Twenty-one native populations (1120 individuals) of maize from Northern Argentina were studied. These populations, which belong to 13 native races, were cultivated at different altitudes (80-3620 m). Nineteen of the populations analyzed showed B chromosome (Bs) numerical polymorphism. The frequency of individuals with Bs varied from 0 to 94%. The number of Bs per plant varied from 0 to 8 Bs, with the predominant doses being 0, 1, 2, and 3. Those populations with varying number of Bs showed a positive and statistically significant correlation of mean number of Bs with altitude. The DNA content, in plants without Bs (A-DNA)(2n = 20), of 17 populations of the 21 studied was determined. A 36% variation (5.0-6.8 pg) in A-DNA content was found. A significant negative correlation between A-DNA content and altitude of cultivation and between A-DNA content and mean number of Bs was found. This indicates that there is a close interrelationship between the DNA content of A chromosomes and doses of Bs. These results suggest that there is a maximum limit to the mass of nuclear DNA so that Bs are tolerated as long as this maximum limit is not exceeded.

No abstract

Genotypes of high (H m ) and low (L m ) male B transmission rate (B-TR) were obtained. B-TR segregation in the F2 is reported, showing that the H m and L m lines differ in a single locus we call mBt (male B transmission), controlling B preferential fertilisation in maize. The egg cells control which one of the sperm nuclei is going to fertilise them, mBt h egg cells being preferentially fertilised by the sperm nucleus carrying the supernumerary B chromosomes (Bs). It is hypothesised that the mBt gene is involved in the normal fertilisation of maize but the parasitic Bs take advantage of the mBt h allele to increase their own transmission. Selection was also carried out when the Bs were transmitted on the female side (H f and L f lines). The F1 hybrids show that the gene(s) that we call fBt (female B transmission), controlling female B-TR, is located on the A chromosomes acting at diploid level, the fBt l allele(s) for low transmission being dominant. This allele causes the loss of Bs at meiosis, which is shown using a specific B molecular probe to determine B presence/absence in microspores of both lines and hybrids. Maize Bs are a nice example of intragenome conflict, because the mBt and fBt loci are a polymorphic system of attack and defence between A and B chromosomes.

In previous work, genotypes for high and low B chromosome transmission rate were selected from a native race of maize. It was demonstrated that the B transmission is genetically controlled. The present work reports the fourth and fifth generations of selection and the F1 hybrids between the lines. The native B is characterized by a constant behaviour, with normal meiosis and nondisjunction in 100% of postmeiotic mitosis. It is concluded that genetic variation for B transmission between the selected lines is due to the preferential fertilization process. The F1 hybrids show intermediate B transmission rate between the lines. They are uniform, the variance of the selected character being one order of magnitude lower than that of the native population. In addition, 0B x 2B and 2B x 2B crosses were made to study the effect of the presence of B chromosomes in the female parent, resulting in non-significant differences. Several crosses were made both in Buenos Aires and in Madrid to compare the possible environmental effect, but significant differences were not found. Our results are consistent with the hypothesis of a single major gene controlling B transmission rate in maize, which acts in the egg cell at the haploid level during fertilization. It is also hypothesized that maize Bs use the normal maize fertilization process to promote their own transmission.

No abstract

In previous papers we found that the frequency of B chromosomes in native races of maize varies considerably in different populations. Moreover, we found genotypes that control high and low transmission rates (TR) of B chromosomes in the Pisingallo race. In the present work crosses were made to determine whether the genes controlling B-TR are located on the normal chromosome set (As) or on the B chromosomes (Bs). We made female f.0B × male m.2B crosses between and within high (H) and low (L) B-TR groups. The Bs were transmitted on the male side in all cases. The mean B-TR from the progeny of f.0B (H) × m.2B (H) and f.0B (H) × m.2B (L) crosses was significantly higher than that from f.0B (L) × m.2B (L) and f.0B (L) × m.2B (H) crosses. The results show that the B-TR of the crosses corresponds to the H or L B-TR of the 0B female parents irrespective of the Bs of the male parent. This indicates that B-TR is genetically controlled by the 0B female parent and that these genes are located on the A chromosomes.

No abstract

We selected genotypes of high and low B chromosome transmission rate (TR) in a native race of maize (Pisingallo) from northwest Argentina. We made 20 female OB X male IB and 20 fIB X m.OB crosses. The former (GOm) showed a large variation of B TR, with a mean of TR ± SE = 0.52 ± 0.06, ranging from 0.17 to 0.98. In the latter (GOf) the mean was TR = 0.47 ± 0.02 ranging from 0.31 to 0.58. Plants showing the highest and the lowest TR were selected to constitute the progenitors of the G I generations. We made 19 fOB X m.2B crosses, studying 24-30 plants per cross. The TR of the high (H) and low (L) lines in Glm (GlmH and GlmL) significantly differed (TRH = 0.65 ± 0.03, TRL = 0.40 ± 0.01), indicating that the Hand L lines are different groups. The large variation in male TR suggests that preferential fertilization of gametes carrying B chromosomes does not always occur. We also selected plants showing high and low TR in the progenies of f.I B X m.OB crosses (GOf), and made 24 f.1 B X m.OB crosses, studying 23-30 plants per cross. The TRs of the Hand L lines in G If (G IfH and G IfL) were significantly different (TRH = 0.48 ± 0.025, TRL = 0.40 ± 0.02). The TRs in GOf and GlfL were significantly different (TR = 0.47 ± 0.02 and 0.40 ± 0.02, respectively), while this was not the case between GOf and G IfH. Our results demonstrate the existence of genotypes controlling B TR in this native population of maize.