Minichromosomes derived from the B chromosome of maize.

Fourteen minichromosomes derived from the B chromosome of maize are described. The centromeric region of the B chromosome contains a specific repetitive DNA element called the B repeat. This sequence was used to determine the transmission frequency of the different types of minichromosomes over several generations via Southern blot analysis at each generation. In general, the minichromosomes have transmission rates below the theoretical 50% frequency of a univalent chromosome. The gross structure of each minichromosome was determined using fluorescence in situ hybridization (FISH) on root tip chromosome spreads. The presence of the B centromeric repeat and of the adjacent heterochromatic knob sequences was determined for each minichromosome. In two cases, the amount of the centromeric knob repeat is increased relative to the progenitor chromosome. Other isolates have reduced or undetectable levels of the knob sequence. Potential uses of the minichromosomes are discussed.

Negative crossover interference in maize translocation heterozygotes.

Negative interference describes a situation where two genetic regions have more double crossovers than would be expected considering the crossover rate of each region. We detected negative crossover interference while attempting to genetically map translocation breakpoints in maize. In an attempt to find precedent examples we determined there was negative interference among previously published translocation breakpoint mapping data in maize. It appears that negative interference was greater when the combined map length of the adjacent regions was smaller. Even positive interference appears to have been reduced when the combined lengths of adjacent regions were below 40 cM. Both phenomena can be explained by a reduction in crossovers near the breakpoints or, more specifically, by a failure of regions near breakpoints to become competent for crossovers. A mathematical explanation is provided.

Nuclear gene dosage effects upon the expression of maize mitochondrial genes.

Each mitochondrion possesses a genome that encodes some of its own components. The nucleus encodes most of the mitochondrial proteins, including the polymerases and factors that regulate the expression of mitochondrial genes. Little is known about the number or location of these nuclear factors. B-A translocations were used to create dosage series for 14 different chromosome arms in maize plants with normal cytoplasm. The presence of one or more regulatory factors on a chromosome arm was indicated when variation of its dosage resulted in the alteration in the amount of a mitochondrial transcript. We used quantitative Northern analysis to assay the transcript levels of three mitochondrially encoded components of the cytochrome c oxidase complex (cox1, cox2, and cox3). Data for a nuclearly encoded component (cox5b) and for two mitochondrial genes that are unrelated to cytochrome c oxidase, ATP synthase alpha-subunit and 18S rRNA, were also determined. Two tissues, embryo and endosperm, were compared and most effects were found to be tissue specific. Significantly, the array of dosage effects upon mitochondrial genes was similar to what had been previously found for nuclear genes. These results support the concept that although mitochondrial genes are prokaryotic in origin, their regulation has been extensively integrated into the eukaryotic cell.

A genetic test of bioactive gibberellins as regulators of heterosis in maize.

This study tested the hypothesis that gibberellin levels were responsible for the superior growth habit of hybrids (i.e., heterosis). If this were true, plants reduced in their capacity to produce gibberellin, such as maize plants homozygous for dwarf1 (d1), should display a lesser heterotic response. The d1 mutation was introgressed into two inbred lines of maize, B73 and Mo17, for seven generations. Plants segregating for the dwarf phenotype were produced both by self-fertilizing the introgressed inbred lines and by making reciprocal crosses between them to produce hybrids. Measurements were made of several physical traits. The results indicated that the hybrid dwarf plants experienced no loss of heterosis relative to their normal siblings. These results exclude the possibility that modulation of bioactive gibberellins is a major underlying basis of the heterotic response.

Maize tertiary trisomic stocks derived from B-A translocations.

Reciprocal translocations between supernumerary B chromosomes and the basic complement of A chromosomes in maize have resulted in a powerful set of tools to manipulate the dosage of chromosomal segments. From 15 B-A reciprocal translocation stocks that have the B-A chromosome genetically marked we have developed tertiary trisomic stocks. Tertiary trisomics are 2n + 1 aneuploids where the extra chromosome is a translocation element, in this case a B-A chromosome. Whereas B-A translocations produce aneuploidy in the sperm, the tertiary trisomic plant efficiently transmits hyperploid gametes maternally. Because the B-A tertiary trisomic stocks and the B-A translocation stocks from which they were derived are introgressed into the W22 inbred line, the effects of maternally and paternally transmitted trisomic B-A chromosomes can be compared. Data are presented on both the male and female transmission rates of the B-A chromosomes in the tertiary trisomic stocks.

A test for a metastable epigenetic component of heterosis using haploid induction in maize.

We conducted a test to detect if there is a heritable epigenetic component to hybrid vigor and/or inbreeding depression. The impetus for this work was a classical study of the effect of homozygosis on the expression of the maize red color ( r1) locus. It had been shown that maintaining R1 mottling alleles in the homozygous state over several generations produces a progressive decrease of their paternally imprinted expression. This effect is reversed by R1/r1 allele heterozygosity. If this behavior were characteristic of many regulatory genes, then such a phenomenon could contribute to inbreeding depression and heterosis. To examine this question, inbreds of Mo17 and B73 and the two reciprocally produced hybrids were crossed by Stock 6 to generate four classes of maternal haploids. The mature haploid plants were measured for several quantitative traits. If inbreeding depression results from an accumulating heritable effect that is reversed by the hybrid state, one would expect the haploids derived from the hybrids to perform better than those derived from the inbred lines. The hybrid-derived haploids did not exhibit greater average performance than the inbred-derived haploids. These data fail to support the hypothesis that inbreeding depression and heterosis have a metastable epigenetic component.

Dosage-dependent gene regulation in multicellular eukaryotes: implications for dosage compensation, aneuploid syndromes, and quantitative traits.

Evidence from a variety of data suggests that regulatory mechanisms in multicellular eukaryotes have evolved in such a manner that the stoichiometric relationship of the components of regulatory complexes affects target gene expression. This type of mechanism sets the level of gene expression and, as a consequence, the phenotypic characteristics. Because many types of regulatory processes exhibit dosage-dependent behavior, they would impact quantitative traits and contribute to their multigenic control in a semidominant fashion. Many dosage-dependent effects would also account for the extensive modulation of gene expression throughout the genome that occurs when chromosomes are added to or subtracted from the karyotype (aneuploidy). Moreover, because the majority of dosage-dependent regulators act negatively, this property can account for the up-regulation of genes in monosomics and hemizygous sex chromosomes to achieve dosage compensation.