Carnivory and resource-based niche differentiation in anuran larvae: implications for food web and experimental ecology

1.  Food webs represent the paths of material and energy flow through organisms in an ecosystem. Anuran larvae are important components of pond food webs: they are abundant, consume large quantities of food and serve as prey for many organisms. However, there are very basic uncertainties about the feeding ecology of anuran larvae; for instance, as to which trophic level they belong and whether species differ in resource use. Because anuran larvae have been employed in model systems in experimental ecology for decades, these uncertainties could lead to misinterpretation of published experiments, or inadequate designs of experiments directed at general, conceptual issues in ecology.
2.  Using ¹³C and ¹⁵N stable isotope and gut content analyses of free-ranging and enclosed tadpoles of four ranid species ( Lithobates sylvaticus , L. pipiens , L. clamitans , L. catesbeianus ) in the food webs of six wetlands, we tested the following null hypotheses: (i) that anuran larvae are strict primary consumers; (ii) that they are non-selective feeders and therefore exhibit little feeding niche differentiation; (iii) that they are opportunistic consumers and (iv) that their diet remains unchanged through ontogeny.
3.  All four species consumed and assimilated substantial amounts of animal food; bullfrog larvae, in particular, appear to be predatory. Significant feeding niche differentiation among species occurred with respect to the sources of carbon, consumption of animal matter and nutritional quality of food ingested. We further documented opportunistic feeding habits and ontogenetic shifts in diet.
4.  Collectively, these studies revealed complex trophic relationships that might require a reconsideration of the role of anuran larvae in pond food webs, as well as a reinterpretation of results of previous studies employing anuran larvae in model experimental systems.     

Oxygen isotope enrichment (Δ18O) reflects yield potential and drought resistance in maize

Measurement of stable isotopes in plant dry matter is a useful phenotypic tool for speeding up breeding advance in C₃ crops exposed to different water regimes. However, the situation in C₄ crops is far from resolved, since their photosynthetic metabolism precludes (at least in maize) the use of carbon isotope discrimination. This paper investigates the use of oxygen isotope enrichment (Δ¹⁸O) as a new secondary trait for yield potential and drought resistance in maize ( Zea mays L). A set of tropical maize hybrids developed by the International Maize and Wheat Improvement Center was grown under three contrasting water regimes in field conditions. Water regimes clearly affected plant growth and yield. In accordance with the current theory, a decrease in water input was translated into large decreases in stomatal conductance and increases in leaf temperature together with concomitant ¹⁸O enrichment of plant matter (leaves and kernels). In addition, kernel Δ¹⁸O correlated negatively with grain yield under well-watered and intermediate water stress conditions, while it correlated positively under severe water stress conditions. Therefore, genotypes showing lower kernel Δ¹⁸O under well-watered and intermediate water stress had higher yields in these environments, while the opposite trend was found under severe water stress conditions. This illustrates the usefulness of Δ¹⁸O for selecting the genotypes best suited to differing water conditions.     

cpFBPaseII, a novel redox-independent chloroplastic isoform of fructose-1,6-bisphosphatase

A full-length FBPase cDNA has been isolated from Fragaria  ×  ananassa (strawberry) corresponding to a novel putative chloroplastic FBPase but lacking the regulatory redox domain, a characteristic of the plastidial isoenzyme (cpFBPaseI). Another outstanding feature of this novel isoform, called cpFBPaseII, is the absence of the canonical active site. Enzymatic assays with cpFBPaseII evidenced clear Mg²⁺-dependent FBPase activity and a K _m for fructose-1,6-bisphosphate (FBP) of 1.3 m m . Immunolocalization experiments and chloroplast isolation confirmed that the new isoenzyme is located in the stroma. Nevertheless, unlike cpFBPaseI, which is redox activated, cpFBPaseII did not increase its activity in the presence of either DTT or thioredoxin f (TRX f ) and is resistant to H₂O₂ inactivation. Additionally, the novel isoform was able to complement the growth deficiency of the yeast FBP1 deletion fed with a non-fermentable carbon source. Furthermore, orthologues are restricted to land plants, suggesting that cpFBPaseII is a novel and an intriguing chloroplastic FBPase that emerged late in the evolution of photosynthetic organisms, possibly because of a pressing need of land plants.     

