Evaluation of maize microsatellite markers for genetic diversity analysis and fingerprinting in sugarcane

Unveiling the predominance of Saccharum spontaneum alleles for resistance to orange rust in sugarcane using genome-wide association

KEY MESSAGE

Six QTLs of resistance to sugarcane orange rust were identified in modern interspecific hybrids by GWAS. For five of them, the resistance alleles originated from S. spontaneum. Altogether, they efficiently predict disease resistance. Sugarcane orange rust (SOR) is a threatening emerging disease in many sugarcane industries worldwide. Improving the genetic resistance of commercial cultivars remains the most promising solution to control this disease. In this study, an association panel of 568 modern interspecific sugarcane hybrids (Saccharum officinarum x S. spontaneum) from Réunion's breeding program was evaluated for its resistance to SOR under natural conditions of infection. Two genome-wide association studies (GWAS) were conducted between disease reactions and 183,842 single nucleotide polymorphism (SNP) markers obtained by targeted genotyping-by-sequencing. Five resistance quantitative trait loci (QTLs), named Oru1, Oru2, Oru3, Oru4 and Oru5, were identified using a single-locus GWAS (SL-GWAS). These five QTLs all originated from the species S. spontaneum. A multi-locus GWAS (ML-GWAS) uncovered an additional but less significant resistance QTL named Oru6, which originated from S. officinarum. All six QTLs had a moderate to major phenotypic effect on disease resistance. Prediction accuracy estimated with linear regression models based on each of the five QTLs identified by SL-GWAS was between 0.16-0.41. Altogether, these five QTLs provided a relatively high prediction accuracy of 0.60. In comparison, accuracies obtained with six genome-wide prediction models (i.e., GBLUP, Bayes-A, Bayes-B, Bayes-C, Bayesian Lasso and RKHS) reached only 0.65. The good prediction accuracy of disease resistance provided by the QTLs and the predominant S. spontaneum origin of their resistance alleles pave the way for effective marker-assisted breeding strategies.

A short review on sugarcane: its domestication, molecular manipulations and future perspectives

Sugarcane (Saccharum spp.) is a special crop plant that underwent anthropogenic evolution from a wild grass species to an important food, fodder, and energy crop. Unlike any other grass species which were selected for their kernels, sugarcane was selected for its high stem sucrose accumulation. Flowering in sugarcane is not favored since flowering diverts the stored sugar resources for the reproductive and developmental energy needs. Cultivars are vegetatively propagated and sugarcane breeding is still essentially focused on conventional methods, since the knowledge of sugarcane genetics has lagged that of other major crops. Cultivar improvement has been extremely challenging due to its polyploidy and aneuploidy nature derived from a few interspecific hybridizations between Saccharum officinarum and Saccharum spontaneum, revealing the coexistence of two distinct genome organization modes in the modern variety. Alongside implementation of modern agricultural techniques, generation of hybrid clones, transgenics and genome edited events will help to meet the ever-growing bioenergy needs. Additionally, there are two common biotechnological approaches to improve plant stress tolerance, which includes marker-assisted selection (MAS) and genetic transformation. During the past two decades, the use of molecular approaches has contributed greatly to a better understanding of the genetic and biochemical basis of plant stress-tolerance and in some cases, it led to the development of plants with enhanced tolerance to abiotic stress. Hence, this review mainly intends on the events that shaped the sugarcane as what it is now and what challenges ahead and measures taken to further improve its yield, production and maximize utilization to beat the growing demands.

