Freethought & Rationalism Archive

DNAunion · 03-17-2002, 07:35 PM

Quote:

Nic: ... a ubiquitous ID mistake is to focus on one particular system (e.g. mitosis in an advanced eukaryote like yeast), and consider it in isolation.

DNAunion: If we don't look at yeast, then what? The most primitive eukaryotes I am familiar with are micropsporidia. But from what I can gather by some quick searches, far more work on mitosis is being done with yeasts than with microsporidia.

First, (for others), what are microsporidia?

Quote:

”Microsporidia are amitochondrial eukaryotic obligate intracellular parasites. They are reported to infect every animal group from protists to vertebrates, including humans.” (Elisa Bigliardi and Luciano Sacchi, Cell biology and invasion of the microsporidia, Microbes and Infection, 2001, 3:5:373-379)

DNAunion: And here is the most related stuff I could get from quickly searching through abstracts.

Quote:

”The mitotic process in microsporidian Encephalitozoon hellem, a known human pathogen, has been studied with the aim of elucidating some ultrastructural aspects of its nuclear division. The presence of a nuclear spindle, of "electrondense spindle plaques" associated with the nuclear envelope and of cytoplasmic double walled vesicles are reported. We suggest that these "electrondense spindle plaques" serve as foci for intranuclear and cytoplasmic microtubule arrangements, similar to the microtubule organizing centers within the centrosomes of animal cells. The extent to which the microsporidial division process is comparable with that of more familiar eukaryotes such as yeast cells is discussed.” (Bigliardi E, Riparbelli MG, Selmi MG, Lanzarini P, Corona S, Gatti S, Scaglia M, Sacchi L, Mechanisms of microsporidial cell division: ultrastructural study on Encephalitozoon hellem, J Eukaryot Microbiol 1998 May-Jun;45(3):347-51)

DNAunion: That actually sounds like an excellent paper. But I don't have full access, and since I treasure my privacy, I won't ask someone else to send me a copy.

Quote:

”Ultrastructural studies were carried out to describe the nuclear division cycle of a strain of Encephalitozoon hellem isolated from an Italian AIDS patient. The nuclear division occurs during the proliferative vegetative phase and it is characterized by the intranuclear mitosis and by the lack of centrioles. The spindle termini are electron dense spindle plaques (ESPs), resembling to the spindle pole bodies (SPBs) of Saccharomycetes. The ESPs are bifacial organella forming microtubules on both nucleoplasic and cytoplasmic faces. In the outer layer of the spindle plaque are present vesicular elements lined by a double membrane of unknown function. The peculiar morphological features of E. hellem ESPs indicate that both intranuclear spindle and cytoplasmic microtubules are involved in the nuclear division.” (Sacchi L, Bigliardi E, Lanzarini P, Corona S, Gatti S, Scaglia M, Ultrastructural features of spindle microtubule organization during the nuclear division of Encephalitozoon hellem, : J Eukaryot Microbiol 1997 Nov-Dec;44(6):80S(

DNAunion: Three interesting things. Intranuclear mitosis (is the pleuromitosis?), intranuclear spindle, and no centrioles (though even plants lack centrioles).

But, there seems to be some evidence that microsporidia may not have branched off during eukaryotic evolution as early as has been thought.

Quote:

”Microsporidia are obligate intracellular parasites that have long been considered to be primitive eukaryotes, both on the basis of morphological features and on the basis of molecular, mainly ribosomal RNA-based, phylogenies. However, accumulating sequence data and the use of more sophisticated tree construction methods now seem to suggest that microsporidia share a common origin with fungi and are therefore most probably just curious fungi. In this paper, we describe the current views on the phylogenetic position of the microsporidia and present additional evidence for a close relationship between fungi and microsporidia on the basis of reanalysed ribosomal RNA data. In this respect, the importance of incorporating detailed knowledge of the substitution pattern of sequences into phylogenetic methods is discussed.” (Yves Van de Peer, Abdelghani Ben Ali and Axel Meyer, Microsporidia: accumulating molecular evidence that a group of amitochondriate and suspectedly primitive eukaryotes are just curious fungi, Gene, 2000, 246:1-2:1-8)

Nic Tamzek · 03-17-2002, 08:14 PM

Yeah, microsporidia being basal is out-of-date. They are widely agreed to group within fungi on the basis of protein trees.

