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Amplified Fragment Length Microsatellites (AFLM) might be used to develop microsatellite markers in organisms with limited amounts of DNA applied to Arbuscular Mycorrhizal (AM) fungi

     Department of Plant Pathology, University of California at Davis, Davis, California 95616

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 

Developing microsatellite markers for organisms with limited amounts of DNA can be difficult because sequence information is needed. To overcome this problem in the arbuscular mycorrhizal (AM) fungi Glomus etunicatum and Gigaspora gigantea, global amplification of the genomes of each species was performed with linker-adaptor-PCR from single spores. Amplified fragments were enriched for microsatellite motifs with 5'-biotinylated oligonucleotides and recovered by magnetic streptavidin beads. The recovered fragments were reamplified and separated on denaturing polyacrylamide gels, and 16 selected bands were excised, cloned and sequenced. Seven microsatellite motifs were detected from six clones (efficiency rate of 43.8%). Primers were designed for all putative microsatellite loci and most were successfully amplified from three single-spore preparations and from pools of five, 10 and 20 spores after global amplification. This approach, termed amplified fragment-length micosatellites (AFLM), might aid investigations of organisms that cannot or are not readily cultured in vitro and where DNA is a limiting factor for genetic studies. However, the technique also can be used to isolate microsatellite loci in any organism.

Key words: arbuscular mycorrhizal fungi, enrichment techniques, Gigaspora gigantea, Glomus etunicatum, microsatellites, population genetics

	INTRODUCTION

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Microsatellites quickly are becoming the marker of choice to study gene and genome evolution from the individual to populations or higher-level taxa. Microsatellites are tandemly repeated simple DNA sequences, which are widely dispersed throughout the genomes of eukaryotic and prokaryotic organisms (Tautz et al 1986, Tautz 1993). Microsatellites are highly variable and most are thought to be selectively neutral, making them amendable to population genetic theory. Microsatellite variation can be used to study hybridization, population differentiation (Burg and Croxall 2001), and to indirectly assess levels of gene flow among and within populations (Scribner et al 2001). In addition, some microsatellite loci have been shown to be under selective pressures; examples include triplet repeat loci in humans that cause genetic diseases (Sutherland and Richards 1995) and highly mutable microsatellite tracts that control pathogenicity in bacteria (Moxon et al 1994). Microsatellites also can be used for intraspecific phylogenetic studies (Vander Zwan et al 2000).

AM fungi form symbiotic associations with at least 80% of terrestrial plant species (Smith and Read 1997) and are thought to have been important organisms in facilitating the evolution of plants from an aquatic environment to a terrestrial one (Pirozynski and Mallock 1975, Wilkinson 2001). AM fungi promote plant growth by increasing phosphorus uptake, protecting their host plants against soilborne pathogens, and improving soil structure (Smith and Read 1997). Despite their ecological importance, very little is known about the genetic organization or population genetic structure of AM fungi under natural conditions because they are not easily manipulated under laboratory conditions due to their obligate association with the host plant.

To study AM fungi under field conditions, individuals are isolated with trap plants or soil is sieved to collect individual spores. The large spores of AM fungi possess hundreds to thousands of individual nuclei, with enough DNA within a single spore to apply PCR-based techniques such as randomly amplified polymorphic DNA (RAPD) (Wyss and Bonfante 1993), amplified fragment length polymorphisms (AFLP) (Rosendahl and Taylor 1997), randomly amplified microsatellite (RAM) (Longato and Bonfante 1997), and M13 minisatellite-primed PCR (Zeze et al 1997). However, these techniques suffer from the effects of contaminating microorganisms on or within the spores, which makes collecting data on multiple loci problematic. Therefore, sequence specific markers, such as microsatellites, are desirable.

Microsatellite markers have not been developed for any AM fungus, to our knowledge. The objective of this work was to develop a technique to isolate and detect microsatellite loci in AM fungi from single spores, not to develop a set of specific markers for the AM taxa in this study. To develop this approach, we obtained spores from fresh pot cultures of two AM taxa as model species, one species with small spores (Glomus etunicatum) and another with large spores (Gigaspora gigantea). In our opinion, however, spores derived from monoxenic cultures (Moss and Hepper 1975, Miller-Wideman and Watrud 1985) of isolates collected from an area of interest would be needed to conduct a proper study at the population level. The technique is a modification of the approach used by Hakki and Akkaya (2000), using linker-adaptor-PCR followed by an enrichment step with 5'biotinylated oligonucleotide probes and magnetic bead sequence capture.

