Optimal foraging by an aphid parasitoid affects the outcome of apparent competition

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TAXONOMY AND ZOOGEOGRAPHY OF THE MESOZOIC CYTHERURID OSTRACODA FROM WEST-CENTRAL ARGENTINA

Abstract:  The status of thirty four species of the cytheroidean ostracod family Cytheruridae from the Neuquén Basin, west-central Argentina is reviewed. These species belong to the following nine genera: Eucytherura Müller, ranging in this study from Pliensbachian to Valanginian and represented by fourteen species; Kangarina Coryell and Fields, Aalenian–Bajocian, with a single species; Acrocythere Neale, Hauterivian, one species; Paranotacythere Bassiouni, Berriasian, two species; Procytherura Whatley, Pliensbachian to Hauterivian, twelve species; Cytheropteron Sars, Aalenian–Bajocian, one species: Eocytheropteron Alexander, Hauterivian, one species; Paradoxorhyncha Chapman, Aalenian–Bajocian, one species; and Paracytheridea Müller, Berriasian, one species. Of the three subfamilies of the Cytheruridae, the Cytherurinae, with thirty species are by far the most numerous and abundant throughout the study. The other two subfamilies, the Cytheropterinae are represented by three species and the Paracytherideinae by a single species of the nominative genus. Several of the species are very widely distributed geographically and have, for example, also been recorded from Europe; others indicate close links with South Africa and Australia. The stratigraphical ranges of certain genera have been extended as a result of this study. For example, Kangarina has not previously been recorded below the Cretaceous. The Cytheruridae are clearly the most diverse cytheroidean ostracodes in the Mesozoic of the Neuquén Basin and are more diverse than all other groups of ostracodes combined. Four new species, Eucytherura tessae , Eucytherura yunga , Procytherura amygdala and Eocytheropteron immodicus are described. Eucytherura guillaumeae nom. nov. for Eucytherura tuberculata Brenner and Oertli and Eucytherura paranuda nom. nov. for Eucytherura nuda (Brand) are proposed.     

Comparison of the bud morphology and shoot growth dynamics of four species of Mediterranean subshrub growing along an altitudinal gradient

Bud morphology, bud activity, and shoot growth dynamics were studied over 13 months in four species of Mediterranean subshrubs growing along an altitudinal gradient in north-east Spain. The species selected were representative of the main functional types of subshrubs that grow naturally along altitudinal gradients on Mediterranean mountains. All species bore naked buds as their renewal structures, but the degree of protection of the buds varied between species, increasing with altitude. The period of morphogenesis and the level of shoot pre-formation also varied along the gradient. The species growing at low altitude had partially neoformed shoots, longer shoot growth periods, and started morphogenesis earlier than those growing at high altitude. These differences may indicate different morphological and phenological adaptations of Mediterranean subshrubs in response to increased winter cold along the gradient. At low altitudes, brachyblast-bearing species with partially neoformed shoots may be favoured because of the plastic shoot growth these structures confer. However, as the climate becomes colder and more predictable, brachyblast-bearing species may be replaced by species without brachyblasts, with tightly protected buds and more constrained patterns of shoot development.  © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 151 , 527–539.     

An inter-regional approach to intraspecific variation in habitat association: Rock Buntings Emberiza cia as a case study

The habitat association approach has been increasingly used in ecology to resolve problems in wildlife conservation and management. One problem related to habitat association studies is that they are restricted to small geographical areas within a species' range, and thus they are applicable to only a limited set of environmental conditions utilized by the species. In addition, very few studies address why the preference for specific habitat components may be adaptive for the species in question. The objective of this study was to examine how consideration of populations of a species from two dramatically different environments affects the results of habitat association modelling for a ground-nesting passerine, the Rock Bunting Emberiza cia . At a regional scale, a trend to defending breeding habitat patches with relatively higher stone cover was confined to birds from a temperate region in Slovakia. In contrast, in a semi-arid region in southeastern Spain, Rock Buntings preferred to use breeding habitat patches that had relatively higher grass cover. Combining data from both regions, breeding Rock Buntings showed a general pattern of using habitat patches close to hedges, with low bush cover, high ditch density and a steep slope. Whereas regional habitat association models appear to be sensitive to the particularities of the breeding environment, our study suggests that Rock Bunting breeding habitat association is constrained by the adults' tactics to protect themselves against predators. Although the birds prefer to nest in patches of low vegetation, the better to see nearby predators, these patches are ideally close to taller vegetation that can be used to provide cover when evading predators, and they are also of a rugged profile that helps the birds to approach and leave the nest stealthily.     

