Pronophiline butterflies (Satyridae) of the Cordillera de Mérida, Venezuela

The 36 species of the tribe Pronophilini known from the Cordillera de Merida, the main Andean mountain range in Venezuela, are listed, together with three others probably erroneously described or labelled as from the range. The general ecological characteristics of the tribe are described, using the Merida species as examples. These include its restriction to high altitudes, endemism in mountain 'islands', species partitioning up the altitude gradient, mimicry within the tribe, and seasonality. The evolutionary relationships of the 16 endemic species are analyzed. Two new genera, six new species, two new subspecies and one new form are described. Three new synonymies are established, three new combinations made, new status is given to two taxa and the status of one taxon is revised.     

Evidence for Distinct Aggregation Factors and Receptors in Cells

SINCE the original observations by Wilson¹ that dissociated sponge cells could reassociate in vitro, cell aggregation (or reaggregation) has been widely used as an operational criterion for the study of intercellular adhesion². The introduction of rotation-mediated methods to promote cell aggregation^3,4 led to the possibility of obtaining reproducible quantitative data. In these methods, suspensions of dissociated single cells are shaken under defined conditions of speed and temperature and cell aggregation is measured by either the size of aggregates or the number of single cells. The aggregation of dissociated cells from sponges⁵, chick and mouse embryos⁴ and tissue culture cells⁶ has been investigated with this method. Cells maintained in vitro seemed particularly suitable for studying mechanisms of cell aggregation as they represent a histotypically homogeneous population.     

Dynamic Properties of Giant Muscle Fibers of the Barnacle

Giant muscle fibers of the barnacle give graded, relativelyslow contractions, A plateau level, termed the unit response,occurs with stimuli of 3 msec, but pulses longer than 10–15msec give much greater tension or shortening. Repetitive stimulationwith pulses of 3 msec leads to a tetanus. The magnitude of activestate was determined, and found to be also graded in natureand slow to develop, though early in onset. The full developmentof active state requires 80–120 msec. A high level ofeffective series-elasticity was associated with the sarcomeresthemselves.     

SENSORY THRESHOLDS: ESTIMATION FROM DOSE-RESPONSE CURVES

Three methods of threshold estimation are compared for the detection of 1,8-cineole added to Concord grape juice presented in a semi-ascending paired difference design (Lundahl et al. 1986). Method A tests the significance of obtaining a nonzero response, method B determines the transition point of a 2-phase linear model fitted to the data, method C fits a logistic model to the data and defines threshold as the point of maximum curvature, C_max, of the curve. Method A yields interval estimates of thresholds, the midpoints of which systematically decrease with increasing sample size. Methods B and C yield point estimates of thresholds which do not systematically decrease with increasing sample size, are a feature of the dose-response curve, and are similar in value to each other.     

Isolation Conditions for High Yields of Protoplasts from Laminaria saccharina and L. digitata (Phaeophyceae)

In an investigation of the main factors determining protoplastyield in Laminaria saccharina and L. digitata, protoplasts wereisolated from epidermal, cortical and medullary cells of vegetativethallus by incubation with commercial cellulases, crude andpurified mannuronate lyases and purified guluronate lyases.Treatment of the tissue with the calcium chelator EGTA beforeenzymatic digestion greatly increased the protoplast yield.Preplasmolysis was also necessary to obtain large numbers ofhealthy protoplasts and this was most effective when carriedout during chelation with EGTA. Purification of the mannuronatelyases by ion exchange chromatography reduced the toxicity ofthe crude enzyme preparation. The activities of the wall degradingenzymes were differentially influenced by pH and the optimumfor alginate-lyase activity (8.0) was higher than that for cellulaseactivity (<6.0). Protoplast yield decreased linearly withincreasing pH in the enzyme medium over the range tested (6.0–8.0),and this suggests that cellulases are more critical to walldigestion than alginate-lyases. Ionic osmotica gave improvedyields compared with sugar alcohols or sugars. Increasing thecalcium concentration of the enzyme medium brought about anexponential decrease in protoplast yield and wall digestionwas almost completely inhibited at concentrations exceeding8.0 mol m^–3. However, low levels of calcium (<2.0 molm^–3) were beneficial to protoplast viability. Yields of10⁷ to 10⁸ protoplasts g^–1 fr. wt. were consistently obtainedand 20% to 30% of these regenerated new cell walls within 1–2d of culture. Key words: Laminaria protoplasts, cell wall, alginate-lyases     

Aphid–ant mutualism: how honeydew sugars influence the behaviour of ant scouts

Honeydew is the keystone on which ant–aphid mutualism is built. The present study investigates how each sugar identified in Aphis fabae Scopoli honeydew acts upon the feeding and the laying of a recruitment trail by scouts of the aphid‐tending ant Lasius niger Linnaeus, and thus may enhance collective exploitation by the ant mutualists. The feeding preferences shown by L. niger for honeydew sugars are: melezitose = sucrose = raffinose > glucose = fructose > maltose = trehalose = melibiose = xylose. Although feeding is a prerequisite to the launching of trail recruitment, the reverse is not necessarily true: not all ingested sugar solutions elicit a trail‐laying behaviour among fed scouts. Trail mark laying is only triggered by raffinose, sucrose or melezitose, with the latter sugar being specific to honeydew. By comparing gustatory and recruitment responses of ant foragers to sugar food sources, the present study clarifies the role of honeydew composition both as a source of energy and as a mediator in ant–aphid interactions. Lasius niger feeding preferences can be related to the physiological suitability of each sugar (i.e. their detection by gustatory receptors as well as their ability to be digested and converted into energy). Regarding recruitment, the aphid‐synthesized oligosaccharide (melezitose) could be used by ant scouts as a cue indicative of a long‐lasting productive resource that is worthy of collective exploitation and defence against competitors or aphid predators.     

Genetic variation and bill size dimorphism in a passerine bird, the reed bunting Emberiza schoeniclus

In passerine birds morphological differentiation in bill size within species is not commonly observed. Bill size is usually associated with a trophic niche, and strong differences in it may reflect the process of genetic differentiation and, possibly, speciation. We used both mitochondrial DNA (mtDNA) and nuclear microsatellites to study genetic variation between two subspecies of reed bunting, Emberiza schoeniclus schoeniclus and E.s. intermedia , along their distributional boundary in western Europe. These two subspecies are characterized by a high dimorphism in bill size and, although breeding populations of the two subspecies are found very close to each other in northern Italy, apparently no interbreeding occurs. The observed morphological pattern between the two subspecies may be maintained by geographically varying selective forces or, alternatively, may be the result of a long geographical separation followed by a secondary contact. MtDNA sequences of cytochrome b and ND5 (515 bp) showed little variation and did not discriminate between the two subspecies, indicating a divergence time of less than 500 000 years. The analysis of four microsatellite loci suggested a clear, although weak, degree of genetic differentiation in the large- and small-billed populations, as indicated by F _ST and R _ST values and genetic distances. The correlation between bill size and genetic distance between populations remained significant after accounting for the geographical distances between sampling localities. Altogether, these results indicate a very recent genetic differentiation between the two bill morphs and suggest that a strong selection for large bills in the southern part of the breeding range is probably involved in maintaining the geographical differentiation of this species.