THE ORIGIN OF AFRO-ARABIAN 'DIDELPHIMORPH' MARSUPIALS

Abstract:  New specimens of Peratherium africanum from Early Oligocene deposits of the Fayum, Egypt, provide key information on the relationships of the species. These include the first maxilla to be found and two additional dentaries. The maxilla can be demonstrated to belong to the same species as the holotype dentary by study of the occlusal relationships of upper and lower molars. It can be shown by several synapomorphies that P. africanum is the sister species to European Bartonian–Rupelian Peratherium lavergnense . P. africanum therefore belongs to the 'didelphimorph' family Herpetotheriidae, not to the peradectimorph family Peradectidae. The genus Qatranitherium , previously erected for this species alone, is here synonymized with Peratherium . Comparison with 'didelphimorphian' taxa from early Paleogene deposits of South America suggests more remote relationships, indicating an origin for P. africanum by dispersal from Europe as originally envisaged. The more precise relationships deduced here help to constrain the time interval for dispersal to Afro-Arabia, probably during the earliest Oligocene sea-level low.     

Field and experimental evidence of the effect of Jenynsia multidentata, a small omnivorous–planktivorous fish, on the size distribution of zooplankton in subtropical lakes

1. Small cladocerans, copepod nauplii and rotifers often dominate the zooplankton community in tropical and subtropical lakes. This is probably because of high predation pressure by small omnivorous–planktivorous fish, but experimental evidence is scarce.
2. This study used two approaches to test the effect of the small omnivorous–planktivorous fish species Jenynsia multidentata , which is frequently abundant in (sub)tropical eutrophic lakes in South America, on the size distribution of zooplankton. In Lake Blanca (Uruguay), which lacks any piscivores, we sampled seasonally for both fish and zooplankton. We also conducted an outdoor mesocosm experiment with treatments containing or lacking J. multidentata .
3. Together, the empirical and experimental data suggest that J. multidentata predation plays an important role in modulating the size structure of the zooplankton community in subtropical lakes. In the absence of J. multidentata , stocked large-sized zooplankters like Daphnia obtusa were abundant in the experiments, while small-sized zooplankton dominated in the presence of fish, as they did in the lake itself from spring to the end of the season.     

The relative importance of dispersal and local processes in structuring phytoplankton communities in a set of highly interconnected ponds

1. The recognition that both local and regional processes act together in shaping local communities makes determining their relative roles in natural communities central to understanding patterns in community structure. 2. We investigated the relative influence of these processes on the phytoplankton communities of a highly interconnected pond system. We sampled the phytoplankton communities of 28 ponds concurrently with 20 local environmental variables. 3. We found that phytoplankton community variation, in terms of both phytoplankton community composition (PCC) and diversity, was only significantly explained by local environmental variables. These were mainly associated with the contrasting clear‐water and turbid ecological states of the shallow ponds studied. Clear‐water conditions favoured only a few taxa, resulting in a significantly lower taxon diversity and richness under these conditions. 4. The failure to explain variation in PCC by a dispersal model based on the water flow between ponds points at very effective species sorting. This is attributed to the high population turn‐over rates and sensitivity to environmental conditions of phytoplankton communities. Some evidence was found, however, that dispersal influences local communities through mass effects between neighbouring ponds. 5. Overall, our results emphasize both the strong selection pressure that components of the food web exert on phytoplankton communities and the high potential of these communities to respond to such environmental change, thereby effectively opposing the homogenizing effects of continuous dispersal.     

The Effect of CO2-Concentration on the Occurrence of a Number of Acids from the Citric Acid Cycle in Tomato Leaves

Examination of the effect of CO₂-concentration and time of day on the content of malic acid, citric acid, aconitic acid, isocitric acid, succinic acid and fumaric acid in tomato leaves, revealed that the total content of these acids will rise with the CO₂-concentration up to 0.10 vol% CO₂. In the morning up to 0.22 vol% CO₂ was needed for optimal effect. Samples of leaves picked at 1 a.m. showed the lowest content of these acids. At 9 a.m. the content had increased, and at 4 p.m. the increase was considerable. The content of malic and citric acid constituted 36 and 34% of the total acid content. In the afternoon and the night the aconitic acid represented 14% and in the morning 20% of the total acid content. Isocitric acid, fumaric acid and succinic acid occurred only in relatively small concentrations.     

