B-chromosomes in six Argentine races of maize
--A. M. Chiavarino (1), L. Poggio (1), C. A. Naranjo and J. Cámara Hernández (2)
(1) Also affiliated with Depto. Cs. Biológicas, FCEN, UBA.
(2) Fac. Agr., UBA.

Populations with extra chromosomes, termed B-chromosomes, are frequent in maize. These chromosomes have accumulation mechanisms which involve nondisjunction at a very high frequency in the second pollen mitosis. Pollen grains with B-chromosomes preferentially fertilize the egg cells.

In this investigation we analyze the meiotic behavior of B-chromosomes in several Argentine maize races, which have not been investigated cytologically. These races are: "Altiplano", "Amarillo de ocho", "Capia rosado", "Capia blanco", "Pisingallo" and "Negro".

In "Altiplano", individuals with 0, 1 and 3 B-chromosomes were found. In individuals with 2n=20+1B the B was outside the metaphase plate during metaphase I in 76% of the studied cells and migrated precociously to one pole during anaphase I. In the remaining 24% of the cells the B-univalent split the two chromatids and migrated to both poles during anaphase I. "Altiplano" individuals with 2n=20+3B's showed in diakinesis the configuration 10 II + 1 III (58% of cells) or 11 II + 1 I (42% of cells). In MI the frequency of cells with B's forming trivalents was 36%. In cells with B's forming 1 II + 1 I, the bivalent shows normal migration without separation from A-chromosomes while the univalent tends to move precociously to one pole at anaphase I.

In "Amarillo de ocho", only one individual with 2n=20+2B's was studied. The two B's formed one positive heteromorphic bivalent in pachytene-diplotene. In metaphase I it was indistinguishable from A-bivalents and has normal migration.
 

In "Capia rosado" the meiotic behavior of individuals with 2n=20+1B was analyzed. The B-univalent remains outside the plate in metaphase I, and in anaphase I moves precociously to one pole (Fig. 1A, B) or splits its chromatids, showing normal migration.

In "Capia blanco", individuals with 2n=20+2B's were studied. The most frequent configuration was 11 II and the B's migrate normally, together with the A-chromosomes (Fig. 1C, D). In some cells 10 II + 2 I was observed.

"Pisingallo" has 2n=20, 20+2B's and 20+4B's individuals. The 20+2B's show a meiotic behavior similar to that described for "Capia blanco". The 20+4B's showed 12 II. One of the extra bivalents shows a normal migration without separation from A-chromosomes, while the other migrates to one pole. Very few cells show 1 quadrivalent but the composition of this quadrivalent and its migration has not yet been studied.

In "Negro" one individual of the seven studied has 2n=20+2B. It showed in metaphase I 11 II or 10 II + 2 I. As in the other cases the B's formed bivalents segregating normally and the univalent moved precociously to the poles at anaphase I. This individual showed 2n=20 chromosomes in mitosis of root apex.

Our data indicated that, in the Argentine races, the meiotic behaviour of B-chromosomes to suppress meiotic loss is similar to that described by Carlson and Roseman (1992).

It is very likely that the presence of B-chromosomes modifies the inheritance of biochemical and morphological characters of agronomic importance, and need to be taken into account in plant breeding programs.

Figure 1. A and B: "Capia rosado"; A: metaphase I with 10 II + 1 B; B: anaphase I. C and D: "Capia blanco"; C: metaphase I with 11 II (20+2B's); D: diakinesis with 11 II. Arrows show B-chromosomes. The bar=10µm.


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