Population genetic structure of Sargassum thunbergii (Fucales, Phaeophyta) detected by RAPD and ISSR markers

Genetic variation of four populations of Sargassum thunbergii (Mert.) O. Kuntze and one outgroup of S. fusiforme (Harv.) Setchell from Shandong peninsula of China was studied with random amplified polymorphic DNA (RAPD) and inter-simple sequence repeat (ISSR) markers. A total of 28 RAPD primers and 19 ISSR primers were amplified, showing 174 loci and 125 loci, respectively. Calculation of genetic diversity with different indicators (P%, percentage of polymorphic loci; H, the expected heterozygosity; I, Shannon's information index) revealed low or moderate levels of genetic variations within each S. thunbergii population. High genetic differentiations were determined with pairwise Nei's unbiased genetic distance (D) and fixation index (F-ST) between the populations. The Mantel test showed that two types of matrices of D and FST were highly correlated, whether from RAPD or ISSR data, r=0.9310 (P = 0.008) and 0.9313 (P=0.009) respectively. Analysis of molecular variance (AMOVA) was used to apportion the variations between and within the S. thunbergii populations. It indicated that the variations among populations were higher than those within populations, being 57.57% versus 42.43% by RAPD and 59.52% versus 40.08% by ISSR, respectively. Furthermore, the Mantel test suggested that the genetic differentiations between the four populations were related to the geographical distances (r > 0.5), i.e., they conformed to the IBD (isolation by distance) model, as expected from UPGMA (unweighted pair group method with arithmetic averages) cluster analysis. As a whole, the high genetic structuring between the four S. thunbergii populations along distant locations was clearly indicated in the RAPD and ISSR analyses (r > 0.8) in our study.

EscC is a chaperone for the Edwardsiella tarda type III secretion system putative translocon components EseB and EseD

Edwardsiella tarda is a Gram-negative enteric pathogen that causes disease in both humans and animals. Recently, a type III secretion system (T3SS) has been found to contribute to Ed. tarda pathogenesis. EseB, EseC and EseD were shown to be secreted by the T3SS and to be the major components of the extracellular proteins (ECPs). Based on sequence similarity, they have been proposed to function as the 'translocon' of the T3SS needle structure. In this study, it was shown that EseB, EseC and EseD formed a protein complex after secretion, which is consistent with their possible roles as translocon components. The secretion of EseB and EseD was dependent on EscC (previously named Orf2). EscC has the characteristics of a chaperone; it is a small protein (13 kDa), located next to the translocators in the T3SS gene cluster, and has a coiled-coil structure at the N-terminal region as predicted by COILS. An in-frame deletion of escC abolished the secretion of EseB and EseD, and complementation of Delta escC restored the export of EseB and EseD into the culture supernatant. Further studies showed that EscC is not a secreted protein and is located on the membrane and in the cytoplasm. Mutation of escC did not affect the transcription of eseB but reduced the amount of EseB as measured by using an EseB-LacZ fusion protein in Ed. tarda. Co-purification studies demonstrated that EscC formed complexes with EseB and EseD. The results suggest that EscC functions as a T3SS chaperone for the putative translocon components EseB and EseD in Ed. tarda.

Vast laminated diatom mat deposits from the west low-latitude Pacific Ocean in the last glacial period

Diatoms are one of the predominant contributors to global carbon fixation by accounting for over 40% of total oceanic primary production and dominate export production. They play a significant role in marine biogeochemistry cycle. The diatom mat deposits are results of vast diatoms bloom. By analysis of diatom mats in 136 degrees 00'-140 degrees 00'E, 15 degrees 00'-21 degrees 00'N, Eastern Philippines Sea, we identified the species of the diatoms as giant Ethmodiscus rex (Wallich) Hendey. AMS C-14 dating shows that the sediments rich in diatom mats occurred during 16000-28600 a B.P., which means the bloom mainly occurred during the last glacial period, while there are no diatom mat deposits in other layers. Preliminary analysis indicates that Antarctic Intermediate Water (AAIW) expanded northward and brought silicate-rich water into the area, namely, silicon leakage processes caused the bloom of diatoms. In addition, the increase of iron input is one of the main reasons for the diatom bloom.

