Evaluating the potential of SSR flanking regions for examining taxonomic relationships in the Vitaceae

Complete mitochondrial genome of the endangered Prunus pedunculata (Prunoideae, Rosaceae) in China: characterization and phylogenetic analysis

Introduction

Prunus pedunculata (Prunoideae: Rosaceae), a relic shrub with strong resistance and multiple application values, is endangered in China. Extensive research had been devoted to gene expression, molecular markers, plastid genome analysis, and genetic background investigations of P. pedunculata. However, the mitochondrial genome of this species has not been systematically described, owing to the complexity of the plant mitogenome.

Methods

In the present research, the complete mitochondrial genome of P. pedunculata was assembled, annotated, and characterized. The genomic features, gene content and repetitive sequences were analyzed. The genomic variation and phylogenetic analysis have been extensively enumerated.

Results and discussion

The P. pedunculata mitogenome is a circular molecule with a total length of 405,855 bp and a GC content of 45.63%, which are the smallest size and highest GC content among the known Prunus mitochondrial genomes. The mitogenome of P. pedunculata encodes 62 genes, including 34 unique protein-coding genes (PCGs, excluding three possible pseudogenes), three ribosomal RNA genes, and 19 transfer RNA genes. The mitogenome is rich in repetitive sequences, counting 112 simple sequence repeats, 15 tandem repeats, and 50 interspersed repetitive sequences, with a total repeat length of 11,793 bp, accounting for 2.91% of the complete genome. Leucine (Leu) was a predominant amino acid in PCGs, with a frequency of 10.67%, whereas cysteine (Cys) and tryptophan (Trp) were the least adopted. The most frequently used codon was UUU (Phe), with a relative synonymous codon usage (RSCU) value of 1.12. Selective pressure was calculated based on 20 shared PCGs in the mitogenomes of the 32 species, most of which were subjected to purifying selection (Ka/Ks < 1), whereas ccmC and ccmFn underwent positive selection. A total of 262 potential RNA editing sites in 26 PCGs were identified. Furthermore, 56 chloroplast-derived fragments were ascertained in the mitogenome, ranging from 30 to 858 bp, and were mainly located across IGS (intergenic spacer) regions or rRNA genes. These findings verify the occurrence of intracellular gene transfer events from the chloroplast to the mitochondria. Furthermore, the phylogenetic relationship of P. pedunculata was supported by the mitogenome data of 30 other taxa of the Rosaceae family. Understanding the mitochondrial genome characteristics of P. pedunculata is of great importance to promote comprehension of its genetic background and this study provides a basis for the genetic breeding of Prunus.

Advances in genotyping microsatellite markers through sequencing and consequences of scoring methods for Ceratonia siliqua (Leguminosae)

PREMISE OF THE STUDY

Simple sequence repeat (SSR) or microsatellite markers have been used in a broad range of studies mostly scoring alleles on the basis of amplicon size as a proxy for the number of repeat units of an SSR motif. However, additional sources of variation within the SSR or in the flanking regions have largely remained undetected.

METHODS

In this study, we implemented a next-generation sequencing-based genotyping approach in a newly characterized set of 18 nuclear SSR markers for the carob tree, Ceratonia siliqua. Our aim was to evaluate the effect of three different methods of scoring molecular variation present within microsatellite markers on the genetic diversity and structure results.

RESULTS

The analysis of the sequences of 77 multilocus genotypes from four populations revealed SSR variation and additional sources of polymorphism in 87% of the loci analyzed (42 single-nucleotide polymorphisms and five insertion/deletion polymorphisms), as well as divergent paralog copies in two loci. Ignoring sequence variation under standard amplicon size genotyping resulted in incorrect identification of 69% of the alleles, with important effects on the genetic diversity and structure estimates.

DISCUSSION

Next-generation sequencing allows the detection and scoring of SSRs, single-nucleotide polymorphisms, and insertion/deletion polymorphisms to increase the resolution of population genetic studies.

Micromorphological investigation of leaf epidermis and seeds of Vitaceae from Pakistan using light microscopy and scanning electron microscopy

This present study is the first report on the detailed foliar epidermal anatomy and micromorphology of seeds of five species of Vitaceae from Pakistan using light microscopy (LM) and scanning electron microscopy (SEM). The studied species occur at various localities particularly higher altitude. Qualitative and quantitative leaf micromorphological characters investigated here are related to shape and size of epidermal cells, anticlinal wall pattern, stomatal pore, guard cell, stomatal complex, subsidiary cells, and trichomes. Variations in size and shape were observed in the studied species. Irregular, polygonal and elongated epidermal cells with straight or undulate margins, stomata anomocytic type, and trichomes nonglandular, unicellular to multicellular in all the studied plants. Seed micromorphological qualitative characters studied are shape of margins, integument cell, apical notch, chalaza, anticlinal wall thickness, and quantitative characters are number of seeds per berry, length, and width of seed, length to width ratio. The detailed distribution and microscopic characteristics investigated of family Vitaceae has a distinguished role in identification at genus and species level and may provide evidence in the determination of taxonomic rank of the family in the phylogenetic tree. This study possesses potential for plant taxonomists to further evaluate the species for phytochemical studies and physiology. RESEARCH HIGHLIGHTS: Light microscopy and scanning electron microscopy was used for the micromorphological investigation of family Vitaceae Qualitative and quantitative characteristics were studied Variation in microscopic features of leaf epidermis and seeds Identification of species based on micromorphological characters.

