--L. Poggio, C. Tito, L. Mazoti and C. A. Naranjo
Cytoplasm of Z. mays ssp. mexicana is responsible for several inherited morphological and physiological characters when it is combined with genotypes of Z. mays ssp. mays. Mazoti (1950, 1958, 1963, 1978) pointed out that some of these characters are: a) vegetative period (response to photoperiod), b) yield in grains, c) height of seedling and plants, d) number of spikelets per plant.
In the �Multiple Dominant� line (Z line), with cytoplasm of Z. mays ssp. mexicana (E line), Mazoti and Velazquez (1962) found a greater percentage of pollen sterility, greater variation in the diameter of nucleolus, stickiness among meiotic chromosomes and, in sections of anthers, very frequent intercellular contacts. Mazoti (1975, 1978) showed that Z line has the gene C2-IE7002 in its own cytoplasm, which induces instability, and the cytoplasm of E line could act as an activator of this gene. Mazoti (1987) reported that the knobs in pachytene of the E line have larger size and higher DNA content.
Poggio et al. (MNL 64:71-72, 1990) found that Z and E lines differ significantly in the total DNA content and in their meiotic behavior. These authors found that the E line possesses higher DNA content and heterochromatin than Z line and, in some places of the panicle, a high percentage of meiotic irregularities (desynapsis, cytomixis, cellular fusion, presence of diffuse nucleolar bodies, pseudomultivalents, etc.).
In the present work the DNA content and meiotic behavior is studied in two stable lines of maize (gl ij and c-tester) and these lines with E cytoplasm. The aim of this work is to analyze the question raised by Mazoti (1987) if �the plasmon of Z. mays ssp. mexicana induce knob variation only in presence of the genes of the Multiple Dominant Line�.
The pure lines utilized were maintained in the IFSC since their introduction. Maize c-tester and gl ij lines were introduced in Argentina in 1933 by Ing. Agr. S. Horowitz. The lines gl ij (E) and c-tester (E) were obtained using the pure lines of maize as male recurrent parent during five backcrosses, onto plasmon from Z. mays ssp. mexicana (Florida variety, Huixtla, Mexico).
The meiotic behavior of both lines is indicated in Table 1. Total DNA content is shown in Table 2. The c-tester vs. c-tester (E) and gl ij vs. gl ij (E) did not show any differences in the average of total and closed bivalents. However lines with E cytoplasm showed two groups of five bivalents each with high frequency in prophase I. This separation is more notorious than that reported by Naranjo et al. (Acad. Nac. Cs. Ex. Fix. Nat., Buenos Aires, Monogr. 5:43-53, 1990). Moreover, each group of five bivalents is slightly asynchronic in respect to the other group in the development of meiotic prophase I. The total DNA content is slightly higher in lines with E cytoplasm (Table 2). The quantitative change in DNA content is accompanied by larger C+ bands in mitotic and meiotic metaphases when C banding technique is applied in Z and E lines (Poggio et al., MNL 64:71-72, 1990).
Summarizing, the results obtained suggest that the E cytoplasm promotes, through some still unknown mechanism, an increase of highly repetitive DNA in the zone of the knobs. This effect would be independent of the presence of the �Multiple Dominant Line� genotype since the stable lines gl ij (E) and c-tester (E) have greater DNA content than gl ij and c-tester in their own cytoplasm. Another effect of the nucleus-cytoplasmic interaction seems to be the alteration of the spatial distribution of genomes and the asynchronism detected during the development of prophase I.
Table 1. Meiotic behavior in Metaphase I (MI) and Diakinesis (Diak)
Table
2. Nuclear DNA content
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