紫菜壳孢子萌发过程及其世代差异研究

紫菜是一类海洋藻类研究的模式系统，具有重要的经济价值和理论研究意义。本研究基于紫菜的世代交替生活史，对其壳孢子萌发过程进行了研究，并对丝状孢子体与叶状配子体的世代差异进行了分析，主要包括以下三部分： 1） 对紫菜丝状孢子体与叶状配子体的连接枢纽——壳孢子的发育过程及其光影响进行了研究与讨论，发现壳孢子只有在附着后才能形成细胞壁并发育，说明壳孢子的附着是触发了壳孢子细胞壁的形成以及后期发育的开关，亦即附着是触发紫菜世代交替过程中配子体转录组表达的开关；纤维素酶和果胶酶均能抑制壳孢子附着，但影响机制各不相同，推测果胶质主要介导壳孢子的初始附着，而纤维素则与永久附着相关；波长≥580 nm的高强度（200 μmol•m-2•s-1）可见光有利于壳孢子早期发育。 2） 结合现有藻类数据，对坛紫菜丝状孢子体阶段11000 EST数据进行了大规模的生物信息学分析，结果首次发现坛紫菜丝状孢子体中可能存在PCK型C4光合固碳途径，并筛选出44条在紫菜孢子体中表达上调的代表基因。 3） 结合坛紫菜、条斑紫菜、海带和红毛菜，对红藻和褐藻等大型海藻孢子体与配子体阶段代表基因Rubisco的表达与羧化酶活性差异进行了研究分析，结果表明Rubisco的表达量和初始羧化酶活在其配子体中均显著高于其孢子体世代，即与藻体不同世代的相对复杂度无关，而与染色体倍性相关，说明Rubisco的世代差异极有可能与染色体倍性相连锁，因而可能是海藻世代交替过程中的重要功能基因。

马尾藻种群遗传及早期发育的研究

本研究运用RAPD和ISSR两种分子标记技术，对采自山东半岛4个不同地理位置的鼠尾藻（Sargassum thunbergii）和海黍子（S. muticum）种群进行了遗传多样性和遗传结构的研究，从而对其种群间的地理隔离、基因流动水平及其影响因素做出估计和判断，为马尾藻自然资源的保护和开发提供依据。在室内对鼠尾藻有性生殖幼苗的早期发育和生长进行了研究，了解其繁殖生物学特性，为鼠尾藻人工种苗的培育提供依据。主要研究结果如下： 对4个鼠尾藻（S. thunbergii）地理种群的遗传多样性研究中，筛选出了28条RAPD 引物和19条ISSR引物，分别扩增产生了174和125个位点。选用的三种不同指标，即多态位点比率（P％，percentage of polymorphic loci），平均预期杂合度（H，the expected heterozygosity）和 Shannon's 信息多样性指数（I，Shannon's information index），均可反映出鼠尾藻种群内部的遗传多样性呈较低水平。而群体间遗传距离（D，Nei’s unbiased genetic distance）矩阵和固定化指数（FST,the fixation index）矩阵均反映出群体间高度的遗传分化。通过分子变异分析（AMOVA，Analysis of molecular variance）来区分来自种群内部和种群之间的遗传变异，揭示出多数的遗传变异（57.57% 或59.52%）来自于鼠尾藻种群之间。另外，Mantel分析表明，4个鼠尾藻种群间的遗传分化与地理距离呈正相关（r>0.5），遵循传统的IBD（isolation by distance）模式，UPGMA（unweighted pair group method with arithmetic averages）聚类分析也反映出相似的结果。 对4个海黍子（S. muticum）地理种群遗传结构的研究中，筛选出的24条RAPD 引物和19条ISSR引物分别扩增出164和122个位点。遗传多样性评估结果表明，海黍子种群内部存在较低或者中等水平的遗传多样性，而D矩阵和FST 矩阵均显示种群间存在高水平的遗传分化。并且，发现D和FST 矩阵在RAPD和ISSR分析中均具有高且显著的相关性。AMOVA分析显示，种群之间的遗传变异高于种群内部。Mantel分析和UPGMA聚类分析均发现海黍子种群间的遗传分化遵循IBD模式，即与地理隔离呈正相关（r>0.6）。 并且，RAPD和ISSR分析的结果高度一致（r>0.9，P<0.05），均揭示4个海黍子种群之间存在高度的遗传分化。 对鼠尾藻有性生殖幼苗早期生长发育的研究结果表明，其早期发育过程属于马尾藻科（Sargassaceae）中典型的“8核1卵”型。在一定条件下培养两个月后，产生了1～2个小叶，幼苗的长度达2～3毫米。生长实验发现，温度（10, 15, 20, 25℃）和光照强度（9, 18, 44, 88 µEm-2s-1）对培养第一周幼苗的生长均有显著的影响（ANOVA, P<0.01）。在两个月的培养中，幼苗对温度和光强的耐受范围较宽，在10℃～25℃，9～88 µEm-2s-1条件下均可生长，最适温度和光强为25℃，44 µEm-2s-1；低温（10℃）对幼苗的生长有显著抑制。不同光质对幼苗生长的影响显著（P<0.01），相同光强条件下，蓝光和白光相比较，蓝光显然不能满足鼠尾藻幼苗早期生长的需要。

