Molecular relationships between Australian annual wild rice, Oryza meridionalis, and two related perennial forms

Seed abortion caused by the combination of two duplicate genes in the progeny from the cross between Oryza sativa and Oryza meridionalis

Seed development is an essential phenomenon for all sexual propagative plant species. The functional allele at SEED DEVELOPMENT 1 (SDV1) or SEED DEVELOPMENT 2 (SDV2) loci is essential for seed development for Oryza sativa and Oryza meridionalis. In the present study, we performed fine mapping of SDV1, narrowing down the area of interest to 333kb on chromosome 6. Haplotype analysis around the SDV1 locus of O. meridionalis accessions indicated that they shared the DNA polymorphism, suggesting that they have a common abortive allele at the SDV1 locus. Linkage analysis of the candidate SDV2 gene showed that it was located on chromosome 4. The candidate SDV2 was confirmed using a population in which both the SDV1 and SDV2 genes were segregating. The chromosomal region covering the SDV1 gene was predicted to contain 30 protein-coding genes in O. sativa. Five of these genes have conserved DNA sequences in the chromosomal region of the SDV2 gene on chromosome 4, and not on chromosome 6, of O. meridionalis. These results suggest that these five genes could be candidates for SDV1, and that their orthologous genes located on chromosome 4 of O. meridionalis could be candidates for SDV2.

High-quality Momordica balsamina genome elucidates its potential use in improving stress resilience and therapeutic properties of bitter gourd

Introduction

Momordica balsamina is the closest wild species that can be crossed with an important fruit vegetable crop, Momordica charantia, has immense medicinal value, and placed under II subclass of primary gene pool of bitter gourd. M. balsamina is tolerant to major biotic and abiotic stresses. Genome characterization of Momordica balsamina as a wild relative of bitter gourd will contribute to the knowledge of the gene pool available for improvement in bitter gourd. There is potential to transfer gene/s related to biotic resistance and medicinal importance from M. balsamina to M. charantia to produce high-quality, better yielding and stress tolerant bitter gourd genotypes.

Methods

The present study provides the first and high-quality chromosome-level genome assembly of M. balsamina with size 384.90 Mb and N50 30.96 Mb using sequence data from 10x Genomics, Nanopore, and Hi-C platforms.

Results

A total of 6,32,098 transposons elements; 2,15,379 simple sequence repeats; 5,67,483 transcription factor binding sites; 3,376 noncoding RNA genes; and 41,652 protein-coding genes were identified, and 4,347 disease resistance, 67 heat stress-related, 05 carotenoid-related, 15 salt stress-related, 229 cucurbitacin-related, 19 terpenes-related, 37 antioxidant activity, and 06 sex determination-related genes were characterized.

Conclusion

Genome sequencing of M. balsamina will facilitate interspecific introgression of desirable traits. This information is cataloged in the form of webgenomic resource available at http://webtom.cabgrid.res.in/mbger/. Our finding of comparative genome analysis will be useful to get insights into the patterns and processes associated with genome evolution and to uncover functional regions of cucurbit genomes.

Analysis of Domestication Loci in Wild Rice Populations

The domestication syndrome is defined as a collection of domestication-related traits that have undergone permanent genetic changes during the domestication of cereals. Australian wild rice populations have not been exposed to gene flow from domesticated rice populations. A high level of natural variation of the sequences at domestication loci (e.g., seed shattering, awn development, and grain size) was found in Australian AA genome wild rice from the primary gene pool of rice. This natural variation is much higher than that found in Asian cultivated rice and wild Asian rice populations. The Australian Oryza meridionalis populations exhibit a high level of homozygous polymorphisms relative to domesticated rice, inferring the fixation of distinct wild and domesticated alleles. Alleles of the seed shattering genes (SH4/SHA1 and OsSh1/SH1) present in the shattering-prone O. meridionalis populations are likely to be functional, while the dysfunctional alleles of these seed shattering genes are found in domesticated rice. This confirms that unlike Asian wild rice populations, Australian wild rice populations have remained genetically isolated from domesticated rice, retaining pre-domestication alleles in their wild populations that uniquely allow the impact of domestication on the rice genome to be characterized. This study also provides key information about the domestication loci in Australian wild rice populations that will be valuable in the utilization of these genetic resources in crop improvement and de novo domestication.