Temperature affects the limitation of Daphnia magna by eicosapentaenoic acid,and the fatty acid composition of body tissue and eggs

1. Poikilothermic animals incorporate more polyunsaturated fatty acids (PUFAs) into their cellular membranes as temperature declines, suggesting an increased sensitivity to PUFA limitation in cool conditions. To test this we raised Daphnia magna at different temperatures and investigated the effect of varying dietary PUFA on life history parameters (i.e. growth, reproduction) and the PUFA composition of body tissue and eggs. 2. Upon a PUFA‐rich diet (Cryptomonas sp.) females showed higher concentrations of several ω3 PUFAs in their body tissue at 15 °C than at 20 °C and 25 °C, indicating a greater structural requirement for ω3 PUFAs at low temperature. Their eggs had an equal but higher concentration of ω3 PUFAs than their body tissue. 3. In a life history experiment at 15 and 20 °C we supplemented a diet of a PUFA‐free cyanobacterium with the ω3 PUFA eicosapentaenoic acid (EPA). The growth of D. magna was more strongly EPA limited at low temperature. A greater requirement for structural EPA at 15 °C was indicated by a steeper increase in somatic EPA content with dietary EPA compared to 20 °C. 4. At 20 °C the development of eggs to successful hatching was high when EPA was supplied to the mothers. At 15 °C the hatching success was generally poor, despite of a higher maternal provision of EPA to eggs, compared to that at 20 °C, suggesting that EPA alone was insufficient for proper neonatal development at the low temperature. The growth of offspring from mothers raised at 20 °C without EPA supplementation was very low, indicating that the negative effects of EPA deficiency can be carried on to the next generation. 5. The fatty acid composition of Daphnia sp. in published field studies shows increasing proportions of saturated fatty acids with increasing environmental temperature, whereas ω3 PUFAs and EPA show no clear pattern, suggesting that variations in dietary PUFA may mask temperature‐dependent adjustments in ω3 PUFA concentrations of cladocerans in nature.     

Relative growth rate and biomass allocation in 20 Aegilops (Poaceae) species

This paper analyses relationships between relative growth rate ( rgr ), seed mass, biomass allocation, photosynthetic rate and other plant traits as well as habitat factors (rainfall and altitude) in 20 wild species of Aegilops L. and one closely related species of Amblyopyrum (Jaub. & Spach) Eig., which differ in ploidy level (diploid, tetraploid and hexaploid). The plants were grown hydroponically for 20 d in a growth chamber. The relationships between parameters were calculated either using the phylogenetic information (phylogenetically independent contrasts, PIC) or without using the phylogenetic information (trait values of taxa, TIP). The results using the two approaches were very similar, but there were a few exceptions in which the results were different (e.g. rgr vs. seed mass). Specific leaf area ( sla ) was positively correlated with leaf area ratio ( lar ) and negatively correlated with net assimilation rate ( nar ), which together resulted in the absence of a correlation between sla and rgr . Leaf photosynthetic rates (expressed on a mass or area basis) showed no correlation with rgr . rgr was positively correlated with the stem mass ratio and negatively with root mass ratio. Species with a lower d. wt percentage have a higher rgr . Aegilops species from locations with higher annual rainfall invested less biomass in roots and more in shoots (leaves and stems) and had a higher rgr . Diploid species had a lower seed mass and initial mass than the hybrids (tetraploid and hexaploid species), but there was no correlation of rgr with ploidy level. Polyploid species, which have higher seed mass, occur at a higher altitude than diploid species. Our results show that variation in rgr in Aegilops and Amblyopyrum spp. is associated mainly with variation in biomass allocation (proportion of biomass in stems and roots) and d. wt percentage, and not with variation in sla , leaf photosynthetic rates or seed mass.     

Modeling a Neural Oscillator that Paces Heartbeat in the Medicinal Leech

SYNOPSIS. Heartbeat in the medicinal leech is paced by a neuraloscillator comprising two elemental oscillators whose activityis coordinated intersegmental coordinating fibers. The elementaloscillators each consist of a bilateral pair of heart interneuronslinked by reciprocal inhibitory synapses. The activity cycleof each elemental oscillator consists of alternating burstsof action potentials (plateau/burst phase) and periods inhibition(inactive phase). Oscillation ensues in the reciprocally inhibitorypairs because each neuron is able to escape from the inhibitionits contralateral partner and thus move on to the plateau/burstphase. We have identified and described membrane currents thatcontribute to oscillation and studied graded synaptic transmissionbetween the neurons, using discontinuous current clamp and switchingsingle electrode voltage clamp techniques. A hyperpolarization-activatedinward current, Ih, plays a major role in escape from inhibition,and Ca²⁺ currents produce plateau potentials that support burstformation and mediate graded synaptic transmission. To consolidate our knowledge and guide future research, we haveconstructed a first generation computer model of a neural oscillatorbased on reciprocal inhibition, using Hodgkin-Huxley equationsand a synaptic transfer model, derived from our biophysicalstudies, with Nodus software (De Schutter, 1989). This modelhas confirmed an important role for I_h in sustaining oscillationand has implicated a similarly important role for outward currents(particularly I_A), which remain to be studied. Neural oscillatorsbased on reciprocal inhibition appear to be ubiquitous, andour studies, biophysical and computational, provide insightsinto how they may operate.