A comparison of future increased CO2 and temperature effects on sympatric Heterosigma akashiwo and Prorocentrum minimum

Very little is known about how global anthropogenic changes will affect major harmful algal bloom groups. Shifts in the growth and physiology of HAB species like the raphidophyte Heterosigma akashiwo and the dinoflagellate Prorocentrum minimum due to rising CO2 and temperature could alter their relative abundance and environmental impacts in estuaries where both form blooms, such as the Delaware Inland Bays (DIB). We grew semi-continuous cultures of sympatric DIB isolates of these two species under four conditions: (1) 20 degrees C and 375 ppm CO2 (ambient control), (2)20 degrees C and 750 ppm CO2 (high CO2),(3) 24 degrees C and 375 ppm CO2 (high temperature), and (4) 24 degrees C and 750 ppm CO2 (combined). Elevated CO2 alone or in concert with temperature stimulated Heterosigma growth, but had no significant effect on Prorocentrum growth. P-Bmax (the maximum biomass-normalized light-saturated carbon fixation rate) in Heterosigma was increased only by simultaneous CO2 and temperature increases, whereas P-Bmax in Prorocentrum responded significantly to CO2 enrichment, with or without increased temperature. CO2 and temperature affected photosynthetic parameters alpha, Phi(max), E-k, and Delta F/F'(m) in both species. Increased temperature decreased and increased the Chl a content of Heterosigma and M Prorocentrum, respectively. CO2 availability and temperature had pronounced effects on cellular quotas of C and N in Heterosigma, but not in Prorocentrum. Ratios of C:P and N:P increased with elevated carbon dioxide in Heterosigma but not in Prorocentrum. These changes in cellular nutrient quotas and ratios imply that Heterosigma could be more vulnerable to N limitation but less vulnerable to P-limitation than Prorocentrum under future environmental conditions. In general, Heterosigma growth and physiology showed a much greater positive response to elevated CO2 and temperature compared to Prorocentrum, consistent with what is known about their respective carbon acquisition mechanisms. Hence, rising temperature and CO2 either alone or in combination with other limiting factors could significantly alter the relative dominance of these two co-existing HAB species over the next century. (c) 2007 Elsevier B.V. All rights reserved.

Response of cyanobacterial carbon concentrating system to light intensity: a simulated analysis

Cyanobacteria possess a delicate system known as the carbon concentrating mechanism (CCM), which can efficiently elevate the intracellular inorganic carbon (Ci) concentration via active transportation. The system requires energy supplied by photosystems; therefore, the activity of the Ci transporter is closely related to light intensity. However, the relationship between CCM and light intensity has rarely been evaluated. Here, we present an improved quantitative model of CCM in which light is incorporated, and developed a CCM model that modified after Fridlyand et al. in 1996. Some equations used in this model were inducted to describe the relationship between transport capacity and light intensity, by which the response of the CCM to light change is simulated. Our results indicate that the efficiency of the carbon concentrating system is sensitive to light intensity. When the external Ci concentration was low, CO2 uptake dominated the total Ci uptake with increasing light intensity, while under high external Ci concentrations HCO3- uptake primarily contributed to the total Ci uptake. Variations in the ratio of energy allocated between the transport systems could markedly affect the operation of CCM. Indeed, our simulations suggest that various combinations of Ci fluxes can provide a possible approach to detect the way by which the cell distributes energy produced by the photosystems to the two active Ci transport processes. The proportion of the energy consumed on CCM to the total energy expenditure for the fixation of one CO2 molecule was determined at 18%-40%.

