In native populations from northern Argentina we found numerical polymorphism for accessory chromosomes (Bs). It is reported that Bs are maintained in populations due to various mechanisms of drive (Carlson and Roseman, Genetics 131: 211-223, 1992), consisting of: 1) the suppression of meiotic loss when Bs are in single dose, 2) nondisjunction at the second pollen grain mitosis, 3) preferential fertilization by the sperm nucleus carrying the Bs produced after the nondisjunction process, 4) competitive ability of B-carrying pollen grains. One of the features of the B mode of inheritance is the variation in their transmission rate (TR) in such a way that in some progenies the Bs tend to be lost and in others they tend to increase in number compared to Mendelian expectation. The B TR was estimated by the quotient between the mean number of Bs transmitted to the progeny and the number of Bs of the parental plant that carried them.

This experiment was carried out in the race Pisingallo (VAV 6313), which is a native population from the northwest of Argentina collected on our own. The progeny (G0) obtained from 20 0B x 1B (male side) and 20 1B x 0B (female side) crosses was analysed. The B-TR variation ranged from 0.17 to 0.98 (mean TR= 0.52) and from 0.31 to 0.58 (mean TR= 0.48) for both male and female sides (Figures 1 and 2). Afterwards, the progenies showing the highest and lowest male and female B-TR were selected through two generations (G1 and G2). The results obtained in G1 indicate the presence of two different groups of plants, high and low B-TR for male and female sides, and demonstrate the existence of polymorphism for genes controlling B-TR (Rosato et al. 1996, Am. J. Bot. 83(9): 1107-1112). In G2 the groups were kept up for both sides, nevertheless it was not possible to obtain more selection progress than that obtained in G1 (Table 1, Figures 1 and 2).

The B univalent precociously migrates to a pole during anaphase I and integrates in one of the nuclei. The B univalent is lost when it lags or divides equationally during anaphase I. Rare meiotic loss of Bs was observed and no significant differences were found when high and low male B-TR plants from the parental population were compared (Table 2).

Generally, in 2B plants, Bs can be observed forming a bivalent and remaining in the metaphase I plate together with the A bivalents; the two homologous Bs have regular meiotic behavior. High and low male B-TR plant groups have no significant differences forming two B univalents in G1 and G2. The low frequency of micronuclei in telophase I - dyad and telophase II - tetrad indicates the scarce meiotic loss of the two Bs (Table 3).

In conclusion, the variation found in male and female B-TR could be due to the presence of few genes controlling B-TR in this population. These genes do not affect the meiotic behavior of Bs in high and low B-TR plants. Therefore it seems that these genes could modify the frequency of differential fertilization by the sperm nuclei carrying 0B or 2Bs in the same pollen grain. So, the maintenance of the numerical polymorphism for Bs could be due to the balance of two opposite strengths in populations (pro B -high B-TR- and anti B genes -low B-TR-).

Table 1. Male and female transmission rate in G1 and G2.
 

	Male transmission		Female transmission
	High TR	Low TR	High TR	Low TR
G1	0.650	0.420	0.485	0.405
G2	0.670	0.485	0.532	0.411

Table 2. Meiotic behavior of 1B in VAV 6313 population plants with different B TR.
 

B TR of plants of VAV 6313 population (1B)	Bs lagging in AI (%)		Bs split mitotically in AI (%)		micronuclei in TI-dyads (%)		micronuclei in TII-tetrads (%)
		cell no.		cell no.		cell no.		cell no.
0.17 (low TR)	0	108	2	108	3	133	0	152
0.17 (low TR)	0	50	0	50	7	48	2	270
0.40 (low TR)	1	322	2	322	8	339	0	155
0.66 (high TR)	2	158	3	158	3	330	0	98
0.98 (high TR)	1	150	1	150	4	130	1	284

Table 3. Meiotic behavior of 2Bs in plants selected for high and low TR.
 

Bs lagging or micronuclei in TI-dyads (%)	Frequency of 2 B in diak and MI (%)		Bs lagging or micronuclei in TI-dyads (%)		Bs lagging or micronuclei in TII-tetrads (%)
		cell no.		cell no.		cell no.
0.27 (low TR)	16.1	267	1.0	231	1.2	85
0.27 (low TR)	2.7	263	9.6	124	4.6	109
0.44 (low TR)	6.4	125	0.0	83	0.0	38
0.28 (low TR)	11.8	136	-----	-----	2.4	41
0.28 (low TR)	16.6	235	3.3	245	2.6	76
0.84 (high TR)	2.1	192	5.7	56	1.4	69
0.84 (high TR)	2.3	88	2.1	48	1.2	82
0.98 (high TR)	8.4	333	4.0	99	1.6	64
0.92 (high TR)	2.9	173	1.7	57	2.4	254
0.92 (high TR)	1.0	386	1.7	121	2.6	190

Figure 1. Male transmission rate of G0, G1 and G2.

Figure 2. Female transmission rate of G0, G1 and G2.

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