Wednesday, May 29, 2019

Spiral to Radial Cleavage Transition

Another index card accompanied the one described in the previous post, suggesting the protostome / deuterostome move of retinal nerve fibers from behind the retina to in front of the retina, can be a result of the dorso-ventral inversion mediated by the Pogonophora.  This additional card was written on 1/28/2003 and included the following two statements.

"Somewhere between 10 PM last night and 11:30, I was thinking about the Asellus egg appendage example of embryonic evolution as a model for the embryonic evolution of protostome to deuterostome.  It occurred to me that it was a better example than I thought, it may have been for embryo survival, not just metabolic speed for development."

"Progress seems to have resumed since I started.  45 years with the other explanation:  If I am so creative, why didn't I think of it before in those terms?  I guess I am not,  I'm like everyone else!"

Why I didn't think of it was almost certainly a result of being overwhelmed with the appendage appearing as a new evolutionary feature of the embryo, and it was not an adult ancestral feature that was incorporated in earlier stages as the abandoned "biogenetic law" would have predicted.  In a way, the appendage did incorporate an ancestral feature such as one found in the more primitive Tasmanian isopod as a yolk-filled bulge evolved into an appendage.  Although the appendage disappears in the adult isopod, its position makes it a model of a possible base for development of wing precursors in ancestors of insects.  The only certain fact is it shows embryonic features can arise without representing a precursor in adult features.

The reasoning for the spiral to radial transition is speculative but thought to be almost certain because other evidence (1) puts Pogonophora between protostomes and deuterostomes on the tree of life; (2) shows loss of features can evolve more rapidly than gain of features; (3) the deep sea nutrient input is impoverished; (4) reduced egg-shell thickness and loss of early cell specificity are both energy saving developments of deuterostome development. 

Point one above shows us the protostome condition was ancestral.  The deuterostome condition evolved by loss of specificity in early cleavage divisions of the fertilized eggs; this may have been due to weakened thin or missing eggshells not providing rigid spatial relationships and signals for development.  Loss of such signals made each early division product express the entire genome needed for the "twinning" seen in deuterostome eggs.  Cell fate is eventually determined in some deuterostome embryonic tissue as shown by grafts of presumptive leg tissue making additional legs on frogs.  But that development may require an interaction of cells in the tissue since the example of cloning individuals from skin cells indicates each cell must have the entire genome.

More primitive protostomes that have the ability to regenerate tissue or reproduce by fragmentation must have complete genomes in such cells.  Notable exceptions are aschelminths such as nematodes that undergo reduction of the genome in cells other than reproductive tissue cells.

Simplification via pogonophorans is multi-faceted and simultaneously driven by selection for survival in abyssal conditions and survival during episodes of drastic species extinction during periods of asteroid bombardment

A.  The pogonophorans, as noted above provide a rational explanation for major distinction of the origin of deuterostome development.

B.  Pogonophorans provide an answer for the puzzling inversion of systems noted between protostomes and deuterostomes.

C.  Pogonophoran abyssal adaptation makes their survival during major extinction episodes easier to understand, especially due to their extremely low respiratory rate, extreme longevity, and ability to absorb nutrient at levels found in abyssal sediments.

The pogonophorans become an evolutionary bottleneck limiting features of deuterostomes

A.  Hemoglobin is the only respiratory pigment in blood of deuterostomes.

B.  Chitin is reduced in pogonophorans and absent in deuterostomes.

C.  Segmentation is reduced in pogonophorans and absent in deuterostomes, although metamerism is functionally present as a base for deuterostome complexity development almost certainly derived from ancestral segmented features.

D.  The bottleneck entry was via ancestral polychaete annelids.  The bottleneck exit to the chordate line was via hemichordates.

Complexity seen in deuterostomes is probably based on the limited genetic inheritance from the pogonophorans via duplication of gene copies going on to functionally different units.  At the same time some older versions may be eliminated due to the vagaries of natural selection.

The extreme longevity I first suspected, from the depth pogonophoran tubes must reach in abyssal sediments, seems confirmed by the long-branch attraction shown by pogonophorans appearing in genomic clusters of phylogenies of other major groups.

Joseph G. Engemann    Emeritus Professor of Biology, Western Michigan University, Kalamazoo, Michigan   June 29, 2019

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