Which reinforces several points I was making with perhaps limited effectiveness previously:

1) The rRNA tree is not the last word on phylogeny, it it subject to long-branch attraction artefacts ("long-branch attraction" is where rapid sequence divergence, for whatever reason), results in a "long branch" on a group of sequence trees. All of the long branches will, for statistical reasons, tend to branch down near the base of the tree (the "attraction"), and screw up the rooting of the rest of the tree (the root will be "dragged" towards the longest branches).

2) Microspordia is a classic case of #1, and as it is a parasite it falls nicely into one of the 'rapid evolution more likely' categories discussed in a previous post.

There are ways of detecting long-branch artefacts via statistical tests, and testing it via comparison of many different sequences etc., but now we're reaching the point where discussion on UBB boards by amateurs like me is ridiculous and those wishing to know more are encouraged to join their local molecular phylogenetics graduate program.

Giardia is the new favorite basal eukaryote, but personally I'm afraid that similar problems may apply there.

I agree that it's annoying that most of the molecular work on mitosis is being done on a few model organisms -- but of course this is a problem right across molecular biology. There are good reasons for it (e.g. human disease) but it will be misleading for evolution-related thinking unless this data bias is clearly recognized.

Most of the work on alternative mitosis is via old-fashioned microscopy and electron microscopy, and those obscure journals & researchers are the ones where this stuff has been studied, most of it before the internet revolution so a lot of it won't even come up on search engines. This is where people like Cavalier-Smith (and Lynn Margulis, and other 'evolutionary protistologists') come from originally, although they make active use of the molecular data that has become common (again, biased towards model and disease organisms) in the 1990s.

Nic

Nic Tamzek · 03-17-2002, 11:17 PM

Here's a bit on obscure variant mitosis in protists:

<a href="http://www.madsci.org/posts/archives/mar2000/954297447.Cb.r.html" target="_blank">http://www.madsci.org/posts/archives/mar2000/954297447.Cb.r.html</a>

Quote:

I'm asking to help a student who have a project to do and can't find informations about the different types of mitosis.

I know there's other kind of mitosis but I just wanted to know if somebody knows more about these kind of mitosis. (example: cryptomitosis,
endomitosis, metamitosis, pleuromitosis, orthomitosis...)

Thanks for your answer

[...]

I'm not familiar with many of these terms, except endomitosis, which I suspect is also called "closed mitosis" to distinguish it from the typical "open mitosis" of plant and animals cells. (I suspect that some of these terms are not widely used). However, I can tell you about some of the variations of mitosis off the top of my head. The strangest types of mitosis are found in certain protists (single-celled eucaryotes).

In a number of protists (eugenoids, dinoflagellates) the nucl. env. remains
intact. At least two things can happen to pull apart the 2 groups of chromosomes: microtubules form inside the nucleus (euglenoids, fungi) -or-
the nucleus is penetrated by fingers of cytoplasm that grow into tunnels, containing microtubules, stretching from one side of the nucleus to the
other. The chromosomes attach to the inner membrane of the nuclear envelope, and are pulled apart by sliding along the microtubules (unlike
normal mitosis, there is no direct contact between chromosome and microtubule).

Another variation in mitosis is the nature and origin of spindle microtubules. In addition to the typical centrioles at each end of the
spindle, one finds a larger MTOC (microtubule organizing center) in diatoms (described beautifully by Jeremy Pickett-Heaps) where the spindle microtubules are all parallel to each other, and a large, banded, polar spindle body that forms on the nuclear envelope in fungal cells.

Finally, something called "amitosis" involves the macronucleus of ciliates. The macronucleus, oddly enough, is made from a much much smaller
micronucleus that replicates its DNA repeatedly, then cuts it into tiny gene-sized bits, each of which is given 2 telomeres. So, it is clearly
impossible to line up and sort a million mini-chromosomes! During cell division, a replication band sweeps from one end of the macronucleus to the other, then it simply pinches in two.

Some protists have a nucleus associated with 4 flagellae and a complex microtubular ribbon (the Trichomonads, part of the parabasalians) and they
must have their own special mitotic routine. If you can find a good university library, this info is published in journals like Protistologica,
Journal of Protozoology, etc.. If you need more info, I could fax you a few pages from a protistology book by O. Roger Anderson that discusses such variations (it is not terribly easy to understand without pictures) if you
contact me directly: djacobson@whitworth.edu. Also, feel free to send a follow up query to the Mad scientists network.

