共查询到20条相似文献,搜索用时 31 毫秒
1.
Invasive capacity of Tamarix ramosissima in a Mojave Desert floodplain: the role of drought 总被引:1,自引:0,他引:1
Tamarix ramosissima (Tamaricaceae) is a woody phreatophyte that has invaded thousands of hectares of floodplain habitat in the southwestern U.S.
In this study, we examined the response of gas exchange and stem sap flow of Tamarix and three co-occurring native phreatophytes (Pluchea sericea (Asteraceae), Prosopis pubescens (Fabaceae) and Salix exigua (Salicaceae)) to drought conditions in an early successional floodplain community in the Mojave Desert of southern Nevada.
In an analysis of a size/age series of each species across the whole floodplain (both mature and successional stands), stem
growth rate was lowest for Tamarix. However, along the same successional chronosequence, Tamarix came to dominate the 50+ year old stands with dense thickets of high stem density. Xylem sap flow, when expressed on a sapwood
area basis, was highest in Tamarix under early drought conditions, but comparable between the four species toward the end of the summer dry season. Multivariate
analysis of the gas exchange data indicated that the four species differentiated based on water use under early drought conditions
and separated based on plant water potential and leaf temperature (indices of drought effects) at the end of the summer dry
season. This analysis suggests that the invasive Tamarix is the most drought tolerant of the four species, whereas Salix transpires the most water per unit leaf surface area and is the least tolerant of seasonal water stress. Therefore, Salix appears to be well adapted to early successional communities. However, as floodplains in this arid region become more desiccated
with age, Tamarix assumes greater dominance due to its superior drought tolerance relative to native phreatophytes and its ability to produce
high density stands and high leaf area.
Received: 8 August 1996 / Accepted: 29 January 1997 相似文献
2.
3.
Lucyna Kubiak-Martens 《Vegetation History and Archaeobotany》2002,11(1-2):23-32
Roots and tubers, together with other plant storage organs such as rhizomes, bulbs, corms, etc. are known to be a source
of human food. Until very recently, however, remains of root foods were rarely identified from archaeological contexts in
temperate Europe. New evidence for the use of root foods has been recently recovered from the early Erteb?lle settlement at
Halsskov in Denmark. Remains included charred bulbs of Allium cf. ursinum and tubers of Conopodium majus. They were accompanied by charred remains of parenchymatous tissues derived from underground parts of other plants. All parenchymatous
remains were recovered from features that could be interpreted as pit-cooking depressions. A minimum of two periods of root
food gathering can be assumed, spring to early summer and autumn. The presence of charred seeds of Nuphar pumilum suggests that either the seeds or rhizomes (or both) were use as food.
Received September 4, 2001 / Accepted February 27, 2002 相似文献
4.
Summary. Diabetes mellitus is a primary contributor to progressive kidney dysfunction leading to end-stage renal disease (ESRD). In
the early phase of diabetes, prior to the onset of further complications, both kidney size and glomerular filtration rate
(GFR) increase. Glomerular hyperfiltration is considered a risk factor for downstream complications and progression to ESRD.
Abnormalities in vascular control have been purported to account for the glomerular hyperfiltration in early diabetes. In
this review we discuss a tubulo-centric concept in which tubular growth and subsequent hyper-reabsorption contribute to the onset of glomerular hyperfiltration that
demarks the early stage of diabetes. Kidney growth, in this concept, is no longer relegated to a compensatory response to
hyperfiltration, but rather plays a primary and active role in its genesis and progression. As such, components of kidney growth, such as the polyamines, may provide a means of
early detection of diabetic kidney dysfunction and more effective therapeutic intervention. 相似文献
5.
