Rosemary Carpenter

Complementary floral homeotic phenotypes result from opposite orientations of a transposon at the plena locus of antirrhinum

Recessive mutations at the plena (ple) locus result in a homeotic conversion of sex organs to sterile perianth organs in flowers of Antirrhinum majus. A complementary phenotype, in which sex organs replace sterile organs, is conferred by semidominant ovulata mutations. The ple locus was identified and isolated using a homologous gene, agamous from Arabidopsis, as a probe. The expression of ple is normally restricted to the inner two whorls of the flower, where sex organs develop. However, in ovulata mutants, ple is expressed ectopically in the outer two whorls of the flower and in vegetative organs. These mutants correspond to gain-of-function alleles of ple, suggesting that ple is sufficient for promoting sex organ development within the context of the flower. The plena and ovulata phenotypes result from opposite orientations of the transposon Tam3 inserted in the large intron of ple.

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.

The transposable element Tam3 of Antirrhinum majus generates a novel type of sequence alterations upon excision

SummaryThe 3.5 kb transposable element, Tam3, has been shown to cause somatic and germinal instability at the nivea locus, which encodes chalcone synthase, of Antirrhinum majus. Molecular cloning and sequence analysis of the niv-98::Tam3 allele revealed that the termini of Tam3 consist of 12 bp perfect inverted repeats. Tam3 is integrated in the promoter region of the chalcone synthase gene and generates an 8 bp duplication of target sequences upon integration. DNA sequencing of a niv+x revertant, niv-164, revealed a new type of sequence alteration upon excision: the duplications are displaced by ten nucleotides generated from adjacent sequences. Structural similarities of Tam3 and the maize elements Ac/Ds suggest that these elements belong to a common family.

Comparison of genetic behaviour of the transposable element Tam3 at two unlinked pigment loci in Antirrhinum majus

SummaryIn Antirrhinum majus the transposable element Tam3 has been described at two unlinked loci pallida and nivea, both of which are required for the production of anthocyanin pigment in flowers. In each case the element is inserted in the promoter region and gives a variegated phenotype. We show that the rate of Tam3 excision at both loci is greatly affected by temperature, being approximately 1000-fold higher at 15°C compared with 25°C. Tam3 is also controlled by an unlinked gene Stabiliser, which considerably reduces excision rate. We show that the high degree of sensitivity to temperature and Stabiliser is an intrinsic property of Tam3 which is not shared by an unrelated element, Tam1. The Tam3 insertion at nivea gives rise to a series of alleles which confer reduced pigmentation, novel spatial patterns and changed instability. These are probably a result of imprecise excision and rearrangements of the Tam3 element.

The Metamorphosis of Flowers.

One of the unifying theories of plant biology is that the variety of plant forms are simply different modifications of a common growth plan. Different permutations of a few key features of plant growth can generate a bewildering array of seemingly distinct forms. There is perhaps no better illustration of this than the comparison of a flower and a shoot. The idea that these two apparently different structures might be fundamentally equivalent goes back to Goethes treatise on metamorphosis, published in 1790. He concluded, Flowers which develop from lateral buds are to be regarded as entire plants, which are set in the mother plant, as the mother plant is set in the earth (Goethe, 1790). In equating flowers and shoots, four key assertions need to be made. First, the different parts of the flower (sepals, petals, stamens, and carpels) are equivalent to the leaves of a shoot. Second, the organs of both shoot and flower are separated by internodes, but in the case of the flower these are so short as to be barely visible. Third, the organs of shoot and flower usually have a distinct phyllotaxy, or arrangement around the central axis. Finally, the indeterminate growth that so characterizes a shoot is suppressed in the case of a flower, both apically, because it eventually stops producing organs around the central axis, and laterally, because branches do not normally arise in the axils of floral organs. The comparison of flower and shoot therefore highlights four key variables: organ identity, internode length, phyllotaxy, and determinacy. The numerous forms and habits of plants simply reflect different variations and permutations of these four fundamental aspects of growth. What is their developmental basis?

