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981.
982.
David E. Carr Charles B. Fenster Michele R. Dudash 《Evolution; international journal of organic evolution》1997,51(2):363-372
Several recent theoretical considerations of mating-system evolution predict within-population covariation between levels of inbreeding depression and genetically controlled mating-system characters. If inbreeding depression is caused by deleterious recessive alleles, families with characters that promote self-pollination should show lower levels of inbreeding depression than families with characters that promote outcrossing. The converse is expected if inbreeding depression is due to overdominant allelic interactions. Whether these associations between mating-system and viability loci evolve will have important consequences for mating-system evolution. The evolution of selfing within the genus Mimulus is associated with a reduction in stigma-anther separation (i.e., a loss of herkogamy) and high autogamous seed set. In this study we compared families from two M. guttatus populations that differed genetically in their degree of stigma-anther separation. In one of these populations we also compared families that differed genetically in the degree to which they autogamously set seed in a pollinator-free greenhouse. Dams often differed significantly in levels of inbreeding depression for aboveground biomass and flower production, but variation in inbreeding depression was never explained by herkogamy class or autogamy class. Several factors might account for why families with traits associated with selfing did not show lower inbreeding depression, and these are discussed. Our study also demonstrated significant variation among self progeny from a given female likely due to differences in pollination date and position of fruit maturation. The detection of significant dam × sire interactions suggests biparental inbreeding or differences in combining ability for specific pairs of parents. 相似文献
983.
Aurora García-Dorado 《Evolution; international journal of organic evolution》1997,51(4):1130-1139
The empirical distribution of the mean viability of mutation accumulation lines, obtained from three published experiments, was analyzed using minimum-distance estimation. In two cases (Mukai et al. 1972; Ohnishi 1977), mutations were allowed to accumulate in copies of chromosome II protected from natural selection and recombination. In the other one (Fernández and López-Fanjul 1996), they accumulated in inbred lines derived from an isogenic stock. In contrast with currently accepted hypotheses, we consistently estimated low (about 0.01) genomic viability mutation rates, λ, and a small kurtosis of the distribution of mutational effects on viability (a) in the three datasets. Minimum-distance estimates of the per-generation mean viability change due to mutation (λE[a]) were also obtained. These were very similar for both chromosomal datasets, their absolute values being about five times smaller than estimates obtained from the observed change in mean viability during the mutation process. It must be noted that, in both experiments, viability was measured relative to the Cy chromosome of a Cy/Pm stock. Thus, an unnoticed viability increase in this Cy chromosome may have resulted in overestimation of the mean viability reduction in the lines. In parallel, minimum-distance estimation of λE(a) from inbred lines data (where the selective pressure during the accumulation process was larger) was even somewhat smaller, in absolute value, and very close to the estimate obtained by comparing the mean viability of the lines with that of the control isogenic line. The evolutionary importance of these results, as well as their relevance to the solution of the mutational load paradox, is discussed. 相似文献
984.
清醒猴纹状皮层(V1)神经元与注视位置相关的活动 总被引:1,自引:1,他引:0
用单细胞记录方法,研究了不同眼注视位置对清醒猴初级视皮层(V1)神经元自发活动的影响。实验前训练两只猴注视一个小光点,并使注视点在屏幕上顺序移动25个位置。我们观察到:(1)52%的初级视皮层神经元发放频率受眼位置调制,存在着一个能引起细胞最大发放的“注射野”;当注视点位于该细胞的注射野内时,发放频率明显增强。(2)大多数神经元的注视野位于感受野的外侧,多数在对侧视野范围内。(3)受眼位置影响的神 相似文献
985.
