Nuclear DNA amount and doses of B-chromosomes in native populations of maize
--Rosato, M*, Chiavarino, AM*, Naranjo, CA, Poggio, L*
*also affiliated with Depto. de Ciencias Biologicas, FCEN, UBA

A large nuclear DNA variation occurs in the genus Zea, in such a way that DNA C value may be different among species, subspecies, populations, inbreds and open pollinated lines. Differences in the amount of heterochromatin and/or the occurrence of B-chromosomes (Bs) would be the cause of the variation in the DNA content in maize. However, the effects of different doses of B-chromosomes have been scarcely investigated.

With the aim of investigating the relationship between the DNA content and B-chromosomes, we studied the DNA amount in individuals with different doses of Bs in four Argentinian native populations of maize. The DNA content was measured using Feulgen microdensitometry (Rosato et al., Am. J. Bot. 85(2), 1998, in press). The results of total DNA content are summarized in Table 1. Differences of DNA content in these populations containing individuals with different doses of Bs (0, 1, 2, and 3B) were analyzed by means of a two way ANOVA test. This analysis indicated that neither the mean DNA content of the populations (F=1.2397; p=0.3040) nor different doses of Bs (F=3.2259; p=0.0752) differ statistically. On the other hand, only the interaction between doses of Bs and populations were highly significant (F=4.8693; p=0.0001). This means that the variation of the DNA content with the doses of Bs depends on each population.

The results indicated that in the population VAV 6485, with the lowest A-DNA value (5.8 pg in individuals without Bs), the DNA content of individuals with Bs was significantly higher than that in individuals without Bs (Table 1). In the population VAV 6480, the individuals with different doses of Bs did not show any significant difference in DNA content. In the other two populations, VAV 6479 and VAV 6475 with the highest A-DNA values (6.409 pg and 6.448 pg, respectively), the DNA content in individuals without Bs was equal (VAV 6479) or even significantly higher (VAV 6475) than in individuals with 1B (6.074 pg and 5.786 pg respectively). In the latter populations the increase of DNA content with doses of Bs becomes more evident as the number of Bs increases starting from one B (Table 1).

Ayonoadu and Rees (Heredity 27:365-383, 1971) studied the genome size in individuals without Bs and with 8 Bs in the line Black Mexican sweet corn and they calculated that each B increases by about 5% the DNA content in the B-carrying individuals. In view of our results, individuals with Bs do not always have a higher DNA content than individuals without Bs. For this reason, in native populations it is impossible to predict the genome size of individuals with different doses of Bs, considering an increase of the DNA content of 5% per each B as was determined by Ayonoadu and Rees. This fact is due to the variation in the A-DNA content within these populations which masks the increase due to the presence of Bs.

Table 1. DNA content in individuals with different doses of Bs in four populations.
 
Population (Race) Mean DNA content with different doses of Bs (X(ES) (pg) (ind. no.)
  0B 1B 2B 3B
VAV 6485 (Blanco) 5.800±0.094 (6) 6.310±0.072 (3) 6.392±0.062 (4) 6.599±0.146 (4)
VAV 6480 (Am.grande) 6.106±0.102 (8) 6.085±0.083 (4) 6.285±0.097 (4) 6.423±0.016 (3)
VAV 6479 (Blanco) 6.409±0.086 (3) 6.074±0.050 (6) 6.375±0.070 (4) 6.622±0.068 (4)
VAV 6475 (Harinoso) 6.448±0.131 (6) 5.786±0.145 (3) 6.377±0.053 (4) 6.592±0.081 (4)


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