Janet L. Siefert Kirt A. Martin Fadi Abdi William R. Widger George E. Fox 《Journal of molecular evolution》1997,45(5):467-472
Five complete bacterial genome sequences have been released to the scientific community. These include four (eu)Bacteria,
Haemophilus influenzae, Mycoplasma genitalium, M. pneumoniae, and Synechocystis PCC 6803, as well as one Archaeon, Methanococcus jannaschii. Features of organization shared by these genomes are likely to have arisen very early in the history of the bacteria and
thus can be expected to provide further insight into the nature of early ancestors. Results of a genome comparison of these
five organisms confirm earlier observations that gene order is remarkably unpreserved. There are, nevertheless, at least 16
clusters of two or more genes whose order remains the same among the four (eu)Bacteria and these are presumed to reflect conserved
elements of coordinated gene expression that require gene proximity. Eight of these gene orders are essentially conserved
in the Archaea as well. Many of these clusters are known to be regulated by RNA-level mechanisms in Escherichia coli, which supports the earlier suggestion that this type of regulation of gene expression may have arisen very early. We conclude
that although the last common ancestor may have had a DNA genome, it likely was preceded by progenotes with an RNA genome.
Received: 10 March 1996 / Accepted: 20 May 1997 相似文献
6.
In order to test whether an increased export of carbohydrates by leaves and starch mobilization are critical for floral transition
in Arabidopsis thaliana, the Columbia ecotype as well as its starchless mutant pgm and starch-in-excess mutant sex1 were investigated. Induction of flowering was achieved by exposure of plants to either one long day (LD) or one displaced
short day (DSD). The following conclusions were drawn: (i) Both the pgm and sex1 mutants have a late-flowering phenotype in days shorter than 16 h. (ii) When inductive treatments cause a large percentage
of induced plants, there is always a large, early and transient increase in carbohydrate export from leaves. By contrast,
when an inductive treatment results in only a low percentage of induced plants (pgm plants exposed to one DSD), the export of carbohydrates from leaves is not increased, supporting the idea that phloem carbohydrates
have a critical function in floral transition. (iii) Starch mobilization is not required to obtain an increased carbohydrate
export when induction is by one LD (extended period of photosynthesis), but is absolutely essential when induction is by one
DSD (period of photosynthesis unaffected). (iv) Floral induction apparently increases the capability of the leaf phloem-loading
system.
Received: 27 August 1997 / Accepted: 6 March 1998 相似文献
7.
At the end of the 6th millennium B. C. Triticum monococcum (einkorn) and Triticum dicoccum (emmer) were the main cereals of the early Neolithic Bandkeramik culture in central Europe. New archaeobotanical investigations
at relevant Bandkeramik sites reveal the regular occurrence of morphologically two-grained einkorn. Some preliminary thoughts
on the state of research and the possible origin of two-grained einkorn forms are presented here.
Received January 7, 2002 / Accepted August 5, 2002
Correspondence to: Angela Kreuz 相似文献
8.
First archaeological evidence of banana cultivation in central Africa during the third millennium before present 总被引:1,自引:0,他引:1
Christophe Mbida Mindzie Hughes Doutrelepont Luc Vrydaghs Rony L. Swennen Rudy J. Swennen Hans Beeckman Edmond de Langhe Pierre de Maret 《Vegetation History and Archaeobotany》2001,10(1):1-6
Phytoliths recovered from refuse pits excavated in central Cameroon and dated to ca 2500 B.P. have been positively identified
for the first time in Africa as derived from Musa the cultivated banana, after a comparative study of Musa and Ensete phytoliths. This discovery provides archaeologists with unequivocal proof of early agriculture in central Africa. Furthermore,
the presence of banana in Cameroon much earlier than previously assumed could explain how agriculture spread through the rain
forest. Lastly, as Musa is of Asian origin, this study provides the first concrete evidence of contacts across the Indian Ocean a millennium earlier
than currently accepted.
Received July 12, 1999 / Accepted May 4, 2000 相似文献
9.