The Apertospathulidae, a new family of haptorid ciliates (Protozoa, Ciliophora)

We studied the morphology of three rare haptorid ciliates, using live observation and silver impregnation: Apertospathula verruculifera n. sp., Longispatha elegans n. gen., n. sp., and Rhinothrix porculus (Penard, 1922) n. gen., n. comb. Simple ethanol fixation (50-70%, v/v) is recommended to reveal the ciliary pattern of "difficult" ciliates, such as R. porculus, by protargol impregnation. The three genera investigated have a distinct feature in common, viz., a lasso-shaped oral bulge and circumoral kinety, where the right half is slightly to distinctly longer than the left and the circumoral kinety is open ventrally. Thus, they are united in a new spathidiid family, the Apertospathulidae n. fam., which probably evolved from a Bryophyllum-like ancestor by partial reduction of the oral bulge and circumoral kinety. Apertospathula verruculifera has a wart-like process, the palpus dorsalis, at the anterior end of the dorsal brush. The right branch of the circumoral kinety is only slightly longer than the left one. Longispatha elegans has a straight oral bulge and circumoral kinety, the right branch of which extends to the posterior end of the body while the left branch ends in the anterior third of the body. Rhinothrix porculus, a curious ciliate with a snout-like dorsal elongation of the oral bulge, the palpus oralis, has a highly characteristic ciliary pattern: the oral pattern is as in Longispatha, but the bulge and circumoral kinety extend spirally to the posterior end of the body while the somatic kinetics course meridionally. This is achieved by inserting some shortened kinetics in the curves of the oral bulge.

Two trichodinid ectoparasites from marine molluscs in the Yellow Sea, off China, with the description of Trichodina caecellae n. sp (Protozoa : Ciliophora : Peritrichia)

The morphology and infraciliature of two ectoparasitic ciliates, Trichodina caecellae n. sp. and T. ruditapicis Xu, Song & Warren, 2000, parasitising the gills of marine molluscs from the Shandong coast of the Yellow Sea, China, were investigated following wet silver nitrate and protargol impregnation. T. caecellae was found on the small marine sand clam Caecella chinensis Deshayes and is distinguished mainly by the acute triangle-like blade, the very delicate central part and the needle-shaped ray. T. ruditapicis was studied based on four populations from three clams: two populations from Ruditapes philippinarum (Adams) and one each from Saxidomus purpuratus (Sowerby) and Solen grandis Dunker. All four populations fell within the range of morphometry and agreed closely in the overall appearance of the adhesive disc. However, variability was found in the denticle structure, especially in populations from different host clams. Our observations suggest that denticle morphology may be more or less variable between and within populations, and that such minor differences should not be overestimated. It should be emphasised that, except for the denticle morphology, the bright granules or circles in the centre of the adhesive disc represent another important feature facilitating the identification of this trichodinid species.

Comparative study on CO2 emissions from different types of alpine meadows during grass exuberance period

Potentilla fruticosa scrub, Kobresia humilis meadow and Kobresia tibetica meadow are widely distributed on the Qinghai-Tibet Plateau. During the grass exuberance period from 3 July to 4September, based on close chamber-GC method, a study on CO2 emissions from different treatments was conducted in these meadows at Haibei research station, CAS. Results indicated that mean CO2emission rates from various treatments were 672.09+152.37 mgm-2h-1 for FC (grass treatment); 425.41+191.99 mgrn-2h-1 for FJ (grass exclusion treatment); 280.36+174.83 mgrn-2h-1 for FL (grass and roots exclusion treatment); 838.95+237.02 mgm-2h-1 for GG (scrub+grass treatment); 528.48+205.67 mgm-2h-1for GC (grass treatment); 268.97 ±99.72 mgm-2h-1 for GL (grass and roots exclusion treatment); and 659.20±94.83 mgm-2h-1 for LC (grass treatment), respectively (FC, FJ, FL, GG, GC, GL, LC were the Chinese abbreviation for various treatments). Furthermore, Kobresia humilis meadow, Potentilla fruticosa scrub meadow and Kobresia tibetica meadow differed greatly in average CO2 emission rate of soil-plant system, in the order of GG＞FC＞LC＞GC. Moreover, in Kobresia humilis meadow,heterotrophic and autotrophic respiration accounted for 42% and 58% of the total respiration of soil-plant system respectively, whereas, in Potentilla fruticosa scrub meadow, heterotrophic and autotrophic respiration accounted for 32% and 68% of total system respiration from G-G; 49% and 51%from GC. In addition, root respiration from Kobresia humilis meadow approximated 145 mgCO2m-2h-1,contributed 34% to soil respiration. During the experiment period, Kobresia humilis meadow and Potentilla fruticosa scrub meadow had a net carbon fixation of 111.11 grn-2 and 243.89 grn-2,respectively. Results also showed that soil temperature was the main factor which influenced CO2 emission from alpine meadow ecosystem, significant correlations were found between soil temperature at 5 cm depth and CO2 emission from GG, GC, FC and FJ treatments. In addition, soil moisture may be the inhibitory factor of CO2 emission from Kobresia tibetica meadow, and more detailed analyses should be done in further research.

