B chromosome polymorphism in N.W. Argentine populations
--A.M. Chiavarino, M. Rosato, C.A. Naranjo, J. Cámara Hernándéz1 and L. Poggio2
1Cátedra de Botánica Agricola, FA, UBA;
2also affiliated with Depto. Cs. Biológicas, FCEN, UBA

Supernumerary chromosomes (B's) are one of the most frequent forms of naturally occurring chromosomal polymorphisms. These polymorphisms are common in maize populations, however there are few studies at population level. In native races of maize from Argentina there are no studies analyzing the frequency of polymorphism due to B chromosomes.

In the present work the frequency of B chromosomes and the average mean number of B's per individual belonging to six populations from the races Amarillo de Ocho, Capia Rosado, Capia Blanco, Altiplano, Pisingallo is analysed (Table 1, Fig. 1).

Table 1.
 
Population 0B  1B  2B  3B  4-7B  Percent Ind.w/B  B/Ind Num.
Amarillo de Ocho 45 31 11 - - 55 0.9 64
Altiplano 50 18 9 9 14 50 1.18 22
Capia Rosado 52 36 8 - 4 48 0.8 25
Capia Blanco 44 21 20 3 12 56 1.2 34
Pisingallo (I) 54 35 8 3 - 46 0.6 37
Pisingallo (II) 56 28 13 2 1 44 0.62 78

Figure 1. Frequency of B chromosomes among populations.

In all cases the chromosome counts were made in root tips pretreated with 0.02M 8-hydroxyquinoline for 3 hours. They were fixed in ethanol:acetic (3:1), and stained by 2% acetic hematoxylin. The populational frequency of B's was obtained from 182 individuals (22 to 78 in each population, see Table 1). The meiotic behaviour studies were made in male inflorescences fixed and stained in the same form as described above.

The B frequency was found to range from 44% to 56% in the studied populations (% individuals with B's) (Table 1). The G test indicates that there is a significant difference in the frequency of B's among some of the populations. Moreover the frequency of individuals with different doses of B's varies among the studied populations (Table 1, Fig. 1).

The analysis of meiotic behaviour of individuals with different doses of B's during the male gametogenesis indicates that B's are not lost during meiosis. This fact would contribute to the maintenance of the described B's polymorphisms.

The interpopulational variation described could be due to different selection pressures in each of the populations and/or polytypism for genes affecting the B transmission to the next generation. 


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