EVOLUTION: NEMATODE WORMS

Nematode worms are typically long, round, unsegmented, gradually tapered at both ends.

"Nematodes have many species with relatively little difference in body form.  Many are parasitic and it is thought that most species of vertebrates may have one or more parasitic nematode species unique to them.  Nematodes parasitize many other groups of animals and plants.  Many live in the intestines of animals.  One free-living nematode species lives in organic rich soil but can also live as a parasite in humans. Rotting organic matter in soil is not so different from the intestinal contents of some animals.  Both are rich in bacteria that the nematodes can feed upon.  Adapting to the rich soil made them somewhat "pre-adapted" to life as an intestinal parasite. This adaptation included an ability to live in environments with oxygen so limited many other animals could not survive." (from my unpublished 2010 manscript)

The similarity of structure of different species disappears when the mouth end, and often the anal end, are examined microscopically.  Three jaws are present in some.  The pharanyx may have a muscular bulb that probably helps ingestion of food without losing pressure, the body contents act as a hydrostatic skeleton.  The cuticular covering of the body is molted or shed typically several times in early development.  During the process of development portions of the chromosomal material can be ejected from the chromosomes; this is perhaps a result of selection for the small size of ancestors living among the sand grains of soils. In one species the ejection of chromatin occurs in all cells except the stem cell until the 32 cell stage.  Body cells of many achelminths other than nematodes also seem to have the loss of ability to regenerate that is thought to be a result of the reduced chromosomal material in body cell nuclei (or nuclei when tissues are syncytial).  Near constant number of nuclei or cells of the species are present in the tissues of many aschelminths.

I was reviewing some of Libbie Hyman's work on Aschelminthes (not accepted as a valid cluster by many zoologists), but unfortunately she did not have the benefit of knowing about gnathostomulids (first described in 1956) which were later.  Gnathostomulids seem to be descendents of the simple early flatworms that are not flat, but are adapted to living in sediments that are often anoxic.  Reidel, 1969, suggests the gnathostomulids can be placed in either the Platyhelminthes or the Aschelminthes.  The gastrotrichs may be the connecting link to rotifers.  Nematodes may have been the termination of a line orginating early in the cluster of achelminth groups; but they have a complete lack of cilia, a fact that makes them unlikely to have given rise to any other groups since arthropods also lack cilia but are so clearly derived from annelids that do have cilia.  Thus the lack of cilia in nematodes and arthropods is an analagous, not homologous, trait.

The reason I referred to Hyman was to find out about the adhesive glands or pedal glands, commonly paired on most ashelminths' posteriors, but absent in the gnathostomulids.  The glands are very small and difficult to see, especially in nematodes.  I did not see them in some nematodes I had watched in water on a microscope slide at low magnification, but those nematodes were clearly adhering by their tail as the writhed around.  One researcher (Chitwood) divided nematodes into two groups depending on whether they had phasmids at their posterior.  The mouth area and anal areas of nematodes show great variation in microcopic details not conducive to casual observation.

Such fine details can be a great help in identifying species and often show revealing variation suitable for showing evolutionary relationships.  The October 20, 2017, issue of Science has a research report detailing such a fact with feather-like hairs on water-strider feet.  In the case of water-striders, the details are limited to very close relatives.  In arthropods, similar microscopic comparisons can be made of structures limited to closely related species of the same genus and sometimes of different orders.

When the very small and the very large features match, relationship seems more likely.  To determine evolutionary relationships, neither can be ignored.  Over-dependence on one may lead to error and demonstrate why the novice or student may see things the specialist or teacher does not see, a relationship affecting creativity as noted by Tinbergen.

Among the larger features distinctive for nematodes, that show them as a terminal group in an evolutionary sense, are the muscle cells of the body of the intestinal parasite, Ascaris.  All are longitudinal and each passes a muscle cell process to the nerve enervating the muscle.  Other lines of evidence that the Ecdysozoa are an invalid group are indicated by some of the references appended.

Ascaris can grow to a foot long during it time in the intestine.  It has a simple life cycle with transmission of eggs, typically ingested with fecal contaminated food, hatching in the intestine and larve going through tissue and blood to the lungs where they break out and get coughed up, swallowed, and then comlete their life in the intestine.  Another nematode parasite of humans is thought to be the fiery serpent mentioned by Moses.  It has a big name, Dracunculus medinensis, and is known as the guinea worm.  The adult female can be as much as a meter long and live in the subcutaneous tissue under the skin.  The larvae are discharged through a hole in the skin and, if ingested by an aquatic microcrustacean named Cyclops, complete their larval development and, if Cyclops is ingested by a human, eventually reach their location under the skin.

The great variations in size, number of host species needed to complete life cycles, and adaption to a single or limited number of final hosts of most vertebrates, as well as many invertebrates, seems to indicate an ancient origin for nematodes.