Gene Expression in the Developing Seed of Wild and Domesticated Rice

The composition and nutritional properties of rice are the product of the expression of genes in the developing seed. RNA-Seq was used to investigate the level of gene expression at different stages of seed development in domesticated rice (Oryza sativa ssp. japonica var. Nipponbare) and two Australian wild taxa from the primary gene pool of rice (Oryza meridionalis and Oryza rufipogon type taxa). Transcriptome profiling of all coding sequences in the genome revealed that genes were significantly differentially expressed at different stages of seed development in both wild and domesticated rice. Differentially expressed genes were associated with metabolism, transcriptional regulation, nucleic acid processing, and signal transduction with the highest number of being linked to protein synthesis and starch/sucrose metabolism. The level of gene expression associated with domestication traits, starch and sucrose metabolism, and seed storage proteins were highest at the early stage (5 days post anthesis (DPA)) to the middle stage (15 DPA) and declined late in seed development in both wild and domesticated rice. However, in contrast, black hull colour (Bh4) gene was significantly expressed throughout seed development. A substantial number of novel transcripts (38) corresponding to domestication genes, starch and sucrose metabolism, and seed storage proteins were identified. The patterns of gene expression revealed in this study define the timing of metabolic processes associated with seed development and may be used to explain differences in rice grain quality and nutritional value.

A large deletion within intron 20 sequence of single-copy PolA1 gene as a useful marker for the speciation in Oryza AA-genome species

Oryza AA-genome complex comprises five wild species, O. rufipogon, O. barthii, O. longistaminata, O. glumaepatula, and O. meridionalis. Evolutionary relationships among these five wild species have remained contentious and inconclusive. We found that intron 20 of PolA1, a single-copy nuclear gene, was short (S-type: 141-142 bp) in O. rufipogon, O. barthii, and O. glumaepatula, while long (L-type: ca. 1.5 kb) introns were apparent in O. longistaminata and O. meridionalis. Because Oryza species containing BB, CC, EE, FF, and GG genome showed L-type introns, the S-type intron was probably derived from the L-type intron by the deletion of a 1.4 kb fragment through intramolecular homologous recombination between two tandem TTTTGC repeats. Excluding the large deletion sequence, intron 20 sequence of O. barthii was identical to that of O. longistaminata. As more than 3,470 accessions of O. rufipogon and O. sativa also contained the same intron 20 sequence with O. longistaminata except for single T-nucleotide deletion, which was shared with O. glumaepatuala, the deletion of the T-nucleotide probably occurred in the L-type intron 20 of O. logistaminata. Deletions of a large 1.4 kb fragment and single T-nucleotide within the intron 20 of PolA1 gene were considered as useful DNA markers to study the evolutionary relationships among Oryza AA-genome species.

Potential of Genome Editing to Capture Diversity From Australian Wild Rice Relatives

Rice, a staple food worldwide and a model crop, could benefit from the introduction of novel genetics from wild relatives. Wild rice in the AA genome group closely related to domesticated rice is found across the tropical world. Due to their locality outside the range of domesticated rice, Australian wild rice populations are a potential source of unique traits for rice breeding. These rice species provide a diverse gene pool for improvement that could be utilized for desirable traits such as stress resistance, disease tolerance, and nutritional qualities. However, they remain poorly characterized. The CRISPR/Cas system has revolutionized gene editing and has improved our understanding of gene functions. Coupled with the increasing availability of genomic information on the species, genes in Australian wild rice could be modified through genome editing technologies to produce new domesticates. Alternatively, beneficial alleles from these rice species could be incorporated into cultivated rice to improve critical traits. Here, we summarize the beneficial traits in Australian wild rice, the available genomic information and the potential of gene editing to discover and understand the functions of novel alleles. Moreover, we discuss the potential domestication of these wild rice species for health and economic benefits to rice production globally.

Reticulate Evolution in AA-Genome Wild Rice in Australia

The wild rice gene pool, i.e., AA-genome, in Australia is geographically and genetically distinct from that in Asia. Two distinct taxa are found growing together in northern Australia, Oryza meridionalis (including annual and perennial forms) and an Oryza rufipogon like taxa that have been shown to have a chloroplast genome sequence that is closer to that of O. meridionalis than to O. rufipogon from Asia. Rare plants of intermediate morphology have been observed in the wild despite a reported reproductive barrier between these two species. We now report the resequencing of plants from 26 populations including both taxa and putative hybrids. A comparison of chloroplast and nuclear genome sequences indicated re-combinations that demonstrated hybridisation in both directions. Individuals with intermediate morphology had high nuclear genome heterozygosity consistent with a hybrid origin. An examination of specific genes (e.g., starch biosynthesis genes) revealed the presence of heterozygotes with alleles from both parents suggesting that some wild plants were early generation hybrids. These plants may have low cross-fertility preserving the continuation of the two distinct species. Repeated backcrossing of these rare hybrids to one parent would explain the plants exhibiting chloroplast capture. These observations suggest that reticulate evolution is continuing in wild Oryza populations and may have been a key process in rice evolution and domestication.