Robust Phylogeny of Tetrastigma (Vitaceae) Based on Ten Plastid DNA Regions: Implications for Infrageneric Classification and Seed Character Evolution

Tetrastigma (Miq.) Planch. is one of the most species-rich genera of the economically and agronomically important grape family Vitaceae. It includes ca. 95 species widely distributed in the tropics and subtropics of Asia and Australia. Species of Tetrastigma exhibit great diversity in both vegetative and reproductive characters. Here we inferred a well-supported phylogeny of Tetrastigma based on ten chloroplast DNA regions with an expanded taxon sampling of 72 species and two varieties. Our molecular results support six major clades within Tetrastigma and the relationships among these clades were well-resolved. We also documented seed morphology of 44 species covering the six major clades of the genus. Ancestral states of eight characters (seed shape, seed surface rumination pattern, chalaza length/width ratio, chalaza position, ventral infold position, ventral infold divergence, ventral infold depth in cross section, and endosperm shape) were reconstructed in Mesquite and R with four models. Character optimizations suggest that all character states have evolved multiple times except that the irregular-shaped surface rumination has derived only once in Tetrastigma. We evaluated the taxonomic importance of seed morphology and identified potential morphological evidence to support each major clade. Our comprehensive analyses of Tetrastigma shed insights into the infrageneric classification of this morphologically diverse and ecologically important genus in tropical and subtropical Asia.

Are microsatellite fragment lengths useful for population-level studies? The case of Polygala lewtonii (Polygalaceae)

PREMISE OF THE STUDY

Microsatellites, despite being commonly used population-level markers, contain biases because scoring relies solely on fragment length. Their complexity can lead to homoplasy, the effects of which are poorly understood. We investigated the impact of using fragment lengths, repeats, or flanking region sequences on common population-level analyses.

METHODS

Five polymorphic microsatellite markers amplified across the central Florida scrub endemic Polygala lewtonii (Polygalaceae) and its close, widespread congener P. polygama. We genotyped 147 individuals of P. lewtonii and 156 of P. polygama, and sequenced the amplicons of four markers across all individuals. We ran basic statistics, spatial clustering analysis, historical demographics, and migration tests.

RESULTS

One population of intermediate morphology was genetically clearly identified as P. polygama, making it the southernmost population of this species. Statistics were comparable between the fragment length and repeat numbers, with some notable differences. Flanking regions exhibited surprisingly high polymorphism between species, and between geographically distant conspecific populations.

DISCUSSION

The increasing use of markers developed in one species and amplified in another is only a good practice if precautions are taken, notably the sequencing of the fragments between species and populations. Flanking region sequences are a useful marker at the interspecific level.

Control of viruses infecting grapevine

Grapevine is a high value vegetatively propagated fruit crop that suffers from numerous viruses, including some that seriously affect the profitability of vineyards. Nowadays, 64 viruses belonging to different genera and families have been reported in grapevines and new virus species will likely be described in the future. Three viral diseases namely leafroll, rugose wood, and infectious degeneration are of major economic importance worldwide. The viruses associated with these diseases are transmitted by mealybugs, scale and soft scale insects, or dagger nematodes. Here, we review control measures of the major grapevine viral diseases. More specifically, emphasis is laid on (i) approaches for the production of clean stocks and propagative material through effective sanitation, robust diagnosis, as well as local and regional certification efforts, (ii) the management of vectors of viruses using cultural, biological, and chemical methods, and (iii) the production of resistant grapevines mainly through the application of genetic engineering. The benefits and limitations of the different control measures are discussed with regard to accomplishments and future research directions.

Phylogenetic relationships of species of genus Arachis based on genic sequences

The genus Arachis (Fabaceae), which originated in South America, consists of 80 species. Based on morphological traits and cross-compatibility among the species, the genus is divided into nine taxonomic sections. Arachis is the largest section including the economically valuable cultivated peanut (A. hypogaea). Seven genic sequences were utilized to better understand the phylogenetic relationships between species of genus Arachis. Our study displayed four clades of species of Arachis. Arachis triseminata was genetically isolated from all other species of Arachis studied, and it formed the basal clade with A. retusa and A. dardani from the most ancient sections Extranervosae and Heteranthae, respectively. Species of section Arachis formed a separated single clade from all other species, within which species having B and D genome clustered in one subgroup and three species characterized with an A genome grouped together in another subgroup. A divergent clade including species from five sections was sister to the clade of section Arachis. Between the sister clades and the basal clade there was a clade containing species from the more advanced sections. Phylogenetic relationships of all the species of Arachis using multiple genic sequences were similar to the phylogenies produced with single-copy genes.

A phylogenetic analysis of the grape genus (Vitis L.) reveals broad reticulation and concurrent diversification during neogene and quaternary climate change

BACKGROUND

Grapes are one of the most economically important fruit crops. There are about 60 species in the genus Vitis. The phylogenetic relationships among these species are of keen interest for the conservation and use of this germplasm. We selected 309 accessions from 48 Vitis species,varieties, and outgroups, examined ~11 kb (~3.4 Mb total) of aligned nuclear DNA sequences from 27 unlinked genes in a phylogenetic context, and estimated divergence times based on fossil calibrations.

RESULTS

Vitis formed a strongly supported clade. There was substantial support for species and less for the higher-level groupings (series). As estimated from extant taxa, the crown age of Vitis was 28 Ma and the divergence of subgenera (Vitis and Muscadinia) occurred at ~18 Ma. Higher clades in subgenus Vitis diverged 16 - 5 Ma with overlapping confidence intervals, and ongoing divergence formed extant species at 12 - 1.3 Ma. Several species had species-specific SNPs. NeighborNet analysis showed extensive reticulation at the core of subgenus Vitis representing the deeper nodes, with extensive reticulation radiating outward. Fitch Parsimony identified North America as the origin of the most recent common ancestor of extant Vitis species.

