We are interested in the population genetics and meiotic behavior of the native races of maize from Argentina. B chromosomes are frequent in these races (Rosato et al. MNL 15:67; Chiavarino et al., MNL 15:68, 1993). In the present work we analyze the polymorphism frequencies and the meiotic behavior of B's in 78 individuals of the maize native population "Pisingallo" from NW Argentina. B's were found in 43.6% of the individuals, the distribution being: 64% 1B, 30% 2B, 4% 3B and 3% 4B. The meiotic behavior of B's can be summarized as follows:
1B plants. Metaphase I: The B univalent remains outside the plate. Anaphase I: The B chromosome migrates precociously to one pole in 50-95% of cells studied, and is included in the pole or forms a micronucleus; lags at the metaphase plate (0-8%) and forms a micronucleus; or divides equationally (0-7%), and is included in the poles or forms 1 or 2 micronuclei. The percentage of dyads with micronuclei was 5-35%, indicating that the B is frequently lost in the first meiotic division.
2B plants (Fig. 1). Diakinesis: The B's can be observed forming 2 univalents (0-21%), 1 bivalent (65-94%) or 1 pseudobivalent (0-14%). Metaphase I: Univalents remain outside the plate (5-35%) and can migrate to the same or to different poles, or divide equationally in very few cells; in all the cells the two univalents are secondarily associated. Bivalents remain in the metaphase plate together with the A bivalents (92-100%), or outside the plate. In the former case the B's migrate precociously to the poles in a few cells or migrate together with the A's. In the latter the bivalent lags in AI or the B's migrate precociously to the poles. At metaphase II the B's frequently remain outside the plate. Micronuclei were observed.
3B plants. At diakinesis the B's can be associated, forming a trivalent (35%), bivalent plus univalent (60%) or 3 univalents (5%). The bivalent and univalent have secondary association in all the cells studied. Metaphase I: The 3 univalents remain outside the plate, migrating to the poles at random. The bivalent remains in the plate with the A's migrating normally, while the univalent migrates precociously to one pole. The trivalent always remains in the plate, orientating one B to one pole and two to the other.
The percentages of the different meiotic configurations given above are the pooled data scored in the observed individuals. It is worth mentioning that there was large interindividual variation; for example, in some individuals the B's tended to behave like the normal complement, while in others they tended to form more univalents, laggards, equational division, etc. For this reason we include the range of variation in the most remarkable cases. It is interesting that this variation in the meiotic behavior may be due to a genetic variation present in the native race, which may affect the B chromosome transmission and polymorphism, as occurs in other organisms (Puertas et al., Chromosomes Today 11:391-399, 1993).
1. A and B: 2n=20+2B. A=diakinesis; B=metaphase I. Arrows show the
B univalents. Bar=10 um.
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