Flowering time in inbred lines differing in knob composition

During a series of plantings to produce inbred stocks, some of which were used in experiments to standardize a C-banding method (Aguiar-Perecin, Caryologia 38:23, 1985), some routine observations on flowering time were made. Highly inbred stocks of Zapalote Chico, a high-knob Mexican race, were always observed to be very precocious in comparison with inbred stocks of Ceremonial and Entrelaqado, low-knob Brazilian races. This precocity was also observed in comparison with inbred lines derived from the flint variety JD, previously mentioned, with a difference of around 20 days in an experiment in 1987. In fact, the Zapalote Chico race was described as very precocious by Wellhausen et al. (Races of Maize in Mexico, Secr. Agric. Ganad. Mexico, Rockefeller Found., 1951). Due to the racial and certainly genotypic differences among the materials mentioned, it would be difficult from these simple findings to argue for high knob numbers causing precocity, in the conditions of these plantings in the fields of ESALQ/USP, Piracicaba, which were carried out always in summer, with flowering during January and February.

In this context, and regarding some reports in the literature, it would be interesting to investigate correlations between flowering time, the duration of mitotic cycle and knob composition. Cell cycle time has been shown to be species specific, depending mainly on the basic nuclear DNA content (Van't Hof, Exp. Cell Res. 39:48, 1965; Bennett, Proc. R. Soc. London B. 181:109, 1972). On the other hand, it has been shown in some species of Compositae that the amount of heterochromatin per genome is related to the reduction of the cell cycle times in annual plants with large DNA and heterochromatin contents, but that develop faster than the perennials (Nagl, Nature 249:53, 1974). This reduction was also found in the heterochromatin-rich Allium carinatum (Bosen and Nagl, Cell Biology Intern. Reports, Vol. 2:565, 1978). Further, Vosa, (University of Oxford, U.K., personal communication) also observed a species of Allium from Turkey, with extremely high heterochromatin content, to be very precocious. Price and Bachmann (Plant Syst. Evol. 126:323, 1976) reported that some annual species of Compositae with high DNA contents and longer mitotic cycle time yet displayed a rapid developmental rate.

Correlations between higher DNA content, lengthening of cell cycle and late flowering have been observed when B chromosomes are present in some species of cereals (Jones and Rees, Chromosoma 24:158, 1968; Rao et al., Experientia 35:1154, 1979). In a previous experiment, it was found that an increasing number of B's tends to delay the male flowering rather than the female, in inbred lines of Zapalote Chico (Ruas, Aguiar-Perecin and Miranda-Filho, in preparation).

In order to analyze the effects of knobs on development, lines of the JD variety have been selfed, as mentioned in the communication on knob segregation. In 1984, it was observed that one S2 line and respective S3 progenies (family JD-14) differed by a mean of 10 and 5 days respectively from another group of S3 lines (JD-06). The 2 groups and derivatives (S4, S5 and S6) have maintained this difference (5-10 days) in subsequent years. Only male flowering has been recorded in randomized block experiments. The cytological analysis showed that late materials (JD-06 lines) do not display knobs at 7S and 9S. Here again data appeared to show late material having knobs in fewer positions. As the cytological analysis showed differences in the frequencies of heterozygotes and homozygotes in the various stocks, the flowering time of S4, S5 and S6 progenies have been recorded in order to detect groups with conspicuous differences. These groups in fact appeared more consistently during the experiment of January to February, 1987. The estimates of flowering time between 3 families of S6 progenies derived from JD-14-1 showed that one of them, 300-14-1-3-3, was more precocious. In another group of 3 families derived from JD-14-2, one of them, 301-14-2-1-1, showed delay in flowering (mean of 5 days). The histograms of frequencies of days of flowering of these 2 lines are quite distinct. Table 1 shows the knob constitution of the 2 families but data referring to the 6 families are still incomplete. These results are, at present, the only ones that seem to be in agreement with the report of Chughtai and Steffensen (MNL 61:98, 1987), in the sense of the appearance of more precocious progenies with fewer knobs.

The expectation at the present state of this investigation is that the comparison of subgroups within JD-14 families will provide information on the effects of different knob contents and positions, or interactions of knob-genotypes on plant development, evaluated through the flowering time. Selected crosses and their F2 will also be analyzed. Studies on genes affecting flowering have been controversial in estimating the number of genes (Bonaparte, Can. J. Genet. Cytol. 19:251, 1977). The importance of knob constitution in plant adaptation has been emphasized in the literature, as in the survey of knobs in maize races made by McClintock et al. (Chromosome Constitution of Races of Maize in the Americas, Colegio de Post Graduados, Chapingo, Mexico, 1981). Further, the demonstration made by Viotti et al. (TAG 70:234, 1985) that knob sequences may be found interspersed in the genome is also an additional point to be envisaged in this discussion.

Table 1. C-bands corresponding to knob positions in two families of S6 progenies derived from a flint maize variety.

Margarida L.R. de Aguiar Perecin

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