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Fusarium oxysporum is a ubiquitous species complex of soil-borne plant pathogens comprising of many different formae speciales, each characterized by a high degree of host specificity. In the present investigation, we surveyed microsatellites in the available express sequence tags and transcript sequences of three formae speciales of F. oxysporum viz. melonis (Fom), cucumerium (Foc), and lycopersici (Fol). The relative abundance and density of microsatellites were higher in Fom when compared with Foc and Fol. Thirty microsatellite primers were designed, ten from each forma specialis, for genetic characterization of F. oxysporum isolates belonging to five formae speciales. Of the 30 primers, only 14 showed amplification. A total of 28 alleles were amplified by 14 primers with an average of two alleles per marker. Eight markers showed 100% polymorphism. The markers were found to be more polymorphic (47%) in Fol as compared to Fom and Foc; however, polymorphic information content was the maximum (0.899) in FocSSR-3. Nine polymorphic markers obtained in this study clearly demonstrate the utility of newly developed markers in establishing genetic relationships among different isolates of F. oxysporum.
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Fusarium oxysporum is an economically important soil-borne pathogen with worldwide distribution (Santos et al., 2002). The fungus causes vascular wilt in about 80 botanical species by invading epidermal tissues of the root, extends to the vascular bundles, produces mycelia and/or spores in the vessels, and ultimately results in death of the plants (Namiki et al., 1994). Individual pathogenic strain within the species has a limited host range, and strains with similar or identical host range are assigned to intraspecific groups, called forma specialis (Namiki et al., 1994).
To understand the evolutionary history and genomic constituents of the formae speciales within F. oxysporum requires knowledge of the phylogenetic relationships among isolates (Appel & Gordon, 1996). Over the past several years, genetic diversity in F. oxysporum has been examined using various genetic markers, such as isozyme profiles (Bosland & Williams, 1987), restriction fragment length polymorphisms (RFLP) in mitochondria and nuclear DNA (Jacobson & Gordon, 1990) and inter-simple sequence repeat (ISSR), (Baysal et al., 2009). Phylogenetic analyses based on DNA sequences of housekeeping genes such as the mitochondrial small subunit (mtSSU), ribosomal RNA gene, rDNA intergenic spacer (IGS) region, and translation elongation factor (TEF)-1α gene were extensively studied for genetic and evolutionary relationships within and among the formae speciales of F. oxysporum (O'Donnell et al., 1998; Lievens et al., 2009).
Microsatellites or simple sequence repeats (SSRs) are composed of tandemly repeated 1–6 bp long units (Tautz, 1989). Microsatellites markers are having a reputation of highly polymorphic, locus specific, easily transferable, and cost-effective molecular markers distributed throughout the genome (Powell et al., 1996). Together, these characteristics make microsatellite loci, one of the best markers for genetic mapping and diversity studies. These markers have been widely used for investigating genetic diversity among cultivars and genetic resources, for developing genetic maps suitable for quantitative trait locus (QTL) detection studies and marker-assisted selection programs, whereas use of these markers to study diversity and polymorphism in fungi is limited. Genetic diversity could reveal the adaptive potential of pathogenic populations, and sometimes, SSR patterns could reflect the variability up to formae speciales, which make possible to increase the resolution of existing markers to discriminate individual strain or formae speciales. The transcripts and express sequence tags (EST) of F. oxysporum are available in different databases, but any formal analysis of microsatellites within these sequences is yet to be reported.
The aims of this study were (1) to access microsatellite variability in available EST and transcripts of three formae speciales of F. oxysporum and (2) to develop EST-based microsatellite markers for genetic characterization of Fusarium isolates. To accomplish this, an in silico approach has been used to assess the frequency and distribution of SSRs in EST and transcript sequences within three formae speciales, and primers were designed and validated for polymorphism.
