Genetic effects on microsatellite diversity in wild emmer wheat (Triticum dicoccoides) at the Yehudiyya microsite, Israel

Wild Emmer (Triticum turgidum ssp. dicoccoides) Diversity in Southern Turkey: Evaluation of SSR and Morphological Variations

Wild emmer wheat (Triticum turgidum ssp. dicoccoides) is the ancestral species of cultivated tetraploid wheat with BBAA genomes. Because of its full interfertility with domesticated emmer wheat, this wild species can serve as one of the most important genetic resources to improve durum and bread wheat. To clarify the magnitude of genetic diversity between and within populations of Turkish wild emmer wheat, 169 genotypes of ssp. dicoccoides selected from the 38 populations collected from the three sub-regions (East-1, West-1, and West-2) of the Southeast Anatolia Region of Turkey were molecularly and morphologically characterized. The populations showed significant variation in plant height, heading date, flag leaf area, spike length and number, spikelet, peduncle, lemma, palea, glume and anther lengths, glume hull thickness, anther width, and days to maturity. According to the results of nuclear-SSR analysis, the populations collected from the sub-regions East-1 and West-2 were the most genetically distant (0.539), while the populations collected from the sub-regions West-1 and West-2 were the most genetically similar (0.788) populations. According to the results of AMOVA, there was 84% similarity within the populations studied, while the variation between the populations of the three sub-regions was 16%. In the dendrogram obtained by using nuclear-SSR data, the populations formed two main groups. The populations from the sub-region East-1 were in the first group, and the populations from the sub-regions West-1 and West-2 were in the second group. From the dendrogram, it appears that the populations from the sub-region East-1 were genetically distant from the populations from the sub-regions West-1 and West-2. The results highlight the potential diversity in Southeast Anatolia for wild emmer discovery and utilization.

Defects in the GINS complex increase the instability of repetitive sequences via a recombination-dependent mechanism

Faithful replication and repair of DNA lesions ensure genome maintenance. During replication in eukaryotic cells, DNA is unwound by the CMG helicase complex, which is composed of three major components: the Cdc45 protein, Mcm2-7, and the GINS complex. The CMG in complex with DNA polymerase epsilon (CMG-E) participates in the establishment and progression of the replisome. Impaired functioning of the CMG-E was shown to induce genomic instability and promote the development of various diseases. Therefore, CMG-E components play important roles as caretakers of the genome. In Saccharomyces cerevisiae, the GINS complex is composed of the Psf1, Psf2, Psf3, and Sld5 essential subunits. The Psf1-1 mutant form fails to interact with Psf3, resulting in impaired replisome assembly and chromosome replication. Here, we show increased instability of repeat tracts (mononucleotide, dinucleotide, trinucleotide and longer) in yeast psf1-1 mutants. To identify the mechanisms underlying this effect, we analyzed repeated sequence instability using derivatives of psf1-1 strains lacking genes involved in translesion synthesis, recombination, or mismatch repair. Among these derivatives, deletion of RAD52, RAD51, MMS2, POL32, or PIF1 significantly decreased DNA repeat instability. These results, together with the observed increased amounts of single-stranded DNA regions and Rfa1 foci suggest that recombinational mechanisms make important contributions to repeat tract instability in psf1-1 cells. We propose that defective functioning of the CMG-E complex in psf1-1 cells impairs the progression of DNA replication what increases the contribution of repair mechanisms such as template switch and break-induced replication. These processes require sequence homology search which in case of a repeated DNA tract may result in misalignment leading to its expansion or contraction.

