13
Developmental Genetics and
the Diversity of Animal Form:
Hox Genes in Arthropods
MICHAEL AKAM
University Museum of Zoology, Department of Zoology, Downing Street, Cambridge,
CB2 3EJ, England
Abstract
The wonderful diversity of animal forms is built upon a largely conserved set of
genes and developmental mechanisms. The Hox genes provide one of the best studied examples of how such a conserved mechanism underlies a diversity of forms.
The Hox genes encode proteins that serve as molecular labels for the position of
cells along the major body axis. The differential expression of these genes causes
cells that would otherwise be equivalent to adopt different developmental fates in
different regions of the body. This leads to the differentiation of segments in
arthropods, and of regions along the axial skeleton of vertebrates. An analogous
role is probably conserved in most other triploblastic bilaterian phyla.
The family of I-Iox genes diversified at an early stage in the radiation of the
metazoa. The set of Box genes present in each phylum provides phylogcnetic characters that are useful for establishing the relationships between phyla. Characteristics of the 5' (Abd-B related) genes support the grouping of the metazoa into the
same major lineages as the recent rRNA-based phylogeny proposed by Aguinaldo
and colleagues.
The patterns of expression of the Hox genes provide markers to relate the body
plans of distantly related animals. A good example is the comparison of head segmentation in chelicerate and mandibulate arthropods. Patterns of Hox gene expression refute a model of segment loss in chelicerates, and suggest that these very
different arthropods retain a common set of uniquely defined head segments derived from their last common ancestor. No one gene can be taken to define "homology", but in cases such as this where comparisons of several genes provide a consisKey words. homeotic gene, arthropod, development, evolution, segmentation, tagmosis,
Hox gene, chelicerate, Drosophila, protostome, phylogeny, crustacean, body plan, head,
trunk
195
Developmental Genetics and
the Diversity of Animal Form:
Hox Genes in Arthropods
MICHAEL AKAM
University Museum of Zoology, Department of Zoology, Downing Street, Cambridge,
CB2 3EJ, England
Abstract
The wonderful diversity of animal forms is built upon a largely conserved set of
genes and developmental mechanisms. The Hox genes provide one of the best studied examples of how such a conserved mechanism underlies a diversity of forms.
The Hox genes encode proteins that serve as molecular labels for the position of
cells along the major body axis. The differential expression of these genes causes
cells that would otherwise be equivalent to adopt different developmental fates in
different regions of the body. This leads to the differentiation of segments in
arthropods, and of regions along the axial skeleton of vertebrates. An analogous
role is probably conserved in most other triploblastic bilaterian phyla.
The family of I-Iox genes diversified at an early stage in the radiation of the
metazoa. The set of Box genes present in each phylum provides phylogcnetic characters that are useful for establishing the relationships between phyla. Characteristics of the 5' (Abd-B related) genes support the grouping of the metazoa into the
same major lineages as the recent rRNA-based phylogeny proposed by Aguinaldo
and colleagues.
The patterns of expression of the Hox genes provide markers to relate the body
plans of distantly related animals. A good example is the comparison of head segmentation in chelicerate and mandibulate arthropods. Patterns of Hox gene expression refute a model of segment loss in chelicerates, and suggest that these very
different arthropods retain a common set of uniquely defined head segments derived from their last common ancestor. No one gene can be taken to define "homology", but in cases such as this where comparisons of several genes provide a consisKey words. homeotic gene, arthropod, development, evolution, segmentation, tagmosis,
Hox gene, chelicerate, Drosophila, protostome, phylogeny, crustacean, body plan, head,
trunk
195