Cytogenetic study of a tetraploid hybrid between Zea diploperennis and Zea perennis

In April 1980, a hybrid between Zea diploperennis (2n=20) and Zea perennis (2n=40) was obtained. The hybrids are triploid, perennial and sterile. One plot was different from the others because of the morphological resemblance to Zea diploperennis. It had 40 chromosomes, so a possible explanation was suggested. The hybrid probably could have originated from the crossing of one unreduced gamete of Zea diploperennis with one normal gamete of Zea perennis.

Whatever mechanism could have originated this hybrid, it has a chromosome number (2n=40) with two sets of each of their parents so the F1 and F2 are perennial and fertile. The meiotic configuration of F1 plants was analyzed, and the results of examinations of 184 cells in diakinesis are summarized in Table 1. As the chromosomes had been paired totally or partially there were almost no monovalents and trivalents. That is one of the most remarkable differences between this hybrid and the triploid one (which has originated from the same parents), since it had a high percentage of trivalents and monovalents and a most frequent meiotic config- uration of 5III + 5II + 5I. The meiotic configurations and the average of bivalents and tetravalents were similar to those observed in a clone of Zea perennis. That is why, according to its chromosome-set, it was similar to Zea perennis, but considering the morphology it resembled Zea diploperennis (female parent).

At anaphase 1, 20 chromosomes migrated towards each pole in 90% of the cells. In the remaining 10%, different numbers of chromosomes migrated towards each pole. Other abnormalities observed were lagging chromosomes, and exceptionally, chromatid bridges. The tetrads were normal. The percentage of fertile pollen was 56% and fertile seed was 80%.

From this study, it is important to remark that: (1) all the triploid hybrids of Zea perennis and Zea diploperennis are sterile due to the high number of monovalents and trivalents; (2) for the first time, a tetraploid fertile hybrid from crossing Zea diploperennis with Zea perennis, was obtained; (3) apparently, the chromosome behavior in the hybrids varies if it is a triploid hybrid or a tetraploid one; (4) the chromosome studies of Zea diploperennis, Zea perennis and their hybrids provide enough evidence to demonstrate that Zea perennis is not an autotetraploid derivative of Zea diploperennis.

Table 1.

Maria del Carmen Molina

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