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Transcript
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. 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. 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.