Trophic interactions among invertebrates in termitaria in the African savanna: a stable isotope approach

Abstract 1. Termites (Isoptera) in tropical savannas are known as ecosystem engineers, affecting the spatial and temporal distribution of water, carbon, cations, and nutrients through their mound structures. Their mounds, however, also offer habitation to diverse taxa and feeding guilds of other invertebrates; a keystone role that has not been properly quantified. 2. The aim of this study was to explore the ecosystem role of termitaria in determining invertebrate diversity and their potential trophic interactions. We used stable isotopes to distinguish termite‐feeding invertebrates from invertebrates merely living in termite mounds under field conditions. 3. The results suggest that inquiline spiders (Arachnida) do not feed on termites directly, but on other invertebrates within the termitaria that are termitophagous, elevating the spiders three trophic levels higher than the termites. 4. This study is the first to demonstrate food web interactions among inquiline invertebrates with a stable isotope approach. It provides evidence that termites play a keystone role in the system by providing habitat for various, trophically interacting invertebrates. These results illustrate a rather unexplored ecosystem property of savanna termites.     

Mitochondrial gene diversity in Cepaea– population structure, history and positive selection

Previous studies on the geographical distributions of both shell polymorphisms and mitochondrial haplotypes in Cepaea land snails have suggested varying contributions of natural selection, random genetic drift and population history to the origin and persistence of variation. We combine previous studies of polymorphism within two species of Cepaea , with new molecular data from two mitochondrial genes. The distributions of mitochondrial variants suggest that elements of population history may have been influential in creating patterns of diversity. However, some patterns of amino acid substitution that are found in a protein coding gene are also consistent with the action of diversifying selection. This suggests the possibility that in addition to stochastic processes such as repeated founder events, local extinctions and random genetic drift within a structured population, adaptive molecular change may have affected mitochondrial diversity.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 87 , 167–184.     

Functional bases of host-acceptance behaviour in the aphid parasitoid Aphidius ervi

Abstract.  The host acceptance behaviour in Aphidius ervi is investigated, assessing the role of both external and internal host-associated cues, offered to the experimental parasitoids with parafilm-made aphid dummies. The reaction to internal cues present in the host haemolymph is clearly evident, and its intensity is enhanced by external cues. Parasitoid females lay few eggs in aphid dummies filled with host haemolymph. A significant increase in the number of both oviposition reactions (host stinging) and egg laying is observed only when these dummies are coated with cornicle secretion. However, this enhancement is not observed when the aphid dummies contain distilled water. Thus, the host acceptance behaviour of A. ervi females appears to be controlled by the integration of both external and internal chemical cues. The physiological basis of this behavioural response is investigated with a detailed study on the anatomy and ultrastructure of A. ervi ovipositor. The detection of chemical cues present in the host haemolymph that act as kairomones is made possible by the presence of gustatory sensilla on the tip of the ovipositor. These sensilla consist of porous areas, reached by unbranched dendrites running inside both the lower valves (i.e. first valvulae) and the unpaired upper valve (i.e. second valvulae). The mechanosensory function during oviposition appears to be provided mostly by the basiconic sensilla found on the tip of external valves (i.e. third valvulae). A tentative functional model accounting for the observed oviposition behaviour of A. ervi is proposed.     

Female-biased sexual size dimorphism in tinamous: a comparative test fails to support Rensch's rule

Rensch's rule states that degree of sexual dimorphism increases with body size in species with larger males, and decreases with body size in those with larger females. To test this rule, we assessed the pattern of sexual size dimorphism in tinamous using a comparative analysis of independent contrasts. Tinamous are a monophyletic group of primitive birds comprising at least 47 ground dwelling species with prominent or exclusive paternal care of eggs and offspring. Although the size of females exceeded that of males in most considered species, we found an isometric relationship between males and females, instead of the negative allometric one predicted by Rensch's rule. Previous studies in Strigiformes and Falconiformes found positive allometric and isometric relationships respectively, and, considering these findings with our results, we conclude that Rensch's rule is not supported by birds with exclusively female-biased sexual dimorphism in size.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 80 , 519–527     

Heat stress in grapevine: the pros and cons of acclimation

Heat stress is a major limiting factor of grapevine production and quality. Acclimation and recovery are essential to ensure plant survival, and the recovery mechanisms can be independent of the heat response mechanisms. An experimental set up with and without acclimation to heat followed by recovery [stepwise acclimation and recovery (SAR) and stepwise recovery (SR), respectively] was applied to two grapevine varieties, Touriga Nacional (TN), and Trincadeira (TR), with different tolerance to abiotic stress. Major differences were found between leaves of SAR and SR, especially after recovery; in SAR, almost all parameters returned to basal levels while in SR they remained altered. Acclimation led to a swifter and short‐term antioxidative response, affecting the plant to a lesser extent than SR. Significant differences were found among varieties: upon stress, TN significantly increased ascorbate and glutathione reduction levels, boosting the cell's redox‐buffering capacity, while TR needed to synthesize both metabolites, its response being insufficient to keep the redox state at working levels. TR was affected by stress for a longer period and the up‐regulation pattern of antioxidative stress genes was more obvious. In TN, heat shock proteins were significantly induced, but the canonical heat‐stress gene signature was not evident probably because no shutdown of the housekeeping metabolism was needed.