Processes that ensure genome stability are crucial for all organisms to avoid mutations and decrease the risk of diseases. The coordinated activity of mechanisms underlying the maintenance of high-fidelity DNA duplication and repair is critical to deal with the malfunction of replication forks or DNA damage. Repeated sequences in DNA are particularly prone to instability; these sequences undergo expansions or contractions, leading in humans to various neurological, neurodegenerative, and neuromuscular disorders. A mutant form of one of the noncatalytic subunits of active DNA helicase complex impairs DNA replication. Here, we show that this form also significantly increases the instability of mononucleotide, dinucleotide, trinucleotide and longer repeat tracts. Our results suggest that in cells that harbor a mutated variant of the helicase complex, continuation of DNA replication is facilitated by recombination processes, and this mechanism can be highly mutagenic during repair synthesis through repetitive regions, especially regions that form secondary structures. Our results indicate that proper functioning of the DNA helicase complex is crucial for maintenance of the stability of repeated DNA sequences, especially in the context of recently described disorders in which mutations or deregulation of the human homologs of genes encoding DNA helicase subunits were observed.

Living Organisms Author Their Read-Write Genomes in Evolution

Evolutionary variations generating phenotypic adaptations and novel taxa resulted from complex cellular activities altering genome content and expression: (i) Symbiogenetic cell mergers producing the mitochondrion-bearing ancestor of eukaryotes and chloroplast-bearing ancestors of photosynthetic eukaryotes; (ii) interspecific hybridizations and genome doublings generating new species and adaptive radiations of higher plants and animals; and, (iii) interspecific horizontal DNA transfer encoding virtually all of the cellular functions between organisms and their viruses in all domains of life. Consequently, assuming that evolutionary processes occur in isolated genomes of individual species has become an unrealistic abstraction. Adaptive variations also involved natural genetic engineering of mobile DNA elements to rewire regulatory networks. In the most highly evolved organisms, biological complexity scales with "non-coding" DNA content more closely than with protein-coding capacity. Coincidentally, we have learned how so-called "non-coding" RNAs that are rich in repetitive mobile DNA sequences are key regulators of complex phenotypes. Both biotic and abiotic ecological challenges serve as triggers for episodes of elevated genome change. The intersections of cell activities, biosphere interactions, horizontal DNA transfers, and non-random Read-Write genome modifications by natural genetic engineering provide a rich molecular and biological foundation for understanding how ecological disruptions can stimulate productive, often abrupt, evolutionary transformations.

Solar Radiation-Associated Adaptive SNP Genetic Differentiation in Wild Emmer Wheat, Triticum dicoccoides

Whole-genome scans with large number of genetic markers provide the opportunity to investigate local adaptation in natural populations and identify candidate genes under positive selection. In the present study, adaptation genetic differentiation associated with solar radiation was investigated using 695 polymorphic SNP markers in wild emmer wheat originated in a micro-site at Yehudiyya, Israel. The test involved two solar radiation niches: (1) sun, in-between trees; and (2) shade, under tree canopy, separated apart by a distance of 2-4 m. Analysis of molecular variance showed a small (0.53%) but significant portion of overall variation between the sun and shade micro-niches, indicating a non-ignorable genetic differentiation between sun and shade habitats. Fifty SNP markers showed a medium (0.05 ≤ FST ≤ 0.15) or high genetic differentiation (FST > 0.15). A total of 21 outlier loci under positive selection were identified by using four different FST -outlier testing algorithms. The markers and genome locations under positive selection are consistent with the known patterns of selection. These results suggested that genetic differentiation between sun and shade habitats is substantial, radiation-associated, and therefore ecologically determined. Hence, the results of this study reflected effects of natural selection through solar radiation on EST-related SNP genetic diversity, resulting presumably in different adaptive complexes at a micro-scale divergence. The present work highlights the evolutionary theory and application significance of solar radiation-driven natural selection in wheat improvement.

Structure and extent of DNA methylation-based epigenetic variation in wild emmer wheat (T. turgidum ssp. dicoccoides) populations

BACKGROUND

The genetic structure and differentiation of wild emmer wheat suggests that genetic diversity is eco-geographically structured. However, very little is known about the structure and extent of the heritable epigenetic variation and its influence on local adaptation in natural populations.

