Neither the devil nor the deep blue sea: larval mortality factors fail to explain the abundance and distribution of Tischeria ekebladella

Abstract.  1. The distribution, abundance and population dynamics of herbivorous insects may be affected by trophic interactions, by abiotic influences, or by intra-specific processes. Relatively little is known about how trophic influences vary across space. Here, we investigate spatial variation in mortality in the oak-feeding leaf miner Tischeria ekebladella as attributable to individual causal agents.
2. Leaf miners were experimentally introduced on 67 trees on an island 5 km² in area in south-western Finland. On each tree, some larvae were protected by a muslin bag, others by a glue barrier around the branch and some left exposed.
3. In the bagged transplants, 78.4% of larvae survived, compared with only 9.6% in the other two treatments. Most of the mortality was because of airborne agents: mortality on branches sheltered by a glue barrier was as high as on fully exposed branch tips.
4. We consider mortality caused by parasitoid wasps to be the main source of larval death and the primary factor driving general patterns of survival. The effects of bird predation and premature leaf abscission were negligible.
5. We detected spatial aggregation in larval survival and parasitism rates at the level of individual trees, but not across the landscape.
6. Spatial variation in overall leaf miner survival, parasitism and leaf abscission does not suffice to explain patterns of incidence and abundance of wild T. ekebladella on experimental trees. Rather, we identify metapopulation dynamics as a likely determinant of the spatial distribution of T. ekebladella in the landscape.     

Hopkins' 'host selection principle', another nail in its coffin

Abstract. Adult female parasitoids (Aphidius rhopalosiphi De Stef.) (Hymenoptera: Aphidiidae) reared on the aphid Metopolophium dirhodum (Walk.) (Homoptera: Aphididae) changed their responses to odours of two wheat cultivars ('Maris Huntsman' and 'Rapier') when the parasitized aphids had been moved from the former to the latter cultivar at various times after parasitization. If the aphids were moved after less than 6 days, the female parasitoids emerging from mummies by then formed on 'Rapier' responded positively to that cultivar, whereas parasitoids emerging from mummies transferred as still living aphids after 8 days on 'Maris Huntsman', responded positively to that cultivar rather than to 'Rapier' on which they had emerged. This appeared to be evidence for Hopkins' 'host selection principle', which states that chemical experience acquired by the larva of an endopterygote insect can be transferred through the pupal stage to the adult. However, discrimination in favour of either cultivar disappeared when the parasitoids were reared in aphids on both cultivars, and the emerging females were tested following excision of the pupae from the aphid mummies. The previous discrimination shown by adult parasitoids emerging from aphid mummies must therefore be determined by the chemicals contacted by a parasitoid on the skin of the mummy or while biting its way out of the dead aphid.     

Phylogenetics and biogeography of the Balkan ‘sand gobies’ (Teleostei: Gobiidae): vulnerable species in need of taxonomic revision

Within the Atlantic–Mediterranean region, the ‘sand gobies’ are abundant and widespread, and play an important role in marine, brackish, and freshwater ecosystems. They include the smallest European freshwater fish, Economidichthys trichonis, which is threatened by habitat loss and pollution, as are several other sand gobies. Key to good conservation management is an accurate account of the number of evolutionary significant units. Nevertheless, many taxonomic and evolutionary questions remain unresolved within the clade, and molecular studies are lacking, especially in the Balkans. Using partial 12S and 16S mitochondrial ribosomal DNA sequences of 96 specimens of at least eight nominal species (both freshwater and marine populations), we assess species relationships and compare molecular and morphological data. The results obtained do not support the monophyly of Economidichthys, suggesting the perianal organ to be a shared adaptation to hole‐brooding rather than a synapomorphy, and urge for a taxonomic revision of Knipowitschia. The recently described Knipowitschia montenegrina seems to belong to a separate South‐East Adriatic lineage. Knipowitschia milleri, an alleged endemic of the Acheron River, and Knipowitschia cf. panizzae, are shown to be very closely related to other western Greek Knipowitschia populations, and appear conspecific. A distinct Macedonian–Thessalian lineage is formed by Knipowitschia thessala, whereas Knipowitschia caucasica appears as an eastern lineage, with populations in Thrace and the Aegean. The present study combines the phylogeny of a goby radiation with insights on the historical biogeography of the eastern Mediterranean, and identifies evolutionary units meriting conservation attention. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 73–91.     

