Zea mays ssp. mays (2n = 20) is commonly considered as diploid with x=10. Recently, Molina and Naranjo (Theor. Appl. Genet., 73:542, 1987) and Naranjo and Molina (MNL 61:62, 1987) presented new cytological evidence supporting x = 5 as the basic chromosome number of the genus.
Treatments of meiocytes prior to the onset of meiosis with dilute solutions of colchicine can induce intergenomic pairing, revealing cryptic genome homologies (Jackson & Murray, Theor. Appl. Genet. 64:219, 1983). This technique disrupts a bivalent promoting mechanism in Alopecurus and Triticum, and revealed the polyploid origin in "diploid" species of Helianthus (Jackson & Murray, ibid.; Murray et al., Kew Chrom. Confer. 11:165, 1983).
In 10 plants of maize, colchicine solution (0.5 X 10-4M) was applied prior to the onset of meiosis by injection of the solution in the node. This material showed one quadrivalent in about 81% of the cells in every plant examined.
In another experiment the stems of 5 plants were cut and placed in 0.5 x 10-4M colchicine solution for 12h followed by 24h in water before fixation in 3:1 alcohol: acetic acid. The quadrivalent frequency was higher than in the injected material. A detailed study of 155 cells from one plant showed the following results:
In both treatments the control material was fixed without pretreatment and showed only bivalent formation.
These results constitute new evidence to support the Molina and Naranjo hypothesis that the Zea species with 2n = 20 are of polyploid origin. Although the meiotic configurations that maize presents normally are those expected in a typical allotetraploid (AABB), the induced quadrivalent formation by colchicine suggests a higher affinity between the postulated genomes and the existence of a genetic mechanism similar to that of wheat, which promotes bivalent formation. Finally it underlines the agronomic potential of this technique, as pointed out by Jackson and Murray (ibid.), since the induction of intergenomal recombination can give rise to new and extraordinary recombinant genotypes.
Lidia Poggio, Maria del Carmen Molina and Carlos A. Naranjo
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