Academic journal article Genetics

Chromosome Segmental Dosage Analysis of Maize Morphogenesis Using B-A-A Translocations

Academic journal article Genetics

Chromosome Segmental Dosage Analysis of Maize Morphogenesis Using B-A-A Translocations

Article excerpt

ABSTRACT

The B-A-A translocations have enabled us to simultaneously assess the possible dosage-sensitive interactions of two nonhomologous chromosome segments in affecting maize plant development. Maize B- A-A translocations contain segments of two nonhomologous essential A chromosomes in tandem arrangement attached to a segment of the long arm of a supernumerary B chromosome. By utilizing the frequent nondisjunction of the B centromere at the second pollen mitosis we produced plants containing an extra copy of the two A chromosome segments. We compared these hyperploid plants with nonhyperploid plants by measuring leaf width, plant height, ear height, internode length, stalk circumference, leaf length, and tassel-branch number in 20 paired families that involved one of the chromosome arms 1S, 1L, 4L, 5S, and 10L. One or more of the seven measured traits displayed dosage sensitivity among 17 of the 20 B-A-A translocations, which included the involvement of chromosome arms 2L, 3L, 5L, 6L, and 7L. The most obvious effect of an increased dosage of the B-A-A translocation was a significant decrease in the traits in the hyperploid plants. These effects may be either the additive effects of hyperploidy for the two chromosome segments or a result of gene interaction between them.

MAIZE is an especially well-suited species for the study of anueploidy in plants. Maize simple B-A translocations result from reciprocal interchanges between a supernumeraryBchromosome and an arm of an essential A chromosome. By utilizing the collection of simple B-A translocations the dosage of a large distal segment of 18 of the 20 maize chromosome arms, all except the long armof chromosome 2 (2L) and the short arm of chromosome 8 (8S), can be altered (Roman1947; Beckett 1978, 1991). The dosage can be varied so that the endosperm contains two, three, or four copies and the embryo contains one, two, or three copies of the segment. The presence of an extra copy is referred to as hyperploidy while the lack of a copy of a chromosome segment (only two copies in the endosperm or only one copy in the embryo) is referred to as hypoploidy. One consequence of a change in chromosome segment copy number is a change in phenotype. Hypoploidy of any of several chromosome segments results in a reduced endosperm size, "the small kernel effect," which has been extensively analyzed (Lin 1982; Beckett 1983; Birchler and Hart 1987; Birchler 1993). The effects of aneuploidy on maize plant height and other morphological traits have been investigated. When the dose of several chromosome arm segments is individually increased the resulting hyperploid plants are usually altered to a modest degree (Chang 1984; Lee et al. 1996; Neuffer et al. 1997); when any of these 18 chromosome arm segments is individually decreased the resulting hypoploid plants aremuch more severely altered in their appearance and vigor (Chang et al. 1987; Beckett 1991; Lee et al. 1996; Neuffer et al. 1997). It seems, therefore, that the growth and morphogenesis of maize plants is substantially buffered from the effects of aneuploidy when the dosage of a single chromosome segment is increased( hyperploidy),but oftenis strongly reducedwhen there is adecrease(hypoploidy) indosageof that segment.

We have produced a large number of B-A-A translocations (Sheridan and Auger 2006). These compound B-A translocations bear two A chromosome segments in tandem arrangement attached to a segment of the long arm of a B chromosome. Plants that are hyperploid for a B-A-A chromosome therefore contain an extra dose of both of the A chromosome segments; plants hypoploid for a B-A-A chromosome are lacking a dose of both of the A chromosome segments.

During the propagation of the newly constructed B- A-A translocations we have observed strongly modified plant phenotypes of plants hyperploid for many chromosome regions when two of these regions are simultaneously increased in dosage. Here we report on the altered phenotypes and the chromosome regions that appear to produce these changes when these regions are present in three doses. …

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