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Wheat Information Service
Number 72: 76-78 (1991)


Genetic differentiation between two wild tetraploid wheats, Triticum dicoccoides and T. araraticum as revealed by RFLP analysis of organellar and nuclear DNA

N. Mori1, Y. G. Liu2, C. Nakamura1 and K. Tsunewaki2

1 Laboratory of Genetics, Faculty of Agriculture, Kobe University, Kobe 657, Japan
2 Laboratory of Genetics, Faculty of Agriculture, Kyoto University, Kyoto 606, Japan


Two wild tetraploid wheats, Triticum dicoccoides Korn. (2n = 28, genome constitution AABB), and T. araraticum Jakubz. (2n = 28, AAGG) are generally accepted as the ancestral species of the cultivated forms of the emmer and timopheevi groups, respectively. They are the first polyploid species evolved in Triticum. So, it is important to investigate the intra-as well as interspecific variation of these two species in order to clarify the origin of emmer and timopheevi wheats and to understand phylogeny of polyploid species in Triticum. We have studied the intra- and interspecific variations on organellar and nuclear DNAs of these two species by restriction fragment length polymorphism (RFLP) analyses. A large number of accessions of both species, of which collection sites extend across their entire natural distribution areas, were studied (Fig. 1). From these investigations the following points became clear: (1) The chloroplast genome is highly conserved in each species; When ctDNAs were treated with each of four 6-bp-cutters and electrophoresed, only two variant restriction patterns were found among 27 accessions of T. dicoccoides. No ctDNA variant was found among 27 accessions of T. araraticum, when studied in the same manners. (2) Contrary to the rare intraspecific variation in each species, the two species showed clear and distinct differences in their ctDNAs (Mori et al 1988), being identified to belong to Type 7 and Type 5 chloroplast genomes (Ogihara and Tsunewaki 1988). (3) The mitochondrial genomes of the two species are also clearly distinguished from each other by both the restriction endonuclease analysis and Southern hybridization analysis of their mtDNAs; The percentage of common restriction fragments between the mtDNAs of the two species was 69.2% (Table 1). When hybridized with the two specific mtDNA clones as probes, RFLPs were detected between mtDNAs of the two species in their all digests. The average percentage of the common hybrid fragments was 49. 1 % (Table 2). (4) The interspecific variation of nuclear DNAs between the two species was much greater than the intraspecific varitation observed in each species; Total DNAs from 32 accessions of T. dicoccoides and 24 accessions of T. araraticum were treated with each of two 6-bp-cutters and probed to 28 nuclear DNA (or cDNA) clones. In total, 115 hybrid bands were observed per accession and more than two Southern patterns were found in all probe-enzyme combinations. The genetic distannces (d) estimated after Nei (1987) was 0.0128 on the average of 496 comparisons (range, 0.0034 - 0.0173) between T. dicoccoides accessions, whereas it was 0.0065 on the average of 276 comparisons (0.0009 - 0.0083) between T. araraticum accessios. The intraspecific variation of nuclear DNA in the former is about twice as large as that of the latter. On the contrary, the average genetic distance between the two species was 0.05 (0.0424 - 0.0549). Thus, it is clear that the interspecific variation between the two wild tetraploids is much greater than the intraspecific variation in each. A dendrogram constructed based on their genetic distances (Fig. 2) shows that all 32 accessions of T. dicoccoides and a T. durum accession form a cluster and, similarly, all 24 accessions of T. araraticum and a T. timopheevi accession form another cluster. These two clusters are distantly related to each other, indicating clear genetic differentiation between the emmer and timopheevi groups of wheat.

All these facts are in favor of the diphyletic origin of the two wild tetraploid species, and accordingly, of emmer and timopheevi groups of wheat.


Reference

Ogihara Y and Tsunewaki K (1988) Chloroplast DNA diversity and evolution in Triticum and Aegilops. Proc VII Int Wheat Genet Symp Cambridge: 127-132

Nei M (1987) Molecular evolutionary genetics. Columbia Univ Press NY: pp 106-107.

Sokal RR and Michener CD (1958) A statical method for evaluating systematic relationships. Univ Kans Sci Bull 28, 1409-1438

Mori N, Terachi T and Tsunewaki K (1988) Organellar genome differentiation in wild tetraploid wheats, Triticum dicoccoides and T. araraticum. Proc VII Int Wheat Genet Symp Cambridge: 13-19.

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