Joseph G. Engemann    Emeritus Professor of Biology, Western Michigan University, Kalamazoo, Michigan   October 30, 2017

REFERENCES (comments added)

Aguinaldo, Anna Marie A., James M. Turbeville, Lawrence S. Linford, Maria C. Rivera, James R. Garey, Rudolf A. Raff, and James A. Lake.  1997.  Evidence for a clade of nematodes, arthropods and other moulting animals.  Nature, 387:489-493. Unfortunately, textbooks have picked up their grouping of nematodes with arthropods and some other molting animals in a group they named Ecdysozoa; based on 18s ribosomal DNA sequences, it is inadequate to support such a group.  They even say “It was unexpected to find nematodes contained within the Ecdysozoa because in previous molecular studies they diverged deep in the protostome tree, even before the deuterostome-protostome bifurcation.”   -page 491 has discussion of unequal rates found in other nematode studies (documented and ignored) and their search for and choice of slowly evolving representatives [almost guaranteed to put an outgroup in where it doesn’t belong]

Fraser, Hunter B., Aaron E. Hirsh, Lars M. Steinmetz, Curt Sharfe, and Marcus W. Feldman.  2002.  Evolutionary rate in the protein interaction network.  Science, 296:750-752.  (26 Apr 2002)  “We show that the connectivity of well-conserved proteins in the network is negatively correlated with their rate of evolution.”  “interacting proteins evolve at similar rates.” - used “putatively orthologous sequences between Saccharomyces cerevisiae and the nematode Caenorhabditis elegans.” 

Halanych, Kenneth M.  1996.  Testing hypotheses of chaetognath origins: long branches revealed by 18S ribosomal DNA.  Syst. Biol., 45(223-246.   Well-done study but long branches and small sample size make result of relationships beyond the nematode-chaetognath affinity somewhat dubious.

Halanych, Kenneth M., John D. Bacheller, Anna Marie A. Aguinaldo, Stephanie M. Liva, David M. Hillis, and James A. Lake.  1995.  Evidence from 18S ribosomal DNA that the lophophorates are protostome animals.  Science, 267:1641-1643.  “we propose the node-based name (16)[K. de Queiroz and J. Gauthier, Syst. Zool. 39, 307 (1990)] Lophotrochozoa, which is defined as the last common ancestor of the three traditional lophorate taxa, the mollusks, and the annelids, and all of the descendants of that common ancestor.”  Note 10 includes the following statement “Regions that could not be readily aligned were excluded from the analyses.”  Their proposal is ridiculous when all data are considered.

Halanych, Kenneth M., and Yale Passamaneck.  2001.  A brief review of metazoan phylogeny and future prospects in Hox-research.  Amer. Zool., 41:629-639.  maintain Hox gene research supports the earlier ridiculous proposals of ecdysozoans and lophotrochozoans.  Has numerous references.

Hobert, Oliver, and Gary Ruvkun.  1998.  A common theme for LIM homeobox gene function across phylogeny?  Biol. Bull., 195:377-380.  neurogenesis regulatory genes and transcription factors are very similar in vertebrates, insects, and nematodes

Hobmayer, Bert, Fabian Rentzsch, Kerstin Kuhn, Christoph M. Happel, Christoph Cramer von Laue, Petra Snyder, Ute Rothbackerm, & Thomas W. Holstein.  2000.  WNT signaling molecules act in axis formation in the diploblastic metazoan Hydra.  Nature, 407:186-189.  the WNT signaling pathway had been found in nematodes, insects and vertebrates.

Kappen, Claudia.  2000.  Analysis of a complete homeobox gene repertoire: implications for the evolution of diversity.  Proc. Natl. Acad. Sci. USA, 97:4481-4486.  used the nematode, C. elegans

Sarnat, Harvey B.  1984.  Muscle histochemistry of the planarian Dugesia tigrina (Turbellaria: Tricladida): implications in the evolution of muscle.  Trans. Am. Microsc. Soc., 103(3):284-294.  Says striated muscle is in every metazoan phylum except Porifera and adult Platyhelminthes. (oblique striations in nematodes – Rosenbuth 1965, 67  Wright 62) 

Van Auken, Kimberly, Daniel C. Weaver, Lois G. Edgar, and William B. Wood.  2000.  Caenorhabditis elegans embryonic axial patterning requires two recently discovered posterior-group Hox genes.  Proc. Natl. Acad. Sci. USA, 97:4499-4503.  “essential embryonic patterning in C. elegans requires only Hox genes of the anterior and posterior paralog groups, raising interesting questions about evolution of the medial-group genes.” Three Hox genes in the nematode

HISTORY REPEATING

DEJA VU

If you have a short attention span skip to the third paragraph for an essay demonstrating the maxim that "those who ignore history are bound to repeat it."

2/16/2009

With some helpful editing by Nancy I wrote the following.

