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

EGG APPENDAGES OF THE MICHIGAN ISOPOD

THE MICHIGAN ISOPOD

Baker Woodlot Vernal Pool, home of the isopod studied in Michigan

Baker Woodlot on the campus of Michigan State University in East Lansing, Michigan contained several vernal pools.  One by the road on the western boundary of the woodlot was the location where the population of the isopod, Asellus communis, studied for comparison was found.  As compared to the elevation of the pools on Mount Wellington of over 4,000 feet, the elevation is less than 800 feet.

                                  Vernal Pond in Baker Woodlot

The pools were almost equidistant north or south from the equator, but in opposite hemispheres.  The temperature extremes were not measured but the mild temperatures of Hobart at sea level were replaced a few miles away by colder temperatures on Mount Wellington, especially during the winter.  Trees provided an abundant leaf-fall in the woodlot.  Input of similar nutrients were much more limited on the mountain.  Vertebrate predators such as turtles, fish, salamanders, and frogs were not found on the mountain and are relatively few in number in Tasmania.  More species of invertebrates were found in the Michigan pool.

 EGG COMPARISONS

Both species had females with eggs in brood pouches during the breeding season as shown in the figure of Asellus above.  The difference between the 2nd and 3rd pleopod sizes of the Michigan isopods are shown in the drawing; pleopod 3 covers 4 and 5 on the underside of the very short abdomen (in above figure A).

The egg appendage

The photo A below shows two eggs.  All Asellus eggs develop pairs of egg appendages; one, B,  is enlarged for clarity.

The cells forming the tissue of the appendage have characteristics much like the leaf-like branches of the pleopods under the outer protective branch of the 3rd illustrated.  The delicate cellular structure indicated by B and its similarity to the respiratory appendages suggests the appendage has a respiratory function; the same features could be useful in eliminating ammonia and perhaps other water soluble wastes.  It has been suggested that they could be useful for uptake of nutrients from the mother's brood pouch.  That does not seem likely since water from the pool bathes the eggs, and the eggs have a plentiful store of nutrients in the darker yolk filling the egg.

The Tasmanian isopod eggs (upper left in figure above) do not have external appendages.  The Tasmanian isopods have many points indicating a closer relationship to ancestral crustaceans.  The Michigan ones, in addition to having the egg appendage, have dorso-ventral flattening and fusion of abdominal segments.  Fossil evidence suggests the antiquity of the Tasmanian variety.The egg appendage presence clearly illustrates that evolution can occur in developmental stages independent of adult evolution.  So the use of embryological features as evidence of relationships has to be done cautiously.  The appendage seems to have homology with a simple bulge (paired bulges, one indicated by central semicircle broken line) in the same position on the side of the folded embryo in the egg of the older group.  A peculiar fact is that the appendage sometimes persists briefly (at the point on the adult indicated by the arrow) on newly hatched isopods as illustrated in Sars' 19th century work on The Crustacea of Norway.  The bulge on the embryo of a South African species similar to the Tasmanian one persisted briefly after hatching in one studied by Barnard.

THE DEMISE OF THE BIOGENETIC LAW

In summary, the egg appendage of some isopods is good evidence that the old biogenetic law - that the development of an individual repeats stages in the evolution of the group - was correctly abandoned. Adaptive features can develop in any stage of the life cycle independently from development of other features.  Although  the theory has been abandoned, the stages often do give clues to relationships when viewed in the proper context.

Joseph G. Engemann     June 5, 2014