CONCLUSIONS

Phylogenetic patterns suggested origination of the genus in North America, fragmentation of an ancestral range during the Miocene, formation of extant species in the late Miocene-Pleistocene, and differentiation of species in the context of Pliocene-Quaternary tectonic and climatic change. Nuclear SNPs effectively resolved relationships at and below the species level in grapes and rectified several misclassifications of accessions in the repositories. Our results challenge current higher-level classifications, reveal the abundance of genetic diversity in the genus that is potentially available for crop improvement, and provide a valuable resource for species delineation, germplasm conservation and use.

Genetic diversity and population structure assessed by SSR and SNP markers in a large germplasm collection of grape

BACKGROUND

The economic importance of grapevine has driven significant efforts in genomics to accelerate the exploitation of Vitis resources for development of new cultivars. However, although a large number of clonally propagated accessions are maintained in grape germplasm collections worldwide, their use for crop improvement is limited by the scarcity of information on genetic diversity, population structure and proper phenotypic assessment. The identification of representative and manageable subset of accessions would facilitate access to the diversity available in large collections. A genome-wide germplasm characterization using molecular markers can offer reliable tools for adjusting the quality and representativeness of such core samples.

RESULTS

We investigated patterns of molecular diversity at 22 common microsatellite loci and 384 single nucleotide polymorphisms (SNPs) in 2273 accessions of domesticated grapevine V. vinifera ssp. sativa, its wild relative V. vinifera ssp. sylvestris, interspecific hybrid cultivars and rootstocks. Despite the large number of putative duplicates and extensive clonal relationships among the accessions, we observed high level of genetic variation. In the total germplasm collection the average genetic diversity, as quantified by the expected heterozygosity, was higher for SSR loci (0.81) than for SNPs (0.34). The analysis of the genetic structure in the grape germplasm collection revealed several levels of stratification. The primary division was between accessions of V. vinifera and non-vinifera, followed by the distinction between wild and domesticated grapevine. Intra-specific subgroups were detected within cultivated grapevine representing different eco-geographic groups. The comparison of a phenological core collection and genetic core collections showed that the latter retained more genetic diversity, while maintaining a similar phenotypic variability.

CONCLUSIONS

The comprehensive molecular characterization of our grape germplasm collection contributes to the knowledge about levels and distribution of genetic diversity in the existing resources of Vitis and provides insights into genetic subdivision within the European germplasm. Genotypic and phenotypic information compared in this study may efficiently guide further exploration of this diversity for facilitating its practical use.

RAPD and microsatellite transferability studies in selected species of Prosopis (section Algarobia) with emphasis on Prosopis juliflora and P. pallida

The genus Prosopis (Leguminosae, Mimosoideae), comprises 44 species widely distributed in arid and semi-arid zones. Prosopis pallida (Humb. and Bonpl. ex Willd.) Kunth and P. juliflora (Sw.) DC. are the two species that are truly tropical apart from P. africana, which is native to tropical Africa (Pasiecznik et al. 2004), and they have been introduced widely beyond their native ranges. However, taxonomic confusion within the genus has hampered exploitation and better management of the species. The present study focusses primarily on evaluating the genetic relationship between Prosopis species from the section Algarobia, containing most species of economic importance, though P. tamarugo from section Strombocarpa is also included for comparison. In total, 12 Prosopis species and a putative P. pallida x P. chilensis hybrid were assessed for their genetic relationships based on RAPD markers and microsatellite transferability. The results show that P. pallida and P. juliflora are not closely related despite some morphological similarity. Evidence also agrees with previous studies which suggest that the grouping of series in section Algarobia is artificial.

Transferability and utility of white oat (Avena sativa) microsatellite markers for genetic studies in black oat (Avena strigosa)

Preservation and use of wild oat species germplasm are essential for further improvement of cultivated oats. We analyzed the transferability and utility of cultivated (white) oat Avena sativa (AACCDD genome) microsatellite markers for genetic studies of black oat A. strigosa (A(s)A(s) genome) genotypes. The DNA of each black oat genotype was extracted from young leaves and amplified by PCR using 24 microsatellite primers developed from white oat. The PCR products were separated on 3% agarose gel. Eighteen microsatellite primer pairs amplified consistent products and 15 of these were polymorphic in A. strigosa, demonstrating a high degree of transferability. Microsatellite primer pairs AM3, AM4, AM21, AM23, AM30, and AM35 consistently amplified alleles only in A. sativa, which indicates that they are putative loci for either the C or D genomes of Avena. Using the data generated by the 15 polymorphic primer pairs, it was possible to separate 40 genotypes of the 44 that we studied. The four genotypes that could not be separated are probably replicates. We conclude that A. sativa microsatellites have a high transferability index and are a valuable resource for genetic studies and characterization of A. strigosa genotypes.

Sequence Analysis of SSR-Flanking Regions Identifies Genome Affinities between Pasture Grass Fungal Endophyte Taxa

Fungal species of the Neotyphodium and Epichloë genera are endophytes of pasture grasses showing complex differences of life-cycle and genetic architecture. Simple sequence repeat (SSR) markers have been developed from endophyte-derived expressed sequence tag (EST) collections. Although SSR array size polymorphisms are appropriate for phenetic analysis to distinguish between taxa, the capacity to resolve phylogenetic relationships is limited by both homoplasy and heteroploidy effects. In contrast, nonrepetitive sequence regions that flank SSRs have been effectively implemented in this study to demonstrate a common evolutionary origin of grass fungal endophytes. Consistent patterns of relationships between specific taxa were apparent across multiple target loci, confirming previous studies of genome evolution based on variation of individual genes. Evidence was obtained for the definition of endophyte taxa not only through genomic affinities but also by relative gene content. Results were compatible with the current view that some asexual Neotyphodium species arose following interspecific hybridisation between sexual Epichloë ancestors. Phylogenetic analysis of SSR-flanking regions, in combination with the results of previous studies with other EST-derived SSR markers, further permitted characterisation of Neotyphodium isolates that could not be assigned to known taxa on the basis of morphological characteristics.