RESULTS

The structure and extent of the heritable methylation-based epigenetic variation were assessed within and among natural populations of Triticum turgidum ssp. dicoccoides. We used methylation sensitive amplified polymorphism (MSAP) and transposon methylation display (TMD) techniques, to assess the methylation status of random genomic CCGG sites and CCGG sites flanking transposable elements (TEs), respectively. Both techniques were applied to the DNA of 50 emmer accessions which were collected from five different geographically isolated regions. In order to ensure the assessment of heritable epigenetic variation, all accessions were grown under common garden conditions for two generations. In all accessions, the difference in methylation levels of CCGG sites, including CCGG sites that flanked TEs, were not statistically significant and relatively high, ranging between 46 and 76 %. The pattern of methylation was significantly different among accessions, such that clear and statistically significant population-specific methylation patterns were observed.

CONCLUSION

In this study, we have observed population-unique heritable methylation patterns in emmer wheat accessions originating from five geographically isolated regions. Our data indicate that methylation-based epigenetic diversity might be eco-geographically structured and might be partly determined by climatic and edaphic factors.

Natural selection causes adaptive genetic resistance in wild emmer wheat against powdery mildew at "Evolution Canyon" microsite, Mt. Carmel, Israel

Background

“Evolution Canyon” (ECI) at Lower Nahal Oren, Mount Carmel, Israel, is an optimal natural microscale model for unraveling evolution in action highlighting the basic evolutionary processes of adaptation and speciation. A major model organism in ECI is wild emmer, Triticum dicoccoides, the progenitor of cultivated wheat, which displays dramatic interslope adaptive and speciational divergence on the tropical-xeric “African” slope (AS) and the temperate-mesic “European” slope (ES), separated on average by 250 m.

Methods

We examined 278 single sequence repeats (SSRs) and the phenotype diversity of the resistance to powdery mildew between the opposite slopes. Furthermore, 18 phenotypes on the AS and 20 phenotypes on the ES, were inoculated by both Bgt E09 and a mixture of powdery mildew races.

Results

In the experiment of genetic diversity, very little polymorphism was identified intra-slope in the accessions from both the AS or ES. By contrast, 148 pairs of SSR primers (53.23%) amplified polymorphic products between the phenotypes of AS and ES. There are some differences between the two wild emmer wheat genomes and the inter-slope SSR polymorphic products between genome A and B. Interestingly, all wild emmer types growing on the south-facing slope (SFS=AS) were susceptible to a composite of Blumeria graminis, while the ones growing on the north-facing slope (NFS=ES) were highly resistant to Blumeria graminis at both seedling and adult stages.

Conclusion/Significance

Remarkable inter-slope evolutionary divergent processes occur in wild emmer wheat, T. dicoccoides at EC I, despite the shot average distance of 250 meters. The AS, a dry and hot slope, did not develop resistance to powdery mildew, whereas the ES, a cool and humid slope, did develop resistance since the disease stress was strong there. This is a remarkable demonstration in host-pathogen interaction on how resistance develops when stress causes an adaptive result at a micro-scale distance.

Analysis of BAC-end sequences in common bean (Phaseolus vulgaris L.) towards the development and characterization of long motifs SSRs

The increasing volume of genomic data on the Phaseolus vulgaris species have contributed to its importance as a model genetic species and positively affected the investigation of other legumes of scientific and economic value. To expand and gain a more in-depth knowledge of the common bean genome, the ends of a number of bacterial artificial chromosome (BAC) were sequenced, annotated and the presence of repetitive sequences was determined. In total, 52,270 BESs (BAC-end sequences), equivalent to 32 Mbp (~6 %) of the genome, were processed. In total, 3,789 BES-SSRs were identified, with a distribution of one SSR (simple sequence repeat) per 8.36 kbp and 2,000 were suitable for the development of SSRs, of which 194 were evaluated in low-resolution screening. From 40 BES-SSRs based on long motifs SSRs (≥ trinucleotides) analyzed in high-resolution genotyping, 34 showed an equally good amplification for the Andean and for the Mesoamerican genepools, exhibiting an average gene diversity (H E) of 0.490 and 5.59 alleles/locus, of which six classified as Class I showed a H E ≥ 0.7. The PCoA and structure analysis allowed to discriminate the gene pools (K = 2, FST = 0.733). From the 52,270 BESs, 2 % corresponded to transcription factors and 3 % to transposable elements. Putative functions for 24,321 BESs were identified and for 19,363 were assigned functional categories (gene ontology). This study identified highly polymorphic BES-SSRs containing tri- to hexanucleotides motifs and bringing together relevant genetic characteristics useful for breeding programs. Additionally, the BESs were incorporated into the international genome-sequencing project for the common bean.

