Distribution of Multivalents and Genotypic Control of Chromosome Pairing in Setcreasea Species and Hybrids

Chromosome associations during metaphase I of meiosis were analysed in Setcreasea pallida, S. brevifolia, S. ovata, S. purpurea and interspecific hybrids, back-crosses and F2 populations between these species. Multivalents were observed besides bivalents in the tetraploid S. brevifolia, S. ovata, S. purpurea and interspecific hybrids, backcrosses and F2 populations between these species. The frequencies of cells with 0, 1, 2 ... 6 multivalents were tested against the expected binomial in triploid and tetraploid species and hybrids.Each of the six sets of four chromosomes in Setcreasea brevifolia var. brevifolia, S. purpurea and S. ovata were capable of pairing as a quadrivalent. The distribution of multivalents was binomial. Autopolyploid origin of these species was proposed.In most of the interspecific hybrids between the abovesaid species, the distribution of multivalents was binomial. It was proposed that the differentiation between the species of Setcreasea is due mainly to gene mutations rather than due to changes in chromosome structure.A genetic mechanism causing chromosomes to pair mostly as bivalents was proposed. The cytogenetic significance of predominant bivalent formation was discussed.