Glossary

developmental regulatory gene homologous homologue homology homoplasy modularity

A gene encoding a transcription factor or a signaling protein, which is expressed during development in specific patterns, and is able to regulate expression of other genes and control patterning and morphogenesis of specific body parts. Exhibiting biological homology; having the same relative position (topologically), embryological origin and common ancestor. For animal or body parts having the same basic organization plan, topo-logical position of homologous morphogenetic fields and radial his-togenetic divisions is generally identical; however, for identifying homologous cell groups, embryolo-gical origin is a prevalent criterion, since their topological position may differ between animals due to distinct migration.

The same organ in different animals under every variety of form and function, inherited from a common ancestor. See also 'homologous'. A similarity attributed to common evolutionary origin. See also 'homologous'.

A similarity not due to common evolutionary origin, but to convergent or parallel evolution. Principle by which biological systems are made up of discrete units or modules, which are module morphogenetic field radial histogenetic division self-regulated and dynamic entities able to change independently of the rest of the body during development and evolution.

Each unit of a modular organism or organ, from genetic cascades to cells and cell fields. A module is a self-regulated and dynamic entity that can change independently of the rest of the body without lethal consequences, producing phenotypic variations.

Higher-order module, specified by a particular combination of developmental regulatory genes, that gives rise to a particular body division or subdivision. It constitutes a major unit of development and evolution, and the natural comparison character for homology considerations. A radially arranged region or territory of the brain, whose neurons primarily derive from a specific morphogenetic field (i.e., from a restricted ventricular sector of the neural tube). The radial feature of brain histogenetic divisions is based on the predominant glial fiber-guided migration of immature neurons in their way from the ventricular zone to the mantle during development. Nevertheless, radial histogenetic divisions can contain immigrant cells coming from other fields by tangential migration. The fact that the majority of the cells in each radial histogenetic brain division originate in a specific morphogenetic field, makes these divisions natural comparison units for field homology considerations in adult animals. This does not mean that all cells in a particular division in one animal are necessarily homologous to the cells of the same division of other animals.

telencephalon Bilateral evaginations of the rostral forebrain. It shows two major divisions in all vertebrates: pallium and subpallium.

topology Geometric configuration of any given structure (such as the brain) according to internal coordinates, which remain unaltered, independent of deformations or differential growth of subdivisions that occur during development. According to this, the topological position of any division/subdivision within the structure, and its relation to neighbors, remain the same throughout ontogeny. Further, in organisms sharing the same configuration and basic organization plan (e.g., vertebrates), the topological position of homologous divisions/subdivisions should be the same across species. In contrast, some homologous cell groups may vary their topological position by differential migration in different animals.

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