Nuclear DNA content in Zea

Microdensitometry measurements revealed significant differences in 4C DNA content between and within taxa in the genus Zea. Rayburn et al. (Am. J. Bot. 72:1610, 1985) and Laurie and Bennett (Heredity 55:307, 1985) demonstrated that there are variations in DNA content among varieties of Zea mays ssp. mays and ssp. mexicana. Moreover, Rayburn et al. (ibid.) have shown that there is a significant positive correlation between C-band number, % of heterochromatin, and genome size (DNA content). They also show that DNA content decreases with increasing latitude. Their results indicate that intraspecific variation in DNA content in maize is largely caused by differences in the amount of heterochromatin and is significantly correlated with geographical distribution, suggesting that these characteristics have adaptative significance and are, therefore, of potential agricultural interest (Laurie and Bennett; Bennett, New Phytol. 106:171, 1987).

In the present work we have studied the 2C nuclear DNA content in 4 taxa of the genus Zea and in 3 interspecific hybrids by microdensitometry of Feulgen-stained root-tip telophase cells. The technique was done according to Poggio and Hunziker (J. Hered. 77:43, 1986). The optimum time of hydrolysis occurred after 30 min in 5N HCl at 20C. Allium cepa var Ailsa Craig was used as standard. The results are summarized in Table 1.

Rayburn et al., and Laurie and Bennett, have demonstrated that there is a highly significant variation in 2C DNA content in maize, ranging from 4.92pg in the Seneca 60 hybrid to 6.74pg for a Zapalote Chico accession. The 9063 line analyzed in this work has 2C DNA content values similar to those of the races of Mexican maize regarded as "primitive"; namely Chapalote (5.82pg) and Nan-Tel (5.96pg), reported by Laurie and Bennett. The 9063 line has a high value when compared with Seneca 60 (2C = 4.92pg) or Knobless Tama Flint (2C = 5.14pg) values published by Laurie and Bennett. The same authors found significant differences among races of annual teosintes (ssp. mexicana). The line of ssp. mexicana analyzed in this work shows a very high 2C DNA content (2C = 6.73pg) considering that the highest value reported by Laurie and Bennett was 2 C = 6.44pg for the line K65-1. The artificial hybrid between both lines studied here shows an intermediate value between them and differs significantly from both progenitors.

Variation in the amount of heterochromatin (number of C+ bands) would appear to be an important cause of differences in DNA content, and significant differences in number of C+ bands in metaphase and prophase were found between Z. mays ssp. mays (9063) and ssp. mexicana (4024). The 9063 line presents 12 C+ bands distributed in 10 chromosomes. The 4024 line has a higher number of C+ bands (17-18) and only 2 chromosomes are devoid of bands.

Preliminary studies in another line of ssp. mexicana (4025) show the presence of heterozygosity for terminal C+ bands. This fact could account for differences in karyotypes, DNA content and morphological characters in hybrids with the same progenitors. These data indicate that the DNA content of ssp. mays (9063), compared with the higher DNA content of ssp. mexicana (4024) could be attributed to a smaller amount of heterochromatin. The hybrid presents, as is expected, an intermediate number of C+ bands in metaphase and C+ chromocenters in interphase (Poggio et al., in preparation).

Laurie and Bennett reported 2C = 5.28pg for Z. diploperennis. The line studied here shows 2C = 6.55pg (Z. diploperennis, Mexico Jalisco, Sierra de Mananthan Occidental, 2Km E of Las Joyas, Leg. Rafael Guzman & M.A. de Guzman - No. 1120, Nov. 1980). The same authors reported that Z. perennis had a DNA content close to twice that of Z. diploperennis (Z. perennis, Mexico, Jalisco, Ciudad Guzman. Leg. Dra Prywer, cultivated in "Instituto Fitotecnico de Santa Catalina" since 1962). In the present work Z. diploperennis shows less DNA content per basic genome than Z. perennis (Table 1). This difference is statistically significant. This fact is not surprising. Although in polyploids of recent origin the DNA content is directly proportional to the DNA content of diploid progenitors, there are cases in which this proportionality does not occur (Poggio and Hunziker, J. Hered. 77:43, 1986; Martinez and Ginzo, Can. J. Genet. Cytol. 27:766, 1985; Bennett, New Phytol. 106:171, 1987; Poggio et al., Darwiniana 27:25, 1986).

The hexaploid hybrid Z. diploperennis X Z. perennis has an intermediate value between the progenitors. The octoploid hybrid probably originated by the fertilization of an unreduced egg cell from Z. diploperennis by a normal male gamete from Z. perennis. The hypothetical genomic constitutions of these species and hybrids are, according to Molina and Naranjo (Theor. Appl. Genet. 73:542, 1987):

The expected result is that the octoploid hybrid will have more DNA content (about 6%) than the octoploid Z. perennis. In fact, the hybrid has more DNA than Z. perennis although the difference is statistically nonsignificant. The expected difference is small and perhaps of a magnitude difficult to detect by Feulgen cytophotometric method.

Finally, it is interesting to point out that the differences found in DNA content and C+ band number in Z. mays ssp. mays and ssp. mexicana do not interfere with normal pairing since the hybrid shows 20 bivalents (II) in meiosis and normal fertility (Naranjo and Molina, MNL 61:62, 1987). The Z. diploperennis X Z. perennis hybrids (2n = 30 and 40) show the expected pairing according to the genomic constitution postulated by Molina and Naranjo.

Table 1. 2C nuclear DNA content in the genus Zea and artificial hybrids.

Claudia M. Tito, Lida Poggio and Carlos A. Naranjo


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