Few parasites,and no evidence for Wolbachia infections,in a freshwater ostracod inhabiting temporary ponds

Biological systems with asexual reproduction have often attracted research on parasites and host immune defence, because parasites are expected to be better able to exploit genetically less diverse populations. In addition, maternally inherited parasitic microorganisms such as Wolbachia can directly alter the reproductive systems of their hosts and induce parthenogenesis. In the freshwater ostracod Eucypris virens, both sexual and asexual reproduction is known, and we speculated that parasite pressures might help to explain their co‐existence. This species complex inhabits shallow, often eutrophic temporary water bodies, conditions that should provide ample opportunities for parasite infections. We surveyed natural populations of E. virens throughout its Europe‐wide range for natural parasites, and particularly tested for the presence of intracellular Wolbachia bacteria. Surprisingly, the results indicate that very few E. virens populations support parasite infections. We also found no evidence for the presence of Wolbachia in the populations screened. The results therefore show that parasitic infections do not play a role in the maintenance of sex in this system. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 207–216.     

Colony kin structure and host‐parasite relatedness in the barnacle goose

Conspecific brood parasitism (CBP), females laying eggs in the nest of other ‘host’ females of the same species, is a common alternative reproductive tactic among birds. For hosts there are likely costs of incubating and rearing foreign offspring, but costs may be low in species with precocial chicks such as waterfowl, among which CBP is common. Waterfowl show strong female natal philopatry, and spatial relatedness among females may influence the evolution of CBP. Here we investigate fine‐scale kin structure in a Baltic colony of barnacle geese, Branta leucopsis, estimating female spatial relatedness using protein fingerprints of egg albumen, and testing the performance of this estimator in known mother‐daughter pairs. Relatedness was significantly higher between neighbour females (nesting ≤ 40 metres from each other) than between females nesting farther apart, but there was no further distance trend in relatedness. This pattern may be explained by earlier observations of females nesting close to their mother or brood sisters, even when far from the birth nest. Hosts and parasites were on average not more closely related than neighbour females. In 25 of 35 sampled parasitized nests, parasitic eggs were laid after the host female finished laying, too late to develop and hatch. Timely parasites, laying eggs in the host’s laying sequence, had similar relatedness to hosts as that between neighbours. Females laying late parasitic eggs tended to be less related to the host, but not significantly so. Our results suggest that CBP in barnacle geese might represent different tactical life‐history responses.     

Variations in the dorso-ventral organization of leaf structure and Kranz anatomy coordinate the control of photosynthesis and associated signalling at the whole leaf level in monocotyledonous species

Photosynthesis and associated signalling are influenced by the dorso-ventral properties of leaves. The degree of adaxial/abaxial symmetry in stomatal numbers, photosynthetic regulation with respect to light orientation and the total section areas of the bundle sheath (BS) cells and the surrounding mesophyll (M) cells on the adaxial and abaxial sides of the vascular bundles were compared in two C₄[ Zea mays (maize) and Paspalum dilatatum ] and one C₃[ Triticum turgidum (Durum wheat)] monocotyledonous species. The C₃ leaves had a higher degree of dorso-ventral symmetry than the C₄ leaves. Photosynthetic regulation was the same on each side of the wheat leaves, as were stomatal numbers and the section area of the BS relative to that of the M cells (BS/M section area ratio). In contrast, photosynthetic regulation in maize and P. dilatatum leaves showed a marked surface-specific response to light orientation. Compared to the adaxial sides of the C₄ monocotyledonous leaves, the abaxial surfaces had more stomata and the BS/M section area ratio was significantly higher. Differences in dorso-ventral structure, particularly in Kranz anatomy, serve not only to maximize photosynthetic capacity with respect light orientation in C₄ monocotyledonous leaves but also allow adaxial and abaxial-specific signalling from the respective M cells.     

