| A synthetic hexaploid wheat with fragile rachis1)
E.R. SEARS U.S. Department of Agriculture and Department ot Agronomy, University of Missouri, Columbia, Missouri, U.S.A. It is generally believed that no wild form of hexaploid wheat exists - that this species is completely dependent on man for its dispersal. Whereas the wild diploid and tetraploid wheats have a fragile rachis that allows the spike to shatter into segments when ripe, the spike of the common wheats and even the presumed ancestral hexaploid wheats, the spelta wheats, is non-fragile. The hexaploids thus lack an adequate means of seed dispersal. Since hexaploid wheat evidently did not originate until after man had already begun to cultivate diploid and tetraploid wheat, it might be assumed that no wild type ever existed. Strong support for this view was provided by the report of MCFADDEN and SEARS (1947) that the amphiploid between wild emmer and T. tauschii (=Aegilops squarrosa), which reconstitutes spelta wheat, has a non-fragile rachis. Such a result was not unexpected in view of SEARS'S previous finding (1941) that the amphiploid between wild einkorn and T. tauschii has a relatively tough rachis. The rachies breaks in different places in the two parents, and the amphiploid is intermediate in its tendency to break at each place. Since wild emmer has the same type of fragility as wild einkorn, its amphiploid with T. tauschii could also be expected to be non-fragile. However, DEKAPRELEVICH (1961) reported finding a hexaploid, fragile-rachised wheat growing wild in Russian Georgia. Subsequently, it became clear that the supposed wild emmer used by MCFADDEN and SEARS in resynthesizing spelta wheat was not really wild at all but had a tough rachis and was therefore a cultivated emmer. This led to the question whether an amphiploid between a truly wild emmer and T.tauschii would have a fragile rachis even though wild einkorn x T. tauschii does not. With the B genome present and presumably acting toward the same type of fragility as that determined by the A genome, it seemed possible that the fragility of the wild emmer might be epistatic in the amphiploid between it and T. tauschii. It was this emmer type of fragility that DEKAPRELEVICH found in the Georgian wheat. If the amphiploid wild emmer x T. tauschii has a fragile rachis, then as KUCKUCK (1964) has suggested, the wheat of DEKAPRELEVICH may have originated as such an amphiploid, independently of the cultivated hexaploids, which are believed to have involved cultivated emmer as their tetraploid parent. Both wild emmer and T. tauschii are native to the area concerned. When 40 spikes of a true wild emmer (from Israel) were pollinated by T. tauschii, two viable seeds were obtained. These were kindly germinated on nutrient agar by Dr. W. F. SHERIDAN. The two plants obtained proved semi-lethal, too weak for treatment with colchicine. Both eventually flowered, however, and although they were completely sterile, the spikes could be observed to be fragile (Fig. 1). Since the effect of sterility is to make the rachis less, not more, fragile, it seems safe to conclude that a fertile hybrid between wild emmer and T. tauschii would have fragile spikes. Whether DEKAPRELEVICH'S wild hexaploid actually originated in this way or as a segregate from a cross of a cultivated hexaploid with wild emmer remains uncertain. Seven other synthetic hexaploids reported by MCFADDEN and SEARS (1941) had the supposedly wild tetraploid wheat as one parent. One involved T. speltoides (=Ae. speltoides) and had a "relatively nonfragile" rachis. Presumably if a truly wild tetraploid had been the wheat parent, this amphiploid would have been fragile. Literature Cited DEKAPRELEVlCH, L. 1961. Die Art Triticum macha Dek. et Men. im Lichte neuester Untersuchungen uber die Herkunft der hexaploiden Weizen. Z. Pflanzenzuchtg. 45 : 17-30. KUCKUCK, H. 1964. Experimentelle Untersuchungen zur Enstehung der Kulturweizen. I. Die Variation des Iranischen Spelzweizens und seine genetischen Beziehungen zu Triticum aestivum spp. vulgare (VILL.. HOST) MAC KEY, ssp. spelta (L.) THELL. und ssp.macha (DEK. et MEN.) MAC KEY mit einem Beitrag zur Genetik des Spelta-Komplexes. Z. Pflanzenzuchtg. 51 : 97-140. MCFADDEN, E.S. and E.R. SEARS 1947. The genome approach in radical wheat breeding. J. Amer. Soc. Agron. 39 : 1011-1026. SEARS, E.R. 1941. Amphidiploids in the seven-chromosome Triticinae. Mo. Agr. Exp. Sta. Res. Bul. 336 : 46 pp. |
| 1) Cooperative investigations of the Agricultural Research Service, U.S. Department of Agriculture, and the Missouri Agricultural Experiment Station. Journal Paper No.7410 of the Missouri Station. |