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The frequency of repeat motifs in the consensus EST sequences and annotated transcripts was assessed, and both perfect and compound SSRs were selected with a minimum acceptable length of 12 bp for di, tri, and tetra-nucleotide motifs (Garnica et al., 2006). Only SSRs with a minimum of three repeats were included in the analyses of penta- and hexa-nucleotide repeats. Maximum number of SSR (1679) was identified in Fol followed by Foc (313) and Fom (204). The higher number of SSRs in Fol was expected because the total size covered by transcripts sequences of Fol (21.7 Mb) was much higher than that of ESTs of Fom (1.3 Mb) and Foc (2.4 Mb). To compare the SSR count between all three formae speciales, the complete length of each set of sequences was analyzed, and thus, total relative abundance and total relative density were calculated and depicted in Table 1. It was found that relative abundance of SSRs in Fom (157) was higher than Foc (130) and Fol (77). Similarly, the relative density of SSR was also higher in Fom (2117) in comparison with Foc (1680) and Fol (1071) (Table 1; Fig. 1a and b).
Figure 1. Graphical representation of relative density (a) and relative abundance (b) of different SSR type found in ESTs and transcripts of Fusarium oxysporum.
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Table 1. Number and distribution of SSRs in consensus ESTs and transcript sequences of different formae speciales of Fusarium oxysporum
| ||Fusarium oxysporum (59.9 Mb)|
|Melonis (ESTs)||Cucumerium (ESTs)||Lycopersici (Transcripts)|
|Number of sequence examined||2641||6448||17708|
|Total size covered by examined sequences (Mb)||1.3 (2.1%)||2.4 (4.0%)||21.7 (36.2%)|
|Number of SSR identified||204||313||1758|
|Perfect SSRs||192 (95.1%)||295 (94.2%)||1679 (95.5%)|
|Compound SSRs||12 (4.9%)||18 (5.8%)||79 (4.5%)|
|Total length of SSRs (bp)||2752||4033||23239|
|Relative density (bp per Mb)||2117||1680||1071|
|Relative abundance (SSR per Mb)||157||130||77|
The maximum frequency of SSRs among all three sequence sets was of trinucleotide repeats (62.7%), while the dinucleotide repeats represented < 3% (Table 2). Tetranucleotide repeats constituted the second most frequent motif (16.7%) followed by hexanucleotide (13.11%) and pentanucleotide (4.91) repeat motifs in sequences of all three formae speciales. However, the percentage of di and pentanucleotide repeat was higher in Fom. This agrees with the results from other eukaryotes, where trinucleotide repeats are overrepresented in coding region (Garnica et al., 2006).
Table 2. Percentage, relative abundance, and relative density of SSRs in sequence sets of different formae speciales of Fusarium oxysporum
| ||Motif length||Count||Percentage||Relative abundance||Relative density|
Out of 30, a total of 14 SSR markers (six from Fom, three from Foc, and five from Fol) amplified easily scorable bands ranged from 70 to 400 bp in all the isolates. Of the 14 markers, three amplified dinucleotide repeats, ten amplified trinucleotide repeats, and only one marker were able to amplify tetranucleotide repeat. We used three indexes (percentage of polymorphic SSRs, number of alleles per locus and PIC value) to indicate SSR polymorphism level. Among all the markers, nine markers (64.3%) were polymorphic, whereas rest five markers (35.7%) were monomorphic. A total of 28 alleles were amplified by 14 markers. We detected 1–4 alleles per microsatellite locus with an average of two alleles per marker. FomSSR primers amplified 10 alleles with 1.8 allele per locus, whereas FocSSR primers detected 4.0 alleles with 1.3 alleles per locus and FolSSR primers detected 14 alleles with 2.8 alleles per locus. Maximum numbers of alleles (4) were amplified by FolSSR-7, while minimum one allele was amplified with five markers viz. FomSSR-3, FomSSR-5, FomSSR-9, FocSSR-5, and FocSSR-6. Three markers namely FomSSR-8, FolSSR-3, and FolSSR-6 amplified three alleles, while five markers namely FomSSR-2, FomSSR-6, FocSSR-3, FolSSR-2, and FolSSR-10 amplified two alleles (Table 3). Of nine polymorphic markers, eight showed 100% polymorphism and one showed 66% (FolSSR-6). On comparison of polymorphism potential of markers derived from each forma specialis, of six SSR markers from Fom and three SSR markers from Foc, only three (50%) and one (65%) markers were found polymorphic, respectively (Table 4). FolSSR markers exhibited highest percentage of polymorphism (100%), all the five markers were found polymorphic. Among the polymorphic markers, the maximum PIC value was obtained with FocSSR-5 (0.899) and minimum with FolSSR-6 (0.023), the average being 0.517.
