5 Metazoan Complexity
157
a prominent example of a conserved antero-posterior patterning system in Bilateria,
where the position of genes within the cluster reflects the relative expression
site along the antero-posterior axis in the fly and vertebrate trunk (McGinnis
and Krumlauf 1992). Bilaterian Hox genes are commonly grouped according to
their relatedness into an “anterior” group (Hox1-2), a Hox3 group, a “central”
group (Hox4-Hox8) and a “posterior” group (Hox9-13) (Garcia-Fernàndez 2005).
Although cnidarians possess putative Hox gene homologs, their genomic structure
and expression differ significantly from bilaterians:
• Nematostella possesses clear orthologs of “anterior” Hox genes, but no Hox3
ortholog. In earlier studies, some Nematostella Hox genes were assigned to “posterior” Hox genes (Hox9-13) (Finnerty and Martindale 1999, Finnerty et al.
2004), but more recent studies indicate that these genes have affinities to both
“central” (Hox4-8) and “posterior” Hox genes without any clear orthology
relationship (Chourrout et al. 2006, Ryan et al. 2006).
• The chromosomal linkage in Nematostella between a Hox gene and the nonHox genes Evx, Mnx and rough is more or less conserved in bilaterians, but all
other clustered Hox genes in the Nematostella genome appear to be the result of
lineage-specific duplications (Chourrout et al. 2006, Kamm et al. 2006). Hence,
a Hox cluster comparable to bilaterians is absent in Nematostella. Although there
is local scrambling, the Nematostella genome shows a strong conservation of
linkage groups with Bilateria (Putnam et al. 2007). It therefore seems unlikely
that the absence of a Hox cluster is due to a secondary, lineage-specific disruption
of an ancestral Hox cluster in Cnidaria.
• Most cnidarian Hox genes show endodermal expression (Finnerty et al. 2004,
Ryan et al. 2007). In contrast, most bilaterian Hox genes are expressed in the
ectoderm.
• Most Nematostella Hox genes show staggered expression – at most – along the
directive axis, while only a few are differentially expressed along the oral-aboral
axis (Finnerty et al. 2004, Ryan et al. 2007). Also, the expression of orthologous genes does not appear to be evolutionarily conserved in the planulae of the
anthozoan Nematostella and the hydrozoans Podocoryne and Eleutheria; orthologues can be expressed at opposite poles of the oral-aboral axis (Finnerty et al.
2004, Kamm et al. 2006, Masuda-Nakagawa et al. 2000, Yanze et al. 2001). This
refutes an evolutionarily conserved mode of Hox patterning along the cnidarian
oral-aboral axis.
5.5.4 The Homology of Body Axes Between Cnidaria and Bilateria
These fundamental differences between bilaterian and cnidarian Hox genes do not
rule out a general role for cnidarian Hox genes in axial patterning, but they do not
support a direct homology of the bilaterian antero-posterior axis to the oral-aboral
or the directive axis in cnidarians. Similarly, other bilaterian axial markers such
as BMPs and their antagonists (see above), the dorsal-anterior marker goosecoid,
157
a prominent example of a conserved antero-posterior patterning system in Bilateria,
where the position of genes within the cluster reflects the relative expression
site along the antero-posterior axis in the fly and vertebrate trunk (McGinnis
and Krumlauf 1992). Bilaterian Hox genes are commonly grouped according to
their relatedness into an “anterior” group (Hox1-2), a Hox3 group, a “central”
group (Hox4-Hox8) and a “posterior” group (Hox9-13) (Garcia-Fernàndez 2005).
Although cnidarians possess putative Hox gene homologs, their genomic structure
and expression differ significantly from bilaterians:
• Nematostella possesses clear orthologs of “anterior” Hox genes, but no Hox3
ortholog. In earlier studies, some Nematostella Hox genes were assigned to “posterior” Hox genes (Hox9-13) (Finnerty and Martindale 1999, Finnerty et al.
2004), but more recent studies indicate that these genes have affinities to both
“central” (Hox4-8) and “posterior” Hox genes without any clear orthology
relationship (Chourrout et al. 2006, Ryan et al. 2006).
• The chromosomal linkage in Nematostella between a Hox gene and the nonHox genes Evx, Mnx and rough is more or less conserved in bilaterians, but all
other clustered Hox genes in the Nematostella genome appear to be the result of
lineage-specific duplications (Chourrout et al. 2006, Kamm et al. 2006). Hence,
a Hox cluster comparable to bilaterians is absent in Nematostella. Although there
is local scrambling, the Nematostella genome shows a strong conservation of
linkage groups with Bilateria (Putnam et al. 2007). It therefore seems unlikely
that the absence of a Hox cluster is due to a secondary, lineage-specific disruption
of an ancestral Hox cluster in Cnidaria.
• Most cnidarian Hox genes show endodermal expression (Finnerty et al. 2004,
Ryan et al. 2007). In contrast, most bilaterian Hox genes are expressed in the
ectoderm.
• Most Nematostella Hox genes show staggered expression – at most – along the
directive axis, while only a few are differentially expressed along the oral-aboral
axis (Finnerty et al. 2004, Ryan et al. 2007). Also, the expression of orthologous genes does not appear to be evolutionarily conserved in the planulae of the
anthozoan Nematostella and the hydrozoans Podocoryne and Eleutheria; orthologues can be expressed at opposite poles of the oral-aboral axis (Finnerty et al.
2004, Kamm et al. 2006, Masuda-Nakagawa et al. 2000, Yanze et al. 2001). This
refutes an evolutionarily conserved mode of Hox patterning along the cnidarian
oral-aboral axis.
5.5.4 The Homology of Body Axes Between Cnidaria and Bilateria
These fundamental differences between bilaterian and cnidarian Hox genes do not
rule out a general role for cnidarian Hox genes in axial patterning, but they do not
support a direct homology of the bilaterian antero-posterior axis to the oral-aboral
or the directive axis in cnidarians. Similarly, other bilaterian axial markers such
as BMPs and their antagonists (see above), the dorsal-anterior marker goosecoid,