Role of NGS and SNP genotyping methods in sugarcane improvement programs

Sugarcane (Saccharum spp.) is one of the most economically significant crops because of its high sucrose content and it is a promising biomass feedstock for biofuel production. Sugarcane genome sequencing and analysis is a difficult task due to its heterozygosity and polyploidy. Long sequence read technologies, PacBio Single-Molecule Real-Time (SMRT) sequencing, the Illumina TruSeq, and the Oxford Nanopore sequencing could solve the problem of genome assembly. On the applications side, next generation sequencing (NGS) technologies played a major role in the discovery of single nucleotide polymorphism (SNP) and the development of low to high throughput genotyping platforms. The two mainstream high throughput genotyping platforms are the SNP microarray and genotyping by sequencing (GBS). This paper reviews the NGS in sugarcane genomics, genotyping methodologies, and the choice of these methods. Array-based SNP genotyping is robust, provides consistent SNPs, and relatively easier downstream data analysis. The GBS method identifies large scale SNPs across the germplasm. A combination of targeted GBS and array-based genotyping methods should be used to increase the accuracy of genomic selection and marker-assisted breeding.

Assessing genetic diversity and population structure of sugarcane cultivars, progenitor species and genera using microsatellite (SSR) markers

Sugarcane is one among the most important commercial crops used to produce sugar, ethanol, and other byproducts, which significantly contributes in the GDP of India and many other countries around the world. Genetic diversity is a platform for any breeding program of a plant species. Estimation of the genetic variability and population structure play a vital role for conservation planning and management of plant genetic resources. Genetic variability serves as a source of noble alleles responsible for key agronomic and quality traits, which ultimately form basis for identification and selection of promising parents for breeding programs. In the present study genetic diversity and population structure of 139 accessions of the genus Saccharum, allied genera of family Poaceae and cultivars were assessed using informative microsatellite (SSR) markers. A sum of 427 alleles was produced using 61 polymorphic primers and number of alleles generated was ranged from 2 to 13 with an average of 7 alleles per locus. PIC values were ranged from 0.35 to 0.90, with a mean value of 0.66 for all the markers evaluated. Cluster analysis based on UPGMA method revealed three major clusters which were further subdivided into nine subclusters. Population structure analysis also established three subpopulations of used accession set, however there were no correlation of sub-groupings with that of place of origin. AMOVA analysis also confirmed that 83% and 17% of total variations were attributed to the within- and between-populations, correspondingly, demonstrating greater exchange of gene pool across places of origin. The principal component analysis (PCA) demonstrated the distribution of accessions in the scatter-plot was substantially dispersed, revealing rich genetic diversity among accessions of different species. The findings from this study will be useful in breeding programs for introgression of noble alleles into modern cultivars by exploiting natural genetic variation existing in sugarcane genetic resources.

Genetic diversity and population structure analysis of Saccharum and Erianthus genera using microsatellite (SSR) markers

In order to understand the genetic diversity and structure within and between the genera of Saccharum and Erianthus, 79 accessions from five species (S. officinarum, S. spontaneum, S. robustum, S. barberi, S. sinense), six accessions of E. arundinaceus, and 30 Saccharum spp. hybrids were analyzed using 21 pairs of fluorescence-labeled highly poloymorphic SSR primers and a capillary electrophoresis (CE) detection system. A total of 167 polymorphic SSR alleles were identified by CE with a mean value of polymorphic information content (PIC) of 0.92. Genetic diversity parameters among these 115 accessions revealed that Saccharum spp. hybrids were more diverse than those of Saccharum and Erianthus species. Based on the SSR data, the 115 accessions were classified into seven main phylogenetic groups, which corresponded to the Saccharum and Erianthus genera through phylogenetic analysis and principle component analysis (PCA). We propose that seven core SSR primer pairs, namely, SMC31CUQ, SMC336BS, SMC597CS, SMC703BS, SMC24DUQ, mSSCIR3, and mSSCIR43, may have a wide appicability in genotype identification of Saccharum species and Saccharum spp. hybrids. Thus, the information from this study contibites to manage sugarcane genetic resources.