The effect of seasonal variation in abiotic factors on fish community structure in temporary and permanent pools in a tropical wetland

1. Fish community structure depends on biotic interactions and abiotic variables. Abiotic variables appear to gain importance in highly variable freshwater systems, such as tropical wetlands where a marked seasonal hydroperiod (dry and wet seasons) modifies water quality and quantity, differentially affecting fish survival and, consequently, modifying species richness and abundance. 2. We evaluated the relationship between abiotic variables and fish community structure in variable (temporary) and stable (permanent) pools that were interconnected in a tropical wetland with marked annual dry and wet seasons. 3. All fish species were able to occupy any of the studied pools, but our results showed distinctive fish community structures in permanent and temporary pools. Community structure was related to temperature, depth, pH and macrophyte coverage. Total fish abundance in the wetland was negatively related to water depth and positively related to macrophyte coverage. 4. Null models of co‐occurrence indicated a non‐random pattern at the wetland scale and a random pattern within groups of pools with similar characteristics, suggesting that fish communities are structured according to habitat features. We conclude that seasonal abiotic variation and habitat characteristics in this highly variable pristine wetland play major roles in structuring fish communities.     

Phylogenetic analysis of the Mexican freshwater crabs of the tribe Pseudothelphusini (Decapoda: Brachyura: Pseudothelphusidae)

The Mexican pseudothelphusid crabs are classified in one subfamily, three tribes, and 13 genera. Up to now, 56 species have been recognized, distributed in a strictly Neotropical pattern, with some of them reaching the state of Sonora on the western slope of Mexico. The tribe Pseudothelphusini is the most diverse, with five genera and 35 species, all of them endemic to Mexico: the two most species‐rich genera are Pseudothelphusa, with 23 species, and Tehuana, with eight species; Epithelphusa includes two species, whereas Disparithelphusa and Smalleyus are monotypic. The Pseudothelphusini lack an updated systematic revision, which could serve as a framework to analyse the monophyletic origin of the group, to clarify the relationships among genera and species, as well as to resolve the taxonomic status of various species complexes. In the present study, an exhaustive morphological revision was conducted using somatic and sexual characters. A phylogenetic analysis was performed using 77 characters and 183 character states, taken from 41 species. Ten trees of the same length were obtained using PAUP 4.0 through a heuristic search. The results show that the tribe as it is actually known constitutes a paraphyletic group, in which the species of Epithelphusa and Pseudothelphusa puntarenas are excluded from the internal group. According to the obtained results, the tribe Pseudothelphusini s.s. includes five genera: Smalleyus, Pseudothelphusa, Tehuana, and two new ones to accommodate Pseudothelphusa galloi and Pseudothelphusa sulcifrons, respectively. This new arrangement considers the provisional suppression of the genus Disparithelphusa, which remained as another species of Pseudothelphusa throughout the cladistic analysis. The phylogenetic results show a strong congruence with the distribution of the species, in several cases grouping species that form morphological clines along a geographical gradient. The previously proposed southern origin of the tribe Pseudothelphusini gains support with the results obtained. © 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 160 , 457–481     

Variation and covariation of life history traits in aphids are related to infection with the facultative bacterial endosymbiont Hamiltonella defensa

Host–symbiont associations play an important role in insects. In aphids, facultative symbionts affect host plant use and increase thermal tolerance and resistance to natural enemies. In spite of these beneficial effects on aphid fitness, the frequency of facultative symbionts in aphids ranges from low to intermediate. Tradeoffs induced by symbionts could prevent the fixation of symbionts in aphid populations. Therefore, we studied the life history traits and correlations between them in 21 clones of the black bean aphid, Aphis fabae, seven of which were infected with the facultative endosymbiont Hamiltonella defensa. We found that clones harbouring H. defensa exhibited significantly higher body mass at maturity and offspring production, and a marginally higher intrinsic rate of increase. However, development time and offspring body size did not differ between symbiont‐free and infected clones. In addition, body mass at maturity was positively correlated with offspring production, offspring body size and intrinsic rate of increase, whereas development time was negatively correlated with body mass at maturity, offspring production and offspring body size. Excluding infected clones had little effect on these correlations; only correlations between body mass at maturity and offspring production, and between development time and offspring body size, became nonsignificant. Therefore, we did not find any evidence for tradeoffs between life history traits induced by symbiont infection. In fact, infected clones had higher overall fitness than symbiont‐free clones under the conditions of our experiment, suggesting that symbionts do not impose costs on aphids harbouring them. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 237–247.     

