Instability of the maize B chromosome

Abstract

The B (9) chromosome of maize exhibits a very ordered type of instability at the second pollen mitosis, when nondisjunction may reach a level of 95%. Much less commonly the chromosome is unstable during early development of the kernel. Instability in the kernel produces recessive sectors in either the endosperm or the sporophyte, reflecting the absence of dominant markers carried by the B (9). The causes of B (9) loss in the endosperm and the sporophyte were investigated for the two observable classes of sectoring: fractional loss (single event) and multiple loss (mosaic pattern). The fractional class represents isochromosome formation by the B (9) (Carlson, 1970, 1971). Data presented here suggest that the isochromosome is a by-product of telocentric formation at the second pollen mitosis, and does not arise directly from the B (9) chromosome. The chromosomal basis for the mosaic pattern of B (9) loss is not completely known. However, one class of mosaic kernels displays a heritable instability of the B (9) chromosome which apparently results from ring chromosome formation by the B (9). The time of origin of the ring B (9) chromosome is prior to the second pollen mitosis, since the unstable chromosome generated in the male parent is transmitted to both the endosperm and the sporophyte. Finally, a genetic factor controlling B (9) stability in the developing endosperm has been found. A single plant (1818-1), crossed as a female parent to a B (9)-containing stock, induced a mosaic pattern of B (9) loss in the endosperm at a very high rate. The characteristics of this plant are being investigated.

Minichromosomes derived from the B chromosome of maize

“…Also, they often lag at metaphase II and in a fraction of cases split again. This latter action might include those cases in which meiosis I division was not equational, but could also include a fraction that are undergoing misdivision of the centromere (Darlington, 1939;Carlson, 1970Carlson, , 1973Carlson and Chou, 1981).…”

The B chromosome of maize

“…The method arose from parallel studies of B A variegation by Ghidoni 59 and Carlson. 60 They analyzed tertiary trisomie crosses of TB-4Sa and TB-9Sb, respectively. With TB-4Sa, the su (sugary) endosperm marker was used to follow B 4 (Su) instability.…”

B chromosomes in plants

“…There is thus evidence of resourcefulness in the transmission of Bs, some of which can be understood and some not. Further evidence of this opportunism comes from the work of Carlson & Roseman (1992). The use of a translocation between the B and chromosome 9 of maize enabled these workers to identify components of the B tbat serve to suppress its meotic loss when unpaired.…”