From hybrid studies carried out among Zea mays, Z. perennis and Z. diploperennis a hypothesis on fertility and chromosome matching in hybrid species of Zea was reached, that these are determined fundamentally by the chromosome number. When the hybrid has 20 or 40 chromosomes, it will have high fertility and chromosome matching, regardless of the species it is crossed with, whereas when the chromosome number is 2n=30 there remains a high number of unmatched chromosomes, which causes the hybrid to be highly sterile.
In order to demonstrate that if the hybrid has 2n=40 chromosomes it is fertile regardless of the species hybridized, a cross was effected in 1982 between a Z. mays tetraploid (2n=40) which had as a marker the sugary gene, and a hybrid tetraploid of Z. diploperennis x Z. perennis (2n=40), obtaining a new hybrid with the following essentials:
Z. m. tetraploid su su x (Z. d. x Z. p.)
F1 = 2n=40 Z. m. t. x (Z. d. x Z. p.)
The cross was viable in 90% of the cases, all seeds being fertile. The F1 plants are annual and similar morphologically to Z. mays - very prolific, medium tassel, uncovered grain, with large coriaceous glumes.
From the chromosome study it was determined that the chromosomes match as bi- or tetravalents and only exceptionally as mono- or trivalents (Table 1). In anaphase an equal chromosome number migrates to each pole, which results in fertile balanced gametes, and only rarely have inverted bridges been observed. The pollen fertility is 98% and fertile seeds are obtained in a 95% proportion.
The results of this work are:
1) The chromosomes of Z. mays, perennis and diploperennis are homologous or homeologous so it could be considered that the three species might have had the same origin, subsequently diverging in some of their genes, which accounts for their morphological and protein differences, but they maintain nevertheless their chromosome homology.
2) Owing to the fact that the chromosomes of the three species of this hybrid have been matched and have a high fertility, it has been demonstrated that if the hybrid has a chromosome number of 2n=40 it is fertile no matter what species of Zea are hybridized.
Table 1: Meiotic configurations of the tetraploid hybrid of Zea mays x (Z. diploperennis x Z. perennis).
Maria del Careen Molina
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