New Hybrid Spikelet Sterility Gene Found in Interspecific Cross between Oryza sativa and O. meridionalis

Various kinds of reproductive barriers have been reported in intraspecific and interspecific crosses between the AA genome Oryza species, to which Asian rice (O. sativa) and African rice (O. glaberrima) belong. A hybrid seed sterility phenomenon was found in the progeny of the cross between O. sativa and O. meridionalis, which is found in Northern Australia and Indonesia and has diverged from the other AA genome species. This phenomenon could be explained by an egg-killer model. Linkage analysis using DNA markers showed that the causal gene was located on the distal end of chromosome 1. Because no known egg-killer gene was located in that chromosomal region, this gene was named HYBRID SPIKELET STERILITY 57 (abbreviated form, S57). In heterozygotes, the eggs carrying the sativa allele are killed, causing semi-sterility. This killer system works incompletely: some eggs carrying the sativa allele survive and can be fertilized. The distribution of alleles in wild populations of O. meridionalis was discussed from the perspective of genetic differentiation of populations.

Substitution Mapping of a Locus Responsible for Hybrid Breakdown in Populations Derived From Interspecific Introgression Line

Hybrid breakdown, a form of postzygotic reproductive barrier, has been reported to hinder gene flow in many crosses between wild and cultivated rice. Here, the phenomenon of hybrid breakdown was observed as low-tillering (i.e., low tiller number) in some progeny of an interspecific cross produced in an attempt to introduce Oryza meridionalis Ng (W1625) chromosomal segments into Oryza sativa L. ssp. japonica "Taichung 65" (T65). Low-tillering lines were obtained in BC4-derived progeny from a cross between W1625 and "Taichung 65," but the locus for low-tillering could not be mapped in segregating populations. As a second approach to map the locus for low-tillering, we analyzed an F2 population derived from a cross between the low-tillering lines and a high-yielding indica cultivar, "Takanari." A major QTL for low-tillering, qLTN4, was detected between PCR-based markers MS10 and RM307 on the long arm of chromosome 4, with a LOD score of 15.6. The low-tillering phenotype was associated with weak growth and pale yellow phenotype; however, low-tillering plant had less reduction of grain fertility. In an F4 population (4896 plants), 563 recombinant plants were identified and the low-tillering locus was delimited to a 4.6-Mbp region between markers W1 and C5-indel3729. This region could not be further delimited because recombination is restricted in this region of qLTN4, which is near the centromere. Understanding the genetic basis of hybrid breakdown, including the low-tillering habit, will be important for improving varieties in rice breeding.

Chromosomal introgressions from Oryza meridionalis into domesticated rice Oryza sativa result in iron tolerance

Iron (Fe) toxicity is one of the most common mineral disorders affecting rice (Oryza sativa) production in flooded lowland fields. Oryza meridionalis is indigenous to northern Australia and grows in regions with Fe-rich soils, making it a candidate for use in adaptive breeding. With the aim of understanding tolerance mechanisms in rice, we screened a population of interspecific introgression lines from a cross between O. sativa and O. meridionalis for the identification of quantitative trait loci (QTLs) contributing to Fe-toxicity tolerance. Six putative QTLs were identified. A line carrying one introgression from O. meridionalis on chromosome 9 associated with one QTL was highly tolerant despite very high shoot Fe concentrations. Physiological, biochemical, ionomic, and transcriptomic analyses showed that the tolerance of the introgression lines could partly be explained by higher relative Fe retention in the leaf sheath and culm. We constructed the interspecific hybrid genome in silico for transcriptomic analysis and identified differentially regulated introgressed genes from O. meridionalis that could be involved in shoot-based Fe tolerance, such as metallothioneins, glutathione S-transferases, and transporters from the ABC and MFS families. This work demonstrates that introgressions of O. meridionalis into the O. sativa genome can confer increased tolerance to excess Fe.

