13. Hox Genes and Arthropod Diversity
199
among the traditional protostomes; echinoderms, urochordates, cephalochordates
and chordates among the deuterostomes. They have also been sampled in platyhelminths and nematodes among the supposed acoelomate phyla, and most recently,
in brachiopods and priapulids. All of these phyla, with the exception of the nematodes, contain sets of Hox genes with multiple distinct gene classes, clearly more
closely related to orthologous genes in other phyla than to other Hox genes in the
same organism (Arenas-Mena et al. 1998; Snow and Buss 1994; Digregorio et al.
1995; Brooke et a1. 1998; Kmita-Cunisse et al. 1998; de Rosa et a1. 1999). These
include orthologues for the five most "anterior" classes of Hox gene, together with
at least one representative of the central genes, and one of the posterior (Abd-B
related) genes.
These conserved similarities suggest that the Hox proteins diversified extensively in the stem lineage of the metazoa, but then their sequences became highly
constrained, at least in the most critical parts of the molecule in and around the
DNA binding domain, so that the unique characteristics of each subclass remain
distinct in the multiple metazoan phyla.
Recent data emerging from a collaboration between the several labs cloning
Hox genes suggests that, for some of the Hox gene classes, the process of sequence
diversification continued after the basal radiation of the metazoan phyla. However,
subsequent to this early radiation, the sequences have been conserved sufficiently
well to retain a surprisingly strong phylogenetic signal linking groups of phyla (de
Rosa et al. 1999). The posterior group (Abd-B related) Hox genes exhibit this pattern. They are the most variable between phyla, both in number and sequence.
Deuterostomes have multiple, and quite diverse posterior genes, but within the
protostomes, the sequences of these posterior genes fall into three clear classes. The
genes of arthropods, priapulids and nematodes comprise one class, while annelids,
molluscs and brachiopods share two classes of posterior gene, both present in each
phylum. This distribution of genes provides support for the subdivision of the
protostomes into two major lineages, the Ecdysozoa and the Lophotrochozoa, as
proposed by Aguinaldo and colleagues (Aguinaldo et al. 1997). on the basis of
ribosomal RNA sequences.
4 Hox Genes and Body Plans
The products of the Hox genes serve as molecular labels during development to
define specific regions of the body. This suggests that patterns of Hox gene expression may be of use in comparisons between species to define underlying similarities
in body plan that are obscured by morphological divergence. We must expect, of
course, that patterns of gene expression will display both conserved ancestral features and novel, derived characteristics. In this regard, genes are not necessarily
"better" than morphological characters (Abouheif et al. 1997). However, they do
provide a new and rich data set, which is partially independent of morphology, and
which may illuminate both underlying patterns of homology, and the processes that
have led to the diversification of body plans.
199
among the traditional protostomes; echinoderms, urochordates, cephalochordates
and chordates among the deuterostomes. They have also been sampled in platyhelminths and nematodes among the supposed acoelomate phyla, and most recently,
in brachiopods and priapulids. All of these phyla, with the exception of the nematodes, contain sets of Hox genes with multiple distinct gene classes, clearly more
closely related to orthologous genes in other phyla than to other Hox genes in the
same organism (Arenas-Mena et al. 1998; Snow and Buss 1994; Digregorio et al.
1995; Brooke et a1. 1998; Kmita-Cunisse et al. 1998; de Rosa et a1. 1999). These
include orthologues for the five most "anterior" classes of Hox gene, together with
at least one representative of the central genes, and one of the posterior (Abd-B
related) genes.
These conserved similarities suggest that the Hox proteins diversified extensively in the stem lineage of the metazoa, but then their sequences became highly
constrained, at least in the most critical parts of the molecule in and around the
DNA binding domain, so that the unique characteristics of each subclass remain
distinct in the multiple metazoan phyla.
Recent data emerging from a collaboration between the several labs cloning
Hox genes suggests that, for some of the Hox gene classes, the process of sequence
diversification continued after the basal radiation of the metazoan phyla. However,
subsequent to this early radiation, the sequences have been conserved sufficiently
well to retain a surprisingly strong phylogenetic signal linking groups of phyla (de
Rosa et al. 1999). The posterior group (Abd-B related) Hox genes exhibit this pattern. They are the most variable between phyla, both in number and sequence.
Deuterostomes have multiple, and quite diverse posterior genes, but within the
protostomes, the sequences of these posterior genes fall into three clear classes. The
genes of arthropods, priapulids and nematodes comprise one class, while annelids,
molluscs and brachiopods share two classes of posterior gene, both present in each
phylum. This distribution of genes provides support for the subdivision of the
protostomes into two major lineages, the Ecdysozoa and the Lophotrochozoa, as
proposed by Aguinaldo and colleagues (Aguinaldo et al. 1997). on the basis of
ribosomal RNA sequences.
4 Hox Genes and Body Plans
The products of the Hox genes serve as molecular labels during development to
define specific regions of the body. This suggests that patterns of Hox gene expression may be of use in comparisons between species to define underlying similarities
in body plan that are obscured by morphological divergence. We must expect, of
course, that patterns of gene expression will display both conserved ancestral features and novel, derived characteristics. In this regard, genes are not necessarily
"better" than morphological characters (Abouheif et al. 1997). However, they do
provide a new and rich data set, which is partially independent of morphology, and
which may illuminate both underlying patterns of homology, and the processes that
have led to the diversification of body plans.