Several studies indicated that the presence of B chromosomes is associated with the presence of knobs and with DNA content. In indigenous populations of North America, Longley (J. Agric. Res. 56:177-195, 1938) found negative association between the number of knobs and the occurrence of B chromosomes. A similar correlation was reported by Bianchi, Ghatnekar and Ghidoni (Chromosoma 14: 601-617, 1963) in Italian populations.

We found in several populations that the increase of DNA content due to the occurrence of B-chromosomes could be masked by the variation of A-DNA content (masking effect) (Rosato et al. MNL 1998, this volume, and Rosato 1997 Ph.D. thesis). With the aim of examining the cause of this masking effect, we analyzed the fluorescent chromosome banding pattern in individuals with different doses of Bs in two Argentine native populations (VAV 6475 and VAV 6485).

In the population VAV 6475, 152 individuals showed 2, 3, or 4 DAPI bands in chromosome pairs 6 and 9; these are interstitial bands on the long arms of chromosomes 6 and 9. All the studied individuals were homozygous for the DAPI+ band of chromosome 9 (+/+) (Table 1). On the contrary, the DAPI band in chromosome 6 was polymorphic, i.e. individuals were homozygous (presence or absence) and heterozygous for the DAPI band. In pair 6, the frequency of homozygotes (+/+), heterozygotes (+/-) and homozygotes (-/-) was 0.11, 0.36 and 0.53, respectively, in individuals with Bs. Otherwise in individuals without Bs, the frequency of homozygotes (+/+), heterozygotes (+/-) and homozygotes (-/-) was 0.17, 0.54 and 0.29, respectively. Assuming that chromosome 9 does not show any variation in the number of DAPI bands, the presence of this DAPI+ band was not considered in the statistical analysis. We compared the frequency of individuals with bands in pair 6 (+/+ plus +/-) in plants with and without Bs using a test for independence (Chi square). The results indicated that the presence of this band is not independent of the presence or absence of B-chromosomes in this population (Yates X 2 = 7.11, p=0.0077).

In the population VAV 6485 we analyzed 47 individuals showing DAPI bands in the same chromosome pairs (6 and 9). In pair 9, the frequency of homozygotes (+/+) and heterozygotes (+/-) was 0.97 and 0.03 in individuals with Bs, 0.88 and 0.12 in individuals without Bs. In pair 6, the frequency of heterozygotes (+/-) and homozygotes (-/-) was 0.26 and 0.74 in individuals with Bs, 0.19 and 0.81 in individuals without Bs. The frequencies of individuals with DAPI bands in pairs 9 and 6 were similar in plants with and without Bs, even though no statistical test could properly be made because of the small size of the sample.

In the population VAV 6485 there was little variation of mean number of bands between individuals with and without Bs. This is in agreement with the increase of the DNA content with the doses of Bs. On the contrary, in the population VAV 6475 the low mean number of DAPI bands in individuals with Bs could be associated with the masking effect (no detection of DNA content supplied by Bs). This fact could be associated with the existence of a negative association between heterochromatic bands and B-chromosomes to maintain an optimum total DNA content (nucleotype).

Table 1. Number of individuals with DAPI+ bands in chromosome pairs 6 and 9 with different doses of Bs from populations VAV 6476 and VAV 6485.
 

	VAV 6475						VAV 6485
Doses of Bs	chrom. pair 9			chrom. pair 6			chrom. pair 9			chrom. pair 6
	+/+	+/-	-/-	+/+	+/-	-/-	+/+	+/-	-/-	+/+	+/-	-/-
0	61	---	---	10	33	18	14	2	---	---	3	13
1	37	---	---	6	16	15	6	1	---	---	---	7
2	33	---	---	2	11	20	12	---	---	---	6	6
3	19	---	---	2	6	11	8	---	---	---	1	7
4	2	---	---	--	--	2	4	--	--	--	1	3
with Bs	91	---	---	10	33	48	30	1	---	---	8	23

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