New cytological evidences for a basic number x = 5 in the genus Zea

In the last News Letter (MNL 60:77-79, 1986) we presented an introduction and discussion of our first results on cytological evidences for a basic number x = 5 in Zea. The same results were published afterwards in Theor. Appl. Genet., in press (1987). The aim of this article is to present new results that support this working hypothesis and add three new pieces in the puzzle of the x = 5 genomic relationship among taxa.

The new results consist of the analysis of the meiotic configurations of Zea mays ssp. mexicana and two F1 artificial interspecific hybrids (Z. mays ssp. mays x Z. mays ssp. mexicana, 2n = 20; Z. perennis x Z. mays ssp. mexicana, 2n = 30). In Table 1 the results of meiotic studies are detailed. Z. mays ssp. mexicana (2n = 20) and the F1 hybrid Z. mays ssp. mays x Z. mays ssp. mexicana (2n = 20), present a regular meiosis with the formation of 10 bivalents. In both tetraploid taxa secondary associations were found with means of 2.74 and 3.57 pairs of bivalents, respectively. In F1 hybrid Z. perennis x Z. mays ssp. mexicana (2n = 30) 5III + 5II + 5I were formed in ca. 47% of the 104 cells studied and the means were III = 5.27, II = 5.5 and I = 4.58 (Table 1).

In Figure 1 the most frequent meiotic configurations are presented. Our previous results on other taxa, with the purpose of making an integration, are also included in the same figure.

In the 2n = 30 hybrid (Z.m.mx. x Z.per., Fig. 1) the 5III could be formed by autosyndetic pairing of A1' and A1" genomes from Z. perennis and by allosyndetic pairing with the A1' genome from Z. mays ssp. mexicana. The 5II could be formed by autosyndetic pairing between C, and C2 homeologous genomes from Z. perennis. Finally the 5I would belong to the B2' genome from Z. mays ssp. mexicana. As in the hybrids with 2n = 30 previously studied (Molina & Naranjo, 1987), in this new hybrid there is a tendency of A, C and B genome separation in meiotic metaphase I, with the trivalents, bivalents and univalents grouped respectively, as indicated.

The case for an allopolyploid origin of 2n = 20 taxa and autoallopolyploid origin of 2n = 40 taxon, seems strong (Fig. 1). Until this moment our results suggest a minimum of three distinct original genomes (A, B and C) in order to explain the genome constitution and the cytogenetic relationships among the four species studied.

Table 1.

Figure 1.

Carlos A. Naranjo and Maria del Carmen Molina
 
 


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