Families of clustered microsatellites in a conifer genome

Microsatellite clustering may account for genetic maps which do not coalesce into the expected number of linkage groups. Microsatellite organization within the large genome of Pinus taeda (1C = 20,000 Mb) was determined by (1) testing whether repeat motifs were sequestered within the low-copy DNA kinetic component and (2) testing for repeat motif clusters within DNA fragments regardless of copy number. Within the low-copy kinetic component, either (AC)n or (AG)n repeat units were present in 32% of sequences. No repeat motifs were found in the total genome control. Clustered repeat motifs were frequent; the (ATG)n triplet repeat motif was located upstream from a CG-rich trinucleotide microsatellite in 26 out of 44 microsatellite sequences. Fourteen of the clustered (ATG)n sequences could be assembled into four microsatellite sequence families based on similarities in the flanking regions. Consistent with the DNA turnover model, family members shared similar flanking regions but differed in repeat motif composition and length.     

Low-copy microsatellite recovery from a conifer genome

Microsatellite development has been stymied by highly repetitive DNA in the large, highly duplicated conifer genome and by so few genomic conifer sequences in public databases. Recovery of microsatellites from the low-copy component was tested as an efficient approach to marker development. Microsatellites were isolated from Pinus taeda L. via low-copy enrichment and filter-hybridization of tri- and tetra-nucleotide repeat motifs. Efficiency at three phases of marker development was compared for low-copy and total-genome control libraries. In the first phase, enrichment for microsatellites was slightly lower in the low-copy libraries. In the second phase, redundancy was higher in the low-copy libraries. In the third phase, low-copy libraries provided more polymorphic markers than total-genome libraries. Of 418 sequenced low-copy clones, 102 were unique sequences with repeat motifs. Of these unique sequences, twice as many were useful for marker development compared to the total-genome control. Difficulty in microsatellite marker development due to highly repetitive DNA can be abated by low-copy enrichment or circumvented by selecting for specific CG-rich trinucleotide repeat motifs. Sixteen new low-copy and genomic P. taeda microsatellites were given as an example. Received: 19 April 2000 / Accepted: 27 February 2001     

Buccal swabbing as a source of DNA from squamate reptiles

Buccal swabbing was compared with other tissues as a source of DNA for microsatellite genotyping from two squamate reptiles. For both species, the lizard Lacerta agilis and the snake Coronella austriaca, buccal swabbing proved more reliable than tissues including tail tips, toe clips and ventral scale clips.     

Rapid isolation of CA microsatellites from the tilapia genome

We have developed (CA)n microsatellite markers for the cichlid fish, Oreochromis niloticus using a variation of the hybrid capture method. The resulting genomic library was highly enriched in repetitive DNA with 96% of clones containing CA repeats. The number of repeats ranged from four to 45 with an average of 19. Two-thirds of the sequenced clones had 12 or more repeats and sufficient flanking sequence to design primers. The resulting markers were tested in an F2 cross of O. niloticus x O. aureus. Nearly 90% of the markers amplified in this cross and 74% of these were informative. This work demonstrates the importance of minimizing the number of polymerase chain reaction (PCR) amplification cycles before and after the enrichment steps to reduce PCR recombination and the generation of chimaeric clones.     

Isolation and characterization of microsatellites from the canine genome

Microsatellite sequences comprising (dC-dA)_n.(dG-dT)_n repeats have been isolated from canine libraries and sequenced. Oligonucleotide primers have been synthesized to the micro-satellite flanking sequences and used in the polymerase chain reaction to amplify those loci from genomic DNA. The degree of polymorphism of each microsatellite was estimated in a set of unrelated dogs. It is concluded that of the 10 loci studied, nine are sufficiently polymorphic to be useful in genetic studies.     

DNA from uncultured organisms as a source of 2,5-diketo-D-gluconic acid reductases

Total DNA of a population of uncultured organisms was extracted from soil samples, and by using PCR methods, the genes encoding two different 2,5-diketo-D-gluconic acid reductases (DKGRs) were recovered. Degenerate PCR primers based on published sequence information gave internal gene fragments homologous to known DKGRs. Nested primers specific for the internal fragments were combined with random primers to amplify flanking gene fragments from the environmental DNA, and two hypothetical full-length genes were predicted from the combined sequences. Based on these predictions, specific primers were used to amplify the two complete genes in single PCRs. These genes were cloned and expressed in Escherichia coli. The purified gene products catalyzed the reduction of 2,5-diketo-D-gluconic acid to 2-keto-L-gulonic acid. Compared to previously described DKGRs isolated from Corynebacterium spp., these environmental reductases possessed some valuable properties. Both exhibited greater than 20-fold-higher kcat/Km values than those previously determined, primarily as a result of better binding of substrate. The Km values for the two new reductases were 57 and 67 microM, versus 2 and 13 mM for the Corynebacterium enzymes. Both environmental DKGRs accepted NADH as well as NADPH as a cosubstrate; other DKGRs and most related aldo-keto reductases use only NADPH. In addition, one of the new reductases was more thermostable than known DKGRs.     

