plant-herbivore biology varies across macroscale gradients. We argue that increased focus on interaction variability will advance understanding of patterns of life on Earth.

Species specificity and intraspecific variation in the chemical profiles of Heliconius butterflies across a large geographic range

In many animals, mate choice is important for the maintenance of reproductive isolation between species. Traits important for mate choice and behavioral isolation are predicted to be under strong stabilizing selection within species; however, such traits can also exhibit variation at the population level driven by neutral and adaptive evolutionary processes. Here, we describe patterns of divergence among androconial and genital chemical profiles at inter- and intraspecific levels in mimetic Heliconius butterflies. Most variation in chemical bouquets was found between species, but there were also quantitative differences at the population level. We found a strong correlation between interspecific chemical and genetic divergence, but this correlation varied in intraspecific comparisons. We identified "indicator" compounds characteristic of particular species that included compounds already known to elicit a behavioral response, suggesting an approach for identification of candidate compounds for future behavioral studies in novel systems. Overall, the strong signal of species identity suggests a role for these compounds in species recognition, but with additional potentially neutral variation at the population level.

Using a portable hydrogen cyanide gas meter to uncover a dynamic phytochemical landscape

PREMISE

Over 3000 species of plants and animals release toxic hydrogen cyanide (HCN) gas when their tissues are crushed. To investigate the role of cyanogenesis in Passiflora-herbivore interactions, we developed an inexpensive, rapid, sensitive method for measuring HCN emissions from crushed tissues.

METHODS

The method includes crushed tissue confinement in a closed chamber, where cyanogenesis reactions occur, followed by evacuation of gas to a portable HCN meter. Parts per million readings are repeated at 5-min intervals until HCN is depleted. Three versions of the closed reaction chamber apparatus were tested: plastic cup, airtight combination mortar-pestle, and glass desiccator jar.

RESULTS

We calibrated the method by comparing with a closed chamber measurement apparatus. The procedure's repeatability was demonstrated with a standard curve using known quantities of cyanogenic glycoside standard. Data collected with this method were also compared with the conventional colorimetric procedure. We processed over 2000 samples using this technique, revealing diverse elements of cyanogenic variation.

CONCLUSIONS

These methods produced well-defined data with minimal error. Results illustrated a one to four order-of-magnitude variation at organizational levels ranging from individual leaves to the entire Passiflora community. We now have a promising tool for uncovering the HCN phytochemical landscape in unprecedented detail.

Variation in Host Plant Usage and Diet Breadth Predict Sibling Preference and Performance in the Neotropical Tortoise Beetle Chelymorpha alternans (Coleoptera: Chrysomelidae: Cassidinae)

Specialized interactions between insects and the plants that they consume are one of the most ubiquitous and consequential ecological associations on the plant. Decades of investigation suggest that a narrow diet favors an individual phytophagous insect's performance relative to a dietary generalist. However, this body of research has tended to approach questions of diet breadth and host usage from the perspective of temperate plant-insect associations. Relationships between diet breadth, host usage, and variation in tropical insect preference and performance remain largely uninvestigated. Here we characterize how variation in diet breadth and host usage affect oviposition preference, development, survival, and gain in mass of a Neotropical tortoise beetle Chelymorpha alternans Boheman 1854 (Coleoptera: Chrysomelidae), using a split-brood, sibling experimental design. Host performance was measured after splitting broods among four no-choice host diets. Groups consuming single hosts varied among themselves in developmental time and survival from larva to adult. Performance did not vary among groups consuming multiple and single hosts. Oviposition preference was measured in choice and no-choice tests. Females displayed preference for the original host in both experiments. Developmental time and survival of offspring sourced from the no-choice experiment was measured for two complete generations to explore correlations with female oviposition preference. Preference for the original host correlated with high survivorship and an intermediate developmental time. Survivorship and time to develop were also high on an alternative host that was less preferred. Departures from predictions of prevailing preference-performance hypotheses suggest that host usage presents C. alternans with fitness trade-offs.

Male sex pheromone components in Heliconius butterflies released by the androconia affect female choice

Sex-specific pheromones are known to play an important role in butterfly courtship, and may influence both individual reproductive success and reproductive isolation between species. Extensive ecological, behavioural and genetic studies of Heliconius butterflies have made a substantial contribution to our understanding of speciation. Male pheromones, although long suspected to play an important role, have received relatively little attention in this genus. Here, we combine morphological, chemical and behavioural analyses of male pheromones in the Neotropical butterfly Heliconius melpomene. First, we identify putative androconia that are specialized brush-like scales that lie within the shiny grey region of the male hindwing. We then describe putative male sex pheromone compounds, which are largely confined to the androconial region of the hindwing of mature males, but are absent in immature males and females. Finally, behavioural choice experiments reveal that females of H. melpomene, H. erato and H. timareta strongly discriminate against conspecific males which have their androconial region experimentally blocked. As well as demonstrating the importance of chemical signalling for female mate choice in Heliconius butterflies, the results describe structures involved in release of the pheromone and a list of potential male sex pheromone compounds.

