User:Jtwsaddress42/Projects/Project 4/Sections/Chapter 3/Vertebrate origins - Genome duplications and a botched metamorphosis

The British zoologist Walter Garstang (9 February 9, 1868 – 23 February 23, 1949) was one of the first to suggest that the chordate ancestor of vertebrates was a tunicate-like organism that expressed two fundamentally different bodyplans over the course of its life as it metamorphosized from its juvenile form to its adult form. Garstang's Hypothesis proposed that progenesis or neoteny was responsible for the emergence of vertebrates - a situation where the somatic larva sexually matures but does not metamorphosize.

In 1972, the American vertebrate paleontologist and anatomist Alfred Sherwood Romer (December 28, 1894 – November 5, 1973) revived this idea in an attempt to get a clearer understanding of the evolutionary roots of the vertebrate body plan and nervous system structure, but with a twist. Romer adopts Garstang's hypothesis and goes on to describe a somatic mobile juvenile larval bodyplan with a notochord, centralized nervous system, segmented striated musculature, and head-senses that is responsible for dispersal in the ecology such that it can find a place to settle down and metamorphosize into an adult; and, a second body plan, a sessile filter-feeding adult anchored to the seafloor with pharyngeal gill slits, smooth muscle, and an enteric nerve net that runs along the GI tract from the pharynx to the sacral end. Through an accident of evolution, most likely the whole-genome duplication-events that lie at the origin of vertebrates, the two body plans were temporally coexpressed and become fused together.

Romer saw the great adaptive evolutionary challenge of vertebrate anatomy as one of somatovisceral integration between these two divisions. Romer hypothesizes that at the origin of vertebrates these two bodyplans, which were sequentially expressed, came to be expressed simultaneously - and fused only at the hindbrain-gill slits and the sacral nerve. Originally, the only points of communication between the two "animals" was via the unmyelinated neurons of the parasympathetic nervous system. The rest of vertebrate evolution revolves around adaptions that allow the integration of these two bodyplans. Romer describes the gradual emergence of the myelinated sympathetic nervous system with the appearance of jawed vertebrates and its increasingly sophisticated development of control over the enteric nervous system and viscera by the somatic division as we move along the evolutionary progression of vertebrate anatomy and physiology.

The most reasonable cause of disruption to the ancestral protovertebrates life cycle is likely to be the first genome duplication event that lies at the origin of vertebrates. Under normal circumstances, once the tunicate-like ancestral somatic juvenile located a place to settle down and anchor, it would metamorphosize into a sessile filter feeding pharyngeal gill basket as it assume the adult body plan. To make this transition, phagocytes are charged with dismantling and clearing the somatic tissues of the juvenile that are no longer needed. Metamorphosis stimulates the maturation of the rudimentarily established viscera and the formation of the protective tunicate basket as an exoskeleton.

A disruption in the process of phagocytic removal of the larval tissues, leaving them intact, while stimulating the maturation of the adult body plan, may have resulted in the simultaneous expression of both body plans at the same time - fused at the points of common structure. In this case, the hind-brain-pharnygeal gill arch region and the sacral end. Such a union would be an awkward merger and would respond poorly to situations that require globally coordinated actions in the ecology.

The neural crest as the fourth germ layer - Welding together of two life-stages
The Swiss anatomist Wilhelm His (July 9, 1831 – May 1, 1904) was the first to identify the neural crest tissue in the embryos of vertebrates in 1886. In studies published in 1898, observation of jaw cartilage and tooth dentine being formed by neural crest cells was demonstrated by the pioneering neuroembryologist Julia Barlow Platt (September 14, 1857 – 1935). Although unappreciated at the time, Platts work demonstrated the ability of the neural crest to form a vast swath of cell types - something typical of the early germ layers, not a secondary tissue.

In 1983, Glenn Northcutt and Carl Gans published their famous paper The genesis of neural crest and epidermal placodes - a reinterpretation of vertebrate origins in the Quarterly Review of Biology, thereby cementing the status of the neural crest as one of the defining features of vertebrates. In 1998, the developmental biologist Brian K. Hall has proposed that the neural crest is a fourth germ-layer - making vertebrates the only quadroblastic animals to have evolved as of yet.

The neural crest is the primary tissue that is responsible for welding the somatic and visceral divisions together into a cohesive and unified whole. The number of tissue types derived from the neural crest are extensive and not limited to nervous system tissue, giving rise to mesenchymal cell types:


 * neurons
 * cartilage
 * bone
 * connective
 * pigment cells

In the developing vertebrate embryo, the neural crest is organized as four major domains:


 * Vagal/sacral neural crest - participates in the construction of the enteric nervous system and the parasympathetic ganglia. This unmyelinated system is the ancient integratory point of somato-visceral communication.


 * Cranial neural crest: participates in the formation of the craniofacial tissues, cranial/pharyngeal arch nerves and components modifying the pharyngeal arch system.
 * Trunk neural crest: participates in the construction of the sympathetic system.
 * Cardiac neural crest: participates in the construction of the cardiopulmonary loop.

Developmental biologists today, in trying to unravel the mysteries of the neural crest, are finally homing in on the detailed genetic, molecular and cellular mechanisms that Edelman was grappling with and trying to conceptually organize in his morphoregulatory hypothesis with its epithelia-mesechymal transitions via genetically constrained and epigenetically modulated CAM and SAM cycles. Marianne E. Bronner and her colleagues have elucidated a gene regulatory network underlying the development of neural crest tissue in vertebrates.

The relationship between striated muscle tissue and bone formation
The vertebrate central nervous system maps itself to the sensorisheets and the muscle ensembles. Similarly, the bones map themselves to the muscles that connect to them, developing in accordance with the relative orientation and tension they exert at their attachment points on the bone. Muscles, and muscle tension, sculpt the bone that forms from the embryonic cartilages that emerge as the endoskeleton develops.

The consequence of muscle mutations on vertebrate morphology
Because of the relationship between bone formation and muscle attachment, mutations that occur in vertebrate striated muscles can result in significant alteration of regional, and sometimes the global, morphology. The phylogenetically constrained skeletomuscular system of vertebrates is extremely plastic, offering the possibilities of many forms, while maintaining a central organizing principle over evolutionary time. The occasional inheritable genetic mutation in the muscle ensembles provides a pathway to quick evolutionary transformation within the vertebrate body plan.