Do Chaoborus larvae migrate in temporary pools?

A diurnal vertical migration of larvae of Chaoborus punctipennis (Say) was observed in shallow temporary woodland pools in East Texas. In the laboratory, in 153 cm tall columns, the larvae underwent a much greater migration than possible in the shallow pools. We hypothesize that the migratory behavior and transparent body of Chaoborus larvae may have originally evolved in such shallow water habitats. These traits enabled Chaoborus to successfully invade the quite different adaptive zone of predation in the plankton of deep lakes.     

A mechanism of respiration-dependent water uptake enhanced by auxin

Summary There are many contradictory observations on the mechanohydraulic relation of growing higher plant cells and tissues. Graphical analysis of the simultaneous equations which govern irreversible wall yielding and water absorption has made more comprehensive the understanding of this relation when relative growth rate is plotted against turgor pressure. It suggests that some respiration-dependent and auxin sensitive process might regulate the difference of osmotic potential between cells and water source. Based on anatomical and electrophysiological knowledge of the pea stem xylem, we propose the wall canal system as the mechanism of respiration-dependent water uptake which is sensitive to auxin. This system consists of the xylem apoplastic walls, the xylem proton pumps, active solute uptake system and cell membranes. In the simplest case, third-order simultaneous differential equations are involved. Numerical analysis showed that net uptake of solutes enables water to be taken up against an opposing gradient of water potential. The behaviour of this wall canal system describes well the mechano-hydraulic relation of enlarging plant cells and tissues. Recent typical, but incompatible, interpretations of this relation are critically discussed based on our model.Abbreviations V the volume of enlarging symplast - the average extensibility of the wall - Pⁱ turgor pressure - Y the yield threshold of the wall - L the relative hydraulic conductance - the solute reflection coefficient of the plasmamembrane - Cⁱ the osmotic concentration of the symplast cells - C^x the osmotic concentration of the xylem vessels - P^x hydrostatic pressure in the xylem vessels - R the gas constant - T absolute temperature - ^o water potential of xylem fluid - ⁱ water potential of symplast cells     

Electrophoretic analysis of a chromosomal hybrid zone in the grasshopper Podisma pedestris

We have made an extensive allozyme survey of 21 enzyme and protein loci in populations of the alpine grasshopper Podisma pedestris. This species occurs in two races, differing by a chromosomal fusion which separates the ancestral XO/XX race from a derived neo-XY race. These races also differ in DNA content, and hybrids between them have reduced viability. Electrophoresis reveals that the amount of genetic differentiation between these races is no greater than the variation among populations within each race. Both larger-scale surveys and a detailed survey of an area where the races hybridize, show that the chromosomal change is not correlated with gene frequency changes at any of the 21 loci studied. These findings are consistent with recently developed theory concerning the strength of the barrier to gene flow posed by a hybrid zone with characteristics such as those measured experimentally in Podisma. It is argued that hybrid zones in other species which involve allozymic differences do so because of stronger selection against hybrids rather than through mating isolation.     

Assortative mating across a hybrid zone in Chorthippus parallelus (Orthoptera: Acrididae)

Two subspecies of the grasshopper Chorthippus parallelus meet and form a hybrid zone in the Pyrenees. Both strong hybrid dysfunction and a behavioural difference occur, which would seem to make the zone a suitable candidate for speciation by reinforcement. One of the classic ways of looking for this is to test for increased levels of assortative mating between populations from close to the region of contact. Here we show that, with virgin insects, such assortment decreases as one approaches the centre of the zone in C. parallelus. The pattern is different upon remating, with non-virgins showing a pattern more like that predicted by reinforcement. Overall there is little evidence for reinforcement. We argue that other tests of the model may be more appropriate for stable hybrid zones.     

LIFE-HISTORY ADAPTATION AND REPRODUCTIVE ISOLATION IN A GRASSHOPPER HYBRID ZONE

Patterns of life-history adaptation and reproductive isolation were investigated in the acridid grasshoppers Melanoplus sanguinipes and M. devastator, which hybridize along an altitudinal gradient in the Sierra Nevada of California. Melanoplus sanguinipes females crossed with M. devastator males produced eggs that were approximately half as viable as eggs from other crosses. Diminished viability was not attributable either to infection by Wolbachia pipientis or to failure of sperm transfer. When offered an opportunity to choose a mate, females from all populations discriminated against males of the other species, whereas in no-choice tests measuring copulation duration only females from the tails of the clines showed preferences. Melanoplus sanguinipes, found at high elevations where the growing season is short, exhibited faster egg hatch, faster larval development, smaller adult body sizes, and smaller clutch sizes than M. devastator. Melanoplus devastator, from California's Central Valley, endured a hot and dry summer in a reproductive diapause that was absent in M. sanguinipes. Clines in reproductive diapause and clutch size coincided with the region of reproductive incompatibility. Development time, body size, and hatch time also changed across the hybrid zone, but the regions of largest transitions in these traits were either difficult to locate using the limited populations studied here or were not coincident with the zone's center. A method is described for combining ecological and phylogenetic analyses to address the unknown issue of whether life-history divergence has conributed to reproductive isolation in this system.     

