| I. Research Notes Abnormal pollen tetrads in Triticum urartu Harcharan S. DHALIWAL Department of Plant Sciences, University of California, Riverside, CA., U.S.A. 92502 In all the higher plants, the products of microsporogenesis, i.e. microspores, are normally arranged in the form of a tetrahedron (Fig. 1) while the products of megasporogenesis form a linear row of four megaspores. In T. urartu and its F1 hybrids and amphiploids with other related species T. boeoticum and Aegilops speltoides, two types of abnormal pollen tetrads viz. semi-linear and linear (Fig. 1) were observed. The frequency of the abnormal tetrads was too high to be attributed to chance occurrence. Similar abnormal pollen tetrads have been observed in an autotetraploid barley by OKAMOTO (personal comm.). The linear and semi-linear pollen tetrads had an overall frequency of 5.44% in a total of 15,058 tetrads observed from 15 genotypes including T. urartu, the F1 hybrids and amphiploids. The frequency of the abnormal tetrads among different genotypes was highly variable. A highest fiequency of 25.60% was observed in one boeoticum-urartu amphiploid. The T. boeotioum and Ae. speltoides lines had no abnormal tetrads. Often the amphiploids involving same urartu but different boeoticum lines had different proportions of the abnormal tetrads indicating that different boeoticum lines interact differently during the development of the abnormal tetrads. The frequency of the linear tetrads, in all the genotypes studied, was lower than that of the semi-linear tetrads suggesting that both types of abnormal tetrads presumably were the result of a one and the same phenomenon affecting the spindle orientation. The cell plate and furrowing invariably arise in the equatorial plane of the spindle apparatus perpendicular to the plane of cell division. Observation from the pollen mother cells during anaphase II indicated that the semilinear and linear tetrads were the result of abnormal spindle orientation, parallel to that of the metaphase I, in one or both the dyads, respectively. Obviously, factors identical to those regulating the development of linear tetrads in megasporogenesis are operating, however, only with a limited penetrance during microsporogenesis in T. urartu and its hybrids with other species. The abnormal tetrads usually occured in groups indicating that the factors affecting spindle orientation were localized at only a few spaces within the anther sacs. The nature and mode of action of the factors affecting the spindle orientation are, however, unknown. The abnormal spindle orientation in T. urartu, a parent of polyploid wheats (JOHNSON 1975, DHALIWAL and JOHNSON 1976) might be of some significance during the evolution of the polyploids. Literature Cited DHALIWAL, H.S., and B.L. JOHNSON 1976. Anther morphology and the origin of the tetraploid wheats. Amer. J. Bot. 63: 363-368. JOHNSON, B.L. 1975. Identification of the apparent B genome donor of wheat. Can. J. Genet. Cytol. 17: 21-39. |