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Diversity of drought-responsive genes in central and peripheral populations of Trifolium purpureum Loisel. Recibido: 24 de Febrero del Aceptado: 3 de Noviembre del Drought-responsive genes may differ in structure and complexity in native populations of a species established in different ecosystems. Peripheral populations may be a source of genetic variability for breeding cultivated plants for abiotic stresses tolerance and the target for core collections in germplasm preservation programs.
Genetic studies including both peripheral and central populations are still limited. This research evaluated genetic diversity of drought-responsive genes in peripheral and central populations of Trifolium purpureum Loisel. Genomic DNA isolated from leaves of three northern and three southern populations of Israel was digested with restriction enzymes and hybridized with four drought-induced and four drought-repressed gene fragments. Gene diversity of drought-induced genes ranged from 0.
Gene diversity for drought-repressed genes ranged from 0. Purpureum than in central ones. Variation within populations for both drought-induced and drought-repressed genes was the main component of molecular variance. Fixation index F ST for drought-induced genes was between 0. Results of this study show that peripheral population might be a reservoir for drought-responsive genes.
Key words: Trifolium purpureum, abiotic stress, adaptation, arid lands, gene flow, molecular variation. Plant adaptation to the environment involves a wide range of development strategies, morphological features, biochemical mechanisms, and physiological traits under genetic control Yamaguchi-Shinozaki and Shinozaki, Genes involved in the responses of plants to short-term and season-long water deficit have been studied in great detail in many agronomic plants but in much less detail in wild species.
Studies in some cultivated plants have shown that there is a number of genes and gene families expressed under drought stress Wang et al. These drought-induced genes can be modeled to be involved in morphological, physiological and molecular features that make the plants able to deal with water deficit Shinozaki and Yamaguchi-Shinozaki, Drought-responsive gene families may differ in structure and complexity in native populations of a species established in different ecosystems.
The central-marginal model in evolutionary ecology proposes that populations near the center of the range central of a species are highly dense, and show high levels of phenotypic and genetic variation, while populations on the edge of the range peripheral are isolated, bare, and chromosomally monomorphic Brussard, The center of the range of a species will coincide with the geographic region that is the most ecologically favorable. The periphery of a range will usually correspond to an area of extreme marginality for the species Vucetich and Waite, Thus, peripheral populations are expected to be genetically distinct from central populations since they may face different selection conditions.
The differences between central and marginal populations will depend on the differences of the effective population size and the gene flow Eckert et al. Peripheral populations may constitute a source of genetic variability for breeding cultivated plants for tolerance to abiotic stresses. They may also be the target for constructing core collections in germplasm preservation programs. However, genetic studies including both peripheral and central populations are still limited Eckert et al.
Nowadays, peripheral populations are acquiring importance for gene conservation, because they may possess genotypes of future adaptive potential especially under climate change conditions Pandey and Rajora, a. Purple clover Trifolium purpureum Loisel is a species native to Mediterranean regions of Europe.
This species is now found as natural populations in regions with environments more hostile than those from where it was originated. Therefore, some T. Other populations may be called peripheral as they populate unfavorable environments.
The presence of this species in grasslands of temperate and semiarid regions may involve differences in diversity of drought-responsive genes among populations. In this study, several drought-responsive genes were used to perform RFLP analyses on central and peripheral populations of T.
The Northern region from where one group of the seeds was collected has a Mediterranean climate with annual winter rain of mm. Seeds were grouped as central northern located or peripheral southern located according to their origin.
Drought-responsive cDNA clones from T. Drought-induced and drought-repressed cDNA clones were confirmed by Northern analysis and later sequenced. Eight drought-responsive cDNAs were used as probes for the analysis of gene diversity of T. The digested samples were electrophoresed in a 0. Four replica gels were made for each population. DNA was transferred onto Zetabind nylon membranes using alkaline solution 0. The membranes were prehybridized overnight in 0. After hybridization, membranes were washed with 0.
The membranes were exposed to x-ray film Midwest Scientific at o C for varying periods of time 24 hours to 7 days Figure 2B.
The polymorphisms for each of the probes were identified on autoradiograms and scored as binary data Figure 2B. The matrices generated were analyzed with Popgene, version 1.
