弱光照和富营养对苦草生长的影响

本研究通过比较在不同光照和营养(3光照×3营养)水平下栽培的沉水植物苦草(Vallisneria natans L.)的生长及生化指标,探讨了富营养水体中弱光和高营养对苦草生长的影响.结果表明:弱光对苦草生长的抑制作用不受外源营养浓度的影响;而高营养对苦草生长的影响受到弱光胁迫程度的交互作用,表现为在光照较强的45%日光下为抑制作用,光照最弱的2.5%日光下为促进作用,在光强居间的10%日光下没有明显作用.植物组织总氮、总磷、氨态氮及游离氨基酸氮含量随光照减弱而增加,而可溶性总糖和淀粉含量减少;总磷、氨态氮、游离氨基酸氮及淀粉含量随营养增加而增加.因此弱光照和过高营养均对苦草生长产生明显抑制作用,两者具有交互作用,主要表现为弱光影响了高营养的抑制作用.在本研究中,高营养对苦草生长有抑制作用,但尚不能导致铵中毒或储存碳缺乏;可能由于10%和2.5%日光下,弱光胁迫对苦草的代谢已产生很大抑制作用,限制了高营养对苦草的抑制作用.     

养殖水体中的微型裸腹溞对高浓度鱼类化学信息素的反应:以生命表研究为例

本文通过研究微型裸腹溞的生活史,探讨了当东湖养殖的鲢鳙密度进一步增大时,它们释放的化学信息素对微型裸腹溞的间接作用,同时也讨论了微型裸腹溞在高密度鱼类捕食下种群能够延续的可能机制。在淡水生态系统中,无视力的生物能通过化学物质的交流来感知环境中的捕食压力,从而作出相应的调整策略。本研究结果显示高浓度的鱼类化学信息素对微型裸腹溞的生活史并没有显著影响,意味着微型裸腹溞有可能通过与该鱼类的长期共存而适应了这类物质。但是我们的结果与前人的相比还是存在着很大的差异,尤其是明显提高的总的后代数、平均怀卵量和内禀增长力;另外短的世代时间、小的初次繁殖时的体长和年龄有可能也是微型裸腹溞在高密度鱼类捕食压力下占优势的良好证据。     

Conserving Plants in Gene Banks and Nature: Investigating Complementarity with Trifolium thompsonii Morton

A standard conservation strategy for plant genetic resources integrates in situ (on-farm or wild) and ex situ (gene or field bank) approaches. Gene bank managers collect ex situ accessions that represent a comprehensive snap shot of the genetic diversity of in situ populations at a given time and place. Although simple in theory, achieving complementary in situ and ex situ holdings is challenging. Using Trifolium thompsonii as a model insect-pollinated herbaceous perennial species, we used AFLP markers to compare genetic diversity and structure of ex situ accessions collected at two time periods (1995, 2004) from four locations, with their corresponding in situ populations sampled in 2009. Our goal was to assess the complementarity of the two approaches. We examined how gene flow, selection and genetic drift contributed to population change. Across locations, we found no difference in diversity between ex situ and in situ samples. One population showed a decline in genetic diversity over the 15 years studied. Population genetic differentiation among the four locations was significant, but weak. Association tests suggested infrequent, long distance gene flow. Selection and drift occurred, but differences due to spatial effects were three times as strong as differences attributed to temporal effects, and suggested recollection efforts could occur at intervals greater than fifteen years. An effective collecting strategy for insect pollinated herbaceous perennial species was to sample >150 plants, equalize maternal contribution, and sample along random transects with sufficient space between plants to minimize intrafamilial sampling. Quantifying genetic change between ex situ and in situ accessions allows genetic resource managers to validate ex situ collecting and maintenance protocols, develop appropriate recollection intervals, and provide an early detection mechanism for identifying problematic conditions that can be addressed to prevent further decline in vulnerable in situ populations.     

Isolation of an osmotic stress- and abscisic acid-induced gene encoding an acidic endochitinase from Lycopersicon chilense

We have identified one osmotic stress- and abscisic acid-responsive member of the endochitinase (EC 3.2.1.14) gene family from leaves of drought-stressed Lycopersicon chilense plants, a natural inhabitant of extremely arid regions in South America. The 966-bp full-length cDNA (designated pcht28) encodes an acidic chitinase precursor with an amino-terminal signal peptide. The mature protein is predicted to have 229 amino acid residues with a relative molecular mass of 24 943 and pI value of 6.2. Sequence analysis revealed that pcht28 has a high degree of homology with class II chitinases (EC 3.2.1.14) from tomato and tobacco. Expression of the pcht28 protein in Escherichia coli verified that it is indeed a chitinase. Northern blot analysis indicated that this gene has evolved a different pattern of expression from that of other family members reported thus far. It is highly induced by both osmotic stress and the plant hormone abscisic acid. Southern blot analysis of genomic DNA suggested that the pcht28-related genes may form a small multigene family in this species. The efficiency of induction of the gene by drought stress, in leaves and stems, is significantly higher in L. chilense than in the cultivated tomato. It is speculated that, besides its general defensive function, the pcht28-encoded chitinase may play a particular role in plant development or in protecting plants from pathogen attack during water stress.     