	MATERIALS AND METHODS

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Genomic amplification – Spores of Glomus etunicatum (UK210A) and Gigaspora gigantea (MN922A) were obtained from INVAM (West Virginia University, Morgantown, WV). The spores were isolated and washed according to INVAMs standard procedures and suspended in sterile H₂O. A modification of Rosendahl's and Taylor's (1997) AFLP protocol for Glomus spp. was applied to both species to globally amplify the genomes via linker-adapter-PCR. A single spore of each species in three replicates and a pool of five, 10 and 20 spores were placed in a chilled (4 C) microtiter plate containing 5 µL of sterile water. The spores were crushed with fine forceps, followed by the addition of 7.5 µL of sterile water. The solution was transferred to a sterile centrifuge tube and incubated at 100 C for 5 min to inhibit endogenous nuclease activity. DNA was digested with 1.25 U of Eco RI and Tru 91 (Roche, Indianapolis, IN) in the presence of BSA (4.0 µg) and incubated for 1.5 h at 37 C and 1.5 h at 65 C (total volume of 15 µL). Specific adapters were made as described by Vos et al (1995) and ligated onto the ends of the restriction fragments by adding 2.5 µL of ligation mix (5 µM of Eco RI adapter, 50 µM of Tru 9I adapter, 0.25 U of ligase (Invitrogen, Carlsbad, CA), followed by incubation at 37 C for 3 h.

The restriction-ligation mixture was diluted (1:1) with TE (10 mM Tris/HCl, pH 8.0, 0.1 mM EDTA) and 10 µL was used as template in a single + 1 selective amplification reaction using specific Eco RI and Tru 9I primers with the addition of an A and G, respectively, at the 3' end (Vos et al 1995). Each 20 µL reaction contained 1X PCR buffer, 2.5 mM MgCl₂, 2.5 mM each dNTP, 0.5 U of Taq polymerase (Invitrogen, Carlsbad, CA) and 3.75 µM of each primer. Thermocycling conditions consisted of an initial hold at 72 C and 94 C for 1 and 4 min, respectively, followed by 35 cycles of 94 C (30 s), 65 C (30 s), and 72 C (1 min) with a final hold of 72 C for 8 min. The amplification products were separated in 1.5% agarose gels and stained with ethidium bromide. All amplifications were performed in a Robocyler Gradient 96 (Strategene, La Jolla, CA). Upon successful amplification, one single-spore sample from each species was diluted 1:50 in TE and used in subsequent reactions, thus allowing a single spore to be the source of all enrichment procedures as described below. The remaining samples were diluted 1:50 in TE and used in subsequent reactions to detect microsatellite regions from the pool of amplified products.

Microsatellite enrichment – The 5'-Biotinylated tri-nucleotides (AAT)₈, (AAC)₈, (CAT)₈, (AGC)₈, and (CCG)₈ (Operon, Alameda, CA) were used in separate reactions to enrich for putative microsatellite-containing regions by adding 5 µL of +1 selective amplification products with 5 µL of each oligonucleotide (25 pmol) in 40 µL of sterile water. The solutions were mixed and heated to 95 C for 8 min, removed from the heating block and allowed to cool to room temperature. Magnetic streptavidin beads (Dynabeads M-280, Dynal Inc., Lake Success, NY) were washed in bulk three times with TE containing 2.0 M NaCl using a Dynal magnetic particle concentrator (Dynal MPC, Dynal Inc., Lake Success, NY) before being resuspended in the same buffer. Fifty µL of the bead solution (100 mg) were added to each enrichment reaction and incubated 20 min at room temperature (total volume 100 µL, final NaCl concentration of 1.0 M). Centrifuge tubes then were placed in the magnetic concentrator and the unbound solution removed with a pipette and washed three times with 100 µL of TE containing 1.0 M NaCl. Sterile H₂O (50 µL) was added to each tube, mixed and heated 8–10 min at 100 C to dissociate the streptavidin from the biotin. Each tube was placed back into the magnetic concentrator, and the unbound portion was removed and cleaned with a QIAquick PCR clean-up kit (Qiagen, Valencia, CA). The cleaned and enriched products were eluted with 50 µL of TE, and 10 µL of the solution was used as template in subsequent PCR reactions as described above.

