MadSci Network: Development

Re: What are the 'advantages' of developing a deuterostome pattern of embryonic

Date: Thu Feb 4 14:35:23 1999
Posted By: Michael Onken, MadSci Admin
Area of science: Development
ID: 916688050.Dv

First, let's define what Deuterostomia entails, and how this differs from Protostomia. The term "Deuterostomia" was coined by K. Grobben in 1908 as part of an attempt to apply order to the Metazoa (multicellular animals). Grobben created a major division to separate the "higher" animal (i.e. vertebrates like us) from the "lower" animals (i.e. invertebrates like bugs and worms). This division was placed deep in the phylogenetic tree to imply a lack of common ancestry between the "higher" Deuterostomes and the "lower" Protostomes. The division was based on three developmental differences between the groups:

                                  Deuterostomia               Protostomia
1.  Early cleavage:           Radial (random)                  Spiral (directed)

2.  Gut formation:            Anus first                          Mouth first

3.  Mesoderm origin:       Pouch off gut               Migration in from walls
                            (Enterocoelomic)                  (Schizocoelomic)
While the first criterion of radial cleavage holds true for almost all Deuterostomes (Deuterostome eggs generally have more yolk than Protostome eggs, preventing the reorientation of the spindle bodies necessary for spiral cleavage), the other two are phylogenetically meaningless: most Deuterostomes, especially all terrestrial vertebrates, are schizocoelomic, with the mesodermal cells migrating into the blastocoel from the ectoderm; just as many Deuterostomes, including all terrestrial vertebrates, don't form the mouth or the anus first - the blastopore becomes the yolk stalk/umbilical cord (which can almost be considered a mouth, since it is used for the uptake of nutrients).

In fact, there are more similarities between Deuterostomes and Protostomes than most of the early developmental biologists could have ever guessed. Recent genetic and biochemical analyses have demonstrated that there are few differences in the two general body plans. Work done especially on fruit flies (Drosophila) has shown that the homeotic genes that control segmentation, axis formation, and even organ, muscle, and limb development are identical between insects and mammals - suggesting a much closer relation between Deuterostomia and Arthropoda. In fact, recent data confirm that the only real difference between the Protostome and Deuterostome body plans is an inverted Dorso-ventral axis; an idea originally proposed by E. Geoffrey Saint-Hilaire in 1822 and swept under the developmental rug by scientists like Grobben who didn't like the idea of man having a common ancestry with bugs. To further support the relatedness of Deuterostomia to the other clades, Recent evidence from rRNA homologies suggest that Deuterostomia arose from within the "higher invertebrate" group of Annelida, Mollusca, and Arthropoda, being closer to Mollusca and Annelida than to Arthropoda, and thus demoting Deuterostomia to a superphylum within Protostomia.

As far as 'advantages' of Deuterostomia over Protostomia, it is important to remember that Deuterostomes make up less than one percent of all animal species, with at least twice as many non-arthropod Protostomes as all Deuterostomes (Arthropoda accounts for about 85% of all metazoan species). The main feature of Deuterostomes that they have used to their advantages is radial cleavage, or more correctly, no spiral cleavage. The mechanism of spiral cleavage involves realigning the mitotic spindles 90° after each division. This limits the amount of extraneous material that can be stored in the cell, since the free movement of the centrosomes through the cytosol is paramount. By not requiring the rearrangement of the spindles, Deuterostome eggs can be filled with materials, like yolk proteins, that allow the embryo to develop further before hatching, so that the mother can supply the energy needed to develop into an adult, allowing the embryo to bypass a larval stage (direct development). Another advantage/disadvantage to randomizing cleavage is the inability to use the cleavage pattern as a foundation for the body plan. While this requires more elaborate schemes for defining cell lineages, it also opens the door to experimentation for greater evolutionary diversity.

Regarding endoskeletons, both Deuterostomes and Protostomes have experimented with hardened internal support structures. The Protostome phylum of Mollusca contains several cephalopods, including squid and cuttlefish, with internal supports that are unarticulated, and therefore offer strength while constricting movement. The Deuterostome phyla of Echinodermata and Chordata both contain examples of endoskeletons, however the echinoderm endoskeleton is also fairly unyielding, and acts almost as an exoskeleton is several species. The only phylum to use a fully articulated endoskeleton is Chordata, which includes the vertebrates. The advantages of the vertebrate endoskeleton include lighter weight (less material and energy to maintain) and greater range of movement (speed and maneuverability). On the downside, the endoskeleton gives no protection from predation or other environmental conditions, which only works in organisms that can develop directly to adulthood and put their energy into escaping predators rather than building a protective shell.

So advantages of being a vertebrate? Well arguably, the cerebral evolution that resulted in some vertebrates becoming sentient (I won't say who) is an offshot of the extensive brain development associated with the more complex locomotor system and the need for more evasive survival tactics - advantage Deuterostomia. But as was so often pointed out during the Cold War, our intelligence could easily lead to our annihilation, resulting in a planet ruled be insects - advantage Protostomia. I think untimately, our potential to spread our species to other planets gives humanity the advantage, but I would be surprised if some cockroaches didn't hitch a ride to colonize the new worlds, as they have with every other human conquest.

If you're interested in more information, I stongly suggest reading "The Shape of Life" by Rudy Raff, or "Cells, Embryos, and Evolution" by John Gerhart and Marc Kirschner.

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