Monday, June 24, 2013
EVOLUTION - THE PROTOSTOME-DEUTEROSTOME LINK
ORIGIN OF DEUTEROSTOME EMBRYOLOGY
Annelid theory as a working hypothesis
Since Gans and Northcutt (1983) provided evidence of a close relative of annelids having some features of development resembling deuterostomes, as noted in the previous post, it is reasonable to evaluate other evidence. Inversion of systems, the primary evidence supporting the annelid theory, has been deemed inadequate by those who comment. But much other evidence is available. The biggest impediment had been the drastic embryological differences between protostomes and deuterostomes.
My doctoral thesis research included a comparison of development of two species of isopods with very different life cycle rates of development as well as a new embryological structure in one species. The rapidly developing egg of the Michigan species was smaller, had a thinner egg shell, and two appendages on the egg. The Tasmanian one had a thicker egg shell and no appendage; but on the each side of the embryo within the egg was a yolk filled bulge in the position from which the other ones had their egg appendages develop.
Otherwise, both embryos packed the egg fully. They both developed in a folded position, legs outermost. But the flattening differs so the Tasmanian one filled up space with the yolk filled bulge. The Tasmanian species has less change from ancestral crustacean features; they also lack the abundant source of food from deciduous tree leaves as available for the Michigan ones.
The main point of this isopod egg observation is that the evolution of a new feature in the egg goes counter to what many biologists think was perhaps a valid portion of the discredited “biogenetic law”. The law is not absolute, especially as my observation showed me, evolution can occur by the development of new features in the earliest life stages of an organism. Clearly, embryological stages do not faithfully repeat steps in the evolution of the organism.
Since the pogonophorans are likely candidates as intermediates, in spite of the general opinion that they were an evolutionary dead-end, what do they contribute to the story? Well, they have lost the annelid digestive system in the adult, their segmentation has nearly disappeared, and they live in an abyssal world with a very low rate of input of food. Such a regime would drastically select options or mutations that save energy.
Why protostome spiral cleavage became deuterostome radial cleavage
The thinning of the egg shell would not constrain the early dividing cells into the packed spiral pattern of ancestral protostomes but it would conserve resources otherwise used for a strong egg shell. Consequently, the loss of structural integrity of the egg shell would not impose the constraints for efficient use of space as in spiral cleavage.
Limited energy and resulting low reproductive potential puts survival at a premium for the individual. Thus, although the first cell divisions (cleavage) of protostome eggs end the potential of the daughter cells to each develop into individuals, it is possible for each of the early dividing cells of the deuterostome egg. Injury or death of one of the first few cells of a deuterostome egg would not necessarily result in death. In fact, identical twins, triplets, and other genetically identical individuals could not have developed if deuterostomes had retained the features of spiral cleavage. The survival advantage of this feature for pogonophoran species in the nutrient poor abyss should be an obvious benefit.
In both cases it is a loss, loss or reduced production of shell, loss of control of early developmental specification. As discussed in an earlier post, loss can occur more rapidly than gain of a feature. The rates are relative to other factors such as food supply, generation time, and value of the features for survival. But clearly, pogonophorans are excellent candidates for the missing link connecting embryological features of protostomes and deuterostomes. The conclusion is hypothetical. The event described was undoubtedly a Pre-Cambrian occurrence. But the conclusion is based on comparative evidence consistent with similar conclusions that will be presented for other evidence.
Interesting, but somewhat irrelevant to the current discussion is the fact that the Michigan isopod egg appendage had the cellular appearance of adult respiratory tissue and must aide their relatively rapid development.
Engemann, Joseph G. 1963. A Comparison of the Anatomy and Natural History of Colubotelson thomsoni Nicholls, a South Temperate, Fresh-water Isopod and Asellus communis Say, a North Temperate, Fresh-water Isopod. Ph.D. Thesis, Michigan State University, East Lansing. 146 pp.
Gans, Carl, and R. Glenn Northcutt. 1983. Neural crest and the origin of vertebrates: a new head. Science, 220:268-274.
A United States Fulbright Grant for study in Australia, aide of staff and use of facilities at the University of Tasmania and Michigan State University, as well as a Faculty Research Grant at Western Michigan University, were instrumental in my making many observations involved in this series of blogs. Many individuals deserve my thanks as well.
Joseph G. Engemann June 24, 2013