Studies with the B-9 chromosome of the translocation, TB-9Sb, led to the isolation of an apparent isochromosome (Carlson, Chromosoma 30:356, 1970). Subsequently, it was found that the chromosome was, in fact, a pseudoisochromosome (Carlson, Genetics 97:379, 1981). The two arms differ in terms of the presence or absence of centric heterochromatin, as depicted below. (B chromosome regions are solid black).
The pseudoisochromosome is stable during plant development but is unstable when transmitted through the pollen parent. It frequently produces variegated kernels in testcrosses using bz1 or c1 as a marker. When kernels with variegated endosperm phenotypes were germinated, telocentrics were frequently found in the plants. Therefore, variegation is associated with misdivision of the chromosome (Annu. Rev. Gen. 12:5, 1978). The reason for variegation may be the absence of a telomere at the terminal centromere, due to a lack of "healing" in the endosperm. Two types of telocentrics were recovered, corresponding to the two arms of the pseudoisochromosome.
The type 1 and type 2 telocentrics are stable both in plant development and in pollen parent crosses. They seldom produce variegated kernels in testcrosses and misdivide infrequently. Nevertheless, it was possible to produce the type 1 isochromosome by misdivision of the type 1 telocentric (Genetics 97:379, 1981). The type 1 isochromosome is stable during development, but produces large numbers of variegated kernels in pollen parent crosses, just as with the pseudoisochromosome.
Recently, an explanation was found for the difference between the two
isochromosomes and the type 1 telocentric, in terms of stability. This
past summer, a number of crosses were made in which the type 1 telocentric
was univalent in meiosis. The crosses were of the type: bz1 bz1
X 9-B 9-B type 1 telo (Bz1). In the progeny, variegation for purple
and bronze phenotypes (Bz bz) was frequent, suggesting high
levels of misdivision. The rate of Bz bz kernels for three ears
was 48 variegated kernels per 597 total, or 8.0%. The variegated classification
was restricted to kernels in which at least 1/6 of the endosperm phenotype
was recessive. In addition, a number of recessive bz kernels were
found in these crosses. With a normal (standard) B-9 the presence of bz
kernels is expected, since nondisjunction at the second pollen mitosis
frequently "uncovers" the recessive. However, the type 1 telocentric is
incapable of nondisjunction, due to the absence of centric heterochromatin.
Therefore, the bz kernels must have another source. A cytological
study was made of the bz kernels found on the same three ears mentioned
above. The plants derived from bz kernels were classified as follows:
20 chromosomes = 12
21 chromosomes with an isochromosome = 13
21 chromosomes with a telocentric = 1
22 chromosomes with two telocentrics = 0
No cases of true nondisjunction, with 22 chromosomes, were found. Instead, many of the kernel types appear to be cases of misdivision. One explanation is that misdivision in meiosis transmitted a damaged telocentric to the second pollen mitosis. This telocentric replicated or divided improperly to form an isochromosome. The 13 bz kernels with an isochromosome in the plant can be explained by migration of the isochromosome to one pole, giving 0-iso disjunction. The 12 bz kernels with only 20 chromosomes could have resulted from lagging of the isochromosome at anaphase and 0-0 disjunction. (Note: the 20 chromosome class is not a case of self-contamination, since a marker in the pollen parent was present in the seeds). The single case of a plant with one telocentric is less easy to explain. However, it is not the result of simple nondisjunction.
The findings are preliminary, but they appear to invalidate prior speculations on the cause of isochromosome instability. They show that the type 1 telocentric can be just as unstable as the original (pseudo-) isochromosome or the type 1 isochromosome. The required condition is a lack of pairing with another chromosome in meiosis. In retrospect, it appears that the original isochromosome and all its derivatives suffer from the same defect: they are unstable when univalent in meiosis. The isochromosomes are unstable in all their crosses, even when a pairing partner is present, because they tend to self pair. The telocentric is unstable only in those crosses in which a pairing partner has been excluded. The finding of a specific defect in centromere behavior for one set of chromosomes may help explain the functioning of one part of the maize centromere. In addition, the fact that the type 1 telocentric is now known to be unstable when univalent provides a simple system for studying the process of misdivision. It is much simpler to follow, cytologically, misdivision of a chromosome with two chromatid arms (telocentric) than one with four arms (isochromosome).
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