Table 3. Details of locus, primer sequence, Tm, Motif, no. of alleles, alleles size, percentage polymorphism, and PIC value of different primers used to evaluate genetic diversity within Fusarium oxysporum isolates
|Primer Name/locus||Forward primer||Reverse primer||Tm (°C)||Motif||No. of alleles||Allele size||% Polymorphism||PIC|
Table 4. A comparison between Fom,Foc, and Fol markers to estimate the level of polymorphism revealed by them
| ||Fom markers||Foc markers||Fol markers||Total markers|
|Marker amplified||6 (60%)||3 (30%)||5 (50%)||14 (47%)|
|No. of monomorphic markers||3 (50%)||2 (66.7%)||0 (0%)||5 (35.7%)|
|No. of polymorphic markers||3 (50%)||1 (33.3%)||5 (100%)||9 (64.3%)|
|Average PIC value||0.439||0.75||0.51||0.531|
|No. of alleles amplified||10||4||14||28|
|Similarity coefficient value (Avg.)||0.66||0.66||0.57||0.63|
Diversity and cluster analysis
The similarity coefficient values between isolates ranged from 0.14 to 0.96 with a mean of 0.61 for all 276 isolate combinations used in the present study. For microsatellite markers derived from Fom, the similarity coefficient values between isolates ranged from 0.22 to 1.00 with average genetic diversity of 33.1%. Similarly, with Foc-derived SSR markers, the similarity coefficients between isolates ranged from 0.4 to 1.00 with 34.5% genetic diversity. For Fol markers, similarity coefficient value ranged from 0.2 to 1.0 with an average diversity being 42.7% (Table 4).
The highest similarity coefficient (0.96) was observed between Fom isolate Fom-1 and Fom-2, which was closely followed by Foc isolate Foc-1 and Foc-3 (0.92). The similarity was expected because both isolates belong to same forma specialis and geographical region. The most diverse (similarity coefficient value 0.12) isolates were Fol-6 and Foi-2. The dendrogram constructed based on similarity index resulted in two major clusters (Fig. 2). High bootstrap values were recorded with internodes, which indicate the robustness of the clustering. The first major cluster has been exclusively composed of Fom isolates, which is further divided into two subclusters having three Fom isolates each. The second cluster having different subclusters comprises a mix of all the formae speciales taken into this study except Fom.
Figure 2. Dendrogram showing genetic relationship among the Fusarium oxysporum isolated based on 14 microsatellites markers. Scale indicates Jaccard's coefficient of similarity.
