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

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.

Embryological evidence

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.

References

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.

Acknowledgements

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