Altitudinal differences in the leaf fitness of juvenile and mature alpine spruce trees (Picea crassifolia)

In many plant species, leaf morphology varies with altitude, an effect that has been attributed to temperature. It remains uncertain whether such a trend applies equally to juvenile and mature trees across altitudinal gradients in semi-arid mountain regions. We examined altitude-related differences in a variety of needle characteristics of juvenile (2-m tall) and mature (5-m tall) alpine spruce (Picea crassifolia Kom.) trees growing at altitudes between 2501 and 3450 m in the Qilian Mountains of northwest China. We found that stable carbon isotope composition (delta C-13), area- and mass-based leaf nitrogen concentration (N-a, N-m), number of stomata per gram of nitrogen (St/N), number of stomata per unit leaf mass (St/LM), projected leaf area per 100 needles (LA) and leaf mass per unit area (LMA) varied nonlinearly with altitude for both juvenile and mature trees, with a relationship reversal point at about 3 100 m. Stomatal density (SD) of juvenile trees remained unchanged with altitude, whereas SD and stomatal number per unit length (SNL) of mature spruce initially increased with altitude, but subsequently decreased. Although several measured indices were generally found to be higher in mature trees than in juvenile trees, N-m, leaf carbon concentration (C.), leaf water concentration. (LWC), St/N, LA and St/LM showed inconsistent differences between trees of different ages along the altitudinal gradient. In both juvenile and mature trees, VC correlated significantly with LMA, N-m, N-a, SNL, St/LM and St/N. Stomatal density, LWC and LA were only significantly correlated with delta C-13 in mature trees. These findings suggest that there are distinct ecophysiological differences between the needles of juvenile and mature trees that determine their response to changes in altitude in semi-arid mountainous regions. Variations in the fitness of forests of different ages may have important implications for modeling forest responses to changes in environmental conditions, such as predicted future temperature increases in high attitude areas associated with climate change.

Independence of alpine meadow plants from soil organic matter for nitrogen supply: evidence from stable nitrogen isotopes

Stable nitrogen isotope signatures of major sources of mineral nitrogen ( mineralization of soil organic nitrogen, biological N-2 fixation by legumes, annual precipitation and plant litter decomposition) were measured to relatively define their individual contribution to grass assimilation at the Haibei Alpine Meadow Ecosystem, Qinghai, China. The results indicated that delta N-15 values (- 2.40 parts per thousand to 0.97 parts per thousand) of all grasses were much lower than those of soil organic matter (3.4 +/- 0.18 parts per thousand) and mineral nitrogen ( ammonium and nitrate together,7.8 +/- 0.57 parts per thousand). Based on the patterns of stable nitrogen isotopes, soil organic matter (3.4 +/- 0.18 parts per thousand), biological N-2 fixation (0 parts per thousand), and precipitation (- 6.34 +/- 0.24 parts per thousand) only contributed to a small fraction of nitrogen requirements of grasses, but plant litter decomposition (- 1.31 +/- 1.01 parts per thousand) accounted for 67%.