A new species of Macrothrix (Anomopoda: Macrothricidae) from Central Mexico

Macrothrix mexicanus sp. nov. is described from central México, a transition zone between the nearctic and neotropics. All localities where it was found are over 1800 meters above sea level. It shows many resemblances with M. laticornis, M. camjatae and M. rosea but is characterized by a persistent dorsal tooth on the valve keel, a spinous papilla on the basipodite of the antenna, the second thoracic limb with a long conical sensillum between scraper 1 and the gnathobase, the endopod of trunk limb IV having two setae; the postabdomen with the dorsal margin bilobed, and the distal segment of the seta natatoria which is unusually long.Abbreviations used on figures EN Endopodite - EP Epipodite - EX Exopodite - IDL Inner distal lobe - ODL Outer distal lobe - GT Gnatobase - E1 Endite 1 - E2 Endite 2 - E3 Endite 3     

Altitudinal zonation of montane Quercus forests along two transects in Chirripó National Park,Costa Rica

Abiotic and vegetation data were collected along two altitudinal transects through mature montane Quercus forests on the Pacific and Atlantic slopes of Costa Rica's Chirripó Massif. Between 2000 and 3200 m asl twenty-four 0.05 ha forest plots were selected at altitudinal intervals of 100 m, and eight soil profiles were described at intervals of 200 m. A TWINSPAN classification aided in the determination of eight zonal forest communities on the basis of their floristic composition. They are grouped in two sets of four: (i) the palm-rich lauraceous-fagaceous Lower Montane Mollinedia-Quercus Forests (2000–2600 m asl) and (ii) the bamboo-rich myrsinaceous-fagaceous Upper Montane Schefflera-Quercus Forests (2500–3200 m asl), respectively. Vegetation changes seem correlated with two major climatic gradients: (i) a temperature gradient (altitude), and (ii) a moisture gradient (wet Atlantic vs. moist Pacific slope). Most soils are Andepts, and residual, colluvial or derived from volcanic material. Humus layers are thicker on the wetter Atlantic slope. A total of 431 vascular plant species consisted of 86 pteridophytes, 1 gymnosperm, 296 dicots and 48 monocots. Species richness, canopy height and stem diameter decrease with increasing altitude, while the canopy surface becomes more flattend. A comparison with other studies shows that Chirripó's montane Quercus forests fit within the environmental ranges known from altitudinal zonations elsewhere in the Tropics.Abbreviations asl above sea level - dbh diameter at breast height - LM Lower Montane - Mt. Mountain - TWINSPAN two way indicator species analysis - UM Upper Montane - VU code referring to soil profiles as presented in Van Uffelen (1991) This paper is dedicated to the memory of Alwyn H. Gentry, an outstanding and inspiring tropical botanist who tragically died in a plane crash in the mountains of Ecuador on August 3 1993, when surveying possible boundaries for a new tropical cloud forest reserve.     

The management of European estuaries: A comparison of the features,controls and management framework of the Tagus (Portugal) and Humber (England)

The paper describes the local, national, European and wider-area framework, statutes, and formal and voluntary mechanisms for managing European estuaries. These aspects are discussed in relation to two large and representative estuarine systems, the Tagus, Portugal, and the Humber, on the English North Sea coast As estuaries are sites of many activities and uses, most of which are encouraged or at least condoned, management has the role of preventing and resolving conflicts between those uses and users. Accepted uses of estuaries include the discharge and dumping of waste materials, fin and shell-fisheries, conservation, land reclamation, natural usage, abstraction by industry, and recreation. Estuarine management is now being carried out within the constraints of local and regional government planning, planning and activities of water pollution control bodies, fisheries control bodies, and navigation and port authorities The Tagus and Humber estuaries support all of the above activities and uses, and have controls within a European legislative framework but have differing histories of management and planning in order to resolve conflicts. In addition the Humber is subject to controls placed on North Sea areas. The paper discusses the relevant national and European legislation (Directives) and accepted practices for management. Furthermore, the paper discusses the formulation and practice of estuarine management plans as used by various bodies (nature conservation, water quality and regional authority). It is of particular note that the lessons from these two estuaries are relevant to many other European estuaries.     

ASSORTATIVE MATING AND NATURAL SELECTION IN AN IRIS HYBRID ZONE

The phenology of different genotypes and the distribution of genetic variation among flowering plants and their progeny were examined to assess the levels of assortative mating and selection in a hybrid population of Iris. This study and a previous survey of RAPD nuclear markers and chloroplast markers indicate that the population consists of parental genotypes and recombinant hybrid genotypes that are similar to the parental species (I. fulva and I. brevicaulis), although lacking intermediate genotypes. Early in the season only I. fulva genotypes produced flowers, but as flowering in these plants decreased, the hybrid genotypes and I. brevicaulis genotypes began flowering, resulting in a 24-d period of coincidental flowering. The genotypic distribution of seeds produced during the period of flowering overlap contained a high frequency of intermediate genotypes that were not present in the adult generation. The degree of effective assortative mating was examined by comparing the observed progeny genotypic distributions with expected distributions from a mixed-mating model. The model included selfing and random outcrossing to the nearest plants that had pollen-bearing flowers on the day the recipient flower was receptive. The observed genotypic distribution of progeny from plants with I. brevicaulis chloroplast DNA (cpDNA) was not significantly different from the expected distribution. For I. fulva genotypes, however, there were higher than expected frequencies in the extreme genotypic classes, although intermediate genotypes were absent, indicating that these plants were preferentially mating with similar genotypes. Compared with the extreme genotypes, a larger proportion of the intermediate seed progeny produced were aborted, indicating that intermediate genotypes have lower viability. On the basis of the observed progeny genotypes and genetic disequilibria estimates for the adults and the progeny, there appears to be a pattern of effective asymmetrical mating in this population. This asymmetry is most likely due to pollen-style interactions that reduce the fertilization ability of genetically dissimilar pollen, or preferential abortion of genetically intermediate zygotes by I. fulva-like genotypes. The lack of any apparent discrimination by I. brevicaulis-like genotypes creates a directional exchange of nuclear genetic elements that will have implications for introgression and the evolution of hybrid genotypes.