Two divergent chloroplast genome sequence clades captured in the domesticated rice gene pool may have significance for rice production

BACKGROUND

The whole chloroplast genomes of 3018 rice genotypes were assembled from available sequence data by alignment with a reference rice chloroplast genome sequence, providing high quality chloroplast genomes for analysis of diversity on a much larger scale than in any previous plant study.

RESULTS

Updated annotation of the chloroplast genome identified 13 more tRNA genes and 30 more introns and defined the function of more of the genes. Domesticated rice had chloroplast genomes that were distinct from those in wild relatives. Analysis confirms an Australian chloroplast clade as a sister to the domesticated clade. All domesticated rice genotypes could be assigned to one of two main clades suggesting the domestication of two distinct maternal genome clades that diverged long before domestication. These clades were very distinct having 4 polymorphisms between all 1486 accession in clade A and all 1532 accessions in clade B. These would result in expression of 3 proteins with altered amino acid sequences and a tRNA with an altered sequence and may be associated with adaptive evolution of the two chloroplast types. Diversity within these pools may have been captured during domestication with subclades enriched in specific groups such as basmati, tropical japonica and temperate japonica. However the phylogenies of the chloroplast and nuclear genomes differed possibly due to modern rice breeding and reticulate evolution prior to domestication. Indica and aus genotypes were common in both chloroplast clades while japonica genotypes were more likely to be found in the same clade (cladeB).

CONCLUSIONS

The different evolutionary paths of the cytoplasmic and nuclear genomes of rice have resulted in the presence of apparently functional chloroplast genome diversity and the implications for rice crop performance require further investigation.

Molecular and Morphological Divergence of Australian Wild Rice

Two types of perennial wild rice, Australian Oryza rufipogon and a new taxon Jpn2 have been observed in Australia in addition to the annual species Oryza meridionalis. Jpn2 is distinct owing to its larger spikelet size but shares O. meridionalis-like morphological features including a high density of bristle cells on the awn surface. All the morphological traits resemble O. meridionalis except for the larger spikelet size. Because Jpn2 has distinct cytoplasmic genomes, including the chloroplast (cp), cp insertion/deletion/simple sequence repeats were designed to establish marker systems to distinguish wild rice in Australia in different natural populations. It was shown that the new taxon is distinct from Asian O. rufipogon but instead resembles O. meridionalis. In addition, higher diversity was detected in north-eastern Australia. Reproductive barriers among species and Jpn2 tested by cross-hybridization suggested a unique biological relationship of Jpn2 with other species. Insertions of retrotransposable elements in the Jpn2 genome were extracted from raw reads generated using next-generation sequencing. Jpn2 tended to share insertions with other O. meridionalis accessions and with Australian O. rufipogon accessions in particular cases, but not Asian O. rufipogon except for two insertions. One insertion was restricted to Jpn2 in Australia and shared with some O. rufipogon in Thailand.

Relationships between Iraqi Rice Varieties at the Nuclear and Plastid Genome Levels

Due to the importance of the rice crop in Iraq, this study was conducted to determine the origin of the major varieties and understand the evolutionary relationships between Iraqi rice varieties and other Asian rice accessions that could be significant in the improvement of this crop. Five varieties of Oryza sativa were obtained from Baghdad/Iraq, and the whole genomic DNA was sequenced, among these varieties, Amber33, Furat, Yasmin, Buhooth1 and Amber al-Baraka. Raw sequence reads of 33 domesticated Asian rice accessions were obtained from the Sequence Read Archive (SRA-NCBI). The sequence of the whole chloroplast-genome was assembled while only the sequence of 916 concatenated nuclear-genes was assembled. The phylogenetic analysis of both chloroplast and nuclear genomes showed that two main clusters, Indica and Japonica, and further five sub-clusters based upon their ecotype, indica, aus, tropical-japonica, temperate-japonica and basmati were created; moreover, Amber33, Furat, Yasmin and Buhooth1 belonged to the basmati, indica and japonica ecotypes, respectively, where Amber33 was placed in the basmati group as a sister of cultivars from Pakistan and India. This confirms the traditional story that Amber was transferred by a group of people who had migrated from India and settled in southern Iraq a long time ago.