Radiation and diversification within the Ligularia-Cremanthodium-Parasenecio complex (Asteraceae) triggered by uplift of the Qinghai-Tibetan Plateau

The Ligularia-Cremanthodium-Parasenecio (L-C-P) complex of the Tussilagininae (Asteraceae: Senecioneae) contains more than 200 species that are endemic to the Qinghai-Tibetan Plateau in eastern Asia. These species are morphologically distinct; however, their relationships appear complex. A phylogenetic analysis of members of the complex and selected taxa, of the tribe Senecioneae was conducted using chloroplast (ndhF and trnL-F) and nuclear (ITS) sequences. Phylogenetic trees were constructed from individual and combined datasets of the three different sequences. All analyses suggested that Doronicum, a genus that has been included in the Tussilagininae, should be excluded from this subtribe and placed at the base of the tribe Senecioneae. In addition, the Tussilagininae should be broadly circumscribed to include the Tephroseridinae. Within the expanded Tussilagininae containing all 13 genera occurring in eastern Asia, Tussilago and NSPetasites diverged early as a separate lineage, while the remaining I I genera comprise an expanded L-C-P complex clade. We suggest that the L-C-P clade, which is largely unresolved, most likely originated as a consequence of an explosive radiation. The few monophyletic subclades identified in the L-C-P clade with robust support further suggest that some genera of Tussilagininae from eastern Asia require generic re-circumscriptions given the occurrence of subclades containing species of the same genus in different parts of the phylogentic tree due to homoplasy of important morphological characters used to delimit them. Molecular-clock analyses suggest that the explosive radiation of the L-C-P complex occurred mostly within the last 20 million years, which falls well within the period of recent major uplifts of the Qinghai-Tibetan Plateau between the early Miocene to the Pleistocene. It is proposed that significant increases in geological and ecological diversity that accompanied such uplifting, most likely promoted rapid and continuous allopatric speciation in small and isolated populations, and allowed fixation or acquisition of similar morphological characters within unrelated lineages. This phenomenon, possibly combined with interspecific diploid hybridization because of secondary sympatry during relatively stable stages between different uplifts, could be a major cause of high species diversity in the Qinghai-Tibetan Plateau and adjacent areas of eastern Asia. (c) 2005 Elsevier Inc. All rights reserved.

Molecular phylogeny, recent radiation and evolution of gross morphology of the rhubarb genus Rheum (Polygonaceae) inferred from chloroplast DNA trnL-F sequences

Background and Aims Rheum, a highly diversified genus with about 60 species, is mainly confined to the mountainous and desert regions of the Qinghai-Tibetan plateau and adjacent areas. This genus represents a good example of the extensive diversification of the temperate genera in the Qinghai-Tibetan plateau, in which the forces to drive diversification remain unknown. To date, the infrageneric classification of Rheum has been mainly based on morphological characters. However, it may have been subject to convergent evolution under habitat pressure, and the systematic position of some sections are unclear, especially Sect. Globulosa, which has globular inflorescences, and Sect. Nobilia, which has semi-translucent bracts. Recent palynological research has found substantial contradictions between exine patterns and the current classification of Rheum. Two specific objectives of this research were (1) to evaluate possible relationships of some ambiguous sections with a unique morphology, and (2) to examine possible occurrence of the radiative speciation with low genetic divergence across the total genus and the correlation between the extensive diversification time of Rheum and past geographical events, especially the recent large-scale uplifts of the Qinghai-Tibetan Plateau.Methods The chloroplast DNA trnL-F region of 29 individuals representing 26 species of Rheum, belonging to seven out of eight sections, was sequenced and compared. The phylogenetic relationships were further constructed based on the sequences obtained.Key Results Despite the highly diversified morphology, the genetic variation in this DNA fragment is relatively low. The molecular phylogeny is highly inconsistent with gross morphology, pollen exine patterns and traditional classifications, except for identifying all samples of Sect. Palmata, three species of Sect. Spiciformia and a few species of Sect. Rheum as corresponding monophyletic groups. The monotypic Sect. Globulosa showed a tentative position within the clade comprising five species of Sect. Rheum. All of the analyses revealed the paraphyly of R. nobile and R. alexandrae, the only two species of Sect. Nobilia circumscribed by the possession of large bracts. The crude calibration of lineages based on trnL-F sequence differentiation implied an extensive diversification of Rheum within approx. 7 million years.Conclusions Based on these results, it is suggested that the rich geological and ecological diversity caused by the recent large-scale uplifts of the Qinghai-Tibetan Plateau since the late Tertiary, coupled with the oscillating climate of the Quaternary stage, might have promoted rapid speciation in small and isolated populations, as well as allowing the fixation of unique or rare morphological characters in Rheum. Such a rapid radiation, combined with introgressive hybridization and reticulate evolution, may have caused the transfer of cpDNA haplotypes between morphologically dissimilar species, and might account for the inconsistency between morphological classification and molecular phylogeny reported here.

