1. Two species of annual chrysanthemum-
Chrysanthemum coronarium and
C. carinatum were crossed reciprocally and the resulting F
1 and F
2 generations were studied morphologically and cytologically.
2. The F
1 plants resembled one or the other parent in some characters but were intermediate between the two parents in other characters. Over-or partial dominance of some quantitative traits towards positive directions was noted in F
1. The average pollen fertility in F
1 generation of
C. coronarium×C. carinatum cross was 26.5% (range 19 to 37.9%) and that in the F
1 of reciprocal cross was 32.1% (range 23.7 to 47.7%). Seed fertility (i.e.% of fully developed seeds) in all these hybrids was 2.9% on the average (range 0.4 to 11.3%).
3. The 17 F
1 plants had 18 chromosomes. The average frequency of bivalents per cell in F
1 was 6.98 (range 2 to 9). In all these plants one or two quadrivalents were noted in few cells, the average frequency per cell was 0.27. Other multivalents ranging from trivalent to decavalent was noted in 10 F
1 individuals. All the F
1 plants had dicentric bridge with fragment or only fragment in AI and AII in varying number of cells.
4. A total of 318 F
2 plants were studied morphologically. These plants showed segregation in all the studied characters but the segregations did not fit into any ratio. The average pollen and seed fertility in the F
2 plants were 50.9% (range 15.2 to 68.2%) and 39.6% (range 1.0 to 86.7%) respectively. The average and range of pollen and seed fertility in F2 plants of 3 crosses were found to vary considerably. A total of 17 plants in F
2 were completely seed sterile.
5. A total of 20 F
2 plants were analysed cytologically and all had 2n=18 chromosomes. Three plants had exclusively 9 bivalents. In other plants the average frequency of bivalents per cells was 6.85 with a range from 3 to 9 in a cell. One or two quadrivalents were noted in few cells in 17 F
2 plants and the average frequency per cell was 0.36. In all these 17 plants multivalents ranging from trivalent to decavalent was noted in few cells. Dicentric bridges with acentric fragment was noticed in 3 plants out of these 17 individuals.
6. It has been concluded that these two species are isolated from each other by segregational hybrid sterility and hybrid breakdown type of isolating mechanisms. Inversion has played an important role in chromosome differentiation in these two species of annual chrysanthemum.
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