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What is a phylum? Discuss with reference to the chordates.
Ernst Haeckel coined the term ‘phylum’ when he drew up his ‘Pedigree of
Man’ in 1874. He used it to group together similar organisms based on their
morphology, and arrange them on a tree of life with man sitting comfortably at the
top. We may have now moved away from this rather anthropocentric view of
evolution, but many of his phylum names persist to this day – including the one to
which we belong: Chordata. Comprising around 100 000 species, our phylum is one
of the largest of the 35 or so that make up the animal kingdom. But it is also a highly
instructive example when it comes to thinking about what exactly a phylum means,
and the difficulties encountered when defining the taxonomic rank.
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Until recently, classification based on morphology was really the only method.
However, it can be extremely difficult to distinguish misleading convergent,
analogous traits from truly homologous synapomorphies.
In a phylum, we want all constituent organisms to be more closely related to
each other than to any organism not in the phylum, for it to be of any meaning
in terms of phylogenetic relationships. But how distantly related do organisms
need to be to necessitate being placed in separate phyla? Molecular data can tell
us how closely related groups are, but this could be rather pointless as we
would not know where to stop. A phylum, then, needs some observable
phenotypic character that unites the group – and, crucially, that character must
be synapomorphic.
In chordates, the main defining character is the notochord. This is a flexible, rod
shaped structure formed in gastrulation in all chordate embryos. It is the
primitive axis of the embryo and is derived from mesoderm. In most vertebrates
the notochord develops into the vertebral column although some (inlc. hagfish,
lampreys, coelacanths and lungfish) retain a post-embryonic notochord. Other
important chordate characteristics include a dorsal nerve cord, and the presence
of pharyngeal clefts. These have been modified into gills in fish, while in the
invertebrate chordates they form a filter feeding apparatus. In most tetrapods
they have been lost in the adults but are clearly present in the developing
embryo, in which they have a role in organising head and jaw structure. A
fourth diagnostic feature is longitudinal muscle blocks, which work against the
notochord or internal skeleton to facilitate locomotion.
A group of animals, made up of the Enteropneusta (acorn worms) and
Pterobranchia were recognised to possess a notochord-like structure called a
stomochord and were once classified along with the chordates. The erroneous
nature of this assumption was only realised following more detailed study of
how these structures develop. To create the notochord, chordates express the
gene brachyury, which belongs to the T-box family of transcription factors. This
feature unites all chordates. The stomochord however forms in a different way,
from an in-pocketing of the gut during embryo development. Consequently the
acorn worms and pterobranchs are given their own phylum, Hemichordata, and
DNA sequence data suggests their closest relatives are the echinoderms. Its
another example of misleading convergent evolution; the two phyla developed
a rod-shaped support structure independently.
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Confusingly the hemichordates also possess pharyngeal slits like the chordates,
and this may in fact be homologous. Development of these structres in
goverened in both species by expression of the genes Pax1 and Pax9, which, like
brachyury, encode transcription factors. Presumably these two groups inherited
the structures from a common ancestor early in the deuterostome lineage, and
the structure was secondarily lost in the echinoderms.
Within the chordates there are two invertebrate clades, and there have been
difficulties in working out their relationship to the rest of the chordates. One
group are cephalochordates, such as Branchiostoma lanceolatum, or lancelet. The
adults have a persistent notochord flanked by blocks of muscular tissue
(myotomes, formed from the somites) and these features strongly support their
placement in Chordata. The other more obscure group of invertebrate chordates
are urochordates, or tunicates, such as the sea squirt Ciona intestinalis. The adults
look nothing like a ‘conventional’ chordate; they have no notochord or repeated
muscle blocks. However, adult ascidian tunicates develop indirectly from freeswimming tadpole larvae, which possess many of the defining chordate features
such as a notochord, dorsal nervous system, and muscle blocks in the tail.
How do we know this is not another example of analogy rather than homology?
The brachyury gene once again provides the answer. A very closely related gene
is expressed in the notochord of ascidians – if a vertebrate brachyury gene is
inserted into an ascidian tadpole, the notochord still develops. Often we need to
examine different life stages of organisms to decipher their true phylogeny and
find synapomorphic characters.
Urochordates, being more obscure, were once classified as the basal chordate
group, with cephalochordates being the sister group to the vertebrates. A
popular idea was that the sea squirt from was ancestral, with vertebrates and
Branchiostoma having undergone paedomorphosis. Recent phylogenomic studies
have refuted this idea, and instead urochordates are more closely related to the
vertebrates than the cephalochordates. Again, this illustrates the difficulty of
using morphologies alone to determine phylogenetic relationships.
Recent progress in developmental genetics has radically improved our
understanding of animal phyla. If characteristics are derived and not convergent,
the genetic toolkit for that character will be related between the organisms in
question. Therefore we have a powerful tool for testing the assumptions about
animal phyla made in previous centuries: the collaboration of morphology and
molecular data – i.e. the synthesis of evolutionary development, or evo-devo.
Haeckel was not wrong to group organisms based on their appearances. Rather,
this is a risky and often inaccurate method due to similar selection pressures
leading to convergence in distinct groups. The common morphologies must be
synapomorphic, and often this can only be determined by analysis of DNA
sequences or studying the early development and embryology of the species in
question.