4 Phylogeny of Animals
125
4.2.3 Small Ribosomal RNA Gene and the “New View”
of Animal Phylogeny
The inclusion of molecular data has led to a deep reorganization of the metazoan
tree. The first step was the publication, twenty years ago, by Fields et al. (1988)
of a 22 taxa dataset of SSU ribosomal RNA (also called 18S rRNA), which strikingly proposed the polyphyly of metazoans, with independent origins of bilaterians
and cnidarians among eukaryotes. An early divergence of platyhelminthes within
the bilaterians was also stressed (Field et al. 1988). However, despite the pioneering
aspect of this work, these hypotheses were later rejected when the reinterpretation
of the data demonstrated that the distance method employed had likely caused a
long branch attraction artefact, a common systematic error related to the unequal
rates of evolution among taxa (Felsenstein 1978). The use of alternative methods,
such as evolutionary parsimony, applied to the same dataset, recovered the metazoans and deuterostomes as monophyletic clades and identified a close relationship
between annelids and molluscs (Lake 1990). This setting up of new criteria for critical assessment of molecular phylogenies thus opened a new field of investigation
that was going to strongly reshape our view of metazoan phylogeny.
The first major challenge to the historical views was the inclusion of the
three lophophorate phyla within the protostomes. This reassignment was based
on sequencing and analysis of SSU rRNA genes from phoronids and brachiopods
by Halanych et al. (1995). The switch of the lophophorates phyla from deuterostomes to protostomes was not in the agreement with some of their characters,
particularly the structure and development of their coelomic cavities (Eernisse
et al. 1992, Emig 1982). This finding prompted the authors to coin the term
Lophotrochozoa to describe this new clade uniting lophophorates and trochophorebearing animals such as molluscs and annelids. However, the exact status of
both pseudocoelomates (former aschelminthes) and arthropods with respect to
this lophotrochozoan clade remained uncertain, especially because these lineages
exhibit very fast evolutionary rates that still cause long branch attraction (see
Bergsten 2005). In order to overcome this problem, Aguinaldo et al. (1997)
selected the least diverged sequences of SSU genes for representative protostome animals and particularly for the very rapidly evolving nematodes. This
approach allowed them to recover a clade of moulting animals, that they named
Ecdysozoa, which included the arthropods and several former aschelminthes: the
priapulids and the nematodes. Similarly, their analyses argued for the inclusion
of the platyhelminthes within lophotrochozoans, which would indicate a split of
the protostomes into two main clades: the ecdysozoans and the lophotrochozoans.
This topology was later established as the “new view” of animal phylogeny
because such a tree contrasts with former morphology-based schemes, particularly
in the way it splits the former aschelminthes. This result unexpectedly indicated
that organisms with very dissimilar body plans (e.g. nematodes and arthopods or
platyhelminthes and annelids) could have close evolutionary relationships (Adoutte
et al. 2000).
125
4.2.3 Small Ribosomal RNA Gene and the “New View”
of Animal Phylogeny
The inclusion of molecular data has led to a deep reorganization of the metazoan
tree. The first step was the publication, twenty years ago, by Fields et al. (1988)
of a 22 taxa dataset of SSU ribosomal RNA (also called 18S rRNA), which strikingly proposed the polyphyly of metazoans, with independent origins of bilaterians
and cnidarians among eukaryotes. An early divergence of platyhelminthes within
the bilaterians was also stressed (Field et al. 1988). However, despite the pioneering
aspect of this work, these hypotheses were later rejected when the reinterpretation
of the data demonstrated that the distance method employed had likely caused a
long branch attraction artefact, a common systematic error related to the unequal
rates of evolution among taxa (Felsenstein 1978). The use of alternative methods,
such as evolutionary parsimony, applied to the same dataset, recovered the metazoans and deuterostomes as monophyletic clades and identified a close relationship
between annelids and molluscs (Lake 1990). This setting up of new criteria for critical assessment of molecular phylogenies thus opened a new field of investigation
that was going to strongly reshape our view of metazoan phylogeny.
The first major challenge to the historical views was the inclusion of the
three lophophorate phyla within the protostomes. This reassignment was based
on sequencing and analysis of SSU rRNA genes from phoronids and brachiopods
by Halanych et al. (1995). The switch of the lophophorates phyla from deuterostomes to protostomes was not in the agreement with some of their characters,
particularly the structure and development of their coelomic cavities (Eernisse
et al. 1992, Emig 1982). This finding prompted the authors to coin the term
Lophotrochozoa to describe this new clade uniting lophophorates and trochophorebearing animals such as molluscs and annelids. However, the exact status of
both pseudocoelomates (former aschelminthes) and arthropods with respect to
this lophotrochozoan clade remained uncertain, especially because these lineages
exhibit very fast evolutionary rates that still cause long branch attraction (see
Bergsten 2005). In order to overcome this problem, Aguinaldo et al. (1997)
selected the least diverged sequences of SSU genes for representative protostome animals and particularly for the very rapidly evolving nematodes. This
approach allowed them to recover a clade of moulting animals, that they named
Ecdysozoa, which included the arthropods and several former aschelminthes: the
priapulids and the nematodes. Similarly, their analyses argued for the inclusion
of the platyhelminthes within lophotrochozoans, which would indicate a split of
the protostomes into two main clades: the ecdysozoans and the lophotrochozoans.
This topology was later established as the “new view” of animal phylogeny
because such a tree contrasts with former morphology-based schemes, particularly
in the way it splits the former aschelminthes. This result unexpectedly indicated
that organisms with very dissimilar body plans (e.g. nematodes and arthopods or
platyhelminthes and annelids) could have close evolutionary relationships (Adoutte
et al. 2000).