Genetic diversity of sugarcane hybrid cultivars by RAPD markers

Genetic diversity among sugarcane hybrids (Saccharum spp) is pre-requisite for sugarcane improvement through breeding. Twelve decamer oligonucleotide random-amplified polymorphic DNA (RAPD) markers were utilized to investigate the genetic potential among 24 sugarcane cultivars. A total of 120 fragments were originated by 12 RAPD primers. An average number of fragments were obtained as 11.42 fragments per cultivar, which ranged from 4 to 21 fragments. The genetic similarity among 24 sugarcane cultivars ranged from 0.236 to 0.944 with the mean similarity value of 0.508. On the basis of phylogenetic analysis based on dendrogram, the cultivars were clustered into five groups. Two varieties Co 0118 and CoS 07250 were found as highly diverse sugarcane cultivars. Three most popular cultivars viz, Co 0238, Co 1158, and CoS 08272 were clustered a diverse among particular group. These clusters with their diverse genealogy indicated the influence of parental genome contribution to clustering. Diverse varieties developed for east region were grouped in the separate clusters which indicated the influence of adaptation of varieties to particular agro-climatic condition. Hence, these five diverse hybrid cultivars would be used in further breeding program to get the prominent sugarcane clones which may produced higher cane yield and sugar content.

Comparative analysis of genetic variation in kava (Piper methysticum) assessed by SSR and DArT reveals zygotic foundation and clonal diversification

Kava (Piper methysticum) is a major cash crop in the Pacific. The aim of this study was to assess genetic variation among 103 accessions of kava using SSRs and DArTs. Genetic structure was determined using clustering analyses (WPGMA) and principal coordinate analyses (PCA). Thirteen SSR primers and 75 DArT markers were found polymorphic, and the two types of markers generated similar clustering patterns. Genetic distances ranged from 0 to 0.65 with an average of 0.24 using SSRs and from 0 to 0.64 with an average of 0.24 using DArT. Eleven genotypes were identified with SSR while 28 genotypes were identified with DArT markers. By combining the two sets of markers, a total of only 30 distinct genotypes were observed. In the Vanuatu archipelago, noble cultivars originating from different islands clustered together within a very narrow genetic base despite their diversity of morphotypes. SSR and DArT fingerprints allowed the identification of kava cultivars unsuitable for consumption, so called two-days, and clearly differentiated the wild types classified as P. methysticum var. wichmannii from the cultivars as var. methysticum. Molecular data reveals that all noble cultivars evolved by the predominance of clonal selection. Although they are represented by clearly distinct morphotypes, these cultivars are genetically vulnerable and their potential to adapt to forthcoming changes is limited. These newly developed markers provide high resolution and will be useful for kava diversity analyses and quality assessment.

Promoting utilization of Saccharum spp. genetic resources through genetic diversity analysis and core collection construction

Sugarcane (Saccharum spp.) and other members of Saccharum spp. are attractive biofuel feedstocks. One of the two World Collections of Sugarcane and Related Grasses (WCSRG) is in Miami, FL. This WCSRG has 1002 accessions, presumably with valuable alleles for biomass, other important agronomic traits, and stress resistance. However, the WCSRG has not been fully exploited by breeders due to its lack of characterization and unmanageable population. In order to optimize the use of this genetic resource, we aim to 1) genotypically evaluate all the 1002 accessions to understand its genetic diversity and population structure and 2) form a core collection, which captures most of the genetic diversity in the WCSRG. We screened 36 microsatellite markers on 1002 genotypes and recorded 209 alleles. Genetic diversity of the WCSRG ranged from 0 to 0.5 with an average of 0.304. The population structure analysis and principal coordinate analysis revealed three clusters with all S. spontaneum in one cluster, S. officinarum and S. hybrids in the second cluster and mostly non-Saccharum spp. in the third cluster. A core collection of 300 accessions was identified which captured the maximum genetic diversity of the entire WCSRG which can be further exploited for sugarcane and energy cane breeding. Sugarcane and energy cane breeders can effectively utilize this core collection for cultivar improvement. Further, the core collection can provide resources for forming an association panel to evaluate the traits of agronomic and commercial importance.