Analysis of expressed sequence tags from Prunus mume flower and fruit and development of simple sequence repeat markers

Background

Expressed Sequence Tag (EST) has been a cost-effective tool in molecular biology and represents an abundant valuable resource for genome annotation, gene expression, and comparative genomics in plants.

Results

In this study, we constructed a cDNA library of Prunus mume flower and fruit, sequenced 10,123 clones of the library, and obtained 8,656 expressed sequence tag (EST) sequences with high quality. The ESTs were assembled into 4,473 unigenes composed of 1,492 contigs and 2,981 singletons and that have been deposited in NCBI (accession IDs: GW868575 - GW873047), among which 1,294 unique ESTs were with known or putative functions. Furthermore, we found 1,233 putative simple sequence repeats (SSRs) in the P. mume unigene dataset. We randomly tested 42 pairs of PCR primers flanking potential SSRs, and 14 pairs were identified as true-to-type SSR loci and could amplify polymorphic bands from 20 individual plants of P. mume. We further used the 14 EST-SSR primer pairs to test the transferability on peach and plum. The result showed that nearly 89% of the primer pairs produced target PCR bands in the two species. A high level of marker polymorphism was observed in the plum species (65%) and low in the peach (46%), and the clustering analysis of the three species indicated that these SSR markers were useful in the evaluation of genetic relationships and diversity between and within the Prunus species.

Conclusions

We have constructed the first cDNA library of P. mume flower and fruit, and our data provide sets of molecular biology resources for P. mume and other Prunus species. These resources will be useful for further study such as genome annotation, new gene discovery, gene functional analysis, molecular breeding, evolution and comparative genomics between Prunus species.

Evolution and history of grapevine (Vitis vinifera) under domestication: new morphometric perspectives to understand seed domestication syndrome and reveal origins of ancient European cultivars

BACKGROUND AND AIMS

In spite of the abundance of archaeological, bio-archaeological, historical and genetic data, the origins, historical biogeography, identity of ancient grapevine cultivars and mechanisms of domestication are still largely unknown. Here, analysis of variation in seed morphology aims to provide accurate criteria for the discrimination between wild grapes and modern cultivars and to understand changes in functional traits in relation to the domestication process. This approach is also used to quantify the phenotypic diversity in the wild and cultivated compartments and to provide a starting point for comparing well-preserved archaeological material, in order to elucidate the history of grapevine varieties.

METHODS

Geometrical analysis (elliptic Fourier transform method) was applied to grapevine seed outlines from modern wild individuals, cultivars and well-preserved archaeological material from southern France, dating back to the first to second centuries.

KEY RESULTS AND CONCLUSIONS

Significant relationships between seed shape and taxonomic status, geographical origin (country or region) of accessions and parentage of varieties are highlighted, as previously noted based on genetic approaches. The combination of the analysis of modern reference material and well-preserved archaeological seeds provides original data about the history of ancient cultivated forms, some of them morphologically close to the current 'Clairette' and 'Mondeuse blanche' cultivars. Archaeobiological records seem to confirm the complexity of human contact, exchanges and migrations which spread grapevine cultivation in Europe and in Mediterranean areas, and argue in favour of the existence of local domestication in the Languedoc (southern France) region during Antiquity.

Genetic diversity analysis in the section Caulorrhizae (genus Arachis) using microsatellite markers

Diversity in 26 microsatellite loci from section Caulorrhizae germplasm was evaluated by using 33 accessions of A. pintoi Krapov. & W.C. Gregory and ten accessions of Arachis repens Handro. Twenty loci proved to be polymorphic and a total of 196 alleles were detected with an average of 9.8 alleles per locus. The variability found in those loci was greater than the variability found using morphological characters, seed storage proteins and RAPD markers previously used in this germplasm. The high potential of these markers to detect species-specific alleles and discriminate among accessions was demonstrated. The set of microsatellite primer pairs developed by our group for A. pintoi are useful molecular tools for evaluating Section Caulorrhizae germplasm, as well as that of species belonging to other Arachis sections.

Characterization of new polymorphic functional markers for sugarcane

Expressed sequence tags (ESTs) offer the opportunity to exploit single, low-copy, conserved sequence motifs for the development of simple sequence repeats (SSRs). The authors have examined the Sugarcane Expressed Sequence Tag database for the presence of SSRs. To test the utility of EST-derived SSR markers, a total of 342 EST–SSRs, which represent a subset of over 2005 SSR-containing sequences that were located in the sugarcane EST database, could be designed from the nonredundant SSR-positive ESTs for possible use as potential genic markers. These EST–SSR markers were used to screen 18 sugarcane ( Saccharum spp.) varieties. A high proportion (65.5%) of the above EST–SSRs, which gave amplified fragments of foreseen size, detected polymorphism. The number of alleles ranged from 2 to 24 with an average of 7.55 alleles per locus, while polymorphism information content values ranged from 0.16 to 0.94, with an average of 0.73. The ability of each set of EST–SSR markers to discriminate between varieties was generally higher than the polymorphism information content analysis. When tested for functionality, 82.1% of these 224 EST–SSRs were found to be functional, showing homology to known genes. As the EST–SSRs are within the expressed portion of the genome, they are likely to be associated to a particular gene of interest, improving their utility for genetic mapping; identification of quantitative trait loci, and comparative genomics studies of sugarcane. The development of new EST–SSR markers will have important implications for the genetic analysis and exploitation of the genetic resources of sugarcane and related species and will provide a more direct estimate of functional diversity.

