DEVELOPMENT AND GENE EXPRESSION
Developmental methods offer effective ways for distinguishing certain groups of organisms from one another. Species located closer to each other on the phylogenetic tree tend to follow the same developmental pattern for a longer period in their lifetime, meaning that if the embryonic development of two species differ significantly even at a very early step, there is a good chance that one of them branched off early from the evolutionary pathway of the other (probably after that last similar step in their early ontogeny). On the other hand, if two organisms go under the same modifications that follow the same order in their larval or embryonic stages, it is highly unlikely that their strategies came about independently. By this very method we can compare each and every animal’s ontogenetic strategies to then draw a map, showing their relation based on how long they follow the same trend in terms of the process leading to their mature form. I had the intention that this part of the comparison needs special emphasis given the fact that Echinoderms lost several deuterostome characteristics in the course of time, and besides molecular data their developmental strategy is the best argument for their close relationship with Chordata along with other Deuterostome groups. In the followings I am briefly going to go through the ontogenetic development of all multicellular animals (metazoans) and point out where the process tend to show differing attributes. At the end of this short analysis it should come to light that Chordata and Echinodermata develop almost identically until a relatively late stage and their development is more similar to one another’s than to any of the non-deuterostome group’s. To build up a deeper appreciation about this similarity, one should first understand the basics of metazoan development and its various forms.
After the fusion of the gametes – or in other cases, after some kind of asexual reproduction – the first part of the cell’s development into a multicellular adult is called cleavage or segmentation, which is essentially brought forth by a great number of mitotic divisions. This early process gives rise to a hollow bulb-like structure, separated from its environment by a single layer of cells. After a given amount of growth this outer layer goes under a process called invagination through which the outer layer folds into itself, forming a second inner cavity that is called the Archenteron and – in contrast to the former hollow structure – is directly in contact with the outer environment through the blastopore. This stage of embryonic development is referred to as ‘blastula’ and the process leading to it as ‘blastulatuion’. The layer covering the outside of the structure is called the ‘ectoderm’, and the layer covering the inside of it is called the ‘endoderm’ – in other words the outer and inner germ layers respectively. The development of basal metazoans stops at the blastula stage, as in the case of sponges (Porifera), where the ectoderm gives rise to the external epithelia and the primitive nervous system, and the endoderm covers the inner cavity and differentiates into several cell types, including the ones secreting digestive enzymes for extracellular digestion. A somewhat similar body plan can be seen in the case of Cnidarians and Ctenophores, but with an undoubtedly more complex system in terms of the inner cavities. These organisms are often referred to as ‘diploblasts’, which name stands for the possession of only two germ layers.
The phyla mentioned in the followings are collectively called ‘triploblasts’, that almost exclusively contains organisms with bilateral symmetry, or at least organisms that had bilateral symmetry, but lost it somewhere in their evolutionary history. ‘Triplo’ refers to the possession of a third germ layer – mesoderm – that arises through the next developmental step, called gastrulation, in which the infolding of the endoderm usually forms an inner cavity called the coelom. The coelom can be absent (Platyhelminthes: acoelomate) or poorly developed (Nematodes: Pseudocoelomates), which seemed a good enough general trait to classify organisms within triploblasts; however, subsequent comparison of protein-coding gene sequences and Hox gene clustering overthrow this concept and seemed to support a different classification (Noriyuki Satoh et al. 2014). Triploblasts were divided into two major divisions, Protostomes and Deuterostomes – with Chordata and Echinodermata in the latter –. This grouping correlates with some conspicuous differences in early development of Protostomes and Deuterostomes. One of the Protostome clades was named after their unique characteristic of exhibiting spiral cleavage at the very first steps of their early development. These species fall under the clade, ‘Spiralia’ (referring to the way of segmentation). It is debated whether this clade is the equivalent of Lophotrochozoans or not – which latter name refers to the feeding organ of the larvae –, but they are usually interchangeable. Although evidence for spiral cleavage is poor in some phyla within Spirilia, it is commonly accepted that all spiralian phyla derived from an ancestor that underwent spiral cleavage (Andreas Hejnol, 2010). Ecdysozoa is the other large Protostome clade including Arthropoda, Tardigrada and Nematoda amongst others. Members of these groups mainly exhibit radial cleavage, just like Deuterostomes (Chordata and Echinodermata as well), but we can find an even more profound difference in their development, which is the formation of the posterior and anterior openings. In the case of Protostomes the blastopore gives rise to the mouth and the second opening of the gut develops into the anus, whereas in the case of Deuterostomes it happens the other way around. Also, at Protostomes the coelom develops by the splitting of the mesoderm, whereas in Deuterostomes the secondary invagination of the mesoderm is the one giving rise to the coelom.
This overview of metazoan ontogeny clearly demonstrates that Chordates and Echinoderms can be sharply separated from other phyla in terms of their development. Another interesting observation regarding the body plan of Triploblasts suggests a somewhat similar case of relation among the above mentioned groups.
In Arthropods the central nervous system develops on the ventral side, and the digestive tract is located on the dorsal side, whereas in the case of Chordates the two systems develop the other way around (Noriyuki Satoh et al. 2014). The family of genes responsible for the formation of the dorsal and ventral axis
EMBRYOLOGY IS AN EFFECTIVE WAY TO DISTINGUISH CERTAIN GROUPS OF ORGANISMS FROM EACH OTHER. WHY?
EARLY STAGES OF DEVELOPMENT – WHERE DOES CERTAIN ANIMAL GROUPS STOP
METAZOA, EUMETAZOA, BILATERIA, DIPLOBLASTS
AT TRIPLOBLASTS – BLASTOPHORE GIVES RISE TO?
DEUTEROSTOMES, PROTOSTOMES (COELOM)
MOTILE FREE-LIVING LIFESTYLE OF CEPHALOCHORDATES AND CHORDATES WAS BELIEVED TO HAVE EVOLVED FROM A MOTILE LARVAL STAGE OF THE SEDENTARY, TENTACULATE ANCESTOR BY PAEDOMORPHOSIS, IN WHICH THE LARVAL STAGE BECOMES THE SEXUALLY MATURE ONE, REPLACING THE SESSILE MATURE STAGE. TUNICATE LARVACEANS – GOOD EXAMPLE.
(CHORDATE EVOLUTION AND THE THREE-PHYLUM SYTEM, 07.11.2014, NORIYUKI SATOH, DANIEL ROKHSAR, TERUAKI NISHIKAWA)
Andreas Hejnol, Integrative and Comparative Biology, Volume 50, Issue 5, 1 November 2010,
JAMES W. VALENTINE, Cleavage patterns and the topology of the metazoan tree of life, 1997