Is maize B chromosome preferential fertilization controlled by a single gene?

Abstract

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.

Meiotic and mitotic behaviour of B chromosomes of ryegrass

“…The four genetic regions are (a) distal eucromatic tip and (b) distal eucromatin, are trans-acting and essential for nondisjunction; (c) centromeric chromatin is a cisacting receptor for nodisjunction, and (d) short arm and centromere region enhances nondisjunction (CARLSON & ROSEMAN, 1992;JONES & HOUBEN, 2003). Subsequent experiments (CHIAVARINO et al, 2001) determined that a single major gene in an A chromosome controls B-chromosome transmission rate in maize, and that ir acts in the haploid egg cell at the time of fertilization. However, concerning rye, PUERTAS et al (1998) andHOUBEN (2003) described that the genes that control its mode of polymorphism are located in the Bs and suggested that such genes are sites where there are formations of chiasma.…”

One gene determines maize B chromosome accumulation by preferential fertilisation; another gene(s) determines their meiotic loss

“…As the Bs were always transmitted on the male side, we call these lines H m and L m . The selection process was carried out following the method described in Rosato et al (1996) and Chiavarino et al (1998Chiavarino et al ( , 2001, which consists in selecting plants with the highest and nearly the lowest B-TR in 0B Â 2B crosses. Plants with the lowest B-TR cannot be selected because their progenies do not contain sufficient Bs to continue the selection process.…”

“…Thereafter, one H m L m F1 plant with 2Bs was self-pollinated to obtain the F2 segregation of the gene/genes involved. As the H m L m and L m H m F1 hybrids have the same genetic composition and B-TR (Chiavarino et al, 2001), it is sufficient to study the F2 of either one of the hybrids. In this case, the H m L m F2 was used.…”

“…The F1 hybrids between the H m and L m lines are uniform, showing intermediate B-TR and a low variance. These data, together with the fact that nearly all selection gain was obtained in the first generation of selection, allowed us to hypothesise that a single major gene is involved in the control of preferential fertilisation in maize (Chiavarino et al, 2001). …”

“…Variation in nondisjunction frequency was reported by Rusche et al (1997) using a molecular probe to detect the presence, location and frequency of B chromosomes in interphase nuclei of pollen grains in the TB-10 L18 translocation line. However, when Bs are in their native form, the progeny of female 0B Â male 2B crosses has 0B or 2Bs in more than 99% of the cases, demonstrating that non-disjunction occurs nearly always (Rosato et al, 1996;Chiavarino et al, 2001).…”