Meiotic behaviour and total DNA content in a homozygous line of maize with cytoplasm of Z. mays ssp. mexicana --L. Poggio, C. Tito and C. Naranjo A pure line of maize (Multiple Dominant of Dr. Randolph, No. 1877) (Z line) was used as male recurrent parent during 20 backcrosses onto Z. mays ssp. mexicana (Florida variety, Huixtla, Mexico), obtaining the E line. Both lines have the same homozygous genotype but different cytoplasm. Mazoti (1958) reported that the cytoplasm of ssp. mexicana was responsible for several inherited morphological and physiological characters such as precocity, number of ears per plant, etc. Moreover, Mazoti and Velazquez (1962) found that the E line presented a greater percentage of pollen sterility, greater variance in the diameter of the nucleolus, chromosomes in meiosis with stickiness manifestation and a high frequency of intercellular contacts. Later, Mazoti (1982) established that the knobs in the homozygous line of maize in the "plasmon" of ssp. mexicana increase their size and the DNA content.

In the present work the total DNA content was measured in both lines (E and Z) with Feulgen cytophotometry with the aim to elucidate whether the increase in the size of knobs was due to differential packing, or insertion or endoreduplication of DNA. Moreover, the meiotic behaviour of both lines was studied to determine if the partial male sterility of the alloplasmic line E could be due to chromosomal irregularities.
 
Line
Number of cells studied
Bivalents (range) 
Univalents (range)
Z
96
9.22 ± 0.09 (6-9)
1.56 ± 0.18 (2-8)
E
98
8.09 ± 0.16 (2-9)
3.25 ± 0.33 (2-16)

Although Z does not have regular meiosis, it has a mean of univalents significantly lower than E. Moreover, in the E line several meiotic irregularities were observed: desynapsis, cytomixis, nuclear fusion, nucleolar bodies and pseudomultivalents. These abnormalities were only observed in some of the flowers of the panicle. This fact could indicate that the nucleus-cytoplasm interaction alters the canalization of development, although the origin of the disharmony is yet unknown.
 
Line
DNA content (2C) pg
Number of nuclei measured
Z
6.75 ± 0.04
155
E
7.08 ± 0.04
157

There is a quantitative change in the total DNA content, being significantly high in the E line. Moreover, the E line presents bigger knobs in pachytene, and bigger C+ bands in mitotic and meiotic metaphases when C-banding technique is applied. These results could indicate that the cytoplasm of ssp. mexicana produces nucleotypical changes in the nucleus of maize, increasing the highly repetitive DNA in the zone of the knobs. This could be affecting the regulatory system.

It can be concluded that the interaction between the nucleus of maize and the cytoplasm of Z. mays ssp. mexicana may induce a mechanism that generates rapid genome changes which will be manipulated to our advantage when more information about the nature of the phenomena described would be obtained.


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