Transposable elements in Antirrhinum majus: generators of genetic diversity

Abstract Several unstable mutations in Antirrhinum majus are caused by the insertion of transposable elements into genes. Excision and rearrangements of these elements can give rise to quantitative genetic variation and altered spatial patterns of gene expression. Element transposition is controlled by genetic and environmental factors and may make an important contribution to natural variation.

Somatic crossing-over in Antirrhinum majus

SummaryMany aberrant somatic sectors on the corollas of heterozygous Antirrhinum majus can be related to a number of genetic causes while twin-spots are attributed to somatic crossing-over, hitherto, considered a relatively rare occurrence in higher plants. The administration of caffeine increases the frequency of single aberrant sites and also of twin-spots. The inclusion of the unstable genes nivea-recurrens and pallida-recurrens has no effect on the frequency of twin-spots. Homozygosity following somatic crossing-over is presumed for several loci and some evidence is presented for deficiency aberrations. The use of precise precursors in the biosynthetic pathway to anthocyanidin formation provided confirmatory evidence for the identification of the genes involved in the somatic aberrations.

Resurgence of genetic instability in Antirrhinum majus

Abstract The frequent somatic change pallida—recurrens (pal—rec) → Pallida (Pal) in Antirrhinum majus to give full or spotted pigmentation varies inversely with temperature and is regulated by a stabiliser (st—St) system. Some Pal/Pal lines derived from highly unstable pal—rec/pal—rec had many pale sites indicating a resurgence of genetic instability. F1s between these resurgent (R) lines and lines homozygous recessive for different pigment-controlling genes showed numerous acyanic and occasional unstable rec areas. The frequency of these sites on the flowers also varied inversely with temperature and with the pal—rec stabiliser constitution. The application of 0.25% caffeine quadrupled the frequency of pale sites in R/R plants. Unstable somatic rec-type sites occurred de novo in crosses of R/R/ with 4 acyanic (or partially acyanic) genotypes. The administration of chemically defined precursors to the sites identified the genes involved somatically. Two contrasting types of instability in A. majus are described and compared. In the first, a blocked pigment-producing gene resumes its function (e.g. pal—rec → Pal) to give pigmented sites against an acyanic background. In the second, acyanic (or pale) sites are seen against an otherwise fully pigmented background. These occurrences together with the occasional appearance of new rec-type sites in crosses with the resurgent R/R lines are considered in terms of transposition of controlling elements.

De novo activation of the transposable element Tam2 of Antirrhinum majus

SummaryThe nivea locus of Antirrhinum majus encodes the enzyme chalcone synthase required for the synthesis of red anthocyanin pigment. The stable allele niv-44 contains an insertion in the nivea gene (Tam2) which has all the structural features of a transposable element. We have shown that this insertion can excise from the nivea locus when niv-44 is combined with another allele (niv-99) in a heterozygote. Activation of Tam2 excision is caused by a factor tightly linked to the niv-99 allele and may be due to complementation between Tam2 and a related element, Tam1. Factors which repress the excision of Tam2 and Tam1 are also described. Repression is not inherited in a simple mendelian way. Many stable mutations may be due to the insertion of transposable elements. Our data suggest that their stability may be due to the absence in the genome of activating factors and to the presence of repressors.

Phenotypic effects of short-range and aberrant transposition in Antirrhinum majus

We describe two novel ways in which changes in gene expression in Antirrhinum majus may arise as a consequence of the Tam3 transposition mechanism. One involves excision of Tam3 from the nivea gene promoter and insertion of two new Tam3 copies 3.4 kb and 2.1 kb away, on either side of the excision site. One of the new insertions is in the nivea coding region and completely blocks production of an active gene product. This allele probably arose by a symmetrical double transposition, following chromosome replication. The second case involves a small deletion at one end of Tam3 in the pallida gene, flanked by a sequence typical of a Tam3 excision footprint. This suggests that the end of Tam3 was cleaved at an early step in an attempted transposition and re-ligated back to its original flanking sequence. The alteration restores some expression to the pallida gene, suggesting that the ends of the intact Tam3 element contain components which can actively inhibit gene expression. The implications of these findings for the mechanism of Tam3 transposition and for the effects of Tam3 on host gene expression are discussed.