Abstract
The Zea Genus includes various species of gramineae of American origin. Of all those species, the only one which has economical value is Zea mays ssp mays, known as maize, a cereal of high energy value which is cultivated for human and animal consumption.

Wild species, known as teosinte, differ significantly in phenotype with respect to maize, although in some cases, have developed a similar appearance, as response to the selective eradication by farmers who consider it a weed of cultivation of maize.

In order to elucidate the philogenetic relations, ploidy and differentiation in the evolution of the Zea Genus homeologous genomes, Zea species and hybrids were analysed from the phenotypical, genotypical and cytogenetical point of view, inducing the cryptic intergenomic pairing of the homeologous genomes with a diluted solution of colchicine. 

As a result of this study it has been observed that:
The relative chromosome length of the Zea species with 2n=20 does not differ significantly from one to another, showing only small differences of size in some species. In most of the hybrids these small differences were minimized since all of the homeologous chromosomes paired and showed very small differences. These results seem to show that, in the homeologous chromosomes of Zea Genus hybrids, the interchromomeric areas get shorter or longer in order to allow chromomeric pairing. The exceptions were the Zea mays x Zea parviglumis dihybrid and the Zea mays, Zea diploperennis and Zea luxurians trihybrid, where three pairs of homeologous chromosomes of different sizes paired.

In the case of Zea perennis (2n=40), the size of the chromosomes is apparently different to the rest of the species with 2n=20, but if they are grouped and the five chromosome pairs that have the same form and size (1-2, 3-4, 7-8, 9-10 and 15-16) are considered as only one, its chromosomes do not significantly differ from the rest of the Zea complex.

The most considerable difference among the maize and wild species chromosomes was the position of the knobs. While in the maize there were no knobs or they were interstitial, in the wild species they were terminal and could be small, medium or large.

From the analysis of the meiotic configurations of the Zea species and hybrids, grouped according to their ploidy, it was inferred that:

* The species and hybrids bearing the same ploidy had similar meiotic configurations, number of chiasmata, pollen and seed fertility. The exceptions were Z. mays with 2n=40 and the hybrids Z. mays x Z. mays with 2n=30 and Z. parviglumis x Z. diploperennis with 2n=40, which significantly varied their meiotic configurations from the group they belonged to. 
* Among the species with 2n=20, Zea mays and Zea mexicana had the most chomosome stability and were the most fertile.
* The hybrids between the different Zea species with 2n=20 were fertile and it was difficult to differentiate them from the teosinte in their phenotypical aspect and chromosomes.
* A tetravalent, produced by the pairing of two pairs of homeologous chromosomes was occasionally observed only in the dihybrid between Z. mays x Z. parviglumis with 2n=20 and the trihybrid in the Zea mays, Zea diploperennis and Zea luxurians 
* The trihybrid with 2n=20 had less pollen (60-80%) and seed (72%) fertility than the dihybrids bearing the same ploidy.
* In the hybrids with 2n=30, the most frequent meiotic configuration was 5lll+5ll+5l, with an average of 5.46l+5.45ll+4.50lll. The exception in this group was the hybrid between the maize with different ploidy (Z. mays x Z. mays 2n=30), where, since the chromosomes were homeologous, there exists a probability that they paired among themselves, and therefore produced a high number of trivalent, and the most frequent meiotic configuration being 2l+2ll+8lll, with an average of 2.65l+2.56ll+7.41lll.
* Pollen fertility varied between 4 and 83% whereas seed fertility was lower (0-15%). Strangely, the hybrid between maize of different ploidy was the most sterile and the most difficult to obtain.

In the group including the species and hybrids with 2n=40, a number of different situations were found:
* Zea mays with 2n=40 had a high percentage of tetravalent (8.34) and a low percentage of bivalent (3.24), possibly due to the pairing of the homologous chromosomes of both genomes. At the other extreme, was the exceptional hybrid produced by the chromosomic duplication between Z. parviglumis x Z. diploperennis with 2n=40, which gave rise to Z. parviglumis x Z. diploperennis with 2n=40, where it is possible for the homeologous chromosomes of each of the species to preferentially paired, and consequently, a high percentage of bivalents (15.27) and a low percentage of tetravalent (2.02) are produced. Zea perennis and the rest of the hybrids with 2n=40 had similar meiotic configurations with a pairing average of 11.21ll and 4.431V.

The cryptic pairing of Zea mays homeologous chromosomes, induced with diluted colchicine solution, varied according to the ploidy:
* In species and hybrids with 2n=20, homeologous chromosomes of both genomes pairing, obtaining a maximum of up to 5lV. The exception was Zea diploperennis and the hybrids with this species, where the cryptic pairing of homeologous chromosomes was not induced at least with the dose of colchicine, in which all of the Zea species responded favorably.
* In the hybrids with 2n=30, the cryptic pairing of homologous chromosomes increase to a maximum of 10lll, with the exception of the hybrid between Z. mays x Z. mays with 2n=30, where significant differences between the treated and non-treated material were not observed. It seems that in this hybrid, preferential pairing of homologous chromosomes took place and as a result, cryptic pairing of homologous chromosomes was not induced.
* In species and hybrids with 2n=40, cryptic pairing of homeologous chromosomes took place up to a maximum of 10lV, with the exception of Z. mays with 2n=40, where significant differences between treated and non-treated material did not exist.

 By integrating results obtained by other authors with our own, it can be deduced that the evolutionary mechanism of the Zea Genus could have been the following:

1. Starting from a common polyploid ancestor with AAAA genome, the homologous chromosomes seem to have been differentiated giving rise to a diploid species with two genomes A and B. This new species seems to have had more advantages to adapt since it is more stable due to the fact that it is diploid
2. In the next evolutionary stage, the B genome seems to have been differentiated giving rise to the different Zea species.
3. The species that have appeared until today seem to have maintained their ability to hybridize, giving rise to fertile descendants due to the fact that no dramatic changes occurred in the homeologous chromosomes.
4. This evolutionary differentiation without great chromosomic alterations was possibly favored since the species kept themselves geographically isolated.