Cytology of higher polysomics in Coix gigantea (Poaceae): Pentasomy and hexasomy

Among a natural population of Coix gigantea a pentasomic (2n+3; 2n=23) and a hexasomic (2n+4; 2n=24) were isolated and studied cytologically for the first time. The extra homologous chromosomes in the pentasomic plant did not disturb the behaviour of the rest of the bivalents through meiosis except for the rare occurrence of a few univalents. The majority of PMCs at diakinesis showed nine bivalents and a pentavalent. The various anaphase segregations led to the formation of sub- and super-haploid male gametes with from n–1 (n=9) to n+4 (n=14) chromosomal constitutions. The four extra homologous chromosomes in the hexasomic plant, however, disturbed meiosis leading to sticky chromosomal configurations and sterility. At diakinesis, the majority of the PMCs showed nine bivalents and a hexavalent.The origin of these higher polysomics is considered to be through fusion of male and female super-haploid gametes produced by the trisomics that occurred in large numbers in the population. The occurrence and dominance of nullisomics together with other aneuploids in the diploid population of C. gigantea make it quite certain that the 2n=20 form of the species is at a tetraploid level.