Plant Domestication and Crop Evolution in the Near East: On Events and Processes

Reconstructing the evolutionary history of crop plants is fundamental for understanding their adaptation profile and the genetic basis of yield-limiting factors, which in turn are critical for future crop improvement. A major topic in this field is the recent claim for a millennia-long ‘protracted’ domestication process. Here we evaluate the evidence for the protracted domestication model in light of published archaeobotanical data, experimental evidence and the biology of the Near Eastern crops and their wild progenitors. The crux of our discussion is the differentiation between events or ‘domestication episodes’ and the later following crop evolutionary processes under domestication (frequently termed ‘crop improvement stage’), which are by definition, still ongoing. We argue that by assuming a protracted millennia-long domestication process, one needlessly opts to operate within an intellectual framework that does not allow differentiating between the decisive (critical) domestication traits and their respective loci, and those that have evolved later during the crop dissemination and improvement following the episodic domestication event. Therefore, in our view, apart from the lack of experimental evidence to support it, the protracted domestication assumption undermines the resolution power of the study of both plant domestication and crop evolution, from the cultural as well as from the biological perspectives.     

Genetic Relationship in Cicer Sp. Expose Evidence for Geneflow between the Cultigen and Its Wild Progenitor

There is a debate concerning mono- or poly-phyletic origins of the Near Eastern crops. In parallel, some authors claim that domestication was not possible within the natural range of the wild progenitors due to wild alleles flow into the nascent crops. Here we address both, the mono- or poly-phyletic origins and the domestications within or without the natural range of the progenitor, debates in order to understand the relationship between domesticated chickpea (Cicer arietinum L.) and its wild progenitor (C. reticulatum Ladizinsky) with special emphasis on its domestication centre in southeastern Turkey. A set of 103 chickpea cultivars and landraces from the major growing regions alongside wild accessions (C. reticulatum, C. echinospermum P.H Davis and C. bijugum K.H. Rech) sampled across the natural distribution range in eastern Turkey were genotyped with 194 SNPs markers. The genetic affinities between and within the studied taxa were assessed. The analysis suggests a mono-phyletic origin of the cultigen, with several wild accession as likely members of the wild stock of the cultigen. Clear separation between the wild and domesticated germplasm was apparent, with negligible level of admixture. A single C. reticulatum accession shows morphological and allelic signatures of admixture, a likely result of introgression. No evidence of geneflow from the wild into domesticated germplasm was found. The traditional farming systems of southeaster Turkey are characterized by occurrence of sympatric wild progenitor—domesticated forms of chickpea (and likewise cereals and other grain legumes). Therefore, both the authentic crop landraces and the wild populations native to the area are a unique genetic resource. Our results grant support to the notion of domestication within the natural distribution range of the wild progenitor, suggesting that the Neolithic domesticators were fully capable of selecting the desired phenotypes even when facing rare wild-domesticated introgression events.     

Gibberellic acid (GA) increases fibre cell differentiation and secondary cell-wall deposition in spring wheat (Triticum aestivum L.) culms

The role of gibberellic acid (GA) in differentiation and secondary cell-wall deposition of fibre cells of spring wheat (Triticum aestivum) culms was studied using applications of GA and chlormequat (a GA biosynthesis inhibitor). In certain genotypes, higher GA levels may increase the number of cortical fibre cell files by changing cell fate from parenchyma to fibre, and induce thicker secondary cell-walls.     

Construction of BAC and BIBAC libraries and their applications for generation of SSR markers for genome analysis of chickpea, Cicer arietinum L.

Large-insert bacterial artificial chromosome (BAC) libraries, plant-transformation-competent binary BAC (BIBAC) libraries, and simple sequence repeat (SSR) markers are essential for many aspects of genomics research. We constructed a BAC library and a BIBAC library from the nuclear DNA of chickpea, Cicer arietinum L., cv. Hadas, partially digested with HindIII and BamHI, respectively. The BAC library has 14,976 clones, with an average insert size of 121 kb, and the BIBAC library consists of 23,040 clones, with an average insert size of 145 kb. The combined libraries collectively cover ca. 7.0× genomes of chickpea. We screened the BAC library with eight synthetic SSR oligos, (GA)₁₀, (GAA)₇, (AT)₁₀, (TAA)₇, (TGA)₇, (CA)₁₀, (CAA)₇, and (CCA)₇. Positive BACs were selected, subcloned, and sequenced for SSR marker development. Two hundred and thirty-three new chickpea SSR markers were developed and characterized by PCR, using chickpea DNA as template. These results have demonstrated that BACs are an excellent source for SSR marker development in chickpea. We also estimated the distribution of the SSR loci in the chickpea genome. The SSR motifs (TAA)_n and (GA)_n were much more abundant than the others, and the distribution of the SSR loci appeared non-random. The BAC and BIBAC libraries and new SSR markers will provide valuable resources for chickpea genomics research and breeding (the libraries and their filters are available to the public at ).J. Lichtenzveig and C. Scheuring contributed equally to this study.     