An analysis of molecular variance AMOVA was performed taking into account the frequencies of haplotypes generated by each probe.
Drought-induced genes showed great complexity as measured by the number of different fragments in the hybridized Southern blots of T.
Several polymorphic loci were identified by using drought-induced cDNAs as probes. The number of polymorphic loci produced by the hybridization of southern blots containing genomic DNA of T. Drought-repressed genes showed similar results to drought-induced genes regarding the number of loci and the complexity of the genes.
Drought-repressed genes produced 6 to 11 loci when used to hybridize Southern blots containing genomic DNA from T. The number of polymorphic loci may vary depending on the species and markers used.
Thus, Coulibaly et al. The amount of genetic variability of a population is a function of the genetic diversity originally available to the species and of the later influence of processes such as selection, gene flow, and the mating system Despres et al. The differences of average gene diversity between central and peripheral populations were not significant for any of the drought-induced genes used as probes.
Khanlou et al. The AMOVA showed that variation within populations was the main component of the variance for the drought-induced genes. In addition, variation among populations within groups was highly variable in the four drought-induced genes. The difference of variance among populations within groups was significant for all the genes, except for the one that codes for arginine decarboxylase.
In contrast, variation among groups of all drought-induced genes was very low and not significant Table 2. The estimated values of gene flow for drought-induced genes were high with a relatively wide range. The average gene flow for the group of drought-induced genes was 3. Also, Welt et al. Fixation index F ST showed that in three out of the four drought-induced genes there were significant differences among populations, with values ranging from 0.
The fixation index F ST is a suitable measure of genetic differentiation among subpopulations populations within groups. This index measures the overall reduction in average heterozygosity in subpopulations regarding the total heterozygosity, and is the most inclusive measure of population substructure. F ST has a theoretical minimum of 0 when there is not genetic divergence and a maximum of 1 when alternative alleles are fixed in different subpopulations.
Thus, F ST is used as a relative measure of population structure and a comparative estimation of gene flow Miller et al. Gene diversity of drought-induced genes in central and peripheral populations of T. The expected heterozygosity of 12 allozymes in 9 populations of T. The relatively high gene diversity for drought-induced genes observed in T. Cross pollinated species tend to have higher levels of variability within populations but a smaller degree of differentiation among populations than selfing species.
Moreover, genetic differentiation is greater in annuals than in perennials Fisher et al. However, average gene diversity was not significantly different in central or peripheral populations for individual drought-repressed genes.
Gene differentiation among populations, measured as fixation index F ST , was significant for all drought-repressed genes in T. Also, the estimated gene flow was high for all four drought-repressed genes Table 4. Variation among groups central vs.
Hagen and Hamrick found that populations of T. The total genetic variation due to differences among regions and among populations was 0. The main component of variation of drought-responsive genes in central and peripheral populations of T. The high variation within populations may be explained by the mating system of the species and their wide distribution. In general, genetic diversity for drought-responsive genes in T purpureum was high. On the contrary, in some species a very low genetic diversity has been found.
In populations of wild rice Oryza granulata the gene diversity was in the range of 0. The low gene diversity was attributed to geographically narrow distribution of the populations as a colonizing species Gao et al. Some studies show a higher genetic differentiation in peripheral populations than in central populations, such as those on eastern white cedar Thuja occidentalis L.
Pandey and Rajora, b. ABA stress response genes Asr1 and Asr2 had different gene diversity in wild and cultivated populations of Phaseolus vulgaris L. Also, Trejo-Calzada and O'Connell showed that populations of Dactylis glomerata from an arid region of Israel had a greater genetic diversity for drought-responsive genes 0.
Genetic diversity for both drought-induced and drought-repressed genes was high; however, the differences in average gene diversity between central and peripheral populations were not significant for any of the drought-responsive genes. On the contrary, variation among groups for all individual drought-induced and drought-repressed genes was very low and not significant.
Gene differentiation or fixation index F ST was significant for all the genes used as probes in this study, except for the one that code for arginine decarboxylase.
The estimated gene flow was relatively high for drought-induced and drought-repressed genes. Brussard P. Annual Review of Ecology and Systematics Chavarro, S.
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