4 Phylogeny of Animals
135
signature gene, the GMT enzyme, in this phylum. However, their position among
bilaterians remained ambiguous since no clear phylogenetic signal was recovered
for a precise branching of acoels among bilaterians (Fig. 4.4). The peculiar status of
acoel flatworms and their exclusion from platyhelminths was then confirmed by the
mean of phylogenomics.
4.5.4 Deeper into Protostome Relationships
The protostome clade presents the broadest diversity of phyla and body organizations and notably includes the most diverse kind of body organization (Adoutte
et al. 2000). In particular, many minor protostome groups that were formerly
placed within the aschelminthes display morphological features that are very hard
to interpret. Moreover, they have been very difficult to position using classical
molecular phylogenetics because of their fast evolutionary rates (e.g. Syndermates)
(Passamaneck and Halanych 2006). Recently, an impressive sequencing effort was
undertaken with the aim of solving this question of protostome relationships using
the power of phylogenomics. It was expected that the resolution of lophotrochozoan
phylogeny would be improved by the collection of EST data from several new phyla
and from the most slowly diverging species of known phyla (e.g. platyhelminthes,
molluscs) (Dunn et al. 2008, Hausdorf et al. 2007, Struck and Fisse 2008). The
analysis of these new data allowed new relationships to be proposed within the
lophotrochozoans: trochophore bearing animals, molluscs and an extended annelid
clade that includes echiurans and sipunculids were found to be closely allied to
nemerteans and lophophorates (Fig. 4.4) (Dunn et al. 2008). The nemerteans were
surprisingly positioned as lophophorate sister-group. This last result corroborates
the recent finding that the pilidium larva of palaeonemertean Carinoma mutabilis
displays strong similarities with trochophore larvae (Maslakova et al. 2004). This
clade of molluscs, annelids, lophophorates and nemerteans is supported by the
presence of chitinous chaetae and may be corroborated by a palaetonlogical scenario that proposes a common origin for those chitinous chaetae and the calcareous
spicules from which mollusc shell could have derived (Conway Morris and Peel
1995). Platyhelminthes are placed as sister-group of this lophotrochozoan assemblage, but the other phyla that were generally also associated with lophotrochozoans
are more difficult to position accurately (Dunn et al. 2008). The bryozoa phylum (also called ectoprocta) and the entoprocts were proposed to cluster together,
resurrecting an ancient hypothesis (Hausdorf et al. 2007), but this grouping was
not recovered with different marker genes and taxonomic sampling. The position
of these phyla remained thus very ambiguous, just as those of numerous other
groups such as gastrotriches, rotifers or the enigmatic myzostomids, which exhibit
strong “leaf instability”, a tendency to exhibit alternative branchings in the most
likely trees and different bootstrap replicates (Dunn et al. 2008). The assembly
of the protostome tree of life is thus far from achieved but these recent advances
have demonstrated that increased taxon sampling leads to a strong improvement in
phylogenetic resolution.
135
signature gene, the GMT enzyme, in this phylum. However, their position among
bilaterians remained ambiguous since no clear phylogenetic signal was recovered
for a precise branching of acoels among bilaterians (Fig. 4.4). The peculiar status of
acoel flatworms and their exclusion from platyhelminths was then confirmed by the
mean of phylogenomics.
4.5.4 Deeper into Protostome Relationships
The protostome clade presents the broadest diversity of phyla and body organizations and notably includes the most diverse kind of body organization (Adoutte
et al. 2000). In particular, many minor protostome groups that were formerly
placed within the aschelminthes display morphological features that are very hard
to interpret. Moreover, they have been very difficult to position using classical
molecular phylogenetics because of their fast evolutionary rates (e.g. Syndermates)
(Passamaneck and Halanych 2006). Recently, an impressive sequencing effort was
undertaken with the aim of solving this question of protostome relationships using
the power of phylogenomics. It was expected that the resolution of lophotrochozoan
phylogeny would be improved by the collection of EST data from several new phyla
and from the most slowly diverging species of known phyla (e.g. platyhelminthes,
molluscs) (Dunn et al. 2008, Hausdorf et al. 2007, Struck and Fisse 2008). The
analysis of these new data allowed new relationships to be proposed within the
lophotrochozoans: trochophore bearing animals, molluscs and an extended annelid
clade that includes echiurans and sipunculids were found to be closely allied to
nemerteans and lophophorates (Fig. 4.4) (Dunn et al. 2008). The nemerteans were
surprisingly positioned as lophophorate sister-group. This last result corroborates
the recent finding that the pilidium larva of palaeonemertean Carinoma mutabilis
displays strong similarities with trochophore larvae (Maslakova et al. 2004). This
clade of molluscs, annelids, lophophorates and nemerteans is supported by the
presence of chitinous chaetae and may be corroborated by a palaetonlogical scenario that proposes a common origin for those chitinous chaetae and the calcareous
spicules from which mollusc shell could have derived (Conway Morris and Peel
1995). Platyhelminthes are placed as sister-group of this lophotrochozoan assemblage, but the other phyla that were generally also associated with lophotrochozoans
are more difficult to position accurately (Dunn et al. 2008). The bryozoa phylum (also called ectoprocta) and the entoprocts were proposed to cluster together,
resurrecting an ancient hypothesis (Hausdorf et al. 2007), but this grouping was
not recovered with different marker genes and taxonomic sampling. The position
of these phyla remained thus very ambiguous, just as those of numerous other
groups such as gastrotriches, rotifers or the enigmatic myzostomids, which exhibit
strong “leaf instability”, a tendency to exhibit alternative branchings in the most
likely trees and different bootstrap replicates (Dunn et al. 2008). The assembly
of the protostome tree of life is thus far from achieved but these recent advances
have demonstrated that increased taxon sampling leads to a strong improvement in
phylogenetic resolution.