重庆老龙洞二叠系-三叠系界线地层微生物岩研究

The largest mass extinction in the Phanerozoic happened at the end of the Permian. The microbialites formed in the extreme environments after the mass extinction has become a hotspot for geologists and paleontologists throughout the world. The dendroid microbialites that were described for the first time in 1999 from the Permian-Triassic boundary section at Laolongdong, Chongqing, have been studied by many geologists from China and overseas. Two important viewpoints about their origin have been proposed. Some researchers believed that they resemble Quaternary travertine shrubs in form, and may belong to microbialites. Some other researchers proposed that the dendroid structure is composed of clots formed by coccoidal cynaobacteria, and is microbialite. Our detailed survey on the section reveals that: (1) there is an interval of speckled “microbialite” in the section, and it underlies the dendroid “microbialite”, (2) the dendroid “microbialite” does not always have dendroid appearance; they are dendroid only in very local places; they are not dendroid in most places; for this reason, they are not comparable to recent tufa; (3) the volume of the dendroid structure greatly increases toward the top of the dendroid microbialite interval: accounting to 70% of the whole rock in the top part. This distribution pattern implies that the formation of this structure may be related to downward migration of the diagenetic fluid. Examination of thin sections reveals that the dendroid structure or point-like structure in the “microbialite” look as lighter areas in the thin sections and are composed of large blocky clear calcites containing scattered yellow dirty small calcite rhombi and irregular “points” of relict lime mudstone or wackestone or packstone. Their formation is by any one of the following two processes: (1) dissolution → filling of large blocky calcite; (2) dolomitization → dedolomitization → dissolution by meteoric fresh water → filling by large blocky calcites. It has been found that there are at least two sea-level falls during the P-T transition. As the sea level fall, the carbonate deposits came into supratidal environment, and suffered dolomitization caused by evaporative fluid or mixing water of sea water and meteoric water. Since the fluid migrated downward from the top of the deposits and in random pathway, the dolomitization formed dendroid or speckled dolomitic areas. As the deposits came into subaerial environments, the meteoric fresh water migrated along the dendroid or speckled dolomitic area with higher porosity, and dissolution happened, which caused the rock became spongy or alveolate. In later time, after the strata came into phreatic zone, large clear blocky calcites grew in and filled the pores in the spongy areas. The dendroid and speckled structure were formed in this way, rather than composed of clots formed by coccoid cyanobecteria. The microbial fossils in Laolongdong section include two types. The first is the tube-like cyanobecteria in middle Bed 3, which are generally less than 1 mm in length, taper toward one end, and are internally filled by microspars. They are straight or sinuous, with micritic wall 0.005~0.01 mm thick. Since this kind of microbial fossils are abundant in middle Bed 3, this rock belongs to microbialite. The second type occurs in Bed 5 and lower and middle Bed 6. They are irregular globular in shape, generally 0.2 ~ 0.5 mm in size, with several outward progresses, and internally filled by one layer of needle-like calcite cements on the wall and the large blocky calcite in the inner space. According to their shape and preservation way, it is inferred that this kind of fossils were formed from some kind of bacterial colony. The bacterial colony may be cuticle in composition, since it has some hardness as it is indicated by its resistance to deposit loading. These organisms discomposed during diagenetic time, and formed good porosity. In later diagenetic time, these pores were firstly cemented by needle-like calcites and later filled by large blocky calcites. So, the bacterial colony promoted the formation of dendroid and speckled structures. However, they did not always form such structures. On the other hand, even though no bacterial colony or other microbes or any kind of fossils were present, dendroid or speckled structures can form. Bed 4 of Laolongdong section contains abundant gastropods but no microbial fossils, and is not microbialite, even though it is speckled. The top of Bed 6 is dendroid, but contain no microbial fossils, and is not micrbialite.