Genetic structure and diversity among maize inbred lines as inferred from DNA microsatellites

Two hundred and sixty maize inbred lines, representative of the genetic diversity among essentially all public lines of importance to temperate breeding and many important tropical and subtropical lines, were assayed for polymorphism at 94 microsatellite loci. The 2039 alleles identified served as raw data for estimating genetic structure and diversity. A model-based clustering analysis placed the inbred lines in five clusters that correspond to major breeding groups plus a set of lines showing evidence of mixed origins. A "phylogenetic" tree was constructed to further assess the genetic structure of maize inbreds, showing good agreement with the pedigree information and the cluster analysis. Tropical and subtropical inbreds possess a greater number of alleles and greater gene diversity than their temperate counterparts. The temperate Stiff Stalk lines are on average the most divergent from all other inbred groups. Comparison of diversity in equivalent samples of inbreds and open-pollinated landraces revealed that maize inbreds capture <80% of the alleles in the landraces, suggesting that landraces can provide additional genetic diversity for maize breeding. The contributions of four different segments of the landrace gene pool to each inbred group's gene pool were estimated using a novel likelihood-based model. The estimates are largely consistent with known histories of the inbreds and indicate that tropical highland germplasm is poorly represented in maize inbreds. Core sets of inbreds that capture maximal allelic richness were defined. These or similar core sets can be used for a variety of genetic applications in maize.     

Distribution of dinucleotide microsatellites in the Drosophila melanogaster genome.

Microsatellites, a special class of repetitive DNA, have become one of the most popular genetic markers. The progress of various genome projects has made it possible to study the genomic distribution of microsatellites and to evaluate the potential influence of several parameters on their genesis. We report the distribution of dinucleotide microsatellites in the genome of Drosophila melanogaster. When considering only microsatellites with five or more repeat units, the average length of dinucleotide repeats in D. melanogaster is 6.7 repeats. We tested a wide range of parameters which could potentially influence microsatellite density, and we did not detect a significant influence of recombination rate, number of exons, or total length of coding sequence. In concordance with the neutral expectation for the origin of microsatellites, a significant positive correlation between AT content and (AT/TA)n microsatellite density was detected. While this pattern may indicate that microsatellite genesis is a random process, we also found evidence for a nonrandom distribution of microsatellites. Average microsatellite density was higher on the X chromosome, but extreme heterogeneity was observed between different genomic regions. Such a clumping of microsatellites was also evident on a more local scale, as 38.9% of the contiguous sequences analyzed showed a deviation from a random distribution of microsatellites.     

Sequence analysis of trinucleotide repeat microsatellites from an enrichment library of the equine genome.

Microsatellites are useful tools for the construction of a linkage map and parentage testing of equines, but only a limited number of equine microsatellites have been elucidated. Thus, we constructed the equine genomic library enriched for DNA fragments containing (CAG)n repeats. The enriched method includes hybridization-capture of repeat regions using biotin-conjugated oligonucleotides, nucleotide substrate-biased polymerase reaction with the oligonucleotides and subsequent PCR amplification, because these procedures are useful for the cloning of less abundant trinucleotide microsatellites. Microsatellites containing (CAG)n repeats were obtained at the ratio of one per 3-4 clones, indicating an enrichment value about 10(4)-fold, resulting in less time consumption and less cost for cloning. In this study, 66 different microsatellites, (CAG)n repeats, were identified. The number of complete simple CAG repeats in our clones ranged 4-33, with an average repeat length of 8.8 units. The microsatellites were useful as sequence-tagged site (STS) markers. In addition, some clones containing (CAG)n repeats showed homology to human (CAG)n-containing genes, which have been previously mapped. These results indicate that the clones might be a useful tool for chromosome comparison between equines and humans.     

