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
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.
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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