Bin mapping of genomic and EST-derived SSRs in melon (Cucumis melo L.)

We report the development of 158 primer pairs flanking SSR motifs in genomic (gSSR) and EST (EST-SSR) melon sequences, all yielding polymorphic bands in melon germplasm, except one that was polymorphic only in Cucurbita species. A similar polymorphism level was found among EST-SSRs and gSSRs, between dimeric and trimeric EST-SSRs, and between EST-SSRs placed in the open reading frame or any of the 5'- or 3'-untranslated regions. Correlation between SSR length and polymorphism was only found for dinucleotide EST-SSRs located within the untranslated regions, but not for trinucleotide EST-SSRs. Transferability of EST-SSRs to Cucurbita species was assayed and 12.7% of the primer pairs amplified at least in one species, although only 5.4% were polymorphic. A set of 14 double haploid lines from the cross between the cultivar "Piel de Sapo" and the accession PI161375 were selected for the bin mapping approach in melon. One hundred and twenty-one SSR markers were newly mapped. The position of 46 SSR loci was also verified by genotyping the complete population. A final bin-map was constructed including 80 RFLPs, 212 SSRs, 3 SNPs and the Nsv locus, distributed in 122 bins with an average bin length of 10.2 cM and a maximum bin length of 33 cM. Map density was 4.2 cM/marker or 5.9 cM/SSR.

Analysis of expressed sequence tags from Actinidia: applications of a cross species EST database for gene discovery in the areas of flavor, health, color and ripening

Background

Kiwifruit (Actinidia spp.) are a relatively new, but economically important crop grown in many different parts of the world. Commercial success is driven by the development of new cultivars with novel consumer traits including flavor, appearance, healthful components and convenience. To increase our understanding of the genetic diversity and gene-based control of these key traits in Actinidia, we have produced a collection of 132,577 expressed sequence tags (ESTs).

Results

The ESTs were derived mainly from four Actinidia species (A. chinensis, A. deliciosa, A. arguta and A. eriantha) and fell into 41,858 non redundant clusters (18,070 tentative consensus sequences and 23,788 EST singletons). Analysis of flavor and fragrance-related gene families (acyltransferases and carboxylesterases) and pathways (terpenoid biosynthesis) is presented in comparison with a chemical analysis of the compounds present in Actinidia including esters, acids, alcohols and terpenes. ESTs are identified for most genes in color pathways controlling chlorophyll degradation and carotenoid biosynthesis. In the health area, data are presented on the ESTs involved in ascorbic acid and quinic acid biosynthesis showing not only that genes for many of the steps in these pathways are represented in the database, but that genes encoding some critical steps are absent. In the convenience area, genes related to different stages of fruit softening are identified.

Conclusion

This large EST resource will allow researchers to undertake the tremendous challenge of understanding the molecular basis of genetic diversity in the Actinidia genus as well as provide an EST resource for comparative fruit genomics. The various bioinformatics analyses we have undertaken demonstrates the extent of coverage of ESTs for genes encoding different biochemical pathways in Actinidia.

Genetic diversity of cultivated and wild-type peanuts evaluated with M13-tailed SSR markers and sequencing

Thirty-one genomic SSR markers with a M13 tail attached were used to assess the genetic diversity of the peanut mini core collection. The M13-tailed method was effective in discriminating almost all the cultivated and wild accessions. A total of 477 alleles were detected with an average of 15.4 alleles per locus. The mean polymorphic information content (PIC) score was 0.687. The cultivated peanut (Arachis hypogaea L.) mini core produced a total of 312 alleles with an average of 10.1 alleles per locus. A neighbour-joining tree was constructed to determine the interspecific and intraspecific relationships in this data set. Almost all the peanut accessions in this data set classified into subspecies and botanical varieties such as subsp. hypogaea var. hypogaea, subsp. fastigiata var. fastigiata, and subsp. fastigiata var. vulgaris clustered with other accessions with the same classification, which lends further support to their current taxonomy. Alleles were sequenced from one of the SSR markers used in this study, which demonstrated that the repeat motif is conserved when transferring the marker across species borders. This study allowed the examination of the diversity and phylogenetic relationships in the peanut mini core which has not been previously reported.

Morphological and anatomical development in the Vitaceae. IX. Comparative ontogeny and phylogenetic implications of Vitis rotundifolia Michx

The grape family, Vitaceae, is an economically important fruit family. Although the Vitaceae have been used for conservation, viticulture, and horticulture, little is known about the phylogenetic relationships of this family. Recent phylogenetic reconstructions using DNA data have disagreed with traditional classifications and other DNA-based phylogenies by grouping Vitis and Cissus species within one clade and linking Vitis rotundifolia Michx. (muscadine grape) more closely with these Cissus species. This study uses comparative developmental morphology as an independent method to investigate whether V. rotundifolia is more similar to Cissus antarctica Vent. (kangaroo vine) than other Vitis species. Results obtained using epi-illumination light microscopy and histology were compared with those from similar studies of Vitis ‘Ventura’, Vitis riparia Michx., and C. antarctica. Twelve vegetative and floral characters including axillary bud dorsiventrality, uncommitted primordium shape, calyptra, and ring-shaped gynoecial disk placed V. rotundifolia in with other Vitis species. Only two characters, stipule timing and simple tendrils, were common to C. antarctica and V. rotundifolia, suggesting that V. rotundifolia is more similar to Vitis spp. than to C. antarctica, and supporting traditional classifications contrary to ITS1 and trnL DNA phylogenies.

