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101.
Nine accessions ofVicia narbonensis, considered to be the wild progenitor of faba bean (Vicia faba), were investigated to ascertain the nature and extent of intraspecific karyotypic polymorphism. The chromosome complements resolved into four distinct types (A, B, C, D), and the meiotic data of F1 hybrids (A × B, B × C, A × C) revealed that alteration in chromosome morphology is the result of segmental interchanges. The interchange complexes indicate that the parents differ from each other by 1 to 2 interchanges. It is also evident that karyotype B, and not A as previously reported, is the normal karyotype of the species, and A and C are single homozygotes for unequal interchange. The comparative karyomorphology of the parents and the hybrids, and of two interchange heterozygotes of four chromosomes each in F1 hybrids of A × C shows that the chromosomes involved in the single interchange homozygotes (A, C) are not common and the breaks in both interchanges occurred in short and long arms of the involved chromosomes. Identification of the interchanged chromosomes in the complements and the frequency of ring and chain quadrivalents in the heterozygotes enabled location of the breakpoints. The present results provide probably the first example indicating that interchange homozygosity (A) is not only firmly established but also has enabled the species to spread further by adapting to a wide range of habitats. — The genetic relationships between A and D are very different. All seven chromosome pairs in D could be distinguished from A, and for that matter, B and C as well. From the meiotic pairing properties it is also amply clear that genome D is well differentiated from A and possibly B, and C, and deserves special status. 相似文献
102.
The evolution of the Australian flora through the Tertiary has not been reviewed by a megafossil palaeobotanist for almost a century. Based on material available and published studies, the Australian Tertiary can be considered in three units: Eocene, Oligo-Miocene, and Pliocene. Key taxa in the Eocene includeCasuarinaceae, Proteaceae, Fagaceae, Podocarpaceae, andLauraceae. Many known Eocene deposits are interpreted as warm, humid rainforests.Oligo-Miocene floras reflect a climatic deterioration, with greater occurrence of sclerophylly and xerophylly. A reduction of tropical taxa is seen with an increase ofMyrtaceae and earliest records ofMimosaceae, Chenopodiaceae, andPoaceae. These trends continue into the Pliocene, which is not well represented in Australia.Problems needing to be addressed include the discrepancy between the pollen and megafossil records ofNothofagus, the origins of Australia's wet tropical taxa, and the role of fire in the early evolution of the Australian flora. 相似文献
103.
Gymnodinium aeruginosum has the usual fine structure of a dinoflagellate but does not seem to contain a well elaborated peduncle or a microtubular basket. Naked cells are surrounded by a single large amphiesmal vesicle. It houses an endosymbiont with typical blue-green cryptophycean chloroplasts (generally only one), cryptophycean starch grains in the periplastidal cytoplasm without a nucleomorph, and two membranes separating the periplastidal cytoplasm from the cryptophycean cytoplasm which contains mitochondria, ER, vesicles and ribosomes, but no eukaryotic nucleus. The endosymbiont is surrounded by a single membrane. Possible ways of the acquisition of the endosymbiont and the problem of the existence of ribosomes within a compartment without nucleus are discussed.Devoted to Prof. Dr.L. Geitler, the Nestor of phycology and endosymbiosis research, on the occasion of the 90th anniversary of his birthday. 相似文献
104.
105.
The nucleotide sequence and the 5 flanking region of the rbcL gene coding for the large subunit of ribulose bisphosphate-1,5-carboxylase/oxygenase of Pylaiella littoralis, a brown alga, has been determined and the deduced amino-acid sequence has been compared to those of various photosynthetic and chemoautotrophic Eubacteria, of a red alga and of green plastids (Euglena gracilis, green algae and higher plants). Unlike the rbcL genes of green plastids which are more closely related to those of cyanobacteria the P. littoralis rbcL gene is more closely related to that of a -purple bacterium, as was found for the rbcS gene of another chromophytic alga [Boczar et al., Proc Natl Acad Sci USA 86: 4996–4999, 1989]. Matrix data of homology between the rbcL gene of P. littoralis and the same gene of other organisms are presented. Based on our previous report, the gene coding for the 16S rRNA from P. littoralis is closely related to that of E. gracilis (Markowicz et al., Curr Genet 14: 599–608, 1988). We suggest that the large plastid DNA molecule of P. littoralis is a phylogenetically composite genome which probably resulted from mixed endosymbiosis events, or from a horizontal transfer of DNA. 相似文献
106.
