Heterotopy

Heterotopy is the spatial analogue of heterochrony: it is evolutionary change in the site of expression of a phenotypic trait. Gould (1977) attributes the word “heterotopy” to Haeckel, who used it in a more specialized way, to mean evolutionary change in the germ layer from which an organ differentiates. Wray and McClay (1989, p. 810) list several examples of heterotopy, including the origin of muscles in tetrapod forelimbs from different somites, the origin of vertebrate primordial germ cells from different germ layers, and homeotic “heterotopic” mutations that transfer appendages from one body segment to another. A broad definition of heterotopy extends the concept to include spatial patterning, not only transposition from one location to another, but spatial organization of quantitative processes such as growth (Zellditch et al., 1992) or the location of precursors during the development of homologous traits (Wray and McClay, 1988, p. 313). As in other categorizations of transitions, heterotopies could as well be classified in other ways, such as duplication. Severtzoff signaled a general relationship between heterochrony and heterotopy when he wrote that “heterochrony in development is a means of topographic coordination; i.e., new adaptation of the parts to each other.” Many morphological heterochronies in plants produce heterotopic change, since the morphological ontogeny of a plant is recorded in its adult architecture. Thus, changes in timing of expression of juvenile and adult leaf forms result in heterotopic change in architecture of the mature plant, with the juvenile leaves appearing high on the stem, rather than only basally as before. A clear and oft-described example of environmentally mediated heterotopic change was demonstrated in early experiments on melanization in the Himalayan rabbit (Sturtevant, 1913; Iljin, 1927; Iljin and Iljin, 1930; see discussions in Schmalhausen, 1949 [1986]; Huxley, 1942; Levinton, 1988). In the Himalayan rabbit, as in the Siamese cat, pigment normally develops only in the extremities, where skin temperature is below the general body temperature. Individuals raised at temperatures above 30°C develop white extremities.