I KNOW NOTHING

          Not knowing all the answers tends to free one from some of the errors that creep into the thinking of those in charge.  The animal behaviorist, Tinbergen, noted the ability of those new to a field to find solutions to problems the well-versed do not find.  Those well-trained are often stuck in a rut of thinking that only leads to the things already discovered.  The teacher is often exposed to new ideas by students unencumbered by past thinking.  Listening to new ideas carefully will give the experienced a chance to either help the novice grow and avoid an error, or, enable both to grow along with the subject under discussion.

          As one who knows so little about so many things I have found it has the drawback of finding anyone to take what I have to say seriously.  The right answer is often in the middle of a controversy.  Both sides have some truth on their sides.  Somewhere between the extreme views is where the optimal solution is usually found.  Solutions to our economic problems have been victimized by allowing extreme positions to be alleged as proof the other extreme is right.  It happens every day on talk shows of both sides.

          Higher taxes on the billionaire, made one by defrauding workers of their pensions funds or stockholders from reasonable dividends, is neither undemocratic, robbing the rich to pay the poor, socialistic, nor unfair.  It is justice for the crook, and fair repayment to the country which made their success possible, and owed to surviving families of workers and soldiers who worked and died for their country.

          It is unlikely that any proposal adopted for reviving the economy will be the best possible proposal, but it may be the best practical proposal if it is instituted in time.  Tossing out the CEO’s and their staffs that have been in charge would leave a chaotic void with its own problems for us.  Letting them do their own thing without enforceable guidelines could mean more self-serving decisions than proper oversight would allow.

          The current situation is worse than the usual recession and may require more drastic action than usual for many reasons.

1.  Extreme amounts of capital have apparently been lost in leveraged positions by banks and investment vehicles on nearly fictitious investments with borrowed money involving many times the dollar value of the underlying assets.  Clearly, oversight and transparency is needed.

2.  Enron’s energy scam increasing natural gas prices was apparently repeated by part of the financial community using control of oil futures so great profits could be made by making oil four times more expensive than otherwise.

3.  Jobs disappear, interest rates drop for investors, interest rates increase for debtors, and home prices drop along with retirement accounts.

4.  It’s an environment where no one wants to spend money or pay takes, even if one can.

          All told, it will take some fancy dancing on the part of our government to walk the line between a growing depression and runaway inflation.  In such an environment the policies will have to change drastically then gradually, perhaps back and forth repeatedly in both directions to dampen the swing to one least damaging to all concerned.  Don’t worry if it rocks the boat a little; it has been rocking for quite a while.  If the analogy holds, let’s pray and hope it can shift the balance before it tips over from too extreme a position.

          A seven hundred billion dollar bailout amounts to over $2000 for every individual in this country; or, $10,000,000 for each of the 70.000 wealthiest people in the country; or, $100,000 each for the 7,000,000 poorest families in our country.  Which group would be most effective in getting the money back into the economy?  Yes, some balance would be better.

          The hated gas tax Ross Perot favored in his run for the presidency a few years ago could have paid for maintenance of our highway system.  It would have kept the automakers feet to the fire and given us gas efficient cars.  National deficits and decline of our auto industry would have been lessened.  De-regulation of the banking and investment industries in hindsight was an obviously bad move.  Rather than subsidies for domestic industries, a cautious, wisely constructed, federal system more generous than Medicare for physician reimbursements could benefit employees and make companies more competitive exporters.

          Workers deserve a portable retirement program funded by employers and themselves on the model of the TIAA-CREF program available to educators and some other non-profit employees.  It would reduce the incentive for mergers and spin-offs used to raid pension funds by schemers among the corporate and financial insiders.

          Rather than limiting profits, wages, and capital gains use sensible tax policy to remove incentives for blatant rip-offs.  Make companies pay a 10% tax for all total compensation over $300,000 and 50% for all over one million and 90% for all over one billion.  Do the same for sale prices of companies and assets exceeding the rate of inflation: refund half at a rate of five percent of the tax amount to be released each year stock values and employment do not decline during the following ten years.

          Such policies would obviously benefit from careful and fair design.  Even the extremely wealthy would benefit from seeing their wealth maintained rather than declining by market disasters and the inflation that will come to pay for our growing debt.  Most multi-billionaires who lose half their wealth are still multi-billionaires.  Most home owners who lose half their wealth are in bad financial shape, even if they can keep their homes.

          Tax time is coming.  I won’t grumble if I have to pay.  I’m thankful if I can pay.  I’ll spend some time thinking about the security, education, and good things paid for by my taxes.  I won’t spend too much time worrying about the benefits someone else gets, especially if they are poor, they will spend it and help keep our income coming to us.
_______

If you skipped the first two paragraphs, read them now.  The seven year old journal entry had only one change, a comma was moved to make a sentence make better sense and a typo was corrected.  I also [on 10/23/17] substituited "increasing" for "producing" in describing Enron's scam.

Joseph G. Engemann   Kalamazoo, Michigan    April 9, 2016