Australian Wild Rice Populations: A Key Resource for Global Food Security

Rice is one of the most important food crops contributing to the diet of large numbers of people especially in Asia. Rice (Oryza sativa) was domesticated in Asia many thousands of years ago and more recently independently in Africa. Wild rice populations are found around the tropical world. The extensive production of rice in many areas has displaced the wild populations that were the basis of the original domestications by humans. Recent research, reviewed here, has identified wild rice species in northern Australia that have been isolated from the impact of domestication in Asia. Wild rice populations contain novel alleles that are a source of desirable traits such as erect habit, disease resistance, large grain size, and unique starch properties. These populations include the most divergent genotypes within the primary gene pool of rice and more distant wild relatives. Genome sequencing also suggests the presence of populations that are close relatives of domesticated rice. Hybrid populations that demonstrate mechanisms of ongoing evolution of wild Oryza have been identified in the wild. These populations provide options for both new domestications and utilization of novel alleles to improve or adapt domesticated rice using conventional or preferably new breeding technologies. Climate change and growing food demands associated with population and economic growth are major challenges for agriculture including rice production. The availability of diverse genetic resources to support crop adaptation and new crop domestication is critical to continued production, and increased efforts to support in situ and ex situ conservation of wild Oryza and related species are warranted.

Segregation Distortion Observed in the Progeny of Crosses Between Oryza sativa and O. meridionalis Caused by Abortion During Seed Development

Wild rice relatives having the same AA genome as domesticated rice (Oryza sativa) comprise the primary gene pool for rice genetic improvement. Among them, O. meridionalis and O. rufipogon are found in the northern part of Australia. Three Australian wild rice strains, Jpn1 (O. rufipogon), Jpn2, and W1297 (O. meridionalis), and one cultivated rice cultivar Taichung 65 (T65) were used in this study. A recurrent backcrossing strategy was adopted to produce chromosomal segment substitution lines (CSSLs) carrying chromosomal segments from wild relatives and used for trait evaluation and genetic analysis. The segregation of the DNA marker RM136 locus on chromosome 6 was found to be highly distorted, and a recessive lethal gene causing abortion at the seed developmental stage was shown to be located between two DNA markers, KGC6_10.09 and KGC6_22.19 on chromosome 6 of W1297. We name this gene as SEED DEVELOPMENT 1 (gene symbol: SDV1). O. sativa is thought to share the functional dominant allele Sdv1-s (s for sativa), and O. meridionalis is thought to share the recessive abortive allele sdv1-m (m for meridionalis). Though carrying the sdv1-m allele, the O. meridionalis accessions can self-fertilize and bear seeds. We speculate that the SDV1 gene may have been duplicated before the divergence between O. meridionalis and the other AA genome Oryza species, and that O. meridionalis has lost the function of the SDV1 gene and has kept the function of another putative gene named SDV2.

Decoding and analysis of organelle genomes of Indian tea (Camellia assamica) for phylogenetic confirmation

The NCBI database has >15 chloroplast (cp) genome sequences available for different Camellia species but none for C. assamica. There is no report of any mitochondrial (mt) genome in the Camellia genus or Theaceae family. With the strong believes that these organelle genomes can play a great tool for taxonomic and phylogenetic analysis, we successfully assembled and analyzed cp and mt genome of C. assamica. We assembled the complete mt genome of C. assamica in a single circular contig of 707,441 bp length comprising of a total of 66 annotated genes, including 35 protein-coding genes, 29 tRNAs and two rRNAs. The first ever cp genome of C. assamica resulted in a circular contig of 157,353 bp length with a typical quadripartite structure. Phylogenetic analysis based on these organelle genomes showed that C. assamica was closely related to C. sinensis and C. leptophylla. It also supports Caryophyllales as Superasterids.

Genetic diversity among perennial wild rice Oryza rufipogon Griff., in the Mekong Delta

Oryza rufipogon Griff. is a perennial species of wild rice widely distributed along the channels and rivers of the Mekong Delta, Vietnam. This study attempted to find centers of diversity among wild rice populations in this area and their inter-relationships. The highest genetic diversity was found in the Dong Thap population and the lowest in the Can Tho population. Maternal diversity evaluated using chloroplast INDELs detected ten plastid types, five of which were novel relative to other Asian countries. The mitochondrial genome suggested two unique deletions. One 699-bp deletion via short tandem repeats was accompanied by another deletion including orf153. All accessions carrying the mitochondrial type were found in a particular plastid type. This unique maternal lineage was confined to specific channels where it showed vigorous vegetative growth in comparison to upstream areas where various maternal lineages and maximum genetic diversity occurred. This area along the Mekong Delta is a center of not only nuclear but also maternal diversity.

