Cytogenetic studies on Zea perennis

Zea perennis (Euchlaena perennis) was introduced from Mexico into Argentina in 1962, and up to now has been maintained by clonal propagation. Zea perennis has a chromosome number of 2n = 40. According to Longley (J. Agr. Res. 28:673-682, 1924) all the chromosomes were bivalents, but he afterwards (J. Agr. Res. 48:789-806, 1934) pointed out that it was an exceptional case since Zea perennis was an autotetraploid with 10IV.

From the study of 150 cells at metaphase I, it has been determined that the meiotic configurations most frequently found were 10II + 5IV (55% of the cells) and 12II + 4IV (20.74%), with very few monovalents and trivalents. The average of chiasmata per cell was 15.69 in bivalents and 18.87 in tetravalents, totaling 34.56. The pachytene chromosomes had a tendency to stick, identification being difficult. They had very small terminal knobs in one or the other arm, or in both, or they were knobless. At anaphase in 87.5% of the cells 20 chromosomes migrated to each pole; and in the remaining 12.5%, 19 chromosomes migrated to one pole and 21 to the other. The grains of pollen were small, exhibiting a fertility of 89%.

Up to now it has been considered that Zea perennis is an autotetraploid. However, the meiotic configuration most frequently found was 10II + 5IV. In order that this would take place, the chromosomes could have different grades of evolution, since 5IV might have been differentiated in pairs, losing their homology, to give 10II. On the other hand, with 5IV they could not as yet have differentiated.

In hybrids between Zea mays (2n = 20) and Zea perennis (2n = 40) and reciprocals (2n = 30), the meiotic configuration most frequently observed was 5III + 5II + 5I. In hybrids between Zea diploperennis (2n = 20) and Zea perennis (2n = 40) and reciprocals (2n = 30), the meiotic configuration most frequently observed was 5III + 5II + 5I.

From the foregoing it can be asserted that the chromosomes of Zea mays and Zea diploperennis are homologous with Zea perennis, leaving 5 chromosomes without pairing. These could be chromosomes that had been differentiated, or they could have a different origin from the chromosomes of Zea mays and Zea diploperennis, and for this reason they do not pair. Thus, from all accounts Zea perennis could not be an autotetraploid; with some of the chromosomes differentiated, it is then an auto-allopolyploid.

Maria del Carmen Molina


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