Biology and regulation of ectoplasmic specialization, an atypical adherens junction type, in the testis

Anchoring junctions are cell adhesion apparatus present in all epithelia and endothelia. They are found at the cell-cell interface (adherens junction (AJ) and desmosome) and cell-matrix interface (focal contact and hemidesmosome). In this review, we focus our discussion on AJ in particular the dynamic changes and regulation of this junction type in normal epithelia using testis as a model. There are extensive restructuring of AJ (e.g., ectoplasmic specialization, ES, a testis-specific AJ) at the Sertoli-Sertoli cell interface (basal ES) and Sertoli-elongating spermatid interface (apical ES) during the seminiferous epithelial cycle of spermatogenesis to facilitate the migration of developing germ cells across the seminiferous epithelium. Furthermore, recent findings have shown that ES also confers cell orientation and polarity in the seminiferous epithelium, illustrating that some of the functions initially ascribed to tight junctions (TJ), such as conferring cell polarity, are also part of the inherent properties of the AJ (e.g., apical ES) in the testis. The biology and regulation based on recent studies in the testis are of interest to cell biologists in the field, in particular their regulation, which perhaps is applicable to tumorigenesis.     

Constructing more informative plant–pollinator networks: visitation and pollen deposition networks in a heathland plant community

Interaction networks are widely used as tools to understand plant–pollinator communities, and to examine potential threats to plant diversity and food security if the ecosystem service provided by pollinating animals declines. However, most networks to date are based on recording visits to flowers, rather than recording clearly defined effective pollination events. Here we provide the first networks that explicitly incorporate measures of pollinator effectiveness (PE) from pollen deposition on stigmas per visit, and pollinator importance (PI) as the product of PE and visit frequency. These more informative networks, here produced for a low diversity heathland habitat, reveal that plant–pollinator interactions are more specialized than shown in most previous studies. At the studied site, the specialization index was lower for the visitation network than the PE network, which was in turn lower than for the PI network. Our study shows that collecting PE data is feasible for community-level studies in low diversity communities and that including information about PE can change the structure of interaction networks. This could have important consequences for our understanding of threats to pollination systems.     

A TRADE-OFF FOR HOST PLANT UTILIZATION IN THE BLACK BEAN APHID,APHIS FABAE

Many studies on insect herbivores have sought to find trade-offs between utilization of alternate host plants, both to understand the prevalence of specialization and to appreciate the likelihood of sympatric speciation due to disruptive selection. To date, few studies have found trade-offs. Seventy-seven clones of the black bean aphid, Aphis fabae, were collected from field sites in East Anglia, U.K., over an area of about 10,000 km². These clones exhibit a trade-off in fitness between two alternative hosts, broad bean and nasturtium. This pattern is maintained in the F2 generation. The predominance of broad bean in the area, the fact that clones were only sampled from one of these two hosts, and the absence of “master-of-all-trades” genotypes after recombination all point to the importance of antagonistic pleiotropy rather than linkage disequilibrium in maintaining this trade-off. It is concluded that this population presents strong evidence for a fundamental trade-off for host utilization.     

Gender specialization and distyly in hoary puccoon,LITHOSPERMUM CROCEUM (Boraginaceae)

This 2-yr study of gender and its relation to the sizes of pin and thrum plants in Lithospermum croceum was performed in central Michigan. The population was composed of 51-52% thrums, 42-43% pins, and ≈6% homostyles in 1988 and 1989. Pins produced more flowers and fruits than thrums, and a greater proportion of their ovaries produced more than one nutlet. However, the number of fruits produced by pins and thrums in 1988 did not differ when plant size is a covariate in ANCOVA. The number of seeds produced by plants in 1989 is also unaffected by floral morph when size is a covariate. Instead, plant size is the most important predictor of fecundity in both years. The average percent fruit sets (fruits/ovaries) of the morphs did not differ in either year. Herbivory was more common in 1988 than in 1989, but there was no difference in the frequency with which pins and thrums were attacked. Furthermore, the effect of herbivory on fecundity did not differ for the morphs. The fecundity of neither morph was limited by pollen in 1989. Apparent gender specialization was a consequence of subtle differences in the vegetative sizes of the morphs.     

