Variation in the transmission of B chromosomes, 0B x 1B and 1B x 0B crosses
--M. Rosato, A.M. Chiavarino, M.J. Puertas, C.A. Naranjo, and L. Poggio

Longley (J. Agr. Res. 35:769-784, 1927) first described that B chromosomes (B's) are present in unselected races of maize. We have found that B polymorphisms are common in unselected local populations of maize from Argentina and Bolivia. The B's can be maintained by their mechanisms of "drive," which consist in suppression of meiotic loss and nondisjunction at the second pollen grain mitosis (Roman, Genetics 32:391-409, 1947; Carlson and Roseman, Genetics 131:211-223, 1992), together with preferential fertilization of the sperm nucleus carrying the B's (Roman, Proc. Natl. Acad. Sci. USA 34:36-42, 1948; Carlson, Genetics 62:543-554, 1969) and higher competitive ability of pollen grains carrying the B's (Beckett, J. Hered. 73:29-34, 1982).

The nonMendelian behavior of B's produces a variation in their transmission. The genetic control of this variation was demonstrated in Myrmeleotettix maculatus, Pseudococcus affinis, Secale cereale and Aegilops speltoides (Romera et al., Heredity 66:61-65, 1991; Cebriá et al., Amer. J. Bot. 81:1502-1507, 1994; Jiménez et al., 1995).

We are currently investigating whether such a genetic control of B transmission occurs in maize. The present work reports the analysis of the variation of the rate of B transmission in 1B x 0B and 0B x 1B crosses.

A large sample of 145 individuals of the Pisingallo race (from N.W. Argentine) were screened for B number in the root tips. The seeds were grown on wet filter paper, pretreated with 0.02M 8-hydroxyquinoline for 3 hours and fixed with ethanol:acetic, 3:1. They were stained with 2% hematoxyline and ferric citrate as mordant. From this sample we selected 0B and 1B individuals, and made 20 crosses female 1B x male 0B and 20 crosses female 0B x male 1B. The progeny were collected plant by plant and at least 25 individuals per spike were similarly screened for B number. With these results, the mean number of B's transmitted in each cross was calculated.

In the 0B x 1B crosses we found a large range of variation in the mean number of B's transmitted (0.17 - 1, the average mean being 0.52 ± 0.26). The differences among crosses were due to the proportion of 2B vs. 0B progeny because most plants of the progenies showed 0B or 2B, 1B plants being rarely found. This indicates that nondisjunction occurred in nearly 100% of the cases. On the other hand, this also indicates that the mechanism of nondisjunction occurs irrespective of the mean number of B's transmitted. Therefore, both processes seem to be independently controlled.

In the 1B x 0B crosses the range of variation was smaller (0.31 - 0.58, the average mean being 0.47 ± 0.08). The plants of the progenies showed either 0B or 1B, indicating the lack of nondisjunction on the female side (Fig. 1).

Figure 1. B chromosomes transmitted to the progeny.

If the B's were not lost during meiosis, and nondisjunction occurred in all cases at second pollen grain mitosis, the expected frequency of B's transmitted to the progeny in 0B x 1B crosses would be 25%. Figure 1 shows that much higher frequencies (50%) were observed in our experiment. This indicates the occurrence of preferential fertilization by the sperm nucleus carrying 2B (Roman, Proc. Nat'l. Acad. Sci. USA 34:36-42, 1948).

In some 0B x 1B crosses the frequency of transmission was significantly lower (8%). Therefore, there is a variation in the frequency of B transmission. It is possible that preferential fertilization does not always occur.

Our results show a variation in the rate of transmission which can be related to a genetic control on the preferential fertilization, or other processes of male and female gametogenesis.

The differences between the variation in B transmission rate on the male and the female are remarkable. In our opinion this strongly suggests a genetic control of the variation, because if the variation were due only to environmental causes, the expected variations would be the same in both sexes.

The polymorphism for genes controlling the B transmission rate may explain the differences in B frequency in different populations of maize. 

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