Fraction 1 Protein and the Origin of Polyploid Wheats

The nature of the interspecific hybridizations giving rise to the polyploid wheats has been determinedfrom an analysis of the polypeptide compositions offraction I proteins isolatedfrom diploid, tetraploid, and hexaploid wheat species. In the origins of the tetraploid wheat, the B-genome donor provided the chloroplast genome and was therefore the maternal parent in the cross with Triticum monococcum. Subsequently, the tetraploid wheat, T. dicoccum, was the maternal parent in the cross with Aegilops squarrosa, giving rise to the hexaploid wheat, T. aestivum. Ofthe small number ofdiploid species examined, only Ae. speltoides has a chloroplast genome similar to that of the polythis method outside the genus Nicotiana we have examined the fraction I proteins of species implicated in the origins of polyploid wheat. These analyses provide some indications as to the nature of the B-genome donor and further indicate the directions of the interspecific crosses that gave rise to the tetraploid and hexaploid wheats. The wheat species examined were the hexaploid Triticum aestivum, the tetraploids T. timopheevii and T. dicoccoides and its cultivated form T. dicoccum, and the diploids Aegilops speltoides, Ae. squarrosa, T. urartu, and T. boeoticum and its cultivated form T. monococcum. Fraction 1 proteins were isolated from the supernatant fraction obtained by homogenizing 14-day-old leaves in 3 volumes of 0.05M tris-HCI buffer, pH 7.8, containing 0.2M NaCI, 0.5 mM ethylenediaminetetraacetic acid, 10 mM potassium metabisulfite, and 20 percent (weight to volume) Dowexl(x2) and then centrifuging for 60 minutes at 77,000g. Fraction 1 proteins were precipitated from the extract by ammonium sulfate at 50 percent saturation, and then extraneous proteins were removed by passage through a column of Sepharose 4B to which antibodies to fraction 1 protein had been covalently linked (7). Fraction 1 protein bound to the column was then eluted in the form of dissociated subunits by 8M urea in 0.05M tris-HCI, pH 7.8, 0.2M NaCI. The protein was then S-carboxymethylated before analysis by isoelectric focusing in the presence of 8M urea (1). As shown in Fig. 1, all fraction I proteins from the wheat species contain three large subunit polypeptides and a single small subunit polypeptide. The species fall into two groups on the basis of the isoelectric points of the large subunits. One group, having higher isoelectric points, contains T. aestivum, T. dicoccum, and Ae. speltoides, as well as T. timopheevii (not shown) and T. dicoccoides (see Fig. 2). The other group, with lower isoelectric points for the large subunit polypeptides, contains the diploid species, T. boeoticum, T. urartu, and Ae. squarrosa, as well as T. monococcum (not shown). These differences allow some conclusions to be drawn concerning the course of evolution of the polyploid wheats, because the mode of inheritance of the large subunit polypeptides of fraction I protein is the same in wheat as in the Nicotiana species. This is shown in Fig. 2 by an analysis of the polypeptide composition of fraction I proteins from reciprocal interspecific hybrids of T. boeoticum and T. dicoccoides. The large subunit polypeptides of fraction I protein from the hybrid species are inherited solely from the maternal parent, as has been demonstrated previously for interspecific hybrids in the genus Nicotiana (4).