Heterotopia (medicine)

In botany examples of heterotopy include the transfer of bright flower pigments from ancestral petals to leaves that curl and form to mimic petals. In other cases experiments have yielded plants with mature leaves present on the highest shoots. Normal leaf development progresses from the base of the plant to the top: as the plant grows upwards it produces new leaves and lower leaves mature.[1]

One textbook example of heterotopy in animals, a classic in genetics and developmental biology, is the experimental induction of legs in place of antennae in fruit flies, Drosophila. The name for this specific induction is 'antennapedia'. Surprisingly and elegantly, the transfer takes place in the experiment with no other strange pleiotropic consequences. The leg is transplanted and still is able to rotate on the turret-like complex on the fruit fly's head. The leg simply replaced the Antennae. Before this experiment it was thought that anatomical structures were somehow constrained into certain not well understood and undefined domains. Yet the relatively simple modification took place and caused a dramatic change in phenotype.

This further demonstrated that structures that were thought to be homologous at one time and were later modified still retained some modularity, or were interchangeable even millions of years after evolution had sent antennae down a separate path than the other appendages. This is due to the common origin of homeotic genes.[2] Another well-known example is the environmentally induced heterotopic change seen in the melanin of the Himalayan rabbit and the Siamese cat and related breeds.[3] In the Himalayan rabbit pigments in fur and skin are only expressed in the most distal portions, the very ends of limbs. This is similar to the case Siamese cats. In both the placement of fur pigmentation is induced by temperature. The regions furthest from core body heat and with the lowest circulation develop darker as an induced result.[4] Individuals raised at a uniform external temperature above 30 °C do not express melanin in the extremities and as a result the fur on their paws is left white.[5] The specific gene complex determined to be responsible is in the melanin expression series that is also responsible for albinism. This change is not heritable because it is a flexible or Plastic phenotypic change. The heterotopy demonstrated is that colder body regions are marked by expression of melanin.

The Himalayan rabbit and the Siamese cat are examples of artificial selection on heterotopy, developed by breeders incidentally long before the concept was understood.[4] The current theory is that people selected for stereotypical phenotypic patterns (dark extremities) that happened to be repeatedly produced given a typical temperature.[6] This is perhaps the only known example of convergent mechanisms in artificial selection. The common human breeding cultures that breed the rabbits and cats tended to themselves favor the pattern, in a way closely mimicking the way that the underlying genetics that form flexible adaptations can be selected for based on the phenotype they typically produce in an assumed environment in natural selection.

Another example may have happened in the early history of domesticating horses: tail-type hair grew instead of the wild-type short stiff hair still present in the manes of other equids such as donkeys and zebras.

Heterotopy in molecular biology is the name given to the expression or placement of a gene product from what is typically found in one area to another area. It can also be further expanded to a subtle form of exaptation where a gene product used for one underlying purpose in a diverse group of organisms can re-emerge repeatedly to produce seemingly paraphyletic distributions of traits. But actual phylogenetic analysis supports a monophyletic model as does evolutionary theory. Heterotopy is used to explain this and there are so commonly cited examples.

An example is chitin a very durable structural protein used in surgical[7] sutures as well as durable varnishes but is common to many animals especially crustaceans and insects. But is also found in the African clawed frog (Xenopus laevis).[8]

Wagner et al., suggest that chitin might have a microscopic function observed in cell to cell signaling and the manufacture of insect cuticle for example might represent a recurrent change in the location of expression chitin Speculative, but however Chitin synthase is maintained in many lineages where it does not have an obvious macroscopic function.[9]

It is thought that because so many organisms share such a profound degree of genetic and molecular similarity that shifts in the location of expression might be a regular occurrence throughout time.

Molecular analysis shows that proteins that seem to have a single specific function are instead found in many different tissue types. One example of this phenomenon is crystallin, a clear protein that makes up the lens of the eye; it is also has structural functions in the heart.

• Choristoma