James R. Griesemer 《Biology & philosophy》1990,5(1):3-36
Accounts of the relation between theories and models in biology concentrate on mathematical models. In this paper I consider
the dual role of models as representations of natural systems and as a material basis for theorizing. In order to explicate
the dual role, I develop the concept of a remnant model, a material entity made from parts of the natural system(s) under
study. I present a case study of an important but neglected naturalist, Joseph Grinnell, to illustrate the extent to which
mundane practices in a museum setting constitute theorizing. I speculate that historical and sociological analyses of institutions
can play a specific role in the philosophical analysis of model-building strategies. 相似文献
107.
The prokaryotic endosymbionts that became plastids and mitochondria contained genes destined for one of three fates. Genes
required for free-living existence were lost. Most genes useful to the symbiosis were transferred to the nucleus of the host.
Some genes, a small minority, were retained within the organelle. Here we suggest that a selective advantage of movement of
genes to the nucleus is decreased mutation: plastids and mitochondria have high volume-specific rates of redox reactions,
producing oxygen free radicals that chemically modify DNA. These mutations lead to synthesis of modified electron carriers
that in turn generate more mutagenic free radicals—the “vicious circle” theory of aging. Transfer of genes to the nucleus
is also advantageous in facilitating sexual recombination and DNA repair. For genes encoding certain key components of photosynthesis
and respiration, direct control of gene expression by redox state of electron carriers may be required to minimize free radical
production, providing a selective advantage of organelle location which outweighs that of location in the nucleus. A previous
proposal for transfer of genes to the nucleus is an economy of resources in having a single genome and a single apparatus
for gene expression, but this argument fails if any organellar gene is retained. A previous proposal for the retention of
genes within organelles is that certain proteins are organelle-encoded because they cannot be imported, but there is now evidence
against this view. Decreased free radical mutagenesis and increased sexual recombination upon transfer to the nucleus together
with redox control of gene expression in organelles may now account for the slightly different gene distributions among nuclei,
plastids, and mitochondria found in major eukaryote taxa. This analysis suggests a novel reason for uniparental inheritance
of organelles and the evolution of anisogametic sex, and may also account for the occurrence of nitrogen fixation in symbionts
rather than in nitrogen-fixing organelles.
Correspondence to: J.F. Allen 相似文献
108.
Patricia A. McLenachan Peter J. Lockhart H. Rick Faber Brian C. Mansfield 《Journal of molecular evolution》1996,42(2):273-280
The pregnancy-specific 1-glycoproteins (PSG) form a large family of closely related proteins. Using newly developed methods of sequence analysis, in combination with protein modeling, we provide a framework for investigating the evolution and biological function of genes like the PSG. Evolutionary trees, based on C-terminal sequence, group PSG genes in a manner consistent with their genomic organization. Trees constructed using the N-terminal domain sequences are unreliable as an indicator of phylogeny because of non-neutral processes of sequence change. During duplication of the PSG genes, evolutionary pressures have resulted in a gradient of constrained change across each gene. The N-terminal domains show a nonrandom pattern of amino acid substitutions clustered in the immunoglobulin complementarity-determining region (CDR)-like regions, which appear to be important in the function of the protein. 相似文献
109.
Determination of the entire nucleotide sequence of the aphid 28S ribosomal RNA gene (28S rDNA) revealed that it is 4,147 by
in length with a G + C content of 60.3%. Based on the nucleotide sequence, we constructed a presumed secondary-structure model
of the aphid 28S rRNA which indicated that the aphid 28S rRNA is characterized by the length and high G + C content of its
variable regions. The G + C content of the aphid's variable regions was much higher than that of the entire sequence of the
28S rRNA, which formed a striking contrast to those ofDrosophila with the G + C content much lower than the entire 28S molecule. In this respect, the aphid 28S rRNA somewhat resembled those
of vertebrates. This is the third report of a complete large-subunit rRNA sequence from an arthropod, and the first 28S rRNA
sequence for a nondipterous insect.
Correspondence to: H. Ishikawa 相似文献
110.
Using a maximum-likelihood formalism, we have developed a method with which to reconstruct the sequences of ancestral proteins. Our approach allows the calculation of not only the most probable ancestral sequence but also of the probability of any amino acid at any given node in the evolutionary tree. Because we consider evolution on the amino acid level, we are better able to include effects of evolutionary pressure and take advantage of structural information about the protein through the use of mutation matrices that depend on secondary structure and surface accessibility. The computational complexity of this method scales linearly with the number of homologous proteins used to reconstruct the ancestral sequence. 相似文献