Sexual reproduction of Daphnia pulex in a temporary habitat 总被引:1,自引:0,他引:1
David J. Innes 《Oecologia》1997,111(1):53-60
Species of Daphnia (Crustacea: Cladocera) typically reproduce by cyclical parthenogenesis, in which a period of all-female parthenogenetic reproduction
is followed by sexual reproduction. Sex in Daphnia is determined by the environment, with factors such as temperature, photoperiod and crowding stimulating the production of
males and sexual females. Previous studies on Daphnia pulex from temporary pond habitats demonstrated the coexistence of male-producing and non-male-producing (NMP) females, as determined
under crowding in the laboratory. A strong genetic component to this sex allocation variation suggested that sex expression
in D. pulex is better described as a result of genotype-environment interaction. The present study examined the switch from parthenogenetic
to sexual reproduction in two temporary-pond populations of D. pulex. Both populations showed a very early investment in sexual reproduction, independent of population density, by producing
males very soon after the populations were reestablished from resting eggs in the early spring. Approximately 40% of the initial
broods were male. Additional evidence for gender specialization was obtained by observing the sex of two or three successive
broods for 85 individual females. Fifty-eight females produced successive broods of females, 13 females produced successive
broods of males and 14 females produced successive broods which included both male and female broods. Females that produced
successive female broods under natural conditions included a higher frequency of NMP females compared to a random sample of
females, confirming the existence of NMP females. Sexual females were observed in both populations after the first appearence
of males, suggesting that the presence of males may stimulate the production of sexual females. For D. pulex populations in a temporary environment, there appears to be an increased emphasis on sexual reproduction and a decreased
influence of the environment on sex determination, compared to Daphnia populations in more permanent habitats.
Received: 19 February 1996 / Accepted: 20 January 1997 相似文献
10.
Niji Ohta Naoki Sato Hisayoshi Nozaki Tsuneyoshi Kuroiwa 《Journal of molecular evolution》1997,45(6):688-695
The nucleotide sequence of a cluster of ribosomal protein genes in the plastid genome of a unicellular red alga, Cyanidioschyzon merolae, which has been supposed to be the most primitive alga, was determined. The phylogenetic tree inferred from the amino acid
sequence of ribosomal proteins of two rhodophytes, a chromophyte, a glaucophyte, two chlorophytes (land plants), a cyanobacterium,
and three eubacteria suggested a close relationship between the cyanobacterium Synechocystis PCC6803 and the plastids of various species in the kingdom Plantae, which is consistent with the hypothesis of the endosymbiotic
origin of plastids. In this tree, the two species of rhodophytes were grouped with the chromophyte, and the glaucophyte was
grouped with the chlorophytes. Analysis of the organization of the genes encoding the ribosomal proteins suggested that the
translocation of the str cluster occurred early in the lineage of rhodophytes and chromophytes after these groups had been separated from chlorophytes
and glaucophytes.
Received: 2 June 1997 / Accepted: 15 July 1997 相似文献
11.
Storage forms of N were studied in below-ground structures of nine boreal forest understorey plants. The ericaceous shrubs
Vacciniumvitis-idaea and V.myrtillus, the fern Gymnocarpium dryopteris, the grass Deschampsia flexuosa, and the herbs Epilobium angustifolium, Maianthemum bifolium, Solidago virgaurea, Geranium sylvaticum and Trientalis europaea were sampled in early summer and late autumn from plots fertilised with a complete mixture of nutrients and from non-fertilised
control plots. Concentrations of total nitrogen, insoluble and soluble proteins, free amino acids and nitrate were measured,
and changes in absolute and relative concentrations of these N fractions between early summer and late autumn were used to
identify the forms in which the plants store N. In all species studied, the concentration of free amino acids increased both
between summer and autumn and in response to fertilisation, while the concentration of protein N increased only in response
to fertilisation. Thus, free amino acids appear to have a central role in N storage. In all of the species except G. dryopteris, D. flexuosa and S. virgaurea, arginine dominated the pool of free amino acids and thus arginine was the major form of stored N in most species. In D. flexuosa and S. virgurea, however, asparagine and arginine together were the major forms of stored N, while glutamine was the major free amino acid,
and N storage form, in G. dryopteris.