Polyacrylamide electrophoresis and selection of putative microsatellites – After amplification, 5 µL of formamide dye (98% formamide, 10 mM EDTA pH 8.0, 0.25% bromphenol blue, 0.25% xylene cyanole) was added to each reaction and the reaction mix was denatured for 5 min at 94 C and cooled on ice. Each sample (3–4 µL) was loaded into every other well and separated on 4% acrylamide gels buffered in TBE (1.35 M Tris/HCl pH 8.0, 0.45 M boric acid, 25 mM EDTA) for about 3 h at 100W in a Bio-Rad Sequi-Gen GT Sequencing Cell (Bio-Rad Laboratories Inc., Hercules, CA). The gels then were silver-stained by the method of Bassam et al (1991) and dried overnight.

Selected bands were cut out of the gel, placed in 50 µL of sterile water and incubated at 65 C for approximately an hour or frozen overnight at -20 C before being heated. A subsequent amplification reaction was executed as above with 0.5, 2.5 and 5.0 µL of eluted DNA as template. The amplification products were separated in 1.5% agarose gels, stained with ethidium bromide, and successful PCR amplification products were cleaned with a QIAquick PCR clean-up kit and cloned with a TOPO TA cloning system (Invitrogen, Carlsbad, CA). Positive clones were sequenced at the Division of Biological Sciences sequencing facility (University of California at Davis) using primer T7.

Putative AM fungal sequences were identified first by finding the sequences of the flanking adapters. The sequences were edited and perfect microsatellite repeats were identified with the Web-based program Simple Sequence Repeat Identification Tool (SSRT) (http://ars-genome.cornell.edu/cgi-bin/rice/ssrtool.pl); compound and imperfect repeats were sought manually. Once microsatellites were found, primers were designed, flanking the repetitive regions, using the Web-based program Primer3 (http://www-genome.wi.mit.edu/cgi-bin/primer/primer3_www.cgi). The adapter sequence with up to nine additional nucleotides at the 3' ends were used as priming sites, along with a primer flanking the microsatellite on three putative loci, because the available sequence was insufficient to make two unique primers. When the adapter sequence was used as part of the primer, a control PCR amplification was done using only that primer.

	RESULTS

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Positive +1 selective amplification resulted in a similar smear of products on agarose in all three single-spore preparations and from the pools of 5, 10 and 20 spores for G. etunicatum and G. gigantea. After the enrichment and reamplification step, the differences among the enrichment probes were clear once they were separated in denaturing polyacrylamide gels, and the bands were easily located and excised for G. etunicatum (Fig. 1) and G. gigantea (data not shown). Many bands appeared to be unique for the individual probes used, and others appeared to be homologous between probes. Both classes of bands were excised from the gel, but bands of similar mobility were not cut out in parallel to check for sequence homology.

View larger version (70K): [in this window] [in a new window]    FIG. 1. Polyacrylamide gel electrophoresis of G. etunicatum amplified fragments after enrichment with the oligonucleotide probes (CCG)8, (CAT)8, (AAC)8, (AGC)8, (AAT)8, in lanes 1–5, respectively. Letters represent fragments that were excised and sequenced. L = 50 bp ladder (Invitrogen, Carlsbad, CA)

View larger version (105K): [in this window] [in a new window]    FIG. 2. Polyacrylamide gel electrophoresis of a polymorphic microsatellite locus from clone GleA, amplified from three single-spore preparations and from pools of five, 10 and 20 spores after global amplification, in lanes 1–6, respectively. L = 10 bp ladder (Invitrogen, Carlsbad, CA)

	DISCUSSION

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The efficiency of our approach for detecting microsatellites was 43.8%, which is better than more laborious techniques using biotin-labeled oligonucleotides and magnetic separation. For example, Gardner et al (1999) and Kijas et al (1994) reported efficiency rates of approximately 17% and 20%, respectively, but Gardner et al (1999) had to screen 303 positive clones and Kijas et al (1994) had to develop a genomic library before probing for microsatellite motifs. In this study, only 16 clones were sequenced and no library was constructed. However, the hybridization methods of Gardner et al (1999) and Kijas et al (1994) were such that longer repeats were sought than those in this study, which might explain the lower efficiency rate. The efficiency rate for Hakki and Akkaya (2000) was 13.3%.