Morphological and anatomical development in the Vitaceae. X. Comparative ontogeny and phylogenetic implications of Cissus quadrangularis L

Although the grape family, Vitaceae, is of great economic importance, there is little understanding of the relationships between and within the genera. Recent DNA-based phylogenies of the Vitaceae have disagreed with traditional classifications by grouping some Cissus species (e.g., Cissus antarctica Vent.) in a clade with Vitis species, while placing other Cissus species (e.g., Cissus quadrangularis L.) in a monophyletic clade. This study investigates Cissus phylogeny using epi-illumination light microscopy and histological ontogenetic comparisons of vegetative and reproductive characters of Vitis rotundifolia (Forssk.) Vahl, Vitis ‘Ventura’, Vitis riparia Michx., C. antarctica, and C. quadrangularis. Ten characters limited to the Cissus species (connection of stipules, precocious axillary bud initiation, prophyll growth not obviously unequal, timing of mature shoot architecture on axillary buds, lack of spiral inflorescence branching, absence of calyptra, petals closely enveloping anthers, large gynoecial disk, ovary buried in the disk, and production of copious amounts of nectar) support the traditional phylogenies: the monophyly of Vitis and the exclusion of Cissus species from Vitis. The comparison of C. antarctica (Vitis–Cissus clade) and C. quadrangularis (monophyletic Cissus clade) supports the division of Cissus into two clades based on supernumerary buds, stipule connectivity, uncommitted primordium shape, and inflorescence branching patterns.

Characterization and transferability of microsatellite markers of the cultivated peanut (Arachis hypogaea)

BACKGROUND

The genus Arachis includes Arachis hypogaea (cultivated peanut) and wild species that are used in peanut breeding or as forage. Molecular markers have been employed in several studies of this genus, but microsatellite markers have only been used in few investigations. Microsatellites are very informative and are useful to assess genetic variability, analyze mating systems and in genetic mapping. The objectives of this study were to develop A. hypogaea microsatellite loci and to evaluate the transferability of these markers to other Arachis species.

RESULTS

Thirteen loci were isolated and characterized using 16 accessions of A. hypogaea. The level of variation found in A. hypogaea using microsatellites was higher than with other markers. Cross-transferability of the markers was also high. Sequencing of the fragments amplified using the primer pair Ah11 from 17 wild Arachis species showed that almost all wild species had similar repeated sequence to the one observed in A. hypogaea. Sequence data suggested that there is no correlation between taxonomic relationship of a wild species to A. hypogaea and the number of repeats found in its microsatellite loci.

CONCLUSION

These results show that microsatellite primer pairs from A. hypogaea have multiple uses. A higher level of variation among A. hypogaea accessions can be detected using microsatellite markers in comparison to other markers, such as RFLP, RAPD and AFLP. The microsatellite primers of A. hypogaea showed a very high rate of transferability to other species of the genus. These primer pairs provide important tools to evaluate the genetic variability and to assess the mating system in Arachis species.

Allelic configuration and polysomic inheritance of highly variable microsatellites in tetraploid gynodioecious Thymus praecox agg

Polyploidy plays a pivotal role in plant evolution. However, polyploids with polysomic inheritance have hitherto been severely underrepresented in plant population genetic studies, mainly due to a lack of appropriate molecular genetic markers. Here we report the establishment and experimental validation of six fully informative microsatellite markers in tetraploid gynodioecious Thymus praecox agg. Sequence data of 150 microsatellite alleles and their flanking regions revealed high variation, which may be characteristic for polyploids with a reticulate evolutionary history. Understanding the patterns of mutation (indels and substitutions) in microsatellite flanking-sequences was a prerequisite for the development of co-dominant markers for fragment analyses. Allelic segregation patterns among progeny arrays from ten test crosses revealed tetrasomic inheritance in T. praecox agg. No evidence of frequent double reduction was detected. Polymerase chain reaction (PCR) based dosage effects allowed for precise assignment of allelic configuration at all six microsatellite loci. The quantification of allele copy numbers in PCR was verified by comparisons of observed and expected gametic allele frequencies and heterozygosities in test crosses. Our study illustrates how PCR based markers can provide reliable estimates of heterozygosity and, thus, powerful tools for breeding system and population genetic analyses in polyploid organisms.

Cross-species transferability of G. arboreum-derived EST-SSRs in the diploid species of Gossypium

Diploid species with a common Gossypium origin are highly diverse in morphology and have been classified into eight genomic groups designated A-G and K. In this study, the transferability of 207 Gossypium arboreum-derived expressed sequence tag-simple sequence repeat (EST-SSR) primer pairs was examined among 25 different diploid accessions representing 7 genomes and 23 Gossypium species. We found that 124 of the 207 (60%) primer pairs produced amplification products in all 25 accessions. The remaining 83 (40%) primer pairs produced amplification in only a subset of species, ranging from 13 to 22 species, which is consistent with some genome- and species-specific amplification. The cross-species amplification of these EST-SSRs in 22 diploid species was 96.5% in 4,554 combinations (207 SSRsx22 species), indicative of a high transferability among the Gossypium species. Furthermore, a high level of polymorphism with an average number of 6.53 alleles per SSR marker was detected. No correlation was found between the repeat motif type and cross-species amplification. DNA sequencing showed that the high-level polymorphism findings was mainly due to changes in the number of repeat motifs and that the high transferability can be attributed to a higher-level conservation in the flanking regions among these diploid Gossypium species. The transferability among these different diploid species presented here can increase the efficiency of transferring genetic information across species and further enhance their introgression into cultivated cotton species by the molecular tagging of important genes existing in these diploid species using the EST-SSR markers.