Cytogenetics of semi-fertile triploid and aneuploid intergeneric vine cacti hybrids

Crosses between the diploid Hylocereus polyrhizus, as the female parent, and the tetraploid Selenicereus megalanthus, as the male parent, yielded triploid and aneuploid hybrids. The fruits of these hybrids combined the attractive appearance of Hylocereus fruits with the delicious taste of S. megalanthus fruits. The aim of this work was to assess the fertility and breeding potential of the triploid and aneuploid hybrids with a view to developing an improved vine cactus crop. Pollen mother cells at metaphase I revealed univalents, bivalents, trivalents, and occasionally quadrivalents. Chromosome distribution at anaphase I revealed different classes of chromosome segregation as well as lagging chromosomes. At metaphase II, parallel and tripolar spindles were observed. The occurrence of triads was frequent, whereas dyads were rarely observed. Pollen stainability varied among the clones studied ranging from 9.8% to 18.6%. The diameters of the stained pollen grains varied widely, probably as a result of the number of chromosomes. Despite the allotriploid origin of our hybrids, functional female and male gametes were produced in considerable proportions, most likely as a result of balanced chromosome segregation. The triploid and aneuploid clones studied yielded viable seeds whose number per fruit was strongly dependent on the pollen donor.     

Chromosome doubling in vine cacti hybrids

We performed reciprocal crosses between the tetraploid Selenicereus megalanthus and the diploid Hylocereus species, H. undatus and H. polyrhizus. S. megalanthus x H. undatus gave rise to viable hexaploids and 6x-aneuploid hybrids rather than to the expected triploids. No genuine hybrids were obtained in the reciprocal cross. The pollen diameter of the tetraploid S. megalanthus varied widely, indicating the occurrence of unreduced gametes, while that of H. undatus pollen was very uniform, indicating an extremely low frequency of unreduced gametes. This finding suggests that the hexaploids were formed by chromosome doubling after the formation of the hybrid triploid zygote rather than by fusion of unreduced gametes of the two species.     

Genetic relationships among Hylocereus and Selenicereus vine cacti (Cactaceae): evidence from hybridization and cytological studies

BACKGROUND AND AIMS: Hylocereus and Selenicereus are native to tropical and sub-tropical America. Based on its taxonomic status and crossability relations it was postulated that H. megalanthus (syn. S. megalanthus) is an allotetraploid (2n = 4x = 44) derived from natural hybridization between two closely related diploid taxa. The present work aimed at elucidating the genetic relationships between species of the two genera. METHODS: Crosses were performed and the putative hybrids were analysed by chromosome counts and morphological traits. The ploidy level of hybrids was confirmed by fluorescent in situ hybridization (FISH) of rDNA sites. Genomic in situ hybridization (GISH) was used in an attempt to identify the putative diploid genome donors of H. megalanthus and an artificial interploid hybrid. KEY RESULTS: Reciprocal crosses among four diploid Hylocereus species (H. costaricensis, H. monacanthus (syn. H. polyrhizus), H. undatus and Hylocereus sp.) yielded viable diploid hybrids, with regular chromosome pairing. Reciprocal crosses between these Hylocereus spp. and H. megalanthus yielded viable triploid, pentaploid, hexaploid and aneuploid hybrids. Morphological and phenological traits confirm the hybrid origin. In situ detection of rDNA sites was in accord with the ploidy status of the species and hybrid studied. GISH results indicated that overall sequence composition of H. megalanthus is similar to that of H. ocamponis and S. grandiflorus. High sequence similarity was also found between the parental genomes of H. monacanthus and H. megalanthus in one triploid hybrid. CONCLUSIONS: The ease of obtaining partially fertile F1 hybrids and the relative sequence similarity (in GISH study) suggest close genetic relationships among the taxa analysed.     

A critical review of the protracted domestication model for Near-Eastern founder crops: linear regression, long-distance gene flow, archaeological, and archaeobotanical evidence

The recent review by Fuller et al. (2012a) in this journal is part of a series of papers maintaining that plant domestication in the Near East was a slow process lasting circa 4000?years and occurring independently in different locations across the Fertile Crescent. Their protracted domestication scenario is based entirely on linear regression derived from the percentage of domesticated plant remains at specific archaeological sites and the age of these sites themselves. This paper discusses why estimates like haldanes and darwins cannot be applied to the seven founder crops in the Near East (einkorn and emmer wheat, barley, peas, chickpeas, lentils, and bitter vetch). All of these crops are self-fertilizing plants and for this reason they do not fulfil the requirements for performing calculations of this kind. In addition, the percentage of domesticates at any site may be the result of factors other than those that affect the selection for domesticates growing in the surrounding area. These factors are unlikely to have been similar across prehistoric sites of habitation, societies, and millennia. The conclusion here is that single crop analyses are necessary rather than general reviews drawing on regression analyses based on erroneous assumptions. The fact that all seven of these founder crops are self-fertilizers should be incorporated into a comprehensive domestication scenario for the Near East, as self-fertilization naturally isolates domesticates from their wild progenitors.