Somitegoenesis En Serpientes
Páginas: 34 (8338 palabras)
Publicado: 11 de diciembre de 2012
Development 125, 151-160 (1998) Printed in Great Britain © The Company of Biologists Limited 1998 DEV1241
151
REVIEW ARTICLE Somite number and vertebrate evolution
Michael K. Richardson1,*, Steven P. Allen1, Glenda M. Wright2, Albert Raynaud3 and James Hanken4
1Department of Anatomy and Developmental Biology, St George’s Hospital Medical School, Cranmer Terrace, London SW17 0RE, UK2Department of Anatomy and Physiology, Atlantic Veterinary College, University of Prince Edward Island, 550 University Avenue, Charlottetown, Prince Edward Island, C1A 4P3, Canada 3Laboratoire de Zoologie, Université Paul Sabatier, 118 Route de Narbonne, F 31062, Toulouse Cedex, France 4Department of Environmental, Population, and Organismic Biology, University of Colorado, Boulder, CO 80309-0334, USA*Author for correspondence (e-mail: m.richardson@sghms.ac.uk)
Accepted 11 November 1997; published on WWW 17 December 1997
SUMMARY Variation in segment number is an important but neglected feature of vertebrate evolution. Some vertebrates have as few as six trunk vertebrae, while others have hundreds. We examine this phenomenon in relation to recent models of evolution and development.Surprisingly, differences in vertebral number are foreshadowed by different somite counts at the tailbud stage, thought to be a highly conserved (phylotypic) stage. Somite number therefore violates the ‘developmental hourglass’ model. We argue that this is because somitogenesis shows uncoupling or dissociation from the conserved positional field encoded by genes of the zootype. Several other systemsshow this kind of dissociation, including limbs and feathers. Bmp-7 expression patterns demonstrate dissociation in the chick pharyngeal arches. This makes it difficult to recognise a common stage of pharyngeal development or ‘pharyngula’ in all species. Rhombomere number is more stable during evolution than somite number, possibly because segmentation and positional specification in the hindbrainare relatively interdependent. Although developmental mechanisms are strongly conserved, dissociation allows at least some major evolutionary changes to be generated in phylotypic stages.
Key words: Somite number, Segmentation, Hox genes, Evolution, Phylotypic stage, Vertebrate
EVOLUTIONARY CHANGES IN SOMITE NUMBER The vertebrate body plan is made up of repeating and nonrepeating patterns oforgan primordia (Cooke, 1975). Repeating patterns include somites, feathers, pharyngeal arches, rhombomeres and, less obviously, the digits. At least two mechanisms are involved in generating repeating patterns: a segmentation mechanism and, superimposed on this, a positional field for making the segments or units different (Ingham and Martinez Arias, 1992). Raff has pointed out that it is relativelyeasy for evolution to vary the total number of segments in some repeating series (Raff, 1996). A striking example of this is the variation in the number of body segments in vertebrates (Fig. 1). Vertebrae develop from the sclerotomes of the somites, which themselves segregate from mesoderm laid down during gastrulation and tail development (Verbout, 1985; Keynes and Stern, 1988; Christ andWilting, 1992). Our aim in this article is to assess how evolutionary changes in somite number relate to modern concepts of evolution and development such as the developmental hourglass, the phylotypic stage and the zootype.
Developmental mechanisms controlling somite number are discussed. We argue that uncoupling or dissociation between repeating patterns and positional fields, as exemplified bysomite development, is an important feature of development and evolution in a number of systems. Vertebrate segment number: comparisons with invertebrates Only vertebrates and other chordates have a tailbud. After gastrulation, the vertebrate tailbud can continue to make somites (Gont et al., 1993; Tucker and Slack, 1995). Vertebrate segmentation is therefore, in principle, an open-ended system. This...
151
REVIEW ARTICLE Somite number and vertebrate evolution
Michael K. Richardson1,*, Steven P. Allen1, Glenda M. Wright2, Albert Raynaud3 and James Hanken4
1Department of Anatomy and Developmental Biology, St George’s Hospital Medical School, Cranmer Terrace, London SW17 0RE, UK2Department of Anatomy and Physiology, Atlantic Veterinary College, University of Prince Edward Island, 550 University Avenue, Charlottetown, Prince Edward Island, C1A 4P3, Canada 3Laboratoire de Zoologie, Université Paul Sabatier, 118 Route de Narbonne, F 31062, Toulouse Cedex, France 4Department of Environmental, Population, and Organismic Biology, University of Colorado, Boulder, CO 80309-0334, USA*Author for correspondence (e-mail: m.richardson@sghms.ac.uk)
Accepted 11 November 1997; published on WWW 17 December 1997
SUMMARY Variation in segment number is an important but neglected feature of vertebrate evolution. Some vertebrates have as few as six trunk vertebrae, while others have hundreds. We examine this phenomenon in relation to recent models of evolution and development.Surprisingly, differences in vertebral number are foreshadowed by different somite counts at the tailbud stage, thought to be a highly conserved (phylotypic) stage. Somite number therefore violates the ‘developmental hourglass’ model. We argue that this is because somitogenesis shows uncoupling or dissociation from the conserved positional field encoded by genes of the zootype. Several other systemsshow this kind of dissociation, including limbs and feathers. Bmp-7 expression patterns demonstrate dissociation in the chick pharyngeal arches. This makes it difficult to recognise a common stage of pharyngeal development or ‘pharyngula’ in all species. Rhombomere number is more stable during evolution than somite number, possibly because segmentation and positional specification in the hindbrainare relatively interdependent. Although developmental mechanisms are strongly conserved, dissociation allows at least some major evolutionary changes to be generated in phylotypic stages.
Key words: Somite number, Segmentation, Hox genes, Evolution, Phylotypic stage, Vertebrate
EVOLUTIONARY CHANGES IN SOMITE NUMBER The vertebrate body plan is made up of repeating and nonrepeating patterns oforgan primordia (Cooke, 1975). Repeating patterns include somites, feathers, pharyngeal arches, rhombomeres and, less obviously, the digits. At least two mechanisms are involved in generating repeating patterns: a segmentation mechanism and, superimposed on this, a positional field for making the segments or units different (Ingham and Martinez Arias, 1992). Raff has pointed out that it is relativelyeasy for evolution to vary the total number of segments in some repeating series (Raff, 1996). A striking example of this is the variation in the number of body segments in vertebrates (Fig. 1). Vertebrae develop from the sclerotomes of the somites, which themselves segregate from mesoderm laid down during gastrulation and tail development (Verbout, 1985; Keynes and Stern, 1988; Christ andWilting, 1992). Our aim in this article is to assess how evolutionary changes in somite number relate to modern concepts of evolution and development such as the developmental hourglass, the phylotypic stage and the zootype.
Developmental mechanisms controlling somite number are discussed. We argue that uncoupling or dissociation between repeating patterns and positional fields, as exemplified bysomite development, is an important feature of development and evolution in a number of systems. Vertebrate segment number: comparisons with invertebrates Only vertebrates and other chordates have a tailbud. After gastrulation, the vertebrate tailbud can continue to make somites (Gont et al., 1993; Tucker and Slack, 1995). Vertebrate segmentation is therefore, in principle, an open-ended system. This...
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