Heterometamerie

Heterometamerie, also called heterometamerism in English, is a pattern of segmentation in which serially repeated body units (metameres) are not uniform but show regional specialization along the body axis. In organisms with heterometameric segmentation, different segments can have distinct morphologies, functions, or appendages, reflecting regional identity rather than complete sameness among segments. This contrasts with homometamerism, where metameres are relatively indistinguishable.

Developmentally, heterometamerism emerges from differential gene expression that patterns segments into zones or tagmata. Hox genes and other regional regulators establish identities along the anterior-posterior axis, sometimes accompanied by fusion or loss of segments (tagmosis), producing composite body plans with specialized regions.

In terms of distribution, heterometamerism is common in various arthropods, where the body is partitioned into tagmata such as head, thorax, and abdomen with specialized appendages. Some crustaceans show pronounced heterometamerism in which successive regions bear different forms of limbs and mouthparts. In polychaete annelids, different segments may possess different chaetae or parapodial structures, illustrating segmental specialization. By contrast, many annelids and some insects display substantial uniformity across many metameres but still exhibit regional differentiation at higher organizational levels.

Understanding heterometamerism helps explain how modular body plans can evolve by rearranging, adding, or fusing segments, enabling adaptation to diverse ecological niches. See also: metamerism, segmentation, tagmosis, Hox genes, arthropod morphology.