Analysis of Meiotic Chromosome Behaviour in Diploid Individuals of Chrysanthemum zawadskii and Related Species (Asteraceae): Evidence for Chromosome Rearrangements

Abstract

Diploid individuals (2n=18) 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.