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Course Framework: The Craniata
- No vertebrates are more or less 'highly evolved' or 'advanced'!
- some are more complex than others
- humans are actually fancy fish!
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R e l a t i o n s h i p s
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Where do vertebrates fit in the animal kingdom?
- probably originated in the early Cambrian, ~550 mya
- probably originated by paedomorphosis (sexual maturation ofthe larval form) of a urochordate-like ancestor
- underwent a duplication of the Hox genes, which controldevelopment, at the time of their origin
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Vertebrates possess all the defining characteristics of the taxa towhich they belong...
- euchordata: noto chord persists in adult ,myotomes, caudal fin
- Chordata: notochord in embryo, post anal tail, dorsal nerve tube
- deuterostomata: pharyngeal slits
- coelomata: body cavity
- bilateria: bilateral symmetry, 3 germ layers, motile
- Metazoa: multicellular, heterotrophic, gonads, collagen
- BUT, these do not define vertebrates
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What are the defining characteristics of vertebrates?
- Embryonic development is unique, and includes four main steps
- I. Cleavage
- II. Gastrulation
- III. Neurulation
- IV. Organogenesis
- Vertebrate organ systems also have many unique characteristics
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I. Cleavage
- The zygote divides to form ablastula (a fluid-filled ball of cells).
- Cells remain unspecialized.
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II. Gastrulation
- Cells of the blastula move inward at the blastopore to form agastrula, a hollow ball with three cell layers:
- endoderm (which will form the gut lining
- ectoderm(epidermis, nervous system)
- mesoderm(everything else)
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III. Neurulation
- Dorsal mesoderm cells aggregate to form the notochord - a supporting rod
- - Following induction by the notochord, dorsal ectoderm folds into a dorsal hollownerve cord and neural crest cells
- Mesoderm flanking the notochord becomes segmented intosomites, which will form segmental structures.
- Pharyngeal pouches form along the pharynx.
- Ectodermal placodes form at the eye, ear & nostrils
- The pharyngula is very similar among all vertebrates
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IV. Organogenesis
- Embryonic cells specialize, forming tissues, organs andorgan systems
- differential cell growth and death results in different bodyshapes
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