Allelic dropout from a high-quality DNA source

Allelic dropouts are an important source of genotyping error, particularly in studies using non-invasive sampling techniques. This has important implications for conservation biology, as an increasing number of studies are now using non-invasive techniques to study rare species or endangered populations. Previously, allelic dropout has typically been associated with PCR amplification of low quality/quantity template DNA. However, in this study we recorded high levels of allelic dropout (21–57%) at specific loci amplified from a high quality DNA (63.1 ± 7.8 ng/μl) source in the red fox (Vulpes vulpes). We designed a series of experiments to identify the sources of error. Whilst we were able to show that the best method to identify allelic dropout was the dilution of template DNA prior to PCR amplification, our data also showed two specific patterns: (1) allelic dropouts occurred at specific loci; (2) allelic dropouts occurred at specific pair-wise combinations of alleles. These patterns suggest that mechanisms other than low quantity template DNA are responsible for allelic dropout. Further research on the causes of these patterns in this and other studies would further our understanding of genotyping errors and would aid future studies where allelic dropout may be a serious issue.     

The microsatellites and minisatellites in the genome of Fenneropenaeus chinensis.

Through two-time sequencing randomly in Fenneropenaeus chinensis, 2,597,000 bp cumulative length random genomic sequences about occupying 1.23 per thousand of the entire genome are obtained, in which the length of the first time sequencing is 884,000 bp, by cutting the genome DNA with Sau3AI enzyme, and the second is 1,713,000 bp by breaking the genome DNA with the physical method, ultrasonic. Using tandem repeat finder (TRF) soft to analyze the sequences, 4,588 tandem repeats are found, in which the number of microsatellites (1-6 bp) is 3,888, and 700 for minisatellites ( >or= 7 bp). The cumulative length of repeats is 305,555 bp, accounting for 11.72% of total cumulative sequence length, in which the cumulative length of microsatellites is 232,979 bp, accounting for 8.97% of total sequence length, and greater than those of other organisms, such as human and mosquito, etc. The dinucleotide repeat type is dominant in which the dominant repeat class is AT. The second abundant repeat type is trinucleotide, of which the dominant repeat class is AAT. Interestingly, of all of repeat types, the repeat numbers and repeat classes of primer number repeat types, such as pentanucleotide, heptanucleotide, elevennucleotide, etc. are less than those of repeat types beside them. The phenomena may involve the genesis and the evolution of microsatellites and minisatellites.     

Organellar genome,nuclear ribosomal DNA repeat unit,and microsatellites isolated from a small-scale of 454 GS FLX sequencing on two mosses

Recent innovations in high-throughput DNA sequencing methodology (next generation sequencing technologies [NGS]) allow for the generation of large amounts of high quality data that may be particularly critical for resolving ambiguous relationships such as those resulting from rapid radiations. Application of NGS technology to bryology is limited to assembling entire nuclear or organellar genomes of selected exemplars of major lineages (e.g., classes). Here we outline how organellar genomes and the entire nuclear ribosomal DNA repeat can be obtained from minimal amounts of moss tissue via small-scale 454 GS FLX sequencing. We sampled two Funariaceae species, Funaria hygrometrica and Entosthodon obtusus, and assembled nearly complete organellar genomes and the whole nuclear ribosomal DNA repeat unit (18S-ITS1-5.8S-ITS2-26S-IGS1-5S-IGS2) for both taxa. Sequence data from these species were compared to sequences from another Funariaceae species, Physcomitrella patens, revealing low overall degrees of divergence of the organellar genomes and nrDNA genes with substitutions spread rather evenly across their length, and high divergence within the external spacers of the nrDNA repeat. Furthermore, we detected numerous microsatellites among the 454 assemblies. This study demonstrates that NGS methodology can be applied to mosses to target large genomic regions and identify microsatellites.     

Towards construction of a high resolution map of the mouse genome using PCR-analysed microsatellites.

Fifty sequences from the mouse genome database containing simple sequence repeats or microsatellites have been analysed for size variation using the polymerase chain reaction and gel electrophoresis. 88% of the sequences, most of which contain the dinucleotide repeat, CA/GT, showed size variations between different inbred strains of mice and the wild mouse, Mus spretus. 62% of sequences had 3 or more alleles. GA/CT and AT/TA-containing sequences were also variable. About half of these size variants were detectable by agarose gel electrophoresis. This simple approach is extremely useful in linkage and genome mapping studies and will facilitate construction of high resolution maps of both the mouse and human genomes.     

Hypervariability of simple sequences as a general source for polymorphic DNA markers.

Short simple sequence stretches occur as highly repetitive elements in all eukaryotic genomes and partially also in prokaryotes and eubacteria. They are thought to arise by slippage like events working on randomly occurring internally repetitive sequence stretches. This predicts that they should be generally hypervariable in length. I have used the polymerase chain reaction (PCR) process to show that several randomly chosen simple sequence loci with different nucleotide composition and from different species show extensive length polymorphisms. These simple sequence length polymorphisms (SSLP) may be usefully exploited for identity testing, population studies, linkage analysis and genome mapping.