Received: 10 March 1996 / Accepted: 22 December 1996 相似文献
12.
Arthropods are the most diverse and speciose group of organisms on earth. A key feature in their successful radiation is
the ease with which various appendages become readily adapted to new functions in novel environments. Arthropod limbs differ
radically in form and function, from unbranched walking legs to multibranched swimming paddles. To uncover the developmental
and genetic mechanisms underlying this diversification in form, we ask whether a three-signal model of limb growth based on
Drosophila experiments is used in the development of arthropod limbs with variant shape. We cloned a Wnt-1 ortholog (Tlwnt-1) from Triops longicaudatus, a basal crustacean with a multibranched limb. We examined the mRNA in situ hybridization pattern during larval development
to determine whether changes in wg expression are correlated with innovation in limb form. During larval growth and segmentation Tlwnt-1 is expressed in a segmentally reiterated pattern in the trunk. Unexpectedly, this pattern is restricted to the ventral portion
of the epidermis. During early limb formation the single continuous stripe of Tlwnt-1 expression in each segment becomes ventrolaterally restricted into a series of shorter stripes. Some but not all of these
shorter stripes correspond to what becomes the ventral side of a developing limb branch. We conclude that the Drosophila model of limb development cannot explain all types of arthropod proximodistal outgrowths, and that the multibranched limb
of Triops develops from an early reorganization of the ventral body wall. In Triops, Tlwnt-1 plays a semiconservative role similar to that played by Drosophila wingless in segmentation and limb formation, and morphological innovation in limb form arises in part through an early modulation
in the expression of the Tlwnt-1 gene.
Received: 22 September 1998 / Accepted: 12 January 1999 相似文献
13.
A long-standing hypothesis posits that morphological changes may be more likely to result from changes in regulation of gene
expression than from changes in the protein coding sequences of genes. We have compared the expression pattern of the twisted gastrulation (tsg) gene among five Drosophila species: D. melanogaster, D. simulans, D. subobscura, D. mojavensis, and D. virilis. The tsg gene encodes a secreted protein that is required for the specification of dorsal midline fates in the Drosophila early embryo. TSG is unlike other secreted growth and differentiation factors in Drosophila in that its expression pattern
can be experimentally varied and still result in normal development. Because of this, its regulatory region may be freer to
diverge than that of other developmental genes whose misexpression may lead to lethal defects. Thus, the tsg gene may be a good indicator of the frequency and nature of evolutionary changes affecting patterns of gene expression. Over
∼60 million years (Myr), the tsg gene has retained a dorsal-on/ventral-off pattern and a middorsal region of expression; but there have been marked changes
in the middorsal domain of expression as well as the appearance/loss of other domains of expression along the anterior/posterior
axis. Changes between closely related species (∼2–5 Myr since divergence) that are not reflected among more distantly related
species suggest frequent changes in gene expression over evolutionary time. These changes in gene expression may serve as
the raw material for eventual evolutionary changes in morphology.
Received: 24 March 1997 / Accepted: 20 June 1997 相似文献
14.
Hiroshi Suga Daisuke Hoshiyama Shigehiro Kuraku Kazutaka Katoh Kaoru Kubokawa Takashi Miyata 《Journal of molecular evolution》1999,49(5):601-608
Animals evolved a variety of gene families involved in cell–cell communication and developmental control by gene duplication
and domain shuffling. Each family is made up of several subtypes or subfamilies with distinct structures and functions, which
diverged by gene duplications and domain shufflings before the divergence of parazoans and eumetazoans. Since the separation
from protostomes, vertebrates expanded the multiplicity of members (isoforms) in the same subfamily by further gene duplications
in their early evolution before the fish–tetrapod split. To know the dates of isoform duplications more closely, we have conducted
isolation and sequencing cDNAs encoding the fibroblast growth factor receptor, Eph, src, and platelet-derived growth factor receptor subtypes belonging to the protein tyrosine kinase family from Branchiostoma belcheri, an amphioxus, Eptatretus burgeri, a hagfish, and Lampetra reissneri, a lamprey. From a phylogenetic tree of each subfamily inferred from a maximum likelihood (ML) method, together with a bootstrap
analysis based on the ML method, we have shown that the isoform duplications frequently occurred in the early evolution of
vertebrates around or just before the divergence of cyclostomes and gnathostomes by gene duplications and possibly chromosomal
duplications.