Allelic divergence and cultivar-specific SSR alleles revealed by capillary electrophoresis using fluorescence-labeled SSR markers in sugarcane

Though sugarcane cultivars (Saccharum spp. hybrids) are complex aneupolyploid hybrids, genetic evaluation and tracking of clone- or cultivar-specific alleles become possible through capillary electrophoresis (CE) using fluorescence-labeled SSR markers. Twenty-four sugarcane cultivars, 12 each from India and the USA, were genetically assessed using 21 fluorescence-labeled polymorphic SSR markers. These markers primed the amplification of 213 alleles. Of these alleles, 161 were common to both Indian and US cultivars, 25 were specific to the Indian cultivars, and 27 were observed only in the US cultivars. Only 10 alleles were monomorphic. A high level of heterozygosity was observed in both Indian (82.4%) and US (91.1%) cultivars resulting in average polymorphism information content (PIC) values of 0.66 and 0.77 and marker index (MI) values of 5.07 and 5.58, respectively. Pearson correlation between PIC and MI was significant in both sets of cultivars (r = 0.58 and 0.69). UPGMA clustering separated cultivars into three distinct clusters at 59% homology level. These results propose the potential utility of six Indian cultivar-specific SSR alleles (mSSCIR3_182, SMC486CG_229, SMC36BUQ_125, mSSCIR74_216, SMC334BS_154, and mSSCIR43_238) in sugarcane breeding, vis a vis transporting CE-based evaluation in clone or variety identity testing, cross fidelity assessments, and genetic relatedness among species of the genus Saccharum and related genera.

Development of novel simple sequence repeat markers in bitter gourd (Momordica charantia L.) through enriched genomic libraries and their utilization in analysis of genetic diversity and cross-species transferability

Microsatellite or simple sequence repeat (SSR) markers are the preferred markers for genetic analyses of crop plants. The availability of a limited number of such markers in bitter gourd (Momordica charantia L.) necessitates the development and characterization of more SSR markers. These were developed from genomic libraries enriched for three dinucleotide, five trinucleotide, and two tetranucleotide core repeat motifs. Employing the strategy of polymerase chain reaction-based screening, the number of clones to be sequenced was reduced by 81 % and 93.7 % of the sequenced clones contained in microsatellite repeats. Unique primer-pairs were designed for 160 microsatellite loci, and amplicons of expected length were obtained for 151 loci (94.4 %). Evaluation of diversity in 54 bitter gourd accessions at 51 loci indicated that 20 % of the loci were polymorphic with the polymorphic information content values ranging from 0.13 to 0.77. Fifteen Indian varieties were clearly distinguished indicative of the usefulness of the developed markers. Markers at 40 loci (78.4 %) were transferable to six species, viz. Momordica cymbalaria, Momordica subangulata subsp. renigera, Momordica balsamina, Momordica dioca, Momordica cochinchinesis, and Momordica sahyadrica. The microsatellite markers reported will be useful in various genetic and molecular genetic studies in bitter gourd, a cucurbit of immense nutritive, medicinal, and economic importance.

Exploiting EST databases for the development and characterisation of 3425 gene-tagged CISP markers in biofuel crop sugarcane and their transferability in cereals and orphan tropical grasses

Background

Sugarcane is an important cash crop, providing 70% of the global raw sugar as well as raw material for biofuel production. Genetic analysis is hindered in sugarcane because of its large and complex polyploid genome and lack of sufficiently informative gene-tagged markers. Modern genomics has produced large amount of ESTs, which can be exploited to develop molecular markers based on comparative analysis with EST datasets of related crops and whole rice genome sequence, and accentuate their cross-technical functionality in orphan crops like tropical grasses.

