Size-dependent drift responses of mayflies to experimental hydrologic variation: active predator avoidance or passive hydrodynamic displacement?

Summary Larger nymphs within aquatic insect taxa have been frequently observed to be transported down-stream in the stream drift only at night. Others have hypothesized this pattern results primarily from large nymphs' behavioural avoidance of entering drift during daylight, when size-selective, visually-feeding fish predators are most active. This hypothesis assumes that animals can actively control their entry into the drift, which may not be the case under all flow conditions. We experimentally induced streamflow increases and decreases in adjacent riffles in a hydrologically-stable stream during the daytime to examine whether changes in diel patterns of drift abundance and size-distribution of mayflies were consistent with the hypothesis of active avoidance of diurnal drift. We assessed the likelihood of active vs. passive mechanisms of diurnal drift entry and transport for four taxa that differ with respect to body size, morpho-behavioural attributes, microhabitat use, and general propensity to drift. In each of three seasons, diurnal and nocturnal drift samples were collected in three riffles over two diel cycles. Background drift patterns were established on the first day (no flow manipulation). Six h before sunset on the second day, flow was experimentally increased in one riffle, decreased in the second, and not altered in the third (control). Between-day differences in diurnal and nocturnal drift rate and size composition were then compared among the treatment and reference riffles. Responses of two taxa were consistent with active control over drift entry, transport, or both. For Baetis spp., drift-prone mayflies typically preyed upon by fish, diurnal drift rates immediately increased following both flow reduction and flow elevation in all seasons, but only small individuals comprised the drift. Drift by large individuals was delayed until nighttime. Epeorus longimanus also exhibited significant increases in drift rates following flow reduction and elevation, but responses of this large-bodied species were restricted to nighttime. Drift responses for these two taxa were largely independent of direction of hydrologic change, thus indicating a strong behavioural control over drift. By contrast, numbers and sizes of drifting Paraleptophlebia heteronea and Ephemerella infrequens depended strongly on direction of flow change. Drift rates for both species generally declined after flow reduction and increased after flow elevation. Moreover, after flow elevation, larger individuals often drifted diurnally, a finding consistent with expectations under a passive hydrodynamic model. These experiments indicate that size-dependent mayfly drift reflects not only presumed risk from visual fish predators, but also functional attributes of species such as morphology, behaviour, and microhabitat affiliation, which influence aspects of drift entry and transport under variable hydrologic conditions.     

Associative N2-fixation in plants growing in saline sodic soils and its relative quantification based on15N natural abundance

The interactive effect of low P supply (0, 10, 20 and 40 M) and plant age on nodule number, mass and functioning (ureide analysis technique), vegetative growth and pod production were investigated in glasshouse-grown nodulated cowpea (Vigna unguiculata L.cv. Kausband) in sand culture. Compared with 40 M P, P stress (0 M P) or very low (10 M P) supply markedly impaired nodulation, allantoin and amino-N concentrations and weight of N solutes in xylem exudates. Consequently, P stress reduced top growth and pod yields by 48 and 90%, respectively. N solutes in xylem exudates and total plant N assayed by Kjeldahl technique (as estimates of N₂ fixation) responded similarly to P supply. However, the relative ureide index [(ureide-N/ureide N+amino-N)×100] remained constant (99%), irrespective of P supply, indicating the plants' complete dependency on symbiosis for growth, without implying that growth was markedly increased by N₂ fixation. Although P concentrations in plant tops, roots and nodules increased with P supply, N concentrations in these plant tissues were unaffected by P supply. The concentrations of N and P in the nodules were 2–2 1/2 times higher than in plant tops. P application interacted strongly with plant age, with the largest P effect evidently achieved at the early podding stage. The significance and implications of these results are discussed.     

Linkage between isozyme markers and a locus affecting seed size in Phaseolus vulgaris L.

Summary Backcross and F₂ progenies were produced between two bean genotypes, XR-235 and Calima, which differ in seed weight by a factor of two. The small-seeded XR-235 was used as the pistillate and recurrent parent. These genotypes showed polymorphisms at nine isozyme loci and at the phaseolin locus. Seed size parameters (weight, length, width, and thickness) were determined for each BC₁ and F₂ individual, i.e., for seeds harvested from XR-235 after pollination with F₁ and from the F₁ after selfing, respectively. A combination of starch gel electrophoresis and enzyme activity staining was used to determine the genotype of each BC₁ and F₂ individual at the segregating loci. SDS-PAGE and Coomassie blue staining were used to determine geno-type at the phaseolin locus. Tests for independent assortment using two-way contingency and maximum likelihood tables revealed three linkage pairs: Aco-1 — 20 cM — Dia-1; Adh-1 — 2 cM — Got-2; and Est-2 — 11 cM — Pha. Statistical comparisons were made between the means of genotype classes at each segregating locus for all seed size parameters. The results from two independently obtained BC₁s and the F₂ consistently indicated that the Adh-1-Got-2 segment was linked to a locus that affected seed size and overcame maternal control over seed size. This locus has been designated Ssz-1. This gene exhibited additive gene action and accounted for 30–50% of the seed size difference between the parents.Florida Agricultural Experiment Station, Journal Series No. R00696     

Circulation,metabolic rate,and body size in mammals

Summary This paper attempts to explain Kleiber's rule, which relates metabolic rate of mammals to their body mass, from the structure and function of the blood circulation system.Abbreviations a scaling factor - fractal dimension - hydrodynamic conductivity - l _n length of an arterial blood vessel at bifurcation level n - M body mass - N maximal number of bifurcation levels - p pressure - Q flow - r size of Bohr effect - r _n radius of an arterial blood vessel at bifurcation level n - V volume - VO ₂ rate of oxygen unloading - Z _n number of arterial blood vessels at bifurcation level n     

Seed size,pollination costs and angiosperm success

Summary Seed plants capture pollen before seeds are dispersed and abort unpollinated ovules. As a result, each seed is associated with an accessory cost that represents the costs of pollen capture and the costs of aborted ovules. Accessory costs may explain the minimum seed size among species, because these costs are likely to comprise a greater proportion of total reproductive allocation in species with smaller seeds. For very small propagules, the costs of pollination may not be worth the benefits, perhaps explaining the persistence of pteridophytic reproduction at small propagule sizes. The smallest angiosperm seeds are much smaller than the smallest gymnosperm seeds, both in the fossil record and in the modern flora. This suggests that angiosperms can produce pollinated ovules more cheaply than gymnosperms. Pollination becomes less efficient as a species decreases in abundance, and this loss of efficiency is greater for species with a higher accessory cost per seed. We propose that the greater reproductive efficiency of angiosperms when rare can explain why angiosperm-dominated floras were more speciose than the gymnosperm-dominated floras they replaced.     

Genetic variation in relation to demography of peripheral pool frog populations (Rana lessonae)

Summary Genetic variation in an isolated northern metapopulation of the pool frog (Rana lessonae) in Sweden was compared to that of Central European populations using enzyme electrophoresis and literature data. Of the 31 loci scored, two (EST-2 andIDH-2) were polymorphic while no variation occurred in seven of the eight loci which are polymorphic in Central European populations.The heterozygosity level of the Swedish pool frogs is very low compared to that of other anuran populations, but their mean proportion of fertilized eggs within egg masses (97.5%) was not lower than in more heterozygous species, and their body size-specific fecundity did not differ from that of Polish conspecifics. The low genetic variability of the Swedish pool frogs is discussed in relation to features of the local populations such as size (N), calculated effective size (N _e ) reproductive success and probable history. It is concluded that long-term strong fluctuations in population size caused by reporductive failure in cold years have contributed more to the low genetic variability than could a single founder event due to a recent introduction by man.     

Spring development of a Chlamydomonas population in Lake Nimetön,a small humic forest lake in southern Finland

Biomass development and vertical distribution of a Chlamydomonas population in a small humic forest lake was followed by daily sampling in May-June, 1984. Chlamydomonas dominated the phytoplankton spring bloom, forming 71% of the maximum phytoplankton biomass on 18 May. In early May the outflow rate was high and during the 24 hour period when the maximum rate of surface runoff was recorded (8–9 May), 43% of the Chlamydomonas biomass was flushed out of the lake, which delayed the onset of biomass increase. When surface runoff had slowed down Chlamydomonas biomass started increasing and during wax of the population most cells were < 10 µm in diameter. Population maximum lasted for one day (18 May) and there-after Chlamydomonas biomass decreased towards the end of the study. During wane of the population most cells were > 10 µm in diameter.     

Effects of gravel size and peat material concentrations on embryo survival and alevin emergence of brown trout,Salmo trutta L.

Embryo survival and alevin emergence pattern of brown trout were studied in simulated redds with different homogeneous gravel sizes and different concentrations of peat material. Optimal survival (95%) occurred in 18 mm gravel and survival decreased with decreasing gravel size. High concentrations (40%) of peat material resulted in low survival (65%). The proportion of premature emerging alevins increased in finer gravels and at high peat concentrations. Premature alevins had a large yolk sac and are probably very vulnerable to predators.     

Critical nuclear DNA size and distribution associated with S phase initiation

Using HeLa S-3 cells synchronized by selective detachment, in this paper we report a parallel study of nuclear morphology and autoradiography grain patterns between middle G₁ and middle S phases: Our results show two distinct [³H]-thymidine labeling patterns. The first “peripheral” labeling pattern has a characteristic nuclear size distribution, in contrast to the heterogeneous and varying size distributions of Feulgen-stained nuclei, and apparently is characteristic of very early S phase. The sizes of the second labeling pattern—homogeneous or inhomogeneous grain distribution throughout the nucleus—are equal or larger than the first and vary with S phase progression. Together, the corresponding nuclear sizes of the labeled nuclei represent the larger extreme of nuclear areas, and the labeling index closely parallels the fraction of nuclei with areas larger than the minimum size of the labeled nuclei. These results suggest a characteristic nuclear size (reflecting unique intranuclear DNA distribution) as a necessary, if not sufficient, requirement for S phase initiation. Parallel experimentation with rat liver cells—synchronized in vivo by partial hepatectomy and analyzed by thin section autoradiography—confirms the existence of a peripheral labeling pattern in both the very early part and the very late part of S phase, which reconciles our data with previous results and points to the fact that both initiation and termination sites for DNA replication are near the nuclear periphery.