Reconstructing the Genome of the Vertebrate Ancestor

Xiphophorus Genetic Linkage Map: Beginnings of Comparative Gene Mapping in Fishes

“…Preliminary comparisons of linkages of isozyme loci among teleost gene maps have suggested almost complete conservation of syntenic groups in cyprinodontiforms, cypriniforms, siluriforms, and salmoniforms (Andersson et al, 1996;Morizot, 2000). A few additional conservations were identified in perciforms and tetraodontiforms (Morizot, 2000). This trend suggests that expanded maps will allow large scale reconstruction of the genome of the teleost ancestor.…”

“…In striking contrast to the extensive variability of chromosome numbers in mammals, a majority of teleosts possess karyotypes of 24 pairs of acrocentric and telocentric chromosomes; many other species have numbers between 20 and 50 pairs, with lower numbers often produced by Robertsonian fusions (Chiarelli and Capanna, 1973). Preliminary comparisons of linkages of isozyme loci among teleost gene maps have suggested almost complete conservation of syntenic groups in cyprinodontiforms, cypriniforms, siluriforms, and salmoniforms (Andersson et al, 1996;Morizot, 2000). A few additional conservations were identified in perciforms and tetraodontiforms (Morizot, 2000).…”

“…The explosive expansion of gene maps of the mouse and humans has provided strong support for hypotheses first advanced from comparisons of fish and mammalian genomes that the vertebrate genome was derived from multiple ancestral tetraploidizations with subsequent, perhaps preferential, translocations among paralogous chromosomes Lundin, 1993;Morizot, 1990Morizot, , 1994Morizot, , 2000Ohno, 1970;Postlethwait et al, 1998). As gene maps have expanded in zebrafish (Danio rerio, Cypriniformes), medaka (Oryzias latipes, Beloniformes), and platyfishes and swordtails of the genus Xiphophorus (Cyprinidoniformes), numerous examples of conserved syntenic relationships between fishes and mammals have been discovered, usually with gene arrangements in man or mouse (Andersson et al, 1996;Barbazuk et al, 2000;Morizot, 1983Morizot, , 2000Naruse et al, 2000;Postlethwait et al, 1998;Wakefield and Graves, 1996). This result, while at first surprising in light of extensive karyoptyic divergence in mammals, has become generally accepted as indicative of extensive retention of gene arrangements of the ancestor of teleost fishes and tetrapods that lived some 400 million years ago.…”

“…Morizot (2000) concluded from comparison of poeciliid fish maps that linkage of loci for isocritrate dehydrogenase, pyruvate kinase, and glucosephosphate isomerase in Xiphophorus linkage groups (LGs) II and IV (homologous to segments of human chromosomes 15 and 19, respectively) results from the retention of syntenic arrangements from a duplicated ancestral chromosome. Comparative gene mapping of the multiplicity of gene dupliations in fishes thus offers the tantalizing prospect of identifying most or all paralogous chromosome segments in teleost genomes, and perhaps all vertebrate genomes.…”

“…Lundin (1993), comparing mouse and human gene maps, identified groups of four paralogs that he proposed derived from two tetraploidization events in lineages leading to vertebrates. Morizot (1990Morizot ( , 1994Morizot ( , 2000 hypothesized three sets of eight paralogs from comparing fish and mammalian gene maps that he proposed derived from three tetraploidizations in ancestors of teleosts. Most recently, comparisons of zebrafish gene maps with those of mice and humans (Barbazuk et al, 2000;Davidson et al, 1999;Gates et al, 1999;Postlethwait et al, 1998) led Postlethwait to predict two tetraploidization events in vertebrate ancestors, with a third at some point in the evolution of fishes.…”