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The knowledge of abundance and distribution of genetic variability within and among formae speciales of F. oxysporum is a prerequisite to study their genetic relationships (Bruns et al., 1991). In the present study, the relative density and relative abundance of SSRs in Fom was higher. So far, we do not have any strongly supported explanation for this. However, this discrepancy may be occurred because of transfer of lineage-specific (LS) genomic regions in F. oxysporum that include four entire chromosomes and account for a quarter of the genome (Ma et al., 2010). It has been observed from genome-wide study that the distribution of microsatellites in the genome is not random. Coding regions are mostly dominated by tri and hexa-nucleotide repeats, whereas di, tetra, and penta nucleotide repeats are often found in abundance in noncoding region (Kim et al., 2008; Levdansky et al., 2008). Differential distribution in terms of abundance of SSRs has been reported in between intronic and intergenic regions, 5′ and 3′ UTRs, and in different chromosomes and lastly, different species have different frequencies of SSR types and repeat units (Li et al., 2004; Garnica et al., 2006; Lawson & Zhang, 2006). In our study, we observed similar pattern of distribution of SSR in the coding region where tri and hexanucleotide SSRs were predominant. These tri and hexanucleotide SSRs in the coding region are translated into amino-acid repeats, which possibly contribute to the biological function of the protein (Kim et al., 2008). Dinucleotide SSRs are often found in the exonic region of F. oxysporum; however, (GT)n and (AC)n repeats were common in all the three formae speciales. Stallings et al. (1991) reported that (GT)n repeat is able to enhance the gene activity from a distance independent of its orientation. However, more effective transcription enhancement resulted from the GT repeat being closer to promoter region. Similarly, (CA)n repeat can act as a bridge to bring the promoter into close proximity with a putative repressor protein bound downstream of the (CA)n SSR (Young et al., 2000).
The maximum length of dinucleotide repeats was observed in Fom where TC is repeated 34 times, followed by tri nucleotide repeats in Fol where GGA is repeated 15 times. We found that the overall number of repeat motifs are generally low in the transcripts and cDNA sequences, which is in agreement with the earlier findings of Lim et al. (2004). They observed that shorter numbers of repeats (5–7 U) were predominated with around 90% of all motifs. The expansion of microsatellite repeats in the transcribe region of the genome has been limited because of strong evolutionary and functional constrains (Metzgar et al., 2000). It has been reported that longer repeats have high mutation rates and could, therefore, be less stable. Random mutation followed by DNA polymerase slippages is mainly responsible for short microsatellite repeats (Kruglyak et al., 2000).
High numbers of perfect repeats in long microsatellites are more likely to be polymorphic as compared to shorter one because of higher rate of DNA replication slippage. Several studies in other organisms have shown that the number of repeats is a good indicator of the level of variability (Vigouroux et al., 2002). We investigated whether the polymorphism of SSRs could be affected by any of the factors including different repeat units, SSR types, repeat numbers, and total SSR lengths. The results showed that there were no significant differences in PIC scores among these criteria. Locus FocSSR-3 with four repeats and locus FolSSR-3 with 10 repeats showed PIC value of 0.899 and 0.712, respectively, whereas locus FomSSR-2 with 15 repeats exhibited a PIC value of 0.493.
To analyze the overall pattern of polymorphism of the SSRs in the three formae speciales, we strived to select SSRs randomly from these formae speciales. The average PIC value was comparable and found relatively low for SSR markers compared with previous reports in Fusarium. Bogale et al. (2005) have developed nine SSR markers from F. oxysporum having average PIC value of 0.594. These SSR markers were evaluated on 64 isolates belonging to 21 formae speciales. Similarly, Gauthier et al. (2007) observed average PIC value of 0.756 with 15 makers developed from Fusarium graminearum. The low value of PIC in our study may be contributed to the fact that SSRs represent the coding region of genome which is generally conserved. The number of alleles per locus varied according to the origin of the marker. Markers with PIC values of > 0.50, such as FocSSR-3 (0.899), FolSSR-2 (0.554), FolSSR-3 (0.712), FolSSR-7 (0.641), and FolSSR-10 (0.609), will be highly informative for genetic studies and are extremely useful in distinguishing the polymorphism rate of the marker at specific locus. High levels of polymorphism associated with microsatellites are expected because of the unique mechanism responsible for generating microsatellite allelic diversity by replication slippage rather than by simple mutations or insertions/deletions (Tautz, 1989).
To our knowledge, this is the first attempt to extensively develop SSR markers from the coding regions of F. oxysporum. This study clearly demonstrates the utility of newly developed EST–SSR markers in establishing genetic relationships among different isolates of F. oxysporum. Although the number of useful markers was low, all the isolates could be differentiated from each other. These marker can be further utilized for addressing genetic relatedness in other species of Fusarium because EST-derived SSR markers have a reputation of being highly transferable (Datta et al., 2010).