The plant materials used for these studies were: maize inbreds 2n=40 (Zm40) supplied by the Maize Genetic Coop. Stock Center, Urbana, Illinois, USA, the hybrids DP40 (2n=40) obtained from crossing a non reduced gamete of Zea diploperennis (Zd), 2n=20, by Zea perennis (Zp), 2n=40, and MDP 2n=40 obtained by crossing Zm40¥DP40. Zm40 is annual, 1.8 to 2.0 m height, with one or two fertile ears and indifferent to photoperiod. The hybrid DP40 is perennial, tillering, fertile (80%), seeds scatter at maturity and according to its flowering response it is a qualitative short photoperiod plant with a 13 h critical photoperiod. The hybrid MDP is annual, 2.0 to 2.5 m height, without tillers or at most only one or two. It is prolific, fertile seeds are disposed in 4 to 8 rowed distich ears which do not scatter at maturity and flowering results indifferent to photoperiod.
From the cytogenetic study of the hybrids (Table 1) and their parents it can be deduced:
Zm40 revealed 10iv (30%) and 9iv+2ii (23.94%) with an average of 8.34iv+3.24ii and 33.75 chiasma/cell. According to these results and considering the basic chromosome number for the genus Zea as x=5, the following genomic formula can be proposed for Zm40: AmAmAmAm BmBmBmBm.
Zd presents regular meiosis and 10 bivalents could be observed in 73% of the cells studied. The existence of 1iii+8ii+1i or 1iii+7ii+3i in 4% of the cells would point to some homoeologies between A and B genomes and also suggests a probable segmental allotetraploid. The genomic formula proposed for Zd is AdAd BdBd.
Zp has 5iv+10ii (54.47%) and 4iv+12ii (20,15%) with an average of 4.44iv+11.02ii and 34.56 chiasma/cell (15.69 in bivalents and 18.87 in tetravalents). The most frequent chromosomic pairing found in Zp suggests for this species the genomic formula ApApApAp Bp1Bp1 Bp2Bp2.
The hybrid DP40 has a low percentage of univalents and trivalents, most of its chromosomes being paired as open ring bivalents and tetravalents, with an average of 31.42 chiasma per cell (15.96 in bivalents and 15.30 in tetravalents). The hybrid DP40 and Zp are very similar in their meiotic configurations as well as in the average of i, ii, iii and iv. According to these results the genomic formula ApApAdAd Bp1Bp2 BdBd is proposed.
In MDP (Table 2) chromosome pairing is regular forming bi or tetravalents but mono or trivalents were not found. In 43.93 % of the cells 10ii + 5iv pair, in 12.87 % 8ii + 6iv and in 12.87 % of cells 12ii + 4iv. The average number of chiasma/cell is 31.34 (10.24ii + 21.10iv). During anaphase an equal number of chromosomes migrate to each pole and inversion bridges can exceptionally be observed. Pollen fertility ranges from 83% to 95% and seed viability is 90%. The proposed genomic formula is AmAmApAd BmBm BdBp.
Zm40, Zp, DP40 and MDP were treated with diluted solutions of colchicine (0.5 x 10-4M) to analyze chromosome homologies and pairing amongst homoeologous genomes. From the results in Table 3, it can be deduced: a) colchicine does not increase significantly the number of quadrivalents in Zm40, b) in Zp, DP40 and MDP the number of iv results in greatly increased pairing amongst homoeologous chromosomes of genome B, c) homoeologous genomes A pair in all cases, but genomes B only do if there is no other homologous competition during pairing.
Table 1. Meiotic configurations of the
species and hybrids
|Chiasmata/cell||No of PMCs examined|
Table 2. Meiotic configurations of the
hybrid MDP (2n = 40)
|i||ii||iii||iv||%||No of cells studied|
Table 3. Number of iv/cell in colchicine
(0.5 x 10-4M) treated (T) and control plants of Zp; Zm40; DP40 and MDP.
Number of quadrivalent (%)
|No of PMCs Exam.|
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