|Ernst Haeckel's 1879 Tree of Life contained the phyla he identified |
on the basis of shared body plans between their constituent members
Linnaeus identified groups which could be considered equivalent to phyla based on their shared anatomical and morphological features. This is problematic, however, as during development characteristic features may be lost, resulting in an adult form not readily identifiable as part of previously defined taxa.
A solution came in the 19th century when the zoologist Ernst Haeckel examined the embryology of members of different phyla and found sets of developmental patterns common to each of their members. This led him to suggest that body plans should be used as the means to identify different phyla and their members - body plans are the major feature used today.
This can still be problematic, though, as body plans are also highly mutable during development; the loss of the notochord - a defining feature of the chordate body plan - in the tunicates, for example. Features lost in the adult may still be present in larval or embryological forms of problematic species, but their identification may still be difficult to the point that they cannot be readily assigned to a particular taxon. This highlights another problem: defining precisely what constitutes a particular phylum?
Yet a solution has been proposed by an international team of researchers led by Professor Itai Yanai from the Technion-Israel Institute of Technology. The team selected organisms representative of 10 different phyla which represented as a wide a range of body plans as possible. A powerful technique known as CEL-Seq was then used to monitor the activity of all the genes in individual cells of 70 developing embryos from each of the different phyla. They found that each phylum underwent two distinct modules of genetic expression, along with a transitional period characterised by highly conserved patterns of gene expression.
The researchers propose, that on the basis of these shared expression pathways, the definition of a phylum as 'a set of species sharing the same signals and transcription factor networks during the mid-developmental transition.' They then used this definition to create an hourglass model that captures differential gene expression between different phyla during this critical 'phyletic transition' phase.
This has intriguing consequences for how we may go about examining the origin of different phyla in deep time, particularly in terms of using molecular clocks to estimate divergence times. 'The transition we identified may be a hallmark of development only in animals,' the researchers concluded. 'Or, future work may show that this is a general characteristic of development in all multicellular life.'