Changes in sperm tail development associated with Y chromosome meiotic drive leading to an excess of males in the medfly Ceratitis capitata (Diptera: Tephritidae)

The Mediterranean fruit fly Ceratitis capitata (Wied.) normally produces the sexes in equal ratio but strains carrying the Y chromosome meiotic drive MP (male‐producing) factor show an excess of males. This is associated with a loss of sperm, and abnormal sperm structure in terms of multiple axonemes, atypical numbers of mitochondrial derivatives, and sometimes an incorrect initial orientation of paracrystalline bodies to the axoneme. Sperms are bundled together within spermatocysts, and those with depleted content and abnormalities occur in the same MP testes as normal spermatocysts. The maximum number of sperms per cyst in control strains was 256, each with a single axoneme originating from a single centriole (kinetosome). The maximum per cyst in MP strains was also 256 but MP cysts contained up to 300 axonemes, providing evidence of multiplication of centrioles. The structural changes in MP sperm are discussed in relation to similar abnormalities reported in the mosquito Aedes aegypti inheriting the Y chromosome meiotic drive haplotype MD. The evolutionary significance of this phenomenon is considered. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 351–359.     

Subtle topographical differences along a floodplain promote different plant strategies among Paspalum dilatatum subspecies and populations

It was hypothesised that subtle topographical differences might cause the existence of ecotypes along a floodplain. The apomict grass Paspalum dilatatum subspecies dilatatum inhabits flood‐prone lowlands as well as nearby uplands in the floodplains of Argentina, while the sexual P. dilatatum subspecies flavescens almost exclusively inhabits the uplands. The aim of the present study was to identify the different traits that allow these P. dilatatum populations to inhabit different habitats. Plants of P. dilatatum were reciprocally transplanted between uplands and lowlands. Morphophysiological traits related to flooding tolerance were measured during a flood. Subspecies dilatatum from the uplands and subspecies flavescens showed a high physiological performance in the uplands but a considerable decrease in stomatal conductance, net photosynthesis rates and tiller number in the flooded lowlands. In contrast, the subspecies dilatatum from the lowlands showed relatively lower and stable stomatal conductance, photosynthesis rates and leaf water potential at both sites. Subspecies dilatatum from the lowlands outperformed upland populations at the lowland site with respect to tillering. Leaves of subspecies dilatatum from the lowlands that had grown at the lowland habitat had a lower blade/sheath proportion than leaves of plants transplanted to the uplands. This behavior did not occur in both upland populations. Results suggest that dilatatum Lowland plants have the typical strategy of stress‐tolerant genotypes and that the upland populations are adapted to habitats where competitive species are selected. In conclusion, habitats with subtle differences in topographic level can favour both ecotypic differentiations within an apomict subspecies but also the maintenance of morphophysiological similitudes between coexisting upland populations belonging to different subspecies.     

Microsatellite loci in the Canary Islands endemic ground beetle Trechus flavocinctus and their applicability to cave‐dwelling related species

Six polymorphic microsatellite markers have been developed for the Canary endemic ground beetle Trechus flavocinctus, and the performance of primer pairs amplifying these loci in closely related taxa (including two cave‐obliged species) has been tested. The number of alleles in T. flavocinctus samples taken from three distant localities in two islands ranged from four to 14 per locus. Observed heterozygosities were from 0.333 to 0.933. These markers could be useful tools to study the population structure and the genetic consequences of island and cave colonization by ground beetles.     

Physical and chemical cues affect oviposition by Neoleucinodes elegantalis

Recognition and acceptance of a suitable host plant by phytophagous insects requires the integration of visual, physical and chemical cues. The present study investigates the host cues that a specialist insect integrates to optimize oviposition decisions and whether these cues are weighted in a specific way. The study also determines whether the tomato fruit borer Neoleucinodes elegantalis (Guenée) (Lepidoptera: Crambidae), an important pest on Solanaceae in Brazil, shows a preference for oviposition sites that differ in physical and/or chemical cues. When styrofoam balls are provided as artificial fruits, N. elegantalis deposit significantly more eggs on rough artificial fruits than on smooth ones. Hexane fruit extracts applied to the artificial fruits stimulate female oviposition strongly. Physical and chemical cues also affect the oviposition of females when offered together. Furthermore, certain parts of the artificial fruits are prefered, irrespective of the presence of chemical cues. Both physical and chemical cues affect oviposition decisions; hence, the fruit borer relies on cues of different sensory modalities.