Functional stability of stream-dwelling microbial decomposers exposed to copper and zinc stress

1. We investigated the resistance of aquatic microbial decomposers to Cu and Zn stress and their ability to recover after release from metal exposure, by examining leaf mass loss, fungal reproduction and microbial biomass and diversity.
2. Alder leaves, colonised in a reference stream, were exposed in microcosms to copper (Cu) or zinc (Zn), alone or in mixtures, with metals added together or sequentially (at day 0 or after 10 days). After 20 days, half of the microcosms were released from metals.
3. Leaf mass loss and fungal reproduction were reduced in most metal treatments, and the structure of fungal and bacterial communities was altered as indicated by identification of conidia and DNA fingerprinting based on denaturing gradient gel electrophoresis. Metals reduced the biomass of bacteria, but not that of fungi.
4. After release from metal stress, the structure of fungal communities became similar to that of control, and a recovery of microbial activity seemed to occur as shown by the lack of differences in leaf mass loss, bacterial biomass and fungal reproduction between control and metal treatments.     

Molecular identification of brood‐parasitic females reveals an opportunistic reproductive tactic in ruddy ducks

In many taxa, females lay eggs in the nests of other conspecifics. To determine the conditions under which conspecific brood parasitism develops, it is necessary to identify parasitic offspring and the females who produce them; however, for most systems parasitism can be difficult to observe and most genetic approaches have relatively low resolving power. In this study, we used protein fingerprinting from egg albumen and 10 microsatellite loci to genetically match parasitic ducklings to their mothers in a population of ruddy ducks (Oxyura jamaicensis). We found that 67% of nests contained parasitic offspring, and we successfully identified their mothers in 61% of the cases. Of the parasitic females identified, 77% also had nests of their own (i.e. a dual tactic, where females both nest and lay parasitically), and we found no evidence that parasitic females pursued a specialist (parasitism only) tactic. We also found that parasitic egg laying was not influenced by nest loss, predation or female condition. Thus, in contrast to most waterfowl studied to date, female ruddy ducks appear to lay parasitic eggs whenever the opportunity arises.     

Testing the role of coadapted genes versus bet-hedging for mating strategies in colour polymorphic pygmy grasshoppers

Recent investigations of mate choice indicate that the genetic effect of sires on offspring fitness may depend on the interaction between maternal and paternal genotypes and the environmental conditions experienced by the offspring. Alternative colour morphs of the pygmy grasshopper, Tetrix subulata , represent ecological strategies that differ in body size, life history, thermoregulatory behaviour, and habitat selection. The hypothesis that selection promotes behaviours maintaining coadapted gene complexes predicts individuals to mate assortatively with respect to colour morph. On the other hand, the bet-hedging hypothesis predicts that the temporal variability of the environment inhabited by these animals may select for disassortative mating behaviour resulting in heterogeneous offspring. To distinguish between these competing hypotheses, we investigated mating behaviours using dual-choice experiments. Our results were not in agreement with the prediction of assortative mating but suggest instead that matings were random with regard to colour morph. Polyandry was common, and females mated with the second male regardless of whether the first mating was assortative or disassortative. Polyandry also was equally frequent among females in triads in which the two males belonged to different colour morphs as in triads where both males belonged to the same colour morph. A field experiment confirmed that polyandry occurred also among free-ranging individuals, and uncovered variation in mating success among male colour morphs, probably due to indirect effects of coloration on activity or habitat use. The consequences of this random and polyandrous mating strategy for the evolutionary dynamics of the colour polymorphism remain to be explored.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 491–499.