Diverse genetic variation in maternal lineages with high heterogeneity among in situ-conserved wild rice (Oryza rufipogon Griff.) developed in Thailand

Wild rice, Oryza rufipogon, is a genetic resource that can be used to improve cultivated rice, but its populations are now decreasing in terms of both size and number. Extensive research on wild rice has been conducted in Thailand, where two in situ conservation sites have been preserved in natural areas where perennial wild rice predominates. The genetic structure of wild rice populations was investigated by examining both the chloroplast and nucleus genomes at sites of in situ conservation site in Thailand. One accession from an in situ-conserved site was re-sequenced against the chloroplast genome of O. sativa cv. 'Nipponbare' to develop chloroplast insertion/deletion (cpINDEL) markers. These cpINDEL markers revealed unique maternal lineages in the in situ-conserved populations upon comparison with other Asian wild rice accessions. Diverse genetic variation was also detected with SSR markers throughout the genome. Three populations differed from each other and also within single populations. The sub-populations within an in situ-conserved population showed a complex population structure due to their multiple maternal lineages and relatively higher number of haplotypes when they maintained a relatively large population size. Such a heterogeneous population would serve as a unique gene pool for rice breeding.

Diversity and evolution of rice progenitors in Australia

In the thousands of years of rice domestication in Asia, many useful genes have been lost from the gene pool. Wild rice is a key source of diversity for domesticated rice. Genome sequencing has suggested that the wild rice populations in northern Australia may include novel taxa, within the AA genome group of close (interfertile) wild relatives of domesticated rice that have evolved independently due to geographic separation and been isolated from the loss of diversity associated with gene flow from the large populations of domesticated rice in Asia. Australian wild rice was collected from 27 sites from Townsville to the northern tip of Cape York. Whole chloroplast genome sequences and 4,555 nuclear gene sequences (more than 8 Mbp) were used to explore genetic relationships between these populations and other wild and domesticated rices. Analysis of the chloroplast and nuclear data showed very clear evidence of distinctness from other AA genome Oryza species with significant divergence between Australian populations. Phylogenetic analysis suggested the Australian populations represent the earliest-branching AA genome lineages and may be critical resources for global rice food security. Nuclear genome analysis demonstrated that the diverse O. meridionalis populations were sister to all other AA genome taxa while the Australian O. rufipogon-like populations were associated with the clade that included domesticated rice. Populations of apparent hybrids between the taxa were also identified suggesting ongoing dynamic evolution of wild rice in Australia. These introgressions model events similar to those likely to have been involved in the domestication of rice.

Sequencing of Australian wild rice genomes reveals ancestral relationships with domesticated rice

The related A genome species of the Oryza genus are the effective gene pool for rice. Here, we report draft genomes for two Australian wild A genome taxa: O. rufipogon-like population, referred to as Taxon A, and O. meridionalis-like population, referred to as Taxon B. These two taxa were sequenced and assembled by integration of short- and long-read next-generation sequencing (NGS) data to create a genomic platform for a wider rice gene pool. Here, we report that, despite the distinct chloroplast genome, the nuclear genome of the Australian Taxon A has a sequence that is much closer to that of domesticated rice (O. sativa) than to the other Australian wild populations. Analysis of 4643 genes in the A genome clade showed that the Australian annual, O. meridionalis, and related perennial taxa have the most divergent (around 3 million years) genome sequences relative to domesticated rice. A test for admixture showed possible introgression into the Australian Taxon A (diverged around 1.6 million years ago) especially from the wild indica/O. nivara clade in Asia. These results demonstrate that northern Australia may be the centre of diversity of the A genome Oryza and suggest the possibility that this might also be the centre of origin of this group and represent an important resource for rice improvement.

Population Dynamics Among six Major Groups of the Oryza rufipogon Species Complex, Wild Relative of Cultivated Asian Rice

BACKGROUND

Understanding population structure of the wild progenitor of Asian cultivated rice (O. sativa), the Oryza rufipogon species complex (ORSC), is of interest to plant breeders and contributes to our understanding of rice domestication. A collection of 286 diverse ORSC accessions was evaluated for nuclear variation using genotyping-by-sequencing (113,739 SNPs) and for chloroplast variation using Sanger sequencing (25 polymorphic sites).

RESULTS

Six wild subpopulations were identified, with 25 % of accessions classified as admixed. Three of the wild groups were genetically and geographically closely related to the O. sativa subpopulations, indica, aus and japonica, and carried O. sativa introgressions; the other three wild groups were genetically divergent, had unique chloroplast haplotypes, and were located at the geographical extremes of the species range. The genetic subpopulations were significantly correlated (r ² = 0.562) with traditional species designations, O. rufipogon (perennial) and O. nivara (annual), differentiated based on morphology and life history. A wild diversity panel of 95 purified (inbred) accessions was developed for future genetic studies.