Recent structural variations in the Medicago chloroplast genomes and their horizontal transfer into nuclear chromosomes

Medicago is a genus of legumes (Fabaceae) that resemble common clovers with pinnately trifoliate leaves and spirally coiled seed pods, and Medicago sativa is a famous forage crop throughout the world. In this study, we systematically assembled the complete plastid genomes of 18 Medicago species, representing 35 Medicago accessions, whose genome size ranged from ~119 to 125 kb, and identified one novel inverted repeat (IR) in two accessions of Medicago soleirolii (PI537242 and PI537243), albeit of no IRs in the most accessions. We built a phylogenetic tree based on common protein-coding sequences of 55 Medicago accessions in 38 species, which were placed into five clades with a divergence since 9.37 million years ago. Global alignment revealed independent genome evolution events, including eight inversions in nine species and four intron losses (ILs) in 10 species, among which four inversions and two ILs have not been reported previously. Within 109–111 unique genes, ndhA, rpl2, and ycf3 were under positive selection in 54 Medicago accessions. Finally, by aligning chloroplast genes against the nuclear genome assembly of M. sativa cultivar “Zhongmu No.1”, we found that a large number of chloroplast gene fragments were horizontally transferred to nuclear chromosomes in alfalfa, especially on the chr3:47518422–48722257 coordinates of chromosome 3. Our comprehensive exploration of Medicago chloroplast genomes provided insights for the understanding of Medicago diversity and their genomic evolution events.     

Association mapping and gene-gene interaction for stem rust resistance in CIMMYT spring wheat germplasm

The recent emergence of wheat stem rust Ug99 and evolution of new races within the lineage threatens global wheat production because they overcome widely deployed stem rust resistance (Sr) genes that had been effective for many years. To identify loci conferring adult plant resistance to races of Ug99 in wheat, we employed an association mapping approach for 276 current spring wheat breeding lines from the International Maize and Wheat Improvement Center (CIMMYT). Breeding lines were genotyped with Diversity Array Technology (DArT) and microsatellite markers. Phenotypic data was collected on these lines for stem rust race Ug99 resistance at the adult plant stage in the stem rust resistance screening nursery in Njoro, Kenya in seasons 2008, 2009 and 2010. Fifteen marker loci were found to be significantly associated with stem rust resistance. Several markers appeared to be linked to known Sr genes, while other significant markers were located in chromosome regions where no Sr genes have been previously reported. Most of these new loci colocalized with QTLs identified recently in different biparental populations. Using the same data and Q?+?K covariate matrices, we investigated the interactions among marker loci using linear regression models to calculate P values for pairwise marker interactions. Resistance marker loci including the Sr2 locus on 3BS and the wPt1859 locus on 7DL had significant interaction effects with other loci in the same chromosome arm and with markers on chromosome 6B. Other resistance marker loci had significant pairwise interactions with markers on different chromosomes. Based on these results, we propose that a complex network of gene-gene interactions is, in part, responsible for resistance to Ug99. Further investigation may provide insight for understanding mechanisms that contribute to this resistance gene network.     

A consensus map for Ug99 stem rust resistance loci in wheat

Key message

This consensus map of stem rust genes, QTLs, and molecular markers will facilitate the identification of new resistance genes and provide a resource of in formation for development of new markers for breeding wheat varieties resistant to Ug99.

Abstract

The global effort to identify new sources of resistance to wheat stem rust, caused by Puccinia graminis f. sp. tritici race group Ug99 has resulted in numerous studies reporting both qualitative genes and quantitative trait loci. The purpose of our study was to assemble all available information on loci associated with stem rust resistance from 21 recent studies on Triticum aestivum L. (bread wheat) and Triticum turgidum subsp. durum desf. (durum wheat). The software LPmerge was used to construct a stem rust resistance loci consensus wheat map with 1,433 markers incorporating Single Nucleotide Polymorphism, Diversity Arrays Technology, Genotyping-by-Sequencing as well as Simple Sequence Repeat marker information. Most of the markers associated with stem rust resistance have been identified in more than one population. Several loci identified in these populations map to the same regions with known Sr genes including Sr2, SrND643, Sr25 and Sr57 (Lr34/Yr18/Pm38), while other significant markers were located in chromosome regions where no Sr genes have been previously reported. This consensus map provides a comprehensive source of information on 141 stem rust resistance loci conferring resistance to stem rust Ug99 as well as linked markers for use in marker-assisted selection.     

Identification of Molecular Markers Associated with Verticillium Wilt Resistance in Alfalfa (Medicago Sativa L.) Using High-Resolution Melting

Verticillium wilt, caused by the soilborne fungus, Verticillium alfalfae, is one of the most serious diseases of alfalfa (Medicago sativa L.) worldwide. To identify loci associated with resistance to Verticillium wilt, a bulk segregant analysis was conducted in susceptible or resistant pools constructed from 13 synthetic alfalfa populations, followed by association mapping in two F1 populations consisted of 352 individuals. Simple sequence repeat (SSR) and single nucleotide polymorphism (SNP) markers were used for genotyping. Phenotyping was done by manual inoculation of the pathogen to replicated cloned plants of each individual and disease severity was scored using a standard scale. Marker-trait association was analyzed by TASSEL. Seventeen SNP markers significantly associated with Verticillium wilt resistance were identified and they were located on chromosomes 1, 2, 4, 7 and 8. SNP markers identified on chromosomes 2, 4 and 7 co-locate with regions of Verticillium wilt resistance loci reported in M. truncatula. Additional markers identified on chromosomes 1 and 8 located the regions where no Verticillium resistance locus has been reported. This study highlights the value of SNP genotyping by high resolution melting to identify the disease resistance loci in tetraploid alfalfa. With further validation, the markers identified in this study could be used for improving resistance to Verticillium wilt in alfalfa breeding programs.