In evolutionary developmental biology, homeosis is the transformation of one organ into another, arising from mutation in or misexpression of certain developmentally critical genes, specifically homeotic genes. In animals, these developmental genes specifically control the development of organs on their anteroposterior axis. In plants, however, the developmental genes affected by homeosis may control anything from the development of a stamen or petals to the development of chlorophyll. Homeosis may be caused by mutations in Hox genes, found in animals, or others such as the MADS-box family in plants. Homeosis is a characteristic that has helped insects become as successful and diverse as they are. Homeotic mutations work by changing segment identity during development. For example, the Ultrabithorax genotype gives a phenotype wherein metathoracic and first abdominal segments become mesothoracic segments. Another well-known example is Antennapedia: a gain-of-function allele causes legs to develop in the place of antennae. In botany, Rolf Sattler has revised the concept of homeosis (replacement) by his emphasis on partial homeosis in addition to complete homeosis; this revision is now widely accepted. Homeotic mutants in angiosperms are thought to be rare in the wild: in the annual plant Clarkia (Onagraceae), homeotic mutants are known where the petals are replaced by a second whorl of sepal-like organs, originating in a mutation of a single gene. The absence of lethal or deleterious consequences in floral mutants resulting in distinct morphological expressions has been a factor in the evolution of Clarkia, and perhaps also in many other plant groups. Homeotic mechanisms in animals Following the work on homeotic mutants by Ed Lewis, the phenomenology of homeosis in animals was further elaborated by discovery of a conserved DNA binding sequence present in many homeotic proteins. Thus, the 60 amino acid DNA binding protein domain was named the homeodomain, while the 180 bp nucleotide sequence encoding it was named the homeobox. The homeobox gene clusters studied by Ed Lewis were named the Hox genes, although many more homeobox genes are encoded by animal genomes than those in the Hox gene clusters. The homeotic-function of certain proteins was first postulated to be that of a "selector" as proposed by Antonio Garcia-Bellido. By definition selectors were imagined to be (transcription factor) proteins that stably determined one of two possible cell fates for a cell and its cellular descendants in a tissue. While most animal homeotic functions are associated with homeobox-containing factors, not all homeotic