In 1980 in the Instituto Fitotecnico de Santa Catalina, the species Z. diploperennis (2n=20) was crossed with Z. perennis (2n=40), obtaining two types of perennial hybrids:
1) A triploid hybrid (2n=30), extremely sterile (2% fertile seeds), with an elevated number of monovalent and trivalent chromosomes which caused sterile unbalanced gametes to be produced.
2) A tetraploid hybrid (2n=40) of high fertility and meiotic regularity, which was the product of the watching of an unreduced gamete from Z. diploperennis with a normal gamete of Z. perennis.
From this second hybrid an F2 was obtained, which will be analyzed in this work. The hybrid tetravalent F2 has a chromosome number of 2n=40 in all cases. It is exceedingly tillered, with an average of 75 tillers per plant and a high yield in N.V. and M.S., which makes it a promising material to obtain a new perennial forage.
The F1 is the same as Z. diploperennis and Z. perennis; it has distichous tassels, made up by fruit-bearing capsules which disarticulate when ripe, shedding the fruit. The F2 has separated this character, and a percentage of plants have appeared in which the fruit capsules have soldered, which prevents disarticulation when ripe. In addition a new type of tassel has emerged, double row, the grain being slightly uncovered. This segregation in the F2 indicates that one of the two species has recessive genes which cause the capsules to solder, not loosening when the grain is ripe--a freak due to genetic interaction in both species.
From the chromosome study of the F2, it has been determined that the chromosome number of 2n=40 as well as a high meiotic regularity have been maintained uninterrupted, resulting in a decrease in mono- and trivalents (Table 1). The 151 cells analyzed produced an average of 33.63 chiasmas per cell.
In anaphase an equal number of chromosomes migrate to each pole and only rarely were inverted bridges seen. From 40 plants analyzed, the average pollen fertility is 90%, with a maximum of 98.3% and a minimum of 72.3%, the seed fertility being in every case superior to 80%.
From the cytological study of the F1 triploid and tetraploid and the F2 tetraploid, it has been deduced:
a) Fertility and chromosome stability of hybrids are directly influenced by the chromosome number.
b) The chromosomes of Z. diploperennis and Z. perennis are homologous or homeologous since in hybrids they practically match on the whole.
Table 1: Meiotic configuration of F2 the tetraploid hybrid of Zea mays x (Zea diploperennis x Z. perennis).
Maria del Carmen Molina
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