Cytogenetic studies in the genus Zea : 3. DNA content and heterochromatin in species and hybrids.

The nuclear DNA amount and the heterochromatin content in species and hybrids of Zea were analyzed in telophase nuclei (2C) of the root apex of germinating seeds. The results revealed significant differences among taxa and also among lines and races of maize. The hybrids between Z. mays ssp. mays x Z. mays ssp. mexicana (2n=20), Z. diploperennis x Z. perennis (2n=30), and Z. diploperennis x Z.perennis (2n=40) showed DNA content intermediate between that of the parents. The number of chromosomal C-bands and the proportion of the genome comprising C-band heterochromatin were positively related to genome size. In the different lines and races of maize studied, there was a positive correlation between genome size and the interval from germination to flowering. Octoploid Z. perennis (2n=40) showed the smallest DNA content per basic genome and the smallest heterochromatic blocks, suggesting that the DNA lost by this species consisted mainly of repetitive sequences. Considering that the extant species of Zea are tetraploid (2n=20) and octoploid (2n=40) and that the ancestral diploids are extinct, any consideration of the direction (increase or decrease) of the DNA change would be entirely speculative. The extant species could be the product of natural and artificial selection acting on different genotypic and nucleotypical constitutions at the diploid and/or tetraploid levels.