Another advantage to our approach is the flexibility inherent in the technique. For example, the amount of amplified products used to investigate might be reduced by increasing the selective nucleotides on the 3' ends of the initial amplification reactions. This might be more effective because fewer fragments would be within the hybridization solution and therefore offer fewer opportunities for nonspecific binding. The size of the product analyzed is also at the discrimination of the investigator because bands are excised directly from the acrylamide gel, which also could be gel purified after reamplification and directly sequenced. It also is possible to use one or both adapter sequences, depending on the size of the fragment recovered, as part of the priming sites because all amplifications are from the pool of fragments. Moreover, traditional hybridization procedures, such as those of Gardner et al (1999) and Kijas et al (1994), also could be employed to increase the specificity of the probe to the target DNA and longer oligonucleotide probes or probes with different repeats lengths also could be used.

A potential disadvantage to our approach is the reliance on the global-amplification step because the pool of fragments is used as template in PCR. However, the restriction-ligation step and subsequent PCR amplification by traditional AFLP methods has proven to be robust and therefore should not be a problem. As an alternative, the global-amplification step could be taken by non-linker-adapter PCR techniques once the markers are found with our approach; this might be a viable option for organisms in which insufficient DNA per individual is available for linker-adapter PCR. Genomewide amplification has been demonstrated with as little as 15 pg of human DNA (Cheung and Nelson 1996) and from single sperm cells (Zang et al 1992), using degenerate oligonucleotide primed (DOP) PCR and primer extension preamplification (PEP) PCR, respectively. Another potential drawback is the possibility of null alleles due to the loss of a restriction site or sequence differences within the priming sites. This might explain why only three out of six microsatellites from GigQ were successfully reamplified from single-spore and mixed-spore preparations. However, this is also a potential problem with traditional AFLP, microsatellite and restriction fragment length polymorphism (RFLP) techniques.

The most important consideration before AFLMs are developed in AM fungi, or any other organism, is the source of DNA used to develop the markers. This ultimately will depend on the question that is addressed and is especially important for studies of AM fungi because the initial restriction-ligation step does not discriminate between target DNA and DNA from contaminants. Spores of AM fungi collected from the field or from pot cultures will be colonized inheritantly by other microorganisms (Scannerini and Bonfate 1991, Taber 1982, Bianciotto et al 2000, Redecker et al 1999), for example. The spores for this study came from pot cultures maintained by INVAM and likely were not free of contaminants. Therefore, we are not certain that the microsatellite motifs found were from the target organism. However, the objective of this work was to develop a protocol for the development of microsatellite markers for organisms with limited amounts of DNA, not to develop specific markers in the AM fungi used in this study. Monoxenic cultures would have to be developed for the taxa of interest to minimize the effects of contaminating microbes.

Microsatellite markers in AM fungi could prove to be valuable tools in the study of genome organization, such as the genetic heterogeneity in single spores. Heterogeneity within single spores of AM fungi has been tested almost exclusively with rDNA sequences. This involves extensive cloning and sequencing, which is laborious and expensive. With microsatellite variation, it is possible to determine allelic diversity in a single amplification reaction. In addition, no studies have generated unambiguous multilocus genotypic data from any AM fungus. Microsatellites therefore could be important markers to study evolutionary processes of recombination, selection, migration and drift under field conditions. The technique described here, using spores from monoxenic cultures, provides a novel way to detect and study microsatellite variation in AM fungi and should facilitate genetic studies in organisms in which the quantity of DNA is a limiting factor.

	ACKNOWLEDGMENTS

 
This project was financed by the NSF Biocomplexity program (DEB 9981711). We thank Dr. Joe Morton from INVAM for providing spores for this study.

	FOOTNOTES

 
¹ Corresponding author, gwdouhan{at}ucdavis.edu

Accepted for publication August 31, 2002.

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This Article Abstract Full Text (PDF) An erratum has been published Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Douhan, G. W. Articles by Rizzo, D. M. Search for Related Content PubMed Articles by Douhan, G. W. Articles by Rizzo, D. M. Agricola Articles by Douhan, G. W. Articles by Rizzo, D. M.