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Session Schedule & Abstracts

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Saturday 2nd July, 2016

Symposium: Segmentation and serial homology: surprising new insights for long-standing central questions in vertebrate morphology 1

Room: Salon H   9:30 am–11:00 am

Moderator(s): R. Diogo, G. Wagner, & F. Galis
SEG1-1  9:30 am  Serial homology: how does it fit into the picture? Wagner G. P. *, Yale University
Abstract: While biologists have developed a reasonably coherent understanding of homology as it applies to "the same organ in different species regardless of form of function", the concept of serial homology is more controversial. One argument is that, reasonably, serial homology applies to different parts of the same organism, and for that reason the phylogenetic definition of homology does not fit; ergo serial homology is not homology at all. In this contribution I will present another perspective, which applies to both "special" homology (homologous parts of different species) as well as to parts of the same organism, namely the notion of genetic/developmental individualization. For the phylogenetic homology concept to be meaningful it has to be limited to the comparison of developmentally individualized body parts (this point will be argued more in detail in the talk). However, as soon as we include the notion of developmental individuality, we are also freed from the strictures of the phylogenetic definition and can consider the identity of body parts within the same body. In doing so one has to recognize that we need to distinguish between two categories of serially homologous characters: homomorph and paramorph characters. Homomorphs are simple reiterations of the same developmental program, like multiple instances of the same cell type. The term paramorph derives from the concept of a paralog gene, i.e. two body parts are paramorph if they ancestrally derive from a homomorph reiteration of the same character but each acquired developmental individuality such that they are now two individualized body parts. The notion of paramorph characters is particularly clear in the case of cell type evolution according to the sister cell type concept. The notion of paramorph characters also implies that groups of characters have a hierarchical structure of relatedness. Finally I will address the recent hypothesis that tetrapod fore- and hind limbs are not serially homologous.

SEG1-2  10:00 am  Serial homology vs derived similarity of pectoral and pelvic appendages: comparative, genetic, evo-devo and network studies in fish, tetrapods, and human birth defects. Diogo R*, Howard University; Esteve-Altava B, Howard University; Molnar J, Howard University
Abstract: Most evolutionary and medical textbooks state that the pectoral and pelvic appendages of fish, and therefore the fore and hindlimbs of tetrapods, are serial homologues. However, such statements are mainly the consequence of a repetition of older ideas that were formulated based on romantic ideas and/or the notion of an 'archetype', and that were never tested against empirical data. Here we show how such statements are contradicted by regenerative studies of axolotls, developmental studies of tetrapods, and comparative and evolutionary studies of all major vertebrate groups, including recent re-analyses of the appendicular muscles of chondrichthyans, dipnoans and coelacanths contradict this old dogma, a literature review on the available paleontological data, and the use of novel, state-of-the-art systems biology methods such as anatomical network analyses. That is, the integrative analysis of the data available strongly supports the idea that the similarity of the muscles and bones of the fore and hindlimbs of tetrapods such as salamanders and modern humans is not due to serial homology, but is instead the result of independent evolutionary changes (homoplasy) occurred mainly during the origin of tetrapods due to the co-option of similar genes for the development of both limbs. In fact, contrarily to the limbs, and particularly their most distal regions, the pelvic and pectoral girdles were seemingly very different from the moment they appeared, and have remained very different from each other since then, an idea supported by recent studies of both anatomical and genetic networks. More than becoming lost on specific definitions of what is serial homology, the important take-home message is therefore that the similarity between the pectoral and pelvic fins of fish, and particularly of the fore and hindlimbs of tetrapods, is a derived - and not an ancestral, as has been dogmatically assumed for more than 250 years - similarity.

SEG1-3  10:30 am  The evolution of head segmentation in the Phylum Chordata. Holland L*, University of California San Diego; Gilland E, Howard University
Abstract: A major controversy in evolution of vertebrates from their invertebrate ancestors is whether vertebrate head is fundamentally segmented and inherited this segmentation from an amphioxus-like ancestor. In amphioxus, segmented musculature extends the full length of the body. The pharynx also contains a segmental series of gill slits. While it is generally agreed that the head cavities of agnathans (premandibular, mandibular, and hyoid) evolved from the anterior somites of an amphioxus-like ancestor, the relationship between the head cavities of sharks and those of lampreys remains controversial as does the relationship between head muscles of bony vertebrates and the head cavities and head muscles of sharks. Evidence from comparative morphology of vertebrates and amphioxus and from patterns of gene expression is consistent with an evolutionary scenario in which the anterior somites of an amphioxus-like ancestor gave rise to the head cavities of the lamprey and shark, and into the eye and jaw muscles in bony vertebrates.

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