32. Induction of monosomics in rice through pollen irradiation

There are only a few reports of monosomy in rice as it is a basic diploid and tolerance limits for deficiencies are rather narrow. Seshu and Venkataswamy (1958) selected a sterile monosomic plant from F3 progeny of a cross between an indica and a japonica variety. Similarly, Wang et al. (1988) obtained 5 monosomic plants in the progenies of plants pollinated with irradiated pollen. The transmission of the monosomics to the progenies has not been reported. Thus, rice like other diploid species can tolerate deficiencies at sporophytic level but not at gametophytic level.

In order to obtain additional monosomics of rice, we pollinated the emasculated panicles of rice cultivar IR54 with its irradiated pollen, which had been exposed to 5KR of gamma rays. We obtained 2,885 seeds from which 1368 M1 plants could be raised to reproductive stage. M1 plants were examined for variation in morphological and reproductive features. A total of 137 plants were found to be morphological variants. Of those, 41 plants had normal meiosis and fertility and were thus result of point mutations. Forty plants showed chromosomal inter-

Fig. 1. Meiosis in a primary trisomic of rice showing 11 bivalents and one univalent at diakinesis.

Fig. 2. Meiosis in a tertiary monosomic of rice showing 10 bivalents and one trivalent at metaphase I.

changes and another 45 had 23+1 fragment chromosome. Remaining 11 plants had 23 chromosomes and thus were monosomic.

Three of the monosomics showed 11II + 1I at diakinesis and metaphase I of meiosis (Fig. 1) and were thus primary monosomics. The missing chromosome in these monosomics was not identified but the three monosomics were morpho- logically distinct. lt is therefore likely that they were monosomic for different chromosomes. Unfortunately, plants were completely sterile and extremely weak and died before any seeds for progeny test could be obtained through backcrossing.

The tertiary trisomics sho wed modal chromosome configuration of 10II + 1III (Fig. 2) at metaphase I of meiosis. These plants resulted from fertilization by pollen grains in which breakages occurred in two chromosomes, two arms were lost and the other two joined to produce a tertiary chromosome. The tertiary monosomics showed extreme morphological modifications, were highly sterile and slow growing.

Seshu, D.V. and T. Venkataswamy, 1958. A monosomic in rice. Madras Agric. J. 45: 311-314.

Wang, Z., N. Iwata, Y. Sukekiyo, A. Yoshimura and T. Omura, 1988. A trial to induce chromosome deficiencies and monosomics in rice by using irradiated pollen. RGN 5: 64-65,