| Chromosome pairing and chiasma formation in hybrids
(ABRR) derived from 6x Triticale x 2x rye crosses P.K. GUPTA, I.C. TYAGI and J.R. BAHL Department of Ag. Botany, Meerut University, Meerut, India. As a part of a research programme "Cytogenetic Studies in Triticales" sanctioned to us by Indian Council of Agricultural Research, we are planning to synthesize some tetraploid triticales. For this purpose, as earlier done by KROLOW (1973), we made crosses between 6x Triticale and 2x rye. The cross was easily successful and more than 500 seeds could be obtained by using 6x Triticale as the female parent. Seven different Triticale strains (DTS 30, DTS 205, ST 69-1, 6TA701, TL63A, JNK 6T192, JNK 6T214) and a single rye strain (persian rye) were used in the crosses. Meiosis was studied in plants representing hybrids (ABRR) obtained from all the seven Triticale parents and the results are presented in Table 1. In the hybrids ABRR, one would generally expect the presence of 7"+14', the bivalents being contributed by rye chromosomes only. Since rye commonly shows seven ring bivalents (LAMM, 1936/37 and REES, 1955) one would also expect that in ABRR hybrids, the seven bivalents should be largely ring bivalents. The observed results deviated from this expectation and showed some agreement with earlie results of MILLER and RILEY (1972) and LELLEY (1975). It can be seen from Table 1 that although the average frequency of univalents in diflerent hybrid lines ranged from 13.60-15.36 per p.m.c., in individual p.m.c. the number of univalents could be as low as five and as high as 22, the remaining chromosomes being associated as bivalents and multivalents. (Figs. 1-4). It was also conspicuous that most of the bivalents were mainly rod bivalents as against the expectation of the formation of ring bivalents by the rye chromosomes. It is thus obvious that while increased chromosome pairing involving as many as 23 chromosomes was possible due to promotion of homoeologous chromosome pairing, there was also simultaneous reduction in the chiasmata formation in individual chromosome association as revealed by the preponderance of rod bivalents and chain quadrivalents. Such increase in homoeologous chromosome pairing in hybrids was also inferred by RILEY and MILLER (1972) and LELLEY (1975) who attributed it to an increase in rye genome (relative to wheat). LELLEY (1975) also noticed a reduction in chiasma formation in 6x Triticale x 2x rye hybrids and found that this reduction was observed even between homologous rye chromosomes, which is in agreement with our results. It is thus obvious that there are independent control systems for chromosome pairing and chiasmata formation. However, the frequency of multivalents and the total number of chromosomes involved in pairing during the present study was much higher than that observed by LELLEY (1975). Whether or not some of these quadrivalents are due to translocation differences can not be ascertained, but these multivalents were more frequent in some hybrids than others (Table 1). It can also be seen that chromosome pairing conspicuously differed in the different hybrid lines which as earlier explained by Rees and THOMPSON (1956) and LELLEY (1978), can be attributed to a polygenic control of chiasma formation. Literature Cited KROLOW, K.D. (1973). 4x triticale, production and use in triticale breeding. Proc. 4th Int. Wheat Genet. Symp. 237-243. LELLEY,T. (1975). Genetic control of pairing of rye chromosomes in triticale. Z. I-flanzenzuchtg. 75: 24-29. LELLEY,T. (1976). Induction of homoeologous pairing in wheat by genes of rye suppressing chromosome 5B effect. Can. J . Genet. Cytol. 18: 485-489. LAMM, R. (1936/37). Cytological studies on inbred rye. Hereditas 22: 217-24. MILLER, T.E. and R. RILEY (1972). Meiotic chromosome pairing in wheat rye combinations. Genet. Iber. 24: 241-250. REES, H. (1955). Genotypic control of chromosome behaviour in rye. I. Inbred lines. Heredity 9: 93-116. REES, H. and J.B. THOMPSON (1956). Genotypic control of chromosome behaviour in rye III. Chiasma frequency in homozygotes and heterozygotes. Heredity 10: 409-424. |