Mutational bias in penguin microsatellite DNA

Analysis of nucleotide sequence variation at a microsatellite DNA locus revealed extensive size homoplasy of alleles in Adélie penguins (Pygoscelis adeliae). Variation in the flanking regions at this locus allowed discrimination between mechanisms proposed for length changes in microsatellite DNA alleles. We further examined the structure of alleles for the same microsatellite DNA locus across 11 additional species of penguin (Spheniscidae) by mapping allele sequences onto an independent penguin phylogeny. Our analysis indicated that the repeat motifs appear to have evolved independently on several occasions. We observed sequence instability in the region bordering the repeat tract with a transversional bias predominating. We propose that this bias results from inaccurate DNA replication owing to the sequence context of this repeat tract. Because we show that regions flanking repeat sequences exhibit this mutational bias, this cautions against the use of such regions for phylogeny reconstruction.

Development of SSR markers for the phylogenetic analysis of almond trees from China and the Mediterranean region

Expressed sequence tag (EST) derived simple sequence repeats (SSRs, microsatellites) were screened and identified from 3863 almond and 10 185 peach EST sequences, and the spectra of SSRs in the non-redundant EST sequences were investigated after sequence assembly. One hundred seventy-eight (12.07%) almond SSRs and 497 (9.97%) peach SSRs were detected. The EST-SSR occurs every 4.97 kb in almond ESTs and 6.57 kb in peach, and SSRs with di- and trinucleotide repeat motifs are the most abundant in both almond and peach ESTs. Twenty one EST-SSRs were thereafter, developed and used together with 7 genomic SSRs, to study the genetic relationship among 36 almond (P. communis Fritsch.) cultivars from China and the Mediterranean area, as well as 8 accessions of other related species from the genus Prunus. Both EST-derived and genomic SSR markers showed high cross-species transferability in the genus. Out of the 112 polymorphic alleles detected in the 36 cultivated almonds, 28 are specific to Chinese cultivars and 25 to the others. The 44 accessions were clustered into 4 groups in the phylogenetic tree and the 36 almond cultivars formed two distinct subgroups, one containing only Chinese cultivars and one of unknown origin and the other only those originating from the Mediterranean area, indicating that Chinese almond cultivars have a distinct evolutionary history from the Mediterranean almond. Our preliminary results indicated that common almond was more closely related to peach (P. persica (L.) Batsch.) than to the four wild species of almond, (P. mongolica Maxim., P. ledebouriana Schleche, P. tangutica Batal., and P. triloba Lindl.). The implications of these SSR markers for evolutionary analysis and molecular mapping of Prunus species are discussed.

Genic microsatellite markers in plants: features and applications

Expressed sequence tag (EST) projects have generated a vast amount of publicly available sequence data from plant species; these data can be mined for simple sequence repeats (SSRs). These SSRs are useful as molecular markers because their development is inexpensive, they represent transcribed genes and a putative function can often be deduced by a homology search. Because they are derived from transcripts, they are useful for assaying the functional diversity in natural populations or germplasm collections. These markers are valuable because of their higher level of transferability to related species, and they can often be used as anchor markers for comparative mapping and evolutionary studies. They have been developed and mapped in several crop species and could prove useful for marker-assisted selection, especially when the markers reside in the genes responsible for a phenotypic trait. Applications and potential uses of EST-SSRs in plant genetics and breeding are discussed.

Nonrandom distribution and frequencies of genomic and EST-derived microsatellite markers in rice, wheat, and barley

Background

Earlier comparative maps between the genomes of rice (Oryza sativa L.), barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) were linkage maps based on cDNA-RFLP markers. The low number of polymorphic RFLP markers has limited the development of dense genetic maps in wheat and the number of available anchor points in comparative maps. Higher density comparative maps using PCR-based anchor markers are necessary to better estimate the conservation of colinearity among cereal genomes. The purposes of this study were to characterize the proportion of transcribed DNA sequences containing simple sequence repeats (SSR or microsatellites) by length and motif for wheat, barley and rice and to determine in-silico rice genome locations for primer sets developed for wheat and barley Expressed Sequence Tags.

Results

The proportions of SSR types (di-, tri-, tetra-, and penta-nucleotide repeats) and motifs varied with the length of the SSRs within and among the three species, with trinucleotide SSRs being the most frequent. Distributions of genomic microsatellites (gSSRs), EST-derived microsatellites (EST-SSRs), and transcribed regions in the contiguous sequence of rice chromosome 1 were highly correlated. More than 13,000 primer pairs were developed for use by the cereal research community as potential markers in wheat, barley and rice.

Conclusion

Trinucleotide SSRs were the most common type in each of the species; however, the relative proportions of SSR types and motifs differed among rice, wheat, and barley. Genomic microsatellites were found to be primarily located in gene-rich regions of the rice genome. Microsatellite markers derived from the use of non-redundant EST-SSRs are an economic and efficient alternative to RFLP for comparative mapping in cereals.

Molecular markers from the transcribed/expressed region of the genome in higher plants

In recent years, molecular marker technology in higher plants has witnessed a shift from the so-called random DNA markers (RDMs), developed in the past arbitrarily from genomic DNA and cDNA, to the molecular markers representing the transcriptome and the other coding sequences. These markers have been described as gene targeted markers (GTMs). Another specific class of markers includes the so-called functional markers (FMs), which are supposed to have a cause and effect relationship with the traits of interest. In this review, we first describe the development of these markers representing the transcriptome or genes per se; we then discuss the uses of these markers in some detail and finally add a note on the future directions of research and the implications of the wider application of these markers in crop improvement programmes. Using suitable examples, we describe markers of different classes derived from cDNA clones, expressed sequence tags (ESTs), gene sequences and the unique (coding) sequences obtained through methyl filtration or genome normalization (high C(0) t fraction) from gDNA libraries. While we briefly describe RFLPs, SSRs, AFLPs and SNPs developed from the transcriptome (cDNA clones and EST databases), we have discussed in more detail some of the novel markers developed from the transcriptome and specific genes. These novel markers include expressed sequence tag polymorphisms (ESTPs), conserved orthologue set (COS) markers, amplified consensus genetic markers (ACGMs), gene specific tags (GSTs), resistance gene analogues (RGAs) and exon-retrotransposon amplification polymorphism (ERAP). Uses of these markers have been discussed in some detail under the following headings: development of transcript and functional maps, estimations of genetic diversity, marker-assisted selection (MAS), candidate-gene (CG) approach and map-based cloning, genetical genomics and identification of eQTLs, study of genome organization and taxonomic and phylogenetic studies. At the end, we also append a list of websites relevant to further studies on the transcriptome. For want of space, considerable information including voluminous data in the form of 12 tables, and a long list of references cited in these tables, has been placed on the Internet as electronic supplementary material (ESM), which the readers may find useful.