CONCLUSIONS

Our results suggest that the cultivated aus subpopulation is most closely related to an annual wild relative, japonica to a perennial wild relative, and indica to an admixed population of diverse annual and perennial wild ancestors. Gene flow between ORSC and O. sativa is common in regions where rice is cultivated, threatening the identity and diversity of wild ORSC populations. The three geographically isolated ORSC populations harbor variation rarely seen in cultivated rice and provide a unique window into the genetic composition of ancient rice subpopulations.

Chloroplast genomes: diversity, evolution, and applications in genetic engineering

Chloroplasts play a crucial role in sustaining life on earth. The availability of over 800 sequenced chloroplast genomes from a variety of land plants has enhanced our understanding of chloroplast biology, intracellular gene transfer, conservation, diversity, and the genetic basis by which chloroplast transgenes can be engineered to enhance plant agronomic traits or to produce high-value agricultural or biomedical products. In this review, we discuss the impact of chloroplast genome sequences on understanding the origins of economically important cultivated species and changes that have taken place during domestication. We also discuss the potential biotechnological applications of chloroplast genomes.

Genomics of crop wild relatives: expanding the gene pool for crop improvement

Plant breeders require access to new genetic diversity to satisfy the demands of a growing human population for more food that can be produced in a variable or changing climate and to deliver the high-quality food with nutritional and health benefits demanded by consumers. The close relatives of domesticated plants, crop wild relatives (CWRs), represent a practical gene pool for use by plant breeders. Genomics of CWR generates data that support the use of CWR to expand the genetic diversity of crop plants. Advances in DNA sequencing technology are enabling the efficient sequencing of CWR and their increased use in crop improvement. As the sequencing of genomes of major crop species is completed, attention has shifted to analysis of the wider gene pool of major crops including CWR. A combination of de novo sequencing and resequencing is required to efficiently explore useful genetic variation in CWR. Analysis of the nuclear genome, transcriptome and maternal (chloroplast and mitochondrial) genome of CWR is facilitating their use in crop improvement. Genome analysis results in discovery of useful alleles in CWR and identification of regions of the genome in which diversity has been lost in domestication bottlenecks. Targeting of high priority CWR for sequencing will maximize the contribution of genome sequencing of CWR. Coordination of global efforts to apply genomics has the potential to accelerate access to and conservation of the biodiversity essential to the sustainability of agriculture and food production.

Inconsistent diversities between nuclear and plastid genomes of AA genome species in the genus Oryza

AA genome species in the genus Oryza are valuable resources for improvement of cultivated rice. Oryza rufipogon and O. barthii were progenitors of two domesticated rice species, O. sativa and O. glaberrima, respectively. We used chloroplast single-nucleotide repeats (RCt1-10) to evaluate genetic diversity among AA genome species. Higher diversity was detected in the American species O. glumaepatula and the Asian species O. rufipogon. Other chloroplast sequences indicated that O. glumaepatula shares high similarity with O. longistaminata. Insertions of retrotransposable elements, however, showed a close relation between O. barthii and O. glumaepatula. To clarify phylogenetic relationships among AA genomes, whole-genome sequences obtained from different species were used to develop chloroplast INDEL markers. The INDEL patterns clearly showed multiple maternal origins of O. glumaepatula. The complicated origins have resulted in high genetic diversity in this species. In contrast, the Australian endemic species O. meridionalis tended to show narrower diversity than the other species. High variation in O. rufipogon, reconfirmed using the chloroplast INDELs, covered the variation in O. meridionalis and part of the variation in O. glumaepatula. Maternal lineages including O. barthii, O. longistaminata and the remainder of O. glumaepatula were phylogenetically close to each other and carried low genetic diversity. They were separated from independent lineages, suggesting that they had diverged from a single ancestral maternal lineage, but diverged later to keep gene flow within respective species, as SSR compositions suggested. Genetic relationships among AA genome species indicate how these species have evolved and become distributed across four continents.