Findings

Utilising 246,180 Saccharum officinarum EST sequences vis-à-vis its comparative analysis with ESTs of sorghum and barley and the whole rice genome sequence, we have developed 3425 novel gene-tagged markers — namely, conserved-intron scanning primers (CISP) — using the web program GeMprospector. Rice orthologue annotation results indicated homology of 1096 sequences with expressed proteins, 491 with hypothetical proteins. The remaining 1838 were miscellaneous in nature. A total of 367 primer-pairs were tested in diverse panel of samples. The data indicate amplification of 41% polymorphic bands leading to 0.52 PIC and 3.50 MI with a set of sugarcane varieties and Saccharum species. In addition, a moderate technical functionality of a set of such markers with orphan tropical grasses (22%) and fodder cum cereal oat (33%) is observed.

Conclusions

Developed gene-tagged CISP markers exhibited considerable technical functionality with varieties of sugarcane and unexplored species of tropical grasses. These markers would thus be particularly useful in identifying the economical traits in sugarcane and developing conservation strategies for orphan tropical grasses.

Functionally relevant microsatellites in sugarcane unigenes

BACKGROUND

Unigene sequences constitute a rich source of functionally relevant microsatellites. The present study was undertaken to mine the microsatellites in the available unigene sequences of sugarcane for understanding their constitution in the expressed genic component of its complex polyploid/aneuploid genome, assessing their functional significance in silico, determining the extent of allelic diversity at the microsatellite loci and for evaluating their utility in large-scale genotyping applications in sugarcane.

RESULTS

The average frequency of perfect microsatellite was 1/10.9 kb, while it was 1/44.3 kb for the long and hypervariable class I repeats. GC-rich trinucleotides coding for alanine and the GA-rich dinucleotides were the most abundant microsatellite classes. Out of 15,594 unigenes mined in the study, 767 contained microsatellite repeats and for 672 of these putative functions were determined in silico. The microsatellite repeats were found in the functional domains of proteins encoded by 364 unigenes. Its significance was assessed by establishing the structure-function relationship for the beta-amylase and protein kinase encoding unigenes having repeats in the catalytic domains. A total of 726 allelic variants (7.42 alleles per locus) with different repeat lengths were captured precisely for a set of 47 fluorescent dye labeled primers in 36 sugarcane genotypes and five cereal species using the automated fragment analysis system, which suggested the utility of designed primers for rapid, large-scale and high-throughput genotyping applications in sugarcane. Pair-wise similarity ranging from 0.33 to 0.84 with an average of 0.40 revealed a broad genetic base of the Indian varieties in respect of functionally relevant regions of the large and complex sugarcane genome.

CONCLUSION

Microsatellite repeats were present in 4.92% of sugarcane unigenes, for most (87.6%) of which functions were determined in silico. High level of allelic diversity in repeats including those present in the functional domains of proteins encoded by the unigenes demonstrated their use in assay of useful variation in the genic component of complex polyploid sugarcane genome.

Significant comparative characteristics between orphan and nonorphan genes in the rice (Oryza sativa L.) genome

Microsatellites are short tandem repeats of one to six bases in genomic DNA. As microsatellites are highly polymorphic and play a vital role in gene function and recombination, they are an attractive subject for research in evolution and in the genetics and breeding of animals and plants. Orphan genes have no known homologs in existing databases. Using bioinformatic computation and statistical analysis, we identified 19,26 orphan genes in the rice (Oryza sativa ssp. Japanica cv. Nipponbare) proteome. We found that a larger proportion of orphan genes are expressed after sexual maturation and under environmental pressure than nonorphan genes. Orphan genes generally have shorter protein lengths and intron size, and are faster evolving. Additionally, orphan genes have fewer PROSITE patterns with larger pattern sizes than those in nonorphan genes. The average microsatellite content and the percentage of trinucleotide repeats in orphan genes are also significantly higher than in nonorphan genes. Microsatellites are found less often in PROSITE patterns in orphan genes. Taken together, these orphan gene characteristics suggest that microsatellites play an important role in orphan gene evolution and expression.