Comparative mapping between quercus and castanea using simple-sequence repeats (SSRs)

Simple sequence repeat (SSR) markers from Quercus and Castanea were used for comparative mapping between Quercus robur (L.) and Castanea sativa (Mill.). We tested the transferability of SSRs developed in Quercus to Castanea and vice-versa. In total, 47% (25) of the Quercus SSRs and 63% (19) of the Castanea SSRs showed a strong amplification product in the non-source species. From these 44 putative comparative anchor tags, 19 (15 from Quercus and 4 from Castanea) were integrated in two previously established genetic linkage maps for the two genera. SSR loci were sequenced to confirm the orthology of the markers. The combined information from both genetic mapping and sequence analysis were used to determine the homeology between seven linkage groups, aligned on the basis of pairs or triplets of common markers, while two additional groups were matched using a single microsatellite marker. Orthologous loci identified between Q. robur and C. sativa will be useful as anchor loci for comparative mapping studies within the Fagaceae family.

Fine-scale diversity and specificity in the most prevalent lineage of symbiotic dinoflagellates (Symbiodinium, Dinophyceae) of the Caribbean

The success of coral reefs is due to obligate mutualistic symbioses involving invertebrates and photosynthetic dinoflagellate symbionts belonging to the genus Symbiodinium. In the Caribbean, the vast majority of octocorals and other invertebrate hosts associate with Symbiodinium clade B, and more selectively, with a single lineage of this clade, Symbiodinium B1/B184. Although B1/B184 represents the most prevalent Symbiodinium in the Caribbean, there is little evidence supporting fine-scale diversity and host-alga specificity within this lineage. We explored simultaneously the questions of diversity and specificity in Symbiodinium B1/B184 by sequencing the flanking regions of two polymorphic microsatellites from a series of Symbiodinium clade B cultures along with Symbiodinium B1/B184 populations of the octocorals Pseudopterogorgia elisabethae, P. bipinnata and Gorgonia ventalina. Seven unique sequence variants were identified based on concatenation of the two loci. Phylogenetic analyses of these variants, which we refer to as phylotypes, recognized five as belonging to B1/B184, thus providing the first evidence of distinct taxa within this Symbiodinium lineage. Furthermore, sympatric P. elisabethae and P. bipinnata at San Salvador in the Bahamas were found to harbour distinct Symbiodinium B1/B184 phylotypes, demonstrating unequivocally the existence of fine-scale specificity between Caribbean octocorals and these algae. Taken together, this study exemplifies the complex nature of Symbiodinium biodiversity and specificity.

Molecular linkage maps of Vitis vinifera L. and Vitis riparia Mchx

Two linkage maps for grape (Vitis spp.) have been developed based on 81 F(1) plants derived from an interspecific cross between the wine cultivar Moscato bianco (Vitis vinifera L.) and a Vitis riparia Mchx. accession, a donor of pathogen resistance traits. The double pseudotest-cross mapping strategy was applied using three types of molecular markers. The efficiency of SSRs to anchor homologous linkage groups from different Vitis maps and the usefulness of AFLPs in saturating molecular linkage maps were evaluated. Moreover, the SSCP technique was developed based on sequence information in public databases concerning genes involved in flavonoid and stilbene biosynthesis. For the maternal genetic map a total of 338 markers were assembled in 20 linkage groups covering 1,639 cM, whereas 429 loci defined the 19 linkage groups of the paternal map which covers 1,518 cM. The identification of 14 linkage groups common to both maps was possible based on 21 SSR and 19 AFLP loci. The position of SSR loci in the maps presented here was consistent with other published mapping experiments in Vitis.

Development and transferability of apricot and grape EST microsatellite markers across taxa

EST microsatellite markers were developed in apricot (Prunus armeniaca L.) and grape (Vitis vinifera L.). cDNA libraries from either apricot leaves or grape roots were used in an enrichment procedure for GA and CA repeats. The transferability of EST simple sequence repeat (SSR) markers from apricot and grapevine to other related and unrelated species was examined. Overall, grape primers amplified products in most of the Vitaceae accessions while the apricot primers amplified polymorphic alleles only in closely related species of the Rosaceae. In this taxonomic family, ten EST SSR loci were tested, and one single primer pair, PacB22, was amplified across species and sections in the Prunoideae and Maloideae. Sequencing of EST SSR loci in other species and genera confirmed a higher level of conservation in the microsatellite motif and flanking regions in the Vitaceae compared to the Rosaceae. Two distinct fragments of the PacB22 locus amplified across the Malus and Pyrus genera; however, while the coding region was highly conserved, the microsatellite repeat motif was no longer present. The banding pattern was explained by base substitution and insertion/deletion events in the intronic region of PacB22. This study includes the determination of the degree of polymorphism detected among species and genera in two unrelated taxonomic families and the evaluation of the information provided by the microsatellite repeats and the flanking regions.