Received: 28 April 1998 / Accepted: 30 June 1999 相似文献
15.
Isaäc J. Nijman Patrick van Tessel Johannes A. Lenstra 《Journal of molecular evolution》2002,54(1):9-16
SINE retrotransposition events have proven their value as phylogenetic markers in several eukaryotic taxa at different taxonomic
levels. The genomes of ruminants contain three related SINE elements, Bov-tA, Bov-A2, and Bov-B. To estimate the time points
of retrotransposition of individual copies of these SINEs, we designed PCR primers on database sequences containing SINE insertions
in cattle, sheep, or goat genomes and tested for the presence of these copies in the genomes of other ruminants. It was checked
by sequencing whether length variation of the PCR products reflected a SINE retrotransposition. One Bov-B and nine Bov-tA
insertions were shared by cattle, sheep, goat, and giraffe, indicating an early retrotransposition event before the radiation
of the Pecora, while three other Bov-tA and two Bov-B elements were apparently inserted later. The ruminant α-lactalbumine
gene contains a hotspot of early and more recent Bov-tA insertions, a Bov-tA replacement as well as a recent Bov-B insertion.
Three Bov-A2 insertions were found to be shared only by the Bovidae, the Bovini, and the Bos and Bison species, respectively, indicating that most Bov-A2 insertions are relatively recent. The time elapsed since the retrotransposition
was also reflected in the degeneration of the direct repeats that flank SINE inserts. We suggest that retrotransposition of
SINEs may serve as phylogenetic markers in the ruminant families, subfamilies, and even tribes. In addition, sequencing of
SINE insertions revealed several other unique deletions/insertions that also may be informative for phylogenetic reconstructions
of ruminants.
Received: 19 January 2001 / Accepted: 6 June 2001 相似文献
16.
The complete nucleotide sequence of the 18S subunit of ribosomal DNA (rDNA) was determined for the venerid clams Callista chione (Pitarinae) and Venus verrucosa (Venerinae). Comparison of the new sequences with the published sequences of 1 annelid, 2 gastropods, 2 polyplacophorans,
and 19 bivalves showed that when the annelids are used as outgroup the gastropods diverge from the bivalves, which form a
cluster including the polyplacophorans. When the gastropods alone were compared with the bivalves, the latter split in two
groups corresponding to the two subclasses of Heterodonta and Pteriomorpha. The former include two taxa that diverged early,
Galeomma and Tridacna, while the Veneridae and Mactridae form two sister groups. In contrast to previous reports and in line with morphological
data, the Ostreidae are included in the Pteriomorphia and form a monophyletic group.
Received: 16 May 1998 / Accepted: 11 August 1998 相似文献
17.
We describe differences in life history of the intertidal arboreal snail, Littoraria filosa, among patches of mangroves ranging in size from isolated trees to large stands several square kilometres in area. Recruitment
of L. filosa occurred from mid spring (October) to early winter (June), recruits grew rapidly and copulating adults were found during
the following September–April. Populations within large patches of forest were annuals; all or most individuals died between
October–January (spring–midsummer). In contrast, those in smaller peripheral patches were more likely to survive over the
summer but survival differed among patches and years. These differences in life history were caused by a parasitoid fly (genus Sarcophaga) that attacked L. filosa 10 mm and longer and was present in all large patches, but absent from, or rare, in smaller peripheral patches. Experimental
introductions to isolated trees confirmed that the fly could kill L. filosa. Another sarcophagid parasitoid that attacked L. filosa from 4 to less than 10 mm long was also found in every patch. The combined effects of these parasitoids appear to determine
the metapopulation structure of L. filosa. Most adults in large patches were killed by the larger fly during early summer. Summer recruits were often killed by the
smaller fly within a month of settlement and when this happened effective recruitment of L. filosa was reduced to autumn. The planktotrophic larval stage of L. filosa lasts less than 1 month, so the source of autumn recruits to all patches must have been adults that survived the early summer,
most of which were in small patches or on isolated trees. Consequently these ”peripheral sources” are likely to be important
for persistence of the metapopulation of L. filosa. The results of this study demonstrate that metapopulation structure may be determined by complex interactions and that common
models cannot be assumed to apply in all habitats.
