Current problems with the zootype and the early evolution of Hox genes.

"Hox cluster type" genes have sparked intriguing attempts to unite all metazoan animals by a shared pattern of expression and genomic organization of a specific set of regulatory genes. The basic idea, the zootype concept, claims the conservation of a specific set of "Hox cluster type genes" in all metazoan animals, i.e., in the basal diploblasts as well as in the derived triploblastic animals. Depending on the data used and the type of analysis performed, different opposing views have been taken on this idea. We review here the sum of data currently available in a total evidence analysis, which includes morphological and the most recent molecular data. This analysis highlights several problems with the idea of a simple "Hox cluster type" synapomorphy between the diploblastic and triploblastic animals and suggests that the "zootype differentiation" of the Hox cluster most likely is an invention of the triploblasts. The view presented is compatible with the idea that early Hox gene evolution started with a single proto-Hox (possibly a paraHox) gene. J. Exp. Zool. (Mol. Dev. Evol.) 291:169-174, 2001.

[1]  B. Schierwater,et al.  Homeoboxes in cnidarians. , 1991, The Journal of experimental zoology.

[2]  F. Ruddle,et al.  Detection of homeobox genes in development and evolution. , 1991, Proceedings of the National Academy of Sciences of the United States of America.

[3]  W. Gehring.  Exploring the homeobox. , 1993, Gene.

[4]  A. Miles,et al.  Homeobox genes and the zootype , 1993, Nature.

[5]  H. Bode,et al.  Expression of Cnox-2, a HOM/HOX homeobox gene in hydra, is correlated with axial pattern formation. , 1993, Development.

[6]  J. M. W. Slack,et al.  The zootype and the phylotypic stage , 1993, Nature.

[7]  Jack D. Cohen.  Development of the zootype , 1993, Nature.

[8]  B. Degnan,et al.  A hox/hom homeobox gene in sponges. , 1995, Gene.

[9]  C. Freund,et al.  Guidebook to the Homeobox Genes , 1995 .

[10]  G. Balavoine.  Identification of members of several homeobox genes in a planarian using a ligation-mediated polymerase chain reaction technique. , 1996, Nucleic acids research.

[11]  B. Schierwater,et al.  Homeobox genes in the cnidarian Eleutheria dichotoma: evolutionary implications for the origin of Antennapedia-class (HOM/Hox) genes. , 1996, Molecular phylogenetics and evolution.

[12]  J. Finnerty,et al.  Homeobox genes in the Ctenophora: identification of paired-type and Hox homologues in the atentaculate ctenophore, Beroë ovata. , 1996, Molecular marine biology and biotechnology.

[13]  James Hanken,et al.  There is no highly conserved embryonic stage in the vertebrates: implications for current theories of evolution and development , 1997, Anatomy and Embryology.

[14]  Neil W. Blackstone.  Argumentum ad Ignorantiam , 1997, The Quarterly Review of Biology.

[15]  E. Abouheif,et al.  Developmental genetics and homology: a hierarchical approach. , 1997, Trends in ecology & evolution.

[16]  S P Allen,et al.  Somite number and vertebrate evolution. , 1998, Development.

[17]  J. Finnerty.  Homeoboxes in sea anemones and other nonbilaterian animals: implications for the evolution of the Hox cluster and the zootype. , 1998, Current topics in developmental biology.

[18]  N. M. Brooke,et al.  The ParaHox gene cluster is an evolutionary sister of the Hox gene cluster , 1998, Nature.

[19]  J. Finnerty,et al.  The evolution of the Hox cluster: insights from outgroups. , 1998, Current opinion in genetics & development.

[20]  M. Shankland,et al.  Expression of the head gene Lox22-Otx in the leech Helobdella and the origin of the bilaterian body plan. , 1998, Developmental biology.

[21]  D. Martinez,et al.  Cnidarian homeoboxes and the zootype , 1998, Nature.

[22]  A. Collins,et al.  Evaluating multiple alternative hypotheses for the origin of Bilateria: an analysis of 18S rRNA molecular evidence. , 1998, Proceedings of the National Academy of Sciences of the United States of America.

[23]  B. Schierwater,et al.  Homology of Hox genes and the zootype concept in early metazoan evolution. , 1998, Molecular phylogenetics and evolution.

[24]  N. Lartillot,et al.  Animal evolution. The end of the intermediate taxa? , 1999, Trends in genetics : TIG.

[25]  B. Schierwater,et al.  Isolation of Hox genes from the scyphozoan Cassiopeia xamachana: implications for the early evolution of Hox genes. , 1999, The Journal of experimental zoology.

[26]  G. Ruvkun,et al.  Themes from a NASA workshop on gene regulatory processes in development and evolution. , 1999, The Journal of experimental zoology.

[27]  J. Finnerty,et al.  Ancient origins of axial patterning genes: Hox genes and ParaHox genes in the Cnidaria , 1999, Evolution & development.

[28]  L. Buss,et al.  Expression of a Hox gene, Cnox-2, and the division of labor in a colonial hydroid. , 1999, Proceedings of the National Academy of Sciences of the United States of America.

[29]  R. DeSalle,et al.  Development, evolution, and corroboration , 1999, The Anatomical record.

[30]  Sean B. Carroll,et al.  Hox genes in brachiopods and priapulids and protostome evolution , 1999, Nature.

[31]  W. McGinnis,et al.  Activity regulation of Hox proteins, a mechanism for altering functional specificity in development and evolution. , 1999, Proceedings of the National Academy of Sciences of the United States of America.

[32]  S. Carroll,et al.  Early animal evolution: emerging views from comparative biology and geology. , 1999, Science.

[33]  R. Carroll,et al.  Towards a new evolutionary synthesis. , 2000, Trends in ecology & evolution.

[34]  P. Holland,et al.  Vertebrate innovations. , 2000, Proceedings of the National Academy of Sciences of the United States of America.

[35]  C Kappen,et al.  Analysis of a complete homeobox gene repertoire: implications for the evolution of diversity. , 2000, Proceedings of the National Academy of Sciences of the United States of America.

[36]  Christoph M. Happel,et al.  WNT signalling molecules act in axis formation in the diploblastic metazoan Hydra , 2000, Nature.

[37]  M. Akam.  Arthropods: developmental diversity within a (super) phylum. , 2000, Proceedings of the National Academy of Sciences of the United States of America.

[38]  M. Schummer,et al.  Evolution of Antp-class genes and differential expression of Hydra Hox/paraHox genes in anterior patterning. , 2000, Proceedings of the National Academy of Sciences of the United States of America.

[39]  P. Holland,et al.  Evidence for 14 homeobox gene clusters in human genome ancestry , 2000, Current Biology.

[40]  R. DeSalle,et al.  Transformationalism, taxism, and developmental biology in systematics. , 2000, Systematic biology.

[41]  N. Lartillot,et al.  The new animal phylogeny: reliability and implications. , 2000, Proceedings of the National Academy of Sciences of the United States of America.

[42]  M. Shankland,et al.  Evolution of the bilaterian body plan: what have we learned from annelids? , 2000, Proceedings of the National Academy of Sciences of the United States of America.