Intraspecific phytochemical variation shapes community and population structure for specialist caterpillars

Chemically mediated plant-herbivore interactions contribute to the diversity of terrestrial communities and the diversification of plants and insects. While our understanding of the processes affecting community structure and evolutionary diversification has grown, few studies have investigated how trait variation shapes genetic and species diversity simultaneously in a tropical ecosystem. We investigated secondary metabolite variation among subpopulations of a single plant species, Piper kelleyi (Piperaceae), using high-performance liquid chromatography (HPLC), to understand associations between plant phytochemistry and host-specialized caterpillars in the genus Eois (Geometridae: Larentiinae) and associated parasitoid wasps and flies. In addition, we used a genotyping-by-sequencing approach to examine the genetic structure of one abundant caterpillar species, Eois encina, in relation to host phytochemical variation. We found substantive concentration differences among three major secondary metabolites, and these differences in chemistry predicted caterpillar and parasitoid community structure among host plant populations. Furthermore, E. encina populations located at high elevations were genetically different from other populations. They fed on plants containing high concentrations of prenylated benzoic acid. Thus, phytochemistry potentially shapes caterpillar and wasp community composition and geographic variation in species interactions, both of which can contribute to diversification of plants and insects.

Metazoans of redoxcline sediments in Mediterranean deep-sea hypersaline anoxic basins

BACKGROUND

The deep-sea hypersaline anoxic basins (DHABs) of the Mediterranean (water depth ~3500 m) are some of the most extreme oceanic habitats known. Brines of DHABs are nearly saturated with salt, leading many to suspect they are uninhabitable for eukaryotes. While diverse bacterial and protistan communities are reported from some DHAB haloclines and brines, loriciferans are the only metazoan reported to inhabit the anoxic DHAB brines. Our goal was to further investigate metazoan communities in DHAB haloclines and brines.

RESULTS

We report observations from sediments of three DHAB (Urania, Discovery, L'Atalante) haloclines, comparing these to observations from sediments underlying normoxic waters of typical Mediterranean salinity. Due to technical difficulties, sampling of the brines was not possible. Morphotype analysis indicates nematodes are the most abundant taxon; crustaceans, loriciferans and bryozoans were also noted. Among nematodes, Daptonema was the most abundant genus; three morphotypes were noted with a degree of endemicity. The majority of rRNA sequences were from planktonic taxa, suggesting that at least some individual metazoans were preserved and inactive. Nematode abundance data, in some cases determined from direct counts of sediments incubated in situ with CellTracker(TM) Green, was patchy but generally indicates the highest abundances in either normoxic control samples or in upper halocline samples; nematodes were absent or very rare in lower halocline samples. Ultrastructural analysis indicates the nematodes in L'Atalante normoxic control sediments were fit, while specimens from L'Atalante upper halocline were healthy or had only recently died and those from the lower halocline had no identifiable organelles. Loriciferans, which were only rarely encountered, were found in both normoxic control samples as well as in Discovery and L'Atalante haloclines. It is not clear how a metazoan taxon could remain viable under this wide range of conditions.

CONCLUSIONS

We document a community of living nematodes in normoxic, normal saline deep-sea Mediterranean sediments and in the upper halocline portions of the DHABs. Occurrences of nematodes in mid-halocline and lower halocline samples did not provide compelling evidence of a living community in those zones. The possibility of a viable metazoan community in brines of DHABs is not supported by our data at this time.

Dispositional correlates of hypnosis: a phenomenological approach

An attempt was made to construct and validate a questionnaire measure of hypnotic-like experiences based on Shor's (1979) 8-dimension phenomenological analysis of hypnosis. Separate item pools were developed to measure each disposition: Trance, Nonconscious Involvement, Archaic Involvement, Drowsiness, Relaxation, Vividness of Imagery, Absorption, and Access to the Unconscious. Based on preliminary testing (total N = 856), a final questionnaire was produced containing 5 items measuring normal, everyday experiences in each domain. Results from a standardization sample (N = 468) showed that each of the subscales, except for Archaic Involvement, possessed satisfactory levels of internal consistency and test-retest reliability. Factor analysis indicated that 6 subscales loaded highly on a common factor similar to the absorption construct (Tellegen & Atkinson, 1974), while items pertaining to Relaxation and Archaic Involvement formed separate factors. Validation testing on 4 samples receiving the Harvard Group Scale of Hypnotic Susceptibility, Form A (HGSHS:A) of Shor and E. Orne (1962) (total N = 1855) showed that the Absorption and Trance dimensions correlated most strongly with HGSHS:A; the correlations with Drowsiness, Relaxation, and Nonconscious Involvement approached 0. The scales derived from Shor's analysis, however, did not improve the prediction of hypnotizability over that obtained with the absorption scale (Tellegen & Atkinson, 1974).