Cheers,
Dean Jacobson

Here's a newsgroup post with some references:

<a href="http://groups.google.com/groups?q=pleuromitosis&hl=en&ie=ISO-8859-1&oe=ISO-8859-1&selm=6251DC16D8%40zool.umd.edu&rnum=1" target="_blank">http://groups.google.com/groups?q=pleuromitosis&hl=en&ie=ISO-8859-1&oe=ISO-8859-1&selm=6251DC16D8%40zool.umd.edu&rnum=1</a>

Quote:

Promitosis, premitosis, and pleuromitosis are terms that have been used to distinguish eukaryotic nuclear division mechanisms that do
not form a metaphase plate (common in many protists) from the eumitotic mechanisms with the typical 5 stages including metaphase. See my papers on evolution of mitosis in BioSystems 7: 318-325 & Biology of the Cell 53: 41-50.

You are right that the term promitosis is rarely used now.

-Dennis

elpowell@indyvax.iupui.edu on the subject: defn: promitosis
on 7 Sep 95 14:21:48 -0500 asked:

> Can anyone explain the difference between promitosis and regular,
> garden-variety mitosis? I think that promitosis might be an archaic term
> because I'm not finding any mention of it anywhere.
>
>

...we're not the first people with these problems.

Like I said, this stuff is the very definition of obscurity. Generalizations about single-celled eukaryotes are difficult to make except very broadly.

The International Society for Evolutionary Protistology has the kind of people that know about these things:

<a href="http://megasun.bch.umontreal.ca/isep/isep.html" target="_blank">http://megasun.bch.umontreal.ca/isep/isep.html</a>

Protist societies and journals:
<a href="http://megasun.bch.umontreal.ca/protists/pso.html" target="_blank">http://megasun.bch.umontreal.ca/protists/pso.html</a>

...of which the European Journal of Protistology is probably the top publication:

<a href="http://www.urbanfischer.de/journals/frame_template.htm?/journals/ejp/e-protis.htm" target="_blank">http://www.urbanfischer.de/journals/frame_template.htm?/journals/ejp/e-protis.htm</a>

(Another problem: for some reason, a lot of this field has had important contributions from Russian and German-language publications. Good for them, I guess, but it makes our life tougher and has contributed to the terminology chaos in the field, e.g. cilium vs. flagellum vs. undulipodium vs. [insert 5 other suggested terms I can't remember at the moment])

Nic Tamzek · 03-17-2002, 11:31 PM

Here's one of Dennis Goode's articles (not even one he referenced above, I don't think):

Quote:

<a href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=733781 0&dopt=Abstract" target="_blank">http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=733781 0&dopt=Abstract</a>

Biosystems 1981;14(3-4):271-87

Microtubule turnover as a mechanism of mitosis and its possible evolution.

Goode D.

Movement of chromosomes during mitosis appears to be coupled to the unidirectional turnover of spindle microtubules. This paper outlines a directional turnover model of mitosis which hypothesizes that: (1) Unidirectional turnover of tubulin subunits and microtubule-associated proteins occurs from an assembly site at one end to a disassembly site at the other end of mitotic apparatus microtubules. (2) The components of interpolar microtubules are continuously moving toward each pole due to their assembly in the opposite half spindle and disassembly in the near half spindle. (3) Mitotic chromosome movements are coupled to this directional turnover by selective formation of semi-stable lateral interactions or bridges between kinetochore microtubules and parallel interpolar microtubules. (4) The anaphase velocity of kinetochores relative to the poles is determined by the rate that kinetochore microtubules disassemble on their poleward ends minus the rate they assemble at their kinetochore ends. (5) Spindle elongation occurs when assembly of interpolar microtubules is more rapid than their disassembly or when interpolar microtubules slide in an anti-parallel arrangement. (6) The velocity of chromosome separation is the sum of spindle elongation and the poleward movements of sister chromosomes. Evidence for and against these hypotheses and some possible steps in the evolution of this type of mechanism are discussed.

Here's another early article:

Quote:

<a href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=120345 5&dopt=Abstract" target="_blank">