The control of di ploid like meiosis in poly ploid taxa of chrysanthemum 3. deca ploid chrysanthemum crassum

Watanabe, K.

Cytologia 46(3): 515-530


ISSN/ISBN: 0011-4545
Accession: 006620613

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In C. crassum with 2n = 10x .sbd. 1 = 89, 44 II + 1 I was the main meiotic configuration and 1 IV + 42 II + 1 I and 43 II + 3 I were rarely observed. Eighty-nine chromosomes, at mitotic metaphase, varied in length from 5.4-2.5 .mu.m and in arm ratio from 1.0-5.8. Five satellite chromosomes, 2 medium-sized and 6 small chromosomes with extreme subterminal centromeres were distinguishable. Karyomorphologically this species is not an auto-decaploid. Three F1 hybrids with 2n = 6x = 54 beteen diploid C. boreale and C. crassum were obtained by ovary culture. The following meiotic configurations were frequently observed; 2 IV + 23 II, 1 IV + 25 II, 1 III + 25 1 I, 27 II and 26 II + 2 I. The chromosomes derived from C. crassum paired both homoeologously and homologously with the C. boreale chromosomes. F1-hybrids with 2n = 6x = 54 between C. boreale and C. crassum gave rise to tetraploid B1-hybrids by means of ovary culture after being backcrossed with the diploid of C. boreale. In B1-hybrids with 2n = 4x = 36 the following meiotic configurations were frequently observed: 3 IV + 12 II, 2 IV + 14 II, 1 IV + 16 II, 18 II and 17 II + 2 I. The maximum number of quadrivalents per PMC varied from 2-5 among B-hybrids. The data of quadrivalent frequencies conform to the Poisson distribution. Further homoeologous chromosome pairing was observed in this generation. The diploid-like meiosis in C. crassum, its hexaploid F1-hybrids and tetraploid B1-hybrids must be ensured by the following genetic system although all of the constituent genomes of these polyploids are sufficiently homologous to be able to pair each other. Chromosome pairing is initiated at 2 sites, A and B (the zygomeres localize in 2 loci/chromosome). At either site pairing is always 2-by-2, with the pairing initiated at the A site being independent of that initiated at the B site. The initiation of pairing at the A site always precedes to that at the B site. The initiation of pairing at the B site is usually suppressed by multiple- or poly-genic control. The magnitude of release from the suppression of initiation of pairing at the B site depends on the reduction of suppressive gene dosage and seems to be equal, not differential, at each B site.