Received: 15 September 1999 / Accepted: 31 January 2000 相似文献
18.
The archiascomycetous fungus Protomyces pachydermus has two group I introns within the nuclear small subunit (nSSU) rRNA gene. One of these introns has an internal open reading
frame (ORF) that encodes a predicted protein of 228 amino acid residues. On the other hand, Protomyces macrosporus has two group I introns that insert at the same positions as P. pachydermus, which have no ORF. Each alignment was constructed with Protomyces group I introns located in the same position and other introns retrieved by the BLAST Search. Each phylogenetic tree based
on the alignment shows that Protomyces introns are monophyletic but the relationships among fungal introns do not reflect on the fungal phylogeny. Therefore, it
is suggested that two different horizontal transfers of group I introns occurred at the early stage of Protomyces species diversification.
Received: 11 June 1997 / Accepted: 2 September 1997 相似文献
19.
The synthesis of DNA in nuclei and organellar nucleoids at the various stages of somatic embryogenesis in carrot (Daucus carota L. cv. Kurodagosun) was analyzed using anti-5-bromo-2′-deoxyuridine (BrdU) immunofluorescence microscopy. The active syntheses
of both nuclear and organellar DNA started in the cells forming the embryo proper 3 d after the initiation of embryogenesis,
but not in cells forming suspensor-like cell aggregates. In the early globular embryo, active DNA syntheses were continuously
observed in the whole embryo proper, except for the progenitor cells of the root apical meristem (RAM) and shoot apical meristem
(SAM). These were recognized as slowly cycling cells with a non-BrdU-labelled nucleus and strongly BrdU-labelled organellar
nucleoids. At the heart- and torpedo-shaped embryo stages, both nuclear and organellar DNA syntheses were inactive in the
presumptive RAM and SAM. Thus, slowing down of organellar DNA synthesis is not coupled with, but is later than, that of nuclear
DNA synthesis in the progenitor cells of the embryonic RAM and SAM. These findings clearly indicate that the timing of DNA
synthesis is similar in the progenitor cells of both the RAM and SAM in the early stages of somatic embryogenesis.
Received: 18 January 2000 / Accepted: 2 March 2000 相似文献
20.
Summary. Background: Dysphagia and eating difficulties are highly prevalent in long term care patients. Evaluation of their nutritional status
is complicated by comorbidity, frailty and individual patterns of feeding. In previous studies we found vitamin deficiencies
(folic acid B6 and B12) in orally fed elderly in early stages of oropharyngeal dysphagia despite satisfactory nutritional
parameters (BMI, albumin and hemoglobin). The aim of this study is to evaluate the plasma amino acids levels in these hand-oral
fed elderly patients with dysphagia.
Methods: Plasma amino acids were measured in 15 orally fed elderly patients in early functional outcome swallowing scale (FOSS),
stage 2, and compared with those of 15 matched nasogastric-tube-fed counterparts.
Results: The plasma levels of all measured amino acids, ratio of essential to nonessential, levels of conditionally essential and
the immune-enhancing amino acids were similar in both groups and within the normal range of our laboratory. The traditional
nutritional parameters were also similar in both groups and within the normal range.
Conclusions: Plasma levels of amino acids in elderly patients in early stage of FOSS are satisfactory, supporting the view that their
protein intake is adequate. Further studies should concentrate on patients in advanced stages of FOSS. 相似文献