Evolution under environmental stress at macro- and microscales

Environmental stress has played a major role in the evolution of living organisms (Hoffman AA, Parsons PA. 1991. Evolutionary genetics and environmental stress. Oxford: Oxford University Press; Parsons PA. 2005. Environments and evolution: interactions between stress, resource inadequacy, and energetic efficiency. Biol Rev Camb Philos Soc. 80:589–610). This is reflected by the massive and background extinctions in evolutionary time (Nevo E. 1995a. Evolution and extinction. Encyclopedia of Environmental Biology. New York: Academic Press, Inc. 1:717–745). The interaction between organism and environment is central in evolution. Extinction ensues when organisms fail to change and adapt to the constantly altering abiotic and biotic stressful environmental changes as documented in the fossil record. Extreme environmental stress causes extinction but also leads to evolutionary change and the origination of new species adapted to new environments. I will discuss a few of these global, regional, and local stresses based primarily on my own research programs. These examples will include the 1) global regional and local experiment of subterranean mammals; 2) regional experiment of fungal life in the Dead Sea; 3) evolution of wild cereals; 4) “Evolution Canyon”; 5) human brain evolution, and 6) global warming.

Characterization and comparison of gene-based simple sequence repeats across Brassica species

Simple sequence repeats (SSRs) are important components of eukaryotic genomes and may play an important role in regulating gene expression. However, the characteristics of genic SSRs and the effect of interspecific hybridization and polyploidization on genic SSRs seem not to have received desired attention in terms of scientific investigations. To determine the features of genic SSRs and elucidate their role in polyploidization process of the Brassica family, we identified SSRs in Plant Genome Database-assembled unique transcripts (PUTs) of Brassica species. A higher density of SSRs and a greater number of compound motif SSRs and mononucleotide motif types with large average number of repeats were detected in allotetraploid Brassica napus than in the diploid parental species (Brassica rapa and Brassica oleracea). In addition, a greater proportion of SSR-PUTs were found to be associated with the stress response and developmental processes in B. napus than in the parents. A negative correlation between the repeat number and the motif type and the total length, and a positive correlation between the repeat number and the total length of SSRs were observed. PUT-SSR might be generated from A/T-rich regions. The successful development of 123 pairs of SSR primers for Brassica PUTs showed that SSR-PUTs could be exploited as gene-based SSR functional markers for application in Brassica breeding. These results indicate that interspecific hybridization and polyploidization could trigger the amplification of SSRs, and long SSRs might become shorter to enable the plant to adapt to environmental and artificial selection.

A flexible quantitative methodology for the analysis of gene-flow between conventionally bred maize populations using microsatellite markers

Previous studies of gene-flow in agriculture have used a range of physical and biochemical markers, including transgenes. However, physical and biochemical markers are not available for all commercial varieties, and transgenes are difficult to use when trying to estimate gene flow in the field where the use of transgenes is often restricted. Here, we demonstrate the use of simple sequence repeat microsatellite markers (SSRs) to study gene flow in maize. Developing the first quantitative analysis of pooled SSR samples resulted in a high sampling efficiency which minimised the use of resources and greatly enhanced the possibility of hybrid detection. We were able to quantitatively distinguish hybrids in pools of ten samples from non-hybrid parental lines in all of the 24 pair-wise combinations of commercial varieties tested. The technique was used to determine gene flow in field studies, from which a simple model describing gene flow in maize was developed.

