Meiotic behavior of small chromosomes in maize

Abstract

The typical behavior of chromosomes in meiosis is that homologous pairs synapse, recombine, and then separate at anaphase I. At anaphase II, sister chromatids separate. However, studies of small chromosomes in maize derived from a variety of sources typically have failure of sister chromatid cohesion at anaphase I. This failure occurs whether there is pairing of two copies of a minichromosome or not. These characteristics have implications for managing the transmission of the first generation artificial chromosomes in plants. Procedures to address these issues of minichromosomes are discussed.

Keywords: maize; meiosis; minichromosomes; sister chromatid cohesion; synapsis.

FIGURE 1

Initiating the chromosome type BFB cycle to generate minichromosomes. A translocation between the B chromosome and the short arm of chromosome 9, TB-9Sb, was recombined with a reverse duplication of 9S to generate TB-9Sb-Dp9. This chromosome can recombine with itself as depicted which will join the sister centromeres. At meiosis II the sister centromeres will separate and create a chromatin bridge that will rupture. This broken chromosome will fuse, form a bridge, and break through the gametophyte generation but at the second pollen mitosis the B centromere will undergo non-disjunction. This will place two broken chromosomes in a sperm and thus following the subsequent fertilization event, these two broken chromosomes will set up the chromosome type of BFB cycle that will continue throughout development. Using this approach, Zheng et al. (1999) and Han et al. (2007) produced a collection of minichromosomes described in the text. B repeat is the B chromosome specific sequence present in and around the centromere of the B chromosome. Knob is the repeats of heterochromatin found on maize chromosomes including the short arm of chromosome 9.

FIGURE 2

Pachytene stage with minichromosome revealed by FISH. Red is CRM, the maize centromeric retrotransposon, and green is the B-specific sequence (ZmBs); arrow indicates the dicentric minichromosome. Bars = 10 μm.

FIGURE 3

Immunostaining and FISH for minichromosome. (A) Merged image; (B) Green is ZmBs; (C) Red is CENH3, the centromeric histone signals; (D) Blue is DAPI. Arrows indicate that the minichromosome sister chromatids separate in meiosis I and each sister has a functional centromere. Bars = 10 μm.

FIGURE 4

Cytological analysis of two copies of a minichromosome. Arrows indicate that the two dicentric minichromosomes do not pair with each other. ZmBs is red and green is knob heterochromatin. Bars = 10 μm.

FIGURE 5

Cytological analysis of two copies of a minichromosome. ZmBs is labeled in red; knob heterochromatin is labeled in green. Arrows indicate that the separation of sister chromatids at anaphase I. Bars = 10 μm.