5 Metazoan Complexity
167
associated with signalling processes (Samanta et al. 2006). These data thus support
the idea that dramatically different body plans can be generated by the differential
use of the same components, and caution against a simple link between gene content
and complexity of genomic regulatory programs.
5.12 Lancelets and the Chordate Prototype
The lancelets, also known as Amphioxus or Branchiostoma, are a small group
of cephalochordates that have a long tradition as model systems. Due to their
intermediate phylogenetic position between the non-chordate deuterostomes and
the vertebrates, cephalochordates form a very informative group for evolutionary
comparisons, and have traditionally served to trace back the origin of many vertebrate features (reviewed in Garcia-Fernàndez and Bentio-Gutiérrez 2009). Lancelets
share important features with vertebrates, such as the dorsal nerve cord, as well as
a notochord that lends stability to the swimming larvae, but they lack the vertebral column that protects the dorsal nerve cord in vertebrates. Moreover, lancelets
possess a perforated pharynx (pharyngeal slits) and bilateral blocks of segmented
muscle called myomeres that can be well compared with the somitic myotomes in
vertebrates. In turn, limbs, neural crest cells, paired sensory organs and an elaborate anterior brain are missing in amphioxus, consistent with a later evolutionary
emergence of these features.
On the molecular level, the recently published genome of Branchiostoma floridae (Holland et al. 2008, Putnam et al. 2008) continues a very interesting series
of individual studies that shed light on molecular evolution along the deuterostome – chordate – vertebrate lineage: Findings like the discovery of a single Hox
cluster in Branchiostoma (Garcia-Fernàndez and Holland 1994), in comparison to
the four Hox clusters typical for vertebrates, already suggested that cephalochordates still reflect a “primitive” genomic condition before the occurrence of two
rounds of whole-genome duplications (“2R hypothesis”). Likewise, the discovery
of an intact Para-Hox cluster in Branchiostoma (Brooke et al. 1998), in contrast to
degenerated Para-Hox clusters in other systems, provided evidence that the lancelet
genome likely had preserved ancestral characteristics that secondarily changed in
other lineages. This trend has recently been confirmed by the whole genome analysis of Branchiostoma. This analysis uncovered a remarkable degree of syntenic
relationships between lancelet gene loci and their counterparts in the vertebrates.
Among others, the comparison between the lancelet and human genome allows for
the detection of macro-synteny between their chromosomes, suggesting a set of at
least 17 linkage groups in the chordate ancestor (Putnam et al. 2008). The presence
of four vertebrate relatives for each of these regions is additional support for the
“2R hypothesis”, and also emphasizes the status of the Branchiostoma as a good
representative of the more primitive chordate condition.
Another remarkable finding in the Branchiostoma genome was the existence
of more than 50 non-coding elements that were highly conserved with vertebrate
genomes (Putnam et al. 2008). This number already excludes conserved features in
167
associated with signalling processes (Samanta et al. 2006). These data thus support
the idea that dramatically different body plans can be generated by the differential
use of the same components, and caution against a simple link between gene content
and complexity of genomic regulatory programs.
5.12 Lancelets and the Chordate Prototype
The lancelets, also known as Amphioxus or Branchiostoma, are a small group
of cephalochordates that have a long tradition as model systems. Due to their
intermediate phylogenetic position between the non-chordate deuterostomes and
the vertebrates, cephalochordates form a very informative group for evolutionary
comparisons, and have traditionally served to trace back the origin of many vertebrate features (reviewed in Garcia-Fernàndez and Bentio-Gutiérrez 2009). Lancelets
share important features with vertebrates, such as the dorsal nerve cord, as well as
a notochord that lends stability to the swimming larvae, but they lack the vertebral column that protects the dorsal nerve cord in vertebrates. Moreover, lancelets
possess a perforated pharynx (pharyngeal slits) and bilateral blocks of segmented
muscle called myomeres that can be well compared with the somitic myotomes in
vertebrates. In turn, limbs, neural crest cells, paired sensory organs and an elaborate anterior brain are missing in amphioxus, consistent with a later evolutionary
emergence of these features.
On the molecular level, the recently published genome of Branchiostoma floridae (Holland et al. 2008, Putnam et al. 2008) continues a very interesting series
of individual studies that shed light on molecular evolution along the deuterostome – chordate – vertebrate lineage: Findings like the discovery of a single Hox
cluster in Branchiostoma (Garcia-Fernàndez and Holland 1994), in comparison to
the four Hox clusters typical for vertebrates, already suggested that cephalochordates still reflect a “primitive” genomic condition before the occurrence of two
rounds of whole-genome duplications (“2R hypothesis”). Likewise, the discovery
of an intact Para-Hox cluster in Branchiostoma (Brooke et al. 1998), in contrast to
degenerated Para-Hox clusters in other systems, provided evidence that the lancelet
genome likely had preserved ancestral characteristics that secondarily changed in
other lineages. This trend has recently been confirmed by the whole genome analysis of Branchiostoma. This analysis uncovered a remarkable degree of syntenic
relationships between lancelet gene loci and their counterparts in the vertebrates.
Among others, the comparison between the lancelet and human genome allows for
the detection of macro-synteny between their chromosomes, suggesting a set of at
least 17 linkage groups in the chordate ancestor (Putnam et al. 2008). The presence
of four vertebrate relatives for each of these regions is additional support for the
“2R hypothesis”, and also emphasizes the status of the Branchiostoma as a good
representative of the more primitive chordate condition.
Another remarkable finding in the Branchiostoma genome was the existence
of more than 50 non-coding elements that were highly conserved with vertebrate
genomes (Putnam et al. 2008). This number already excludes conserved features in