Genetic studies of "noble cane" for identification and exploitation of genetic markers

Forty genotypes (clones) of sugarcane, including elite lines, commercial cultivars of Saccharum officinarum and clones of S. barberi were fingerprinted with 50 SSR markers using a PCR-based marker assay. Nei's genetic distances for SSR data were determined and relationships between accessions were portrayed graphically in the form of a dendrogram. Genetic distance values ranging from 0.60 to 1.11 were observed among the 40 sugarcane accessions. The shortest genetic distance of 0.60 was seen between genotypes US-804 and US-130. These two genotypes differed from each other only in 10 bands, with 20 primers. The most dissimilar of the accessions were CP-77-400 and US-133, with a genetic distance of 1.11. SSR fingerprints can help sugarcane breeders to clarify the genetic pedigree of commercial sugarcane varieties and evaluate the efficiency of breeding methods.

Application of simple sequence repeat (SSR) markers for molecular diversity and heterozygosity analysis in maize inbred lines

There is an important role of understanding the genetic diversity among and within inbred lines at the molecular level for maize improvement in different breeding programs. The present study was devoted to estimate the level of genetic diversity among the inbred lines of maize using the simple sequence repeat analysis (SSR). The application of six different SSR markers successfully provided the information on similarity or diversity as well as the heterozygosity of the allelic loci for all the eight inbred line of maize.

Informative genomic microsatellite markers for efficient genotyping applications in sugarcane

Genomic microsatellite markers are capable of revealing high degree of polymorphism. Sugarcane (Saccharum sp.), having a complex polyploid genome requires more number of such informative markers for various applications in genetics and breeding. With the objective of generating a large set of microsatellite markers designated as Sugarcane Enriched Genomic MicroSatellite (SEGMS), 6,318 clones from genomic libraries of two hybrid sugarcane cultivars enriched with 18 different microsatellite repeat-motifs were sequenced to generate 4.16 Mb high-quality sequences. Microsatellites were identified in 1,261 of the 5,742 non-redundant clones that accounted for 22% enrichment of the libraries. Retro-transposon association was observed for 23.1% of the identified microsatellites. The utility of the microsatellite containing genomic sequences were demonstrated by higher primer designing potential (90%) and PCR amplification efficiency (87.4%). A total of 1,315 markers including 567 class I microsatellite markers were designed and placed in the public domain for unrestricted use. The level of polymorphism detected by these markers among sugarcane species, genera, and varieties was 88.6%, while cross-transferability rate was 93.2% within Saccharum complex and 25% to cereals. Cloning and sequencing of size variant amplicons revealed that the variation in the number of repeat-units was the main source of SEGMS fragment length polymorphism. High level of polymorphism and wide range of genetic diversity (0.16-0.82 with an average of 0.44) assayed with the SEGMS markers suggested their usefulness in various genotyping applications in sugarcane.

Very close relationship of the chloroplast genomes among Saccharum species

We recently determined the complete sequence of the sugarcane chloroplast genome. Here, we have used the information for a comprehensive phylogenetic analysis of the genus Saccharum, using all six species (13 accessions). The polymorphisms between sugarcane and maize in 26 chloroplast genome regions were used for the analysis. In 18 of the 26 regions (a total of 5,381 bp), we found 41 mutations involving 17 substitutions, three inversions, six insertion/deletion mutations, and 15 simple sequence repeat length polymorphisms. Based on these results, we calculated a phylogenetic tree of the genus Saccharum, in which all six species are clearly separated. By the analysis, (1) S. sinense and S. barberi, which have identical sequences, belong to the same clade, whereas the other four species, S. officinarum, S. robustum, S. edule, and S. spontaneum, form an independent clade; (2) S. spontaneum has a paraphyletic relationship with the other five species; and (3) no or very low intraspecific variation was observed in S. officinarum, S. robustum, S. sinense, S. barberi, and S. edule, whereas higher intraspecific variation was observed in S. spontaneum. Based on the number of nucleotide substitutions, the divergence time between S. officinarum and S. spontaneum, and between S. officinarum and maize were calculated to be about 730-780 thousand years ago and about 5.9 million years ago, respectively. These results suggest that the cytoplasm of Saccharum species are very closely related.