EST-SSR diversity correlated with ecological and genetic factors of wild emmer wheat in Israel

The differentiation of genetic diversity was estimated among 15 wild emmer wheat (Triticum dicoccoides) populations of the macrogeographic scale in Israel by 25 EST-SSR markers. A total of 92 EST-SSR alleles were detected, and the number of alleles ranged from 1 to 7 with an average of 3.68 per locus. Allele numbers and the polymorphic information content (PIC)value of EST-SSR loci on the B genome were higher than those on the A genome. The genetic similarity coefficient (GS)varied from 0.189 to 0.966, and all genotypes were clustered into four major groups. The population Mt. Gerizim had the highest genetic variations, whereas the population Beit-Oren had the lowest genetic variations. Most of genetic variance existed within populations was observed based on the coefficient of gene differentiation (F(ST)=0.355). The value of genetic distance (D) between the populations varied from 0.112 to 0.672 with an average of 0.406, and the results of Mantel test(r=0.104, p=0.809) showed that the estimates of genetic distance were geographically independent. The values of Nei's gene diversity (He) and Shannon's information index (I) correlated negatively with the temperature factor: mean January temperature (Tj), whereas they correlated positively with another factor: mean number of Sharav days (Sh). The correlation matrix between He in the EST-SSRs and climatic variables contained 37 significant (pB0.05) correlations. The present study established that T. dicoccoides in Israel had a considerable amount of genetic variations at EST-SSR loci at least partly correlated with ecological factors. These results suggested that EST-SSR diversity is adaptive by natural selection and influenced by both internal and external factors and their interactions.

Meiosis-driven genome variation in plants

Meiosis includes two successive divisions of the nucleus with one round of DNA replication and leads to the formation of gametes with half of the chromosomes of the mother cell during sexual reproduction. It provides a cytological basis for gametogenesis and nheritance in eukaryotes. Meiotic cell division is a complex and dynamic process that involves a number of molecular and cellular events, such as DNA and chromosome replication, chromosome pairing, synapsis and recombination, chromosome segregation, and cytokinesis. Meiosis maintains genome stability and integrity over sexual life cycles. On the other hand, meiosis generates genome variations in several ways. Variant meiotic recombination resulting from specific genome structures induces deletions, duplications, and other rearrangements within the genic and non-genic genomic regions and has been considered a major driving force for gene and genome evolution in nature. Meiotic abnormalities in chromosome segregation lead to chromosomally imbalanced gametes and aneuploidy. Meiotic restitution due to failure of the first or second meiotic division gives rise to unreduced gametes, which triggers polyploidization and genome expansion. This paper reviews research regarding meiosis-driven genome variation, including deletion and duplication of genomic regions, aneuploidy, and polyploidization, and discusses the effect of related meiotic events on genome variation and evolution in plants. Knowledge of various meiosis-driven genome variations provides insight into genome evolution and genetic variability in plants and facilitates plant genome research.

High somatic instability of a microsatellite locus in a clonal tree, Robinia pseudoacacia

Robinia pseudoacacia L. is a clonal tree species. To investigate a mutation within eight microsatellite loci of R. pseudoacacia, we analyzed DNA samples obtained from different leaf samples within each ramet, leaves from ramets within the genet, and seeds. Of the eight loci, locus Rops15 (AG motif) displayed hypermutability. The mutation rates of Rops15 within each ramet, among ramets within the genet, and offspring were 6.27% (ranging from 0 to 31.1%), 6.11% (from 0 to 25.0%) and 3.78% (from 0 to 10.9%), respectively. The mutation rate increased with allele size (13-71 repeat units). The mutation patterns observed in Rops15 were distinctive in two ways. First, there was a significant bias toward additions over deletions, and both addition and deletion of single repeats were dominant at alleles with lengths less than 232 bp (63 repeats). Second, for the longest allele of 248 bp (71 repeats), the number of losses was higher than the number of gains. These observations suggest that the mutation patterns of microsatellites in R. pseudoacacia may follow a generalized stepwise mutation model, and that the tendency of long alleles to mutate to shorter lengths acts to prevent infinite growth. Finally, the observation of somatic hypermutability at locus Rops15 highlights the need for caution when using highly polymorphic microsatellites for population genetic structure and paternity analysis in tree species.