Determinants of predation on phytophagous insects: the importance of diet breadth

To evaluate the role of predation in the evolution of diet specialization and to determine the effectiveness of various larval defenses, we offered lepidopteran larvae to colonies of the tropical ant Paraponera clavata. We recorded behavioral and physical characteristics of prey items and used log-linear models to analyze their importance as deterrents to predation by P. clavata. The most important determinant of probability of prey rejection by P. clavata was a prey's diet breadth; specialists were rejected by the ants significantly more than generalists. Other less important, but significant, predictors of prey rejection included ontogeny, morphology and chemistry. Late instar caterpillars were rejected more frequently than early instars, hairy caterpillars were rejected more frequently than caterpillars with other morphologies, and one caterpillar species with an unpalatable extract was rejected more frequently than two species with palatable extracts.     

The ant Lasius niger shows no relationship between task efficiency and body size variation among workers

In ants, workers of different sizes may perform various tasks, even in so-called monomorphic species with relatively low body size variation. However, it is unclear if the body size diversity of monomorphic workers correlates with task efficiency, especially in stressful contingencies. Here we tested if the body size variation of workers corresponds with its efficiency in transferring pupae. Transferring brood is a pre-set behavioral response to stress, e.g. suboptimal temperature. Here we applied a laboratory experiment simulating nest damage. The study was performed on the common garden ant (Lasius niger (Linnaeus, 1758)) – a species with no distinct worker subcastes. The efficiency of workers was measured as the latency of transferring pupae from a lit part of the experimental colony to a darkened part, while the body size diversity was expressed as the within-colony coefficient of variation in head width. We did not find any significant correlation between efficiency and body size variation. Summarizing the existing studies and the present results, we propose the hypothesis that the body size diversity of L. niger may have implications for workers’ division of labor but not for their task efficiency in a stressful contingency.     

Vertically stratified frugivore community composition and interaction frequency in a liana fruiting across forest strata

Vertical stratification is a key feature of tropical forests and structures plant–frugivore interactions. However, it is unclear whether vertical differences in plant-frugivore interactions are due to differences among strata in plant community composition or inherent preferences of frugivores for specific strata. To test this, we observed fruit removal of a diverse frugivore community on the liana Marcgravia longifolia in a Peruvian rain forest. Unlike most other plants, Marcgravia longifolia produces fruits across forest strata. This enabled us to study effects of vertical stratification on fruit removal without confounding effects of plant species and stratum. We found a high number of visits of a few frugivore species in the understorey and a low number of visits of many different frugivores in the canopy and midstorey. Whereas partial and opportunistic frugivores foraged across strata with differing frequencies, obligate frugivores were only found eating fruits in the higher strata. Avian frugivores foraging in the canopy were mainly large species with pointed wings, whereas under- and midstorey avian foragers were smaller with rounded wings. Our findings suggest a continuous shift in the frugivore community composition along the vertical gradient, from a few generalized frugivores in the understorey to a diverse set of specialized frugivores in the canopy. This shift in the frugivore community leads to correlated, reciprocal changes from specialized to generalized plant-frugivore interactions. Thus, we conclude that vertical niche differentiation between species in tropical forests persists even when food resources are available across strata. This highlights its role for promoting biodiversity and ecosystem functioning.     

Parasitic specialization ofGeotrichum candidum citrus race

Parasitic specialization ofGeotrichum candidum citrus race, the causal agent of citrus sour rot, was investigated. Of seven isolates tested for pathogenecity, all could infect ten species of citrus fruits and edible parts of five species of noncitrus crops. Only one isolate (Ap2), isolated from soil of an apple orchard, could infect apple fruit.     

Maintenance of two genetic entities by habitat selection

Summary In the laboratory, the two species of copepodsLepeophtheirus thompsoni andLepeophtheirus europaensis, ectoparasites of flatfishes, can meet and mate on at least one host species. In the wild however, these two species are found isolated on their sympatric hosts. Habitat selection theoretically represents a powerful enough mechanism to explain the maintenance of genetic heterogeneity in the wide sense. In this paper, the host colonization process is studied for both parasite species. It is shown that each parasite can develop and reach adult age on each host species. However,L. thompsoni is highly selective; it almost totally refuses to colonize hosts other than its natural one.Lepeophtheirus europaensis, on the contrary, readily infests turbot and brill in single-host experiments, but strongly prefers the brill when it has a choice. It appears that these two genetic entities are sympatrically maintained due to strong habitat selection. Such a pattern could theoretically only occur in a soft-selection context (density dependence). This point is discussed with respect to the different patterns in host use found in the geographical distribution of these parasites.