Studies on the control of diploid-like meiosis in polyploid taxa of Chrysanthemum. I. Hexaploid Ch. japonense Nakai.

Abstract

1) In meiosis of Ch. japonense (Saka strain) 2n=6x=54, 27 II was the main chromosome configuration and 1 IV+25 II and 26 II+2 I were rarely observed. Fifty-four chromosomes, at mitotic metaphase, varied in length from 8.4μm to 4.2μm and in arm ratio from 1.0 to 4.9. Ten satellite chromosomes and six small chromosomes with extreme subterminal centromeres were well distinguishable. Karyomorphologically this species is not an autohexaploid.2) Eight F1-hybrids between hexaploid Ch. japonense (Saka strain) and diploid Ch. boreale, with 2n=4x=36 and 2n=4x+1=37, were obtained. 2 IV+14 II, 1 IV+16 II and 18 II were the main meiotic configurations in the F1-hybrid with 2n=4x=36. The quadrivalent frequencies conform to the Poisson distribution. The chromosomes derived from Ch.japonense (Saka strain) paired both homoeologously and homologously with the Ch. boreale chromosomes.3) F1-hybrids were partially fertile (0%-1.1% in interbreeding and 0.3%-5.9% in intrabreeding) and gave rise to triploid B1-hybrids on backcrossing to diploid Ch. boreale, and to tetraploid F2-hybrids by intrabreeding.4) In eight of the nine B1-hybrids with 2n=3x=27 studied the average trivalent formation per PMC ranged from 4.66 to 5.55. The trivalent frequencies conform to the Poisson or binomial distribution. Further homoeologous chromosome associations were revealed in this generation. In the ninth B1-hybrid, 9 II+9 I was the main chromosome configuration. This plant was indiscriminable morphologically from the others.5) These B1-hybrids with 2n=3x=27 were partially fertile (0%-0.2% in interbreeding and 0%-3.7% in intrabreeding) and gave rise to B2-hybrids with 2n=24, 26, 27 and 36 on backcrossing to the diploid Ch. boreale, and progenies with 2n=30, 32, 33, 34, 35 and 36 by intrabreeding. In B2-hybrids with 2n=26, the following meiotic configurations were frequently observed; 7 III+2 II+1 I, 6 III+3 II+2 I, 5 III+4 II+3 I, 4 III+5 II+4 I and 3 III+6 II+5 I.6) In seven F2-hybrids with 2n=4x=36, the following meiotic configurations were frequently observed; 3 IV+12 II, 2 IV+14 II, 1 IV+16 II and 18 II. There is a significant difference in the frequency of quadrivalent and bivalent formation among F2-hybrids; the maximum number of quadrivalents per PMC varied from four to two. Quadrivalent frequencies conform to the Poisson distribution. The rate of seed setting varied from 0% to 58.7% in F2-hybrid intra-breedings. There was no correlation between the frequency of multivalent formation and the rate of seed setting.7) Nineteen of the twenty F3-hybrids studied had the chromosome number 2n=4x=36 and the twentieth had 2n=4x+1=37. In eleven F3-hybrids with 2n=4x=36 the following meiotic configurations were frequently observed; 3 IV+12 II, 2 IV+ 14 II, 1 IV+16 II and 18 II. There was a significant difference in the frequency of quadrivalent and bivalent formation among F3-hybrids; the maximum number of quadrivalents per PMC varied from eight to two. Quadrivalent frequencies conform to the Poisson distribution.8) In Ch. japonense (Nakamura strain) with 2n=6x=54, 27 II was the main meiotic configuration and 2 IV+23 II, 1 IV+25 II, 1 IV+24 II+2 I and 26 II+2 I were rarely observed. Fifty-four chromosomes, at mitotic metaphase, varied in length from 6.7μm to 3.8μm and in arm ratio from 1.0 to 4.4. Eight satellite chromosomes and six small chromosomes with extreme subterminal centromeres were well distinguishable. Karyomorphologically this species is not an autohexaploid.9) Two F1-hybrids with 2n=4x+1=37 between diploid Ch. boreale and Ch. japonense (Nakamura strain) were obtained by ovary culture.