Polyploidy, which is thought to have played an important role in plant evolution and speciation, is prevalent in Chrysanthemum (x = 9). In fact, polyploid series are known in C. zawadskii (2x, 4x, 6x, 8x, and 10x) and C. indicum (2x, 4x, and 6x), but the mechanism by which polyploidization occurs is unknown. Here we show that in diploid individuals of both C. zawadskii and C. indicum, the fusion between two adjacent pollen mother cells (PMCs) occurs at a frequency of 1.1-1.3% early in the first meiotic division. While possessing the chromosomes of both PMCs, the fused cell or syncyte undertakes subsequent meiotic division processes as a single large PMC, producing four 2n pollen grains that are able to germinate. Despite their low frequency, syncyte formation may have played a major role in the production of infraspecific polyploids in Chrysanthemum.

Summary Diploid individuals (2nϭ18) of Chrysanthemum zawadskii, C. indicum and C. boreale, two or all three of which often occur together in the wild in East Asia, have diverse metaphase karyotypes. We hypothesised that the diverse karyotypes result from chromosome rearrangements. To test this, we investigated chromosomal behaviour during pollen mother cell (PMC) meiosis and pollen fertility, using 33 individuals collected from seven populations in Korea. All individuals contained one or two multivalents (mostly quadrivalents) at late prophase I and metaphase I in some PMCs (approximately 17-37%), and at least one or two bivalents composed of homologous chromosomes of different length and centromere position in all PMCs. This meiotic chromosomal behaviour indicates that chromosome rearrangements, i.e., reciprocal translocation and unequal crossing-over, are prevalent in Chrysanthemum species. Nevertheless, because of successful chromosome segregation in anaphase I and II, they produce ample stainable pollen with different chromosome complements. Fertilization by such gametes should result in diverse somatic karyotypes. We also discuss the possibility that older or relatively recent hybridization events have led to a high rate of irregular meiotic chromosome behaviours and pollen sterility, particularly in C. indicum.