Basis for the diversity of states of controlling elements in maize

Virus-induced mutations in maize: on the nature of stress-induction of unstable loci

Unstable alleles, broken chromosomes and stable mutants have arisen in maize out of infected plants of Barley Striped Mosaic Virus and other viruses. Surprisingly, these same events have appeared out of progenies of these infected plants that themselves do not show any infection. These mutants showing instability have resulted from insertions that are not necessarily related. Two of these insertions (BS1 and TZ86) that have been analysed molecularly have the general characteristics of maize insertions with terminal inverted repeats and host duplication at the terminus of the transposon. In other experiments three of the unstable alleles at the a locus in maize (A locus, chromosome 3, short arm; one of genes for anthocyanin control) that arose in derivative lines of the initially treated plants are responsive to a transposable element, the Uq element. It was determined that the Uq element was not present in this initially treated plant but was present in the untreated female plant. It is proposed that the initial treatment induced events that in turn led to the mobilization of elements and that these events continue to occur in later generations. It seems that genomic events once initiated such as mobility of elements cannot be terminated despite a discontinuation of the treatment (virus) and, like a Frankenstein monster, is not responsive to its maker.

Differential excision patterns of the En-transposable element at the A2 locus in maize relate to the insertion site

Defined mutant alleles with resident transposons display characteristic patterns of germinal and somatic reversion, and heritable changes in the timing and frequency of reversions, which have been termed "change of state" by McClintock, constantly arise. Several mechanisms were proposed to account for these changes. They may be ascribed to the structure and composition of the elements themselves (composition hypothesis) or to their location (position hypothesis). In the current study, insertion positions were determined for three autonomous En-controlled mutable alleles of the A2 locus in maize that show different somatic reversion patterns. A relationship was observed between En insertion positions in the single coding region of the intronless A2 gene and anthocyanin variegation patterns in the aleurone. An insertion in the 5' region of the coding sequence produced a very late somatic variegation pattern, whereas two early variegation patterns were caused by En insertions in the 3' region of the coding sequence.

A transposable element in diverse corn lines, Ubiquitous (Uq): allelism test

Since the discovery of the Uq transposable element system in the early 1980s, studies of Uq distribution have shown that Uq is pervasive in genetic stocks and corn populations including BSSS, the Illinois oil and protein lines, 'Lancaster', 'Jarvis' and several others. The assumption made was that Uq might have provided the variation that has contributed to maize breeding progress. Of the several inbreds tested in previous studies, only IaI159, one of the contributors to BSSS, contained an active Uq. The main question posed in our experiments was that of chromosomal location(s), or allelic relationships, of active Uq elements in diverse corn populations. Results showed that these Uq elements are allelic, linked or independent, though most found from corn populations show quite distant or independent linkage relationships with each other. The lack of a linear linkage map among these Uq elements may be interpreted to have resulted from transposition events and this might have led to the differentiated instability of an individual Uq element that was found to be variable. Other questions also discussed relate to the origin of Uq elements in corn populations and their role(s) in plant breeding.

Newly activated germinal Uq elements in maize are clustered on one linkage group independently of the standard Uq element

Allelism tests between the standard Uq element (Uq1) and five newly activated germinal Uq elements (Uq2, Uq3, UQ4, Uq5, and Uq6) demonstrate that these new Uq elements are independent of Uq1. Gametes that either contain one Uq or various combinations of two different and phenotypically distinguishable Uq elements, have been constructed either with or without the a-ruq reporter allele. Genetic analyses of the progenies of the gametes (using the standard a-ruq tested line as the other parent) have indicated that (i) each Uq element, when present alone, has the capacity to express full activity except when a secondary transposition or loss of activity has occurred; (ii) all five new Uq elements are independent of Uq1 with respect to transposition activity; and (iii) these newly originated Uqs are clustered on one linkage group. Uq2 is allelic to Uq4, and Uq3 is allelic to Uq5, whereas Uq6 is linked to both allelic pairs. A putative linkage map of these Uq elements is presented. In reciprocal crosses there is a striking difference in phenotypic segregation of Uq; when transmitted via the male parent Uq loses full expression capacity.

Phenotypic diversity mediated by the maize transposable elements Ac and Spm

Mutations caused by the insertion of members of the Ac or Spm family of transposable elements result in a great diversity of phenotypes. With the cloning of the mutant genes and the characterization of their products, the mechanisms underlying phenotypic diversity are being deciphered. These mechanisms include (i) imprecise excision of transposable elements, which can result in the addition of amino acids to proteins; (ii) DNA methylation, which has been correlated with the activity of the element; (iii) transposase-mediated deletions within elements, which can inactivate an element or lead to a new unstable phenotype; and (iv) removal of transcribed elements from RNA, which can facilitate gene expression despite the insertion of elements into exons. An understanding of the behavior of the maize elements has provided clues to the function of cryptic elements in all maize genomes.

Transposable elements generate novel spatial patterns of gene expression in Antirrhinum majus

The pallida gene of A. majus encodes a product required for the synthesis of red flower pigment. We have shown that the unstable pallida(recurrens) mutation is due to the insertion of the Tam3 transposable element near the promoter of the gene. Imprecise excision of Tam3 alters pallida gene expression and generates new spatial patterns or different intensities of flower pigmentation. Distinct spatial patterns may also result from rearrangements induced by Tam3 that alter the relative position of the pallida gene. Changes in Tam3 structure or position result in new unstable phenotypes. These findings suggest that genes may be rendered genetically hypervariable as a consequence of transposable element insertion and excision.