Homeotic mutations (q.v.) in which one floral organ has been replaced by another. For example, in Arabidopsis thaliana (q.v.) the apetala 2 mutations have sepals converted into carpels and petals into stamens. In apetala 3 mutants, petals are converted into sepals and stamens into carpels. In the agamous mutant, stamens are converted to petals and carpels to sepals. The ingenious “ABC model” illustrated below explains the bizarre phenotypes of the mutant flowers. In the normal flower (matrix I) a class A gene (APETALA 2) produces an A morphogen, a class B gene (APETALA 3) produces a B morphogen, and a Class C gene (AGAMOUS) produces a C morphogen. Whorl 1 (W1) meristems produce sepals when A morphogen is present. Whorl 2 (W2) meristems produce petals when A and B morphogens are present. W3 meristems produce stamens when B and C are present, and W4 meristems produce carpels when only C is present. The apetala 2 mutant lacks morphogen A (matrix II), the apetala 3 mutant lacks morphogen B (matrix III), and the agamous mutant lacks morphogen C (matrix IV). Furthermore, C genes must inhibit A genes in W3 and W4 (matrix IV), and A genes must inhibit C genes in W1 and W2 (matrix II). Homeotic mutations allow the genes, normally inactivated, to express themselves. The positions in the matrices where this happens are starred. The proteins encoded by homeotic floral identity genes contain a conserved sequence of 58 amino acids. This presumably binds to DNA in much the same way as the homeobox of Hox genes (q.v.). See Chronology, 1996, Krizek and Mayerowitz; Antennapedia, Antirrhinum, cadastral genes, floral organ primordia, meristems.
Subjects: Genetics and Genomics.