Relationships of wild and domesticated rices (Oryza AA genome species) based upon whole chloroplast genome sequences

Rice is the most important crop in the world, acting as the staple food for over half of the world’s population. The evolutionary relationships of cultivated rice and its wild relatives have remained contentious and inconclusive. Here we report on the use of whole chloroplast sequences to elucidate the evolutionary and phylogenetic relationships in the AA genome Oryza species, representing the primary gene pool of rice. This is the first study that has produced a well resolved and strongly supported phylogeny of the AA genome species. The pan tropical distribution of these rice relatives was found to be explained by long distance dispersal within the last million years. The analysis resulted in a clustering pattern that showed strong geographical differentiation. The species were defined in two primary clades with a South American/African clade with two species, O glumaepatula and O longistaminata, distinguished from all other species. The largest clade was comprised of an Australian clade including newly identified taxa and the African and Asian clades. This refined knowledge of the relationships between cultivated rice and the related wild species provides a strong foundation for more targeted use of wild genetic resources in rice improvement and efforts to ensure their conservation.

Direct chloroplast sequencing: comparison of sequencing platforms and analysis tools for whole chloroplast barcoding

Direct sequencing of total plant DNA using next generation sequencing technologies generates a whole chloroplast genome sequence that has the potential to provide a barcode for use in plant and food identification. Advances in DNA sequencing platforms may make this an attractive approach for routine plant identification. The HiSeq (Illumina) and Ion Torrent (Life Technology) sequencing platforms were used to sequence total DNA from rice to identify polymorphisms in the whole chloroplast genome sequence of a wild rice plant relative to cultivated rice (cv. Nipponbare). Consensus chloroplast sequences were produced by mapping sequence reads to the reference rice chloroplast genome or by de novo assembly and mapping of the resulting contigs to the reference sequence. A total of 122 polymorphisms (SNPs and indels) between the wild and cultivated rice chloroplasts were predicted by these different sequencing and analysis methods. Of these, a total of 102 polymorphisms including 90 SNPs were predicted by both platforms. Indels were more variable with different sequencing methods, with almost all discrepancies found in homopolymers. The Ion Torrent platform gave no apparent false SNP but was less reliable for indels. The methods should be suitable for routine barcoding using appropriate combinations of sequencing platform and data analysis.

Exploring natural selection to guide breeding for agriculture

Climate change threatens reduced crop production and poses major challenges to food security. The breeding of climate-resilient crop varieties is increasingly urgent. Wild plant populations evolve to cope with changes in their environment due to the forces of natural selection. This adaptation may be followed over time in populations at the same site or explored by examining differences between populations growing in different environments or across an environmental gradient. Survival in the wild has important differences to the objective of agriculture to maximize crop yields. However, understanding the nature of adaptation in wild populations at the whole genome level may suggest strategies for crop breeding to deliver agricultural production with more resilience to climate variability.

Chloroplast Genome of Novel Rice Germplasm Identified in Northern Australia

Rice (Oryza sativa L.) was probably domesticated from O. rufipogon in Asia in the last 10,000 years. Relatives of cultivated rice (A genome species of Oryza) are found in South America, Africa, Australia and Asia. These A genome species are the close relatives of cultivated rice and represent the effective gene pool for rice improvement. Members of this group in Northern Australia include, an annual species, O. meridionalis, and two recently distinguished perennial taxa, to one of which the name O. rufipogon has been applied and the other a perennial form of O. meridionalis. Comparison of whole chloroplast genome sequences of these taxa has now been used to determine the relationships between the wild taxa and cultivated rice. The chloroplast genomes of the perennials were both found to be distinguished from O. rufipogon from Asia by 124 or 125 variations and were distinguished from each other by 53 variations. These populations have remained isolated from the overwhelming genetic impact of the large domesticated rice populations in Asia and may be unique descendants of the gene pool from which domesticated rice arose. The conservation of this wild genetic resource may be critical for global food security.

Genomics strategies for germplasm characterization and the development of climate resilient crops

Food security requires the development and deployment of crop varieties resilient to climate variation and change. The study of variations in the genome of wild plant populations can be used to guide crop improvement. Genome variation found in wild crop relatives may be directly relevant to the breeding of environmentally adapted and climate resilient crops. Analysis of the genomes of populations growing in contrasting environments will reveal the genes subject to natural selection in adaptation to climate variations. Whole genome sequencing of these populations should define the numbers and types of genes associated with climate adaptation. This strategy is facilitated by recent advances in sequencing technologies. Wild relatives of rice and barley have been used to assess these approaches. This strategy is most easily applied to species for which a high quality reference genome sequence is available and where populations of wild relatives can be found growing in diverse environments or across environmental gradients.