Molina and Naranjo (Theor. Appl. Genet. 73:542, 1987) and Naranjo and Molina (MNL 61:62, 1987) presented cytological evidences for a basic chromosome number x = 5 in Zea:

1. In 2n = 30 hybrids the most frequent meiotic configuration was 5 III + 5 II + 5 I, and in Z. perennis and hybrids with 2n = 40 it was 5 IV + 10 II.

2. In all species and hybrids with 2n = 20, secondary association was observed, and a maximum of 5 groups of 2 bivalents each was frequent at diplotene-diakinesis.

3. In 2n - 30 hybrids, there is a clear tendency for A, B and C genome separation through the respectively grouped trivalents, bivalents and univalents.

As well as the evidences mentioned it has recently been observed that a high frequency of cells showed formation of 2 spindles with 5 bivalents each in metaphase-anaphase I, in all taxa with 2n = 20. In several cells an asynchronous initiation of anaphase was observed. It is interesting to point out that when cut stems were placed in colchicine solution (Poggio et al., this MNL), the 2 spindles were clearly seen in cells without multivalent formation.

More studies are needed to clarify the origin of the double spindle and the position of different genomes in it. The double spindle is, probably, a relictual condition of diploidy.

Maria del Carmen Molina, Carlos A. Naranjo and Lidia Poggio

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