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1.
Summary The response to a single defoliation was studied on three clones of Themeda triandra collected in the short, mid, and tall grassland regions of the Serengeti National Park (Tanzania). These sites represent a gradient of decreasing grazing intensity. Growth, allocation pattern, and several morphometric traits were monitored during an 80-day period. Clipped plants of the short and medium clones fully compensated for the reduction of biomass, while plants of the tall clone showed overcompensation. During the first two weeks after clipping, clipped plants showed lower relative growth rates than unclipped ones, whereas the opposite was observed later on. Clipped plants compensated for the removal of leaf area by producing new leaves with lower specific weights and higher nitrogen content. They also produced more, smaller tillers. Although clipped plants mobilized nonstructural carbohydrates from roots and crowns, this did not account for a significant amount of growth. Relative growth rates of unclipped plants of the short clone were higher. The relative growth rate of the short clone diminished less after clipping, but also exhibited the lowest increase later. The tall clone was the most negatively affected early, but showed the highest compensation later. Compared to the other clones, the short ecotype showed many of the characteristics that defoliation induced in each individual of any clone: higher allocation to leaf area production, higher relative growth rate, higher number but smaller size of tillers, and lower leaf specific weights. 相似文献
2.
Summary Environmental and water relations parameters during fall were monitored for six conifer tree species common to the central Rocky Mountains growing naturally at the same location (Pinus contorta, Pinus ponderosa, Pinus flexilus, Pseudotsuga menziesii, Abies lasiocarpa, Picea engelmannii). Subsequent to what appeared to be the beginning of seasonal stomatal closure, leaf conductance to water vapor declined sharply following the onset of freezing air temperatures at night. A coincident rapid decline in morning xylem pressure potentials (p) also occurred which resulted in values that were considerably below afternoon p. Continuing decreases in maximum leaf conductance during the day were highly correlated with corresponding decreases in minimum nocturnal air temperatures of the preceding night. By mid-December, morning p returned to values very near afternoon p and were only slightly lower than before the onset of subfreezing nights. A preliminary model is proposed which interprets the qualitative interaction between air and soil temperatures, soil and plant water potentials, and leaf conductance during seasonal stomatal closure in fall. 相似文献
3.
D. M. McLean E. Joan Quantz Ruth D. Bach B. Mae Pevzner R. P. B. Larke G. A. McNaughton 《CMAJ》1964,90(25):1390-1393
Virological investigations of 115 children with the aseptic meningitis syndrome during 1963 resulted in the isolation of enteroviruses from cerebrospinal fluid (CSF) and/or feces of 21 of 48 children who had no association with mumps. For the third successive year, Echo 9 was the dominant enterovirus in cases of aseptic meningitis in Toronto children, but no rashes were associated with Echo 9 meningitis during 1963, in contradistinction to previous years. Mumps virus was isolated from CSF of 25 patients by inoculation of rhesus monkey kidney cultures, and rising or elevated mumps antihemagglutinin titres in paired sera from a further 33 cases provided laboratory evidence of infection with mumps virus in 58 of 67 patients with mumps meningoencephalitis. No enlargement of salivary glands was noted in 20 laboratory-proved cases of mumps meningoencephalitis. Enteroviral meningitis occurred principally during summer, but the peak of mumps meningoencephalitis occurred during late winter. 相似文献
4.
The molecular evolution of mammalian Y-linked DNA sequences is of special
interest because of their unique mode of inheritance: most Y- linked
sequences are clonally inherited from father to son. Here we investigate
the use of Y-linked sequences for phylogenetic inference. We describe a
comparative analysis of a 515-bp region from the male sex- determining
locus, Sry, in 22 murine rodents (subfamily Murinae, family Muridae),
including representatives from nine species of Mus, and from two additional
murine genera--Mastomys and Hylomyscus. Percent sequence divergence was
< 0.01% for comparisons between populations within a species and was
0.19%-8.16% for comparisons between species. Our phylogenetic analysis of
12 murine taxa resulted in a single most- parsimonius tree that is highly
concordant with phylogenies based on mitochondrial DNA and allozymes. A
total evidence tree based on the combined data from Sry, mitochondrial DNA,
and allozymes supports (1) the monophyly of the subgenus Mus, (2) its
division into a Palearctic group (M. musculus, M. domesticus, M.
spicilegus, M. Macedonicus, and M. spretus) and an Oriental group (M.
cookii++, M. cervicolor, and M. caroli), and (3) sister-group relationships
between M. spicilegus and M. macedonicus and between M. cookii and M.
cervicolor. We argue that Y- chromosome DNA sequences represent a valuable
new source of characters for phylogenetic inference.
相似文献
5.
Julia C. McNaughton Craig J. Marshall Judith E. Broom Gillian Hughes Wyn A. Jones Peter A. Stockwell George B. Petersen 《Journal of molecular evolution》1995,40(2):127-135
A THE-1 sequence in intron 7 of the human dystrophin gene has been found to represent a new subfamily of THE-1 elements. The sequence is closely related to the MstII family of repetitive sequences and is more like single-copy sequences found in the galago genome than any other THE-1 sequence previously reported. This new THE-1 sequence has been compared with two other complete THE-1 sequences and three related long-terminal repeat elements that we have previously found in intron 7 of the dystrophin gene, and with members of the same family from elsewhere in the primate genome. Parsimony and deletion analysis show that the cluster of THE-1 sequences in intron 7 of the dystrophin gene has arisen from at least three individual insertion events, rather than from the insertion and duplication of a single progenitor sequence.
Correspondence to: G.B. Petersen 相似文献
6.
Summary Mycorrhizal and nonmycorrhizal plants of Panicum coloratum L. were grown in a factorial treatment design under two nitrogen levels and two clipping heights with an unclipped control. The nitrogen concentration in different plant components was determined following 9 weeks of growth under experimental conditions. Mycorrhizal infection increased green leaf and sheath nitrogen concentration by a relatively small, but significant percentage and had no effect on nitrogen allocation to the various plant components. Clipping increased leaf nitrogen concentration but inhibited growth to the extent that, when compared with the unclipped controls, less nitrogen remained in residual plant biomass with up to half of the total nitrogen allocated to offtake (the material removed by clipping). Plants receiving the higher nitrogen fertilization had higher tissue concentration of N and more N allocated to above-ground living tissues. Mycorrhizal infection interacted with clipping height and also with N availability significantly. Infection was unable to ameliorate the negative effects of the most severe clipping regime and of the low nitrogen availability on leaf and sheath N content. This is possibly due to mycorrhizal demand for carbohydrates competing with the carbohydrate requirement of roots for nitrogen uptake. 相似文献
7.
Net charge transport during sodium-dependent calcium extrusion in isolated salamander rod outer segments
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The light-sensitive current and the current associated with the extrusion of internal Ca2+ in exchange for external Na+ have been recorded from detached rod outer segments from the salamander retina by the use of the whole-cell voltage clamp technique. No significant current-carrying mechanisms are present in the outer segment membrane apart from the light-sensitive conductance and the Na:Ca,K exchange, and exchange currents can therefore be recorded directly without the use of subtraction procedures or pharmacological blockers. The charge moved by the exchange was studied by loading outer segments with a known amount of calcium and then recording the exchange current on return to a Na(+)-containing solution. Calcium is not sequestered to any significant extent in a slowly exchanging internal store, as the charge recovered is unaffected if admission of the Na(+)-containing solution is delayed for 40 s. The number of charges flowing into the cell in exchange for each Ca2+ ion extruded was found not to deviate significantly from one over a wide range of ionic conditions and membrane potentials. These results show that the stoichiometry of the exchange is fixed over a wide range of conditions, and that the size of the inward exchange current is therefore directly proportional to the rate of Ca2+ efflux through the carrier. 相似文献
8.
The acrosomic status of spermatozoa prepared for IVF has been evaluated by means of immunofluorescence test from Fenichel and Hsi using calcium A 23187 ionophore as inductor of acrosome reaction (AR). The spontaneous AR remains slight, even after 6 hour-incubation in Menezo B2 (6,8+2,7%). The response to ionophore, moderate before (11,2+9%), frankly increases after a 6h-capacitation (28,9+8,3%) in a group of 25 IVF couples (tubal indication, normal semen, positive fertilization). Nevertheless, it remains slight or null in 4 cases of unexplained repeated failure of fertilization. The response to ionophore A 23187 allows to explore the kinetics of capacitation of spermatozoa and their ability to perform AR. Its significance in terms of fecondance remains to be precised. 相似文献
9.
10.
Jamin Halberstadt Joshua Conrad Jackson David Bilkey Jonathan Jong Harvey Whitehouse Craig McNaughton Stefanie Zollmann 《PloS one》2016,11(3)
Social psychology is fundamentally the study of individuals in groups, yet there remain basic unanswered questions about group formation, structure, and change. We argue that the problem is methodological. Until recently, there was no way to track who was interacting with whom with anything approximating valid resolution and scale. In the current study we describe a new method that applies recent advances in image-based tracking to study incipient group formation and evolution with experimental precision and control. In this method, which we term “in vivo behavioral tracking,” we track individuals’ movements with a high definition video camera mounted atop a large field laboratory. We report results of an initial study that quantifies the composition, structure, and size of the incipient groups. We also apply in-vivo spatial tracking to study participants’ tendency to cooperate as a function of their embeddedness in those crowds. We find that participants form groups of seven on average, are more likely to approach others of similar attractiveness and (to a lesser extent) gender, and that participants’ gender and attractiveness are both associated with their proximity to the spatial center of groups (such that women and attractive individuals are more likely than men and unattractive individuals to end up in the center of their groups). Furthermore, participants’ proximity to others early in the study predicted the effort they exerted in a subsequent cooperative task, suggesting that submergence in a crowd may predict social loafing. We conclude that in vivo behavioral tracking is a uniquely powerful new tool for answering longstanding, fundamental questions about group dynamics. 相似文献