G. Raes J. Van Ginderachter Yuan Qing Liu L. Brys Kristiaan Thielemans Patrick De Baetselier Anja Geldhof 《Cancer immunology, immunotherapy : CII》1997,45(5):257-265
BW-Sp3 is a BW-5147-derived T cell lymphoma with limited immunogenicity since, despite regression of the majority of subcutaneous
tumors, an important fraction of the animals will die from metastases. In the present study, the BW-Sp3 cells were transfected
with genes encoding B7-1 or B7-2, known to be involved in the induction of T cell responses. The resulting transfectants exhibited
a reduced tumorigenicity and did not cause mortality in the syngeneic recipients. Furthermore, immunization with the B7-1
or B7-2 transfectants resulted in an increased generation of cytotoxic T lymphocytes (CTL) that lysed both the transfectants
and the wild-type BW-Sp3 cells. Since the B7 transfectants were completely rejected in syngeneic recipients and induced potent
CTL recognizing the wild-type BW-Sp3 cells, these engineered cells were considered as candidates for immunotherapy. Vaccinations
with the B7-1 or B7-2 transfectants could completely protect the animals from metastatic disease when subsequently challenged
with wild-type BW-Sp3 cells. Furthermore, immunization with the B7 transfectants could prolong the survival time of mice that
had been challenged intravenously with BW-Sp3 cells. Surprisingly, however, when these transfectants, as well as the wild-type
BW-Sp3 cells, were used for vaccination of tumor-bearing animals, the presence of the subcutaneous BW-Sp3 tumors clearly interfered
with the outcome of immunotherapy, resulting in increased malignancy, as reflected by a higher incidence of progressing tumors
and a reduced survival rate. Possible implications for immunotherapy in humans are discussed.
Received: 5 August 1997 / Accepted: 15 August 1997 相似文献
986.
A. -L. Jacquemart 《Plant Systematics and Evolution》1997,207(3-4):159-172
The reproductive success of three co-flowering species ofVaccinium (V. myrtillus, V. vitis-idaea andV. uliginosum) was studied in one heathland of the Upper Ardennes, Belgium, during three years (1988–1990). The purpose was to examine whether pollen limitation, flower position and flowering phenology may influence patterns of fruit set in these three sympatric species. I quantified fruit and seed set following supplementary hand-pollinations and compared this to natural fruit set. On the same plants, I also quantified fruit and seed set in relation to the spatial position of the flowers on the ramet and their temporal sequence of blooming. Hand-pollination had no significant effect on fruit set inV. vitis-idaea andV. uliginosum, but significantly increased seed number per fruit in 1989. InV. myrtillus both fruit and seed set were increased by supplementary pollination, but significantly in only one year. Analyses of position effects revealed that the fruits in the lower positions in the ramet did not mature preferentially and did not contain more seeds inV. uliginosum and inV. vitis-idaea. Flowering phenology also had no significant effect for these species. InV. myrtillus hand-pollinated flowers showed a seasonal decline in seed number, and control (naturally pollinated) flowers showed a seasonal increase in fruit set. Fruit and seed set appear to be pollen-limited rather than resource-limited inV. uliginosum. In the two other species, unfavorable weather (frosts) could be a more important cause of low fruit and seed set. 相似文献
987.
South Georgia has many introduced plant and animal species, a consequence of its long history of human habitation. Introduced
reindeer have a strong effect on the vegetation of the Stromness Bay area by causing the replacement of indigenous species
by grazing-tolerant grasses such as the exotic Poa annua, and in certain circumstances, the indigenous Festuca contracta. Recently it has been argued that an introduced predatory carabid has contributed to declines in the abundance and an increase
in the body size of adults of the indigenous perimylopid beetle Hydromedion sparsutum. However, it also appears that body size of these beetles is smaller in areas where exotic grasses predominate compared to
undisturbed areas. Here we test the hypothesis that by causing the spread of poorer quality grasses, especially the exotic
Poa annua, reindeer may be having an indirect effect on H. sparsutum. To do this we examined the nutritional ecology of H. sparsutum larvae on four grass species which form a major part of its diet, viz. the indigenous Parodiochloa flabellata, Phleum alpinum and Festuca contracta, and the exotic Poa annua. Larvae showed the highest growth rate on Parodiochloa flabellata, followed by Phleum alpinum, F. contracta and Poa annua. These differences are due to poorer absorption of the exotic grass, and poorer utilization of the absorbed material in the
case of F. contracta. Poor growth of larvae on F. contracta appears to be due to its low water and nitrogen contents, whereas in the case of P. annua a combination of low water content and high nitrogen content may be responsible for low growth rates. Low growth rates associated
with poor-quality food may lead either to a prolongation of the life cycle or of the length of feeding bouts of an insect.
Neither option appears to be feasible for H. sparsutum, and this means that the outcome of feeding on poorer-quality foods would be a reduction in final adult size. This has fitness
consequences for the beetle. Hence it appears that by causing the spread of grasses that are unsuitable for growth of H. sparsutum, reindeer may be having an indirect effect on this beetle species.
Received: 18 August 1996 / Accepted: 7 February 1997 相似文献
988.
Effects of small mammals and vertebrate predators on vegetation in the Chilean semiarid zone 总被引:4,自引:0,他引:4
J. R. Gutiérrez P. L. Meserve S. Herrera L. C. Contreras F. M. Jaksic 《Oecologia》1997,109(3):398-406
We monitored the cover and seed bank response of shrubs, perennial herbs, and ephemeral plants to experimental exclusion
of both the principal rodent herbivore, Octodon degus, and its vertebrate predators from 1989 to 1994 in a semiarid Chilean mediterranean site. Although both richness and species
composition of the plant community at the study site were largely determined by abiotic factors (mainly rainfall and soil
nutrients), predator and herbivore exclusion had significant effects on the relative abundance of several plant species. Experimental
exclusion of herbivores was associated with increased cover of some shrubs and a perennial grass, and decreased cover and
seed densities of several ephemerals, especially those exotic or restricted to areas underneath shrubs. Herbivores apparently
reduced shrubs through browsing and indirectly affected herb cover and seed densities by opening up areas under shrubs and/or
modifying physical and chemical conditions of the soil. Plant responses to predator exclusion were less clear. Nevertheless,
higher cover of some shrubs and ephemerals in the presence of predators suggests tritrophic effects through changes in small
mammal densities and/or foraging behavior.
Received: 22 April 1996 / Accepted: 14 August 1996 相似文献
989.
氮离子注入棉花花粉诱发农艺性状变异的研究 总被引:6,自引:0,他引:6
N+注入陆地棉和海岛棉的成熟花粉用于自交和远缘杂交,结果表明:离子注入花粉,在M1代就可表出现较高频率的有益突变,在M1相对成苗率和花粉育性为50%时,花粉辐照M2突变率是种子辐照的1.6倍,用N+离子注入陆地棉和海岛棉花粉进行正反远缘杂交,F2M2农艺性状表现出偏母遗传且后代遗传稳定加快,并观察到父本某些性状的转移。离子注入花粉适宜的剂量范围为2×1015—10×1015N+/cm2。 相似文献
990.
J.E. Fauth 《Ecology letters》1999,2(1):36-43
Identifying keystone species is essential for understanding community dynamics and preserving species richness. However, few studies have used quantitative, a priori methods to identify potential keystone species. Species known to act as keystones in North Carolina (NC) temporary ponds ( Notophthalmus viridescens , eastern newt, and Siren intermedia , lesser siren) were tested to see whether they played the same role in similar habitats in South Carolina (SC). Newts and sirens had no effect on anuran species richness in SC. Instead, another salamander ( Ambystoma talpoideum , mole salamander) absent from the NC ponds was identified as a strong keystone in SC. It functioned independently of environmental factors and the densities of other predators. Larval dragonflies ( Tramea carolina , Carolina saddlebag) were identified as weak, context-dependent keystones in SC, supporting anuran richness in isolated ponds with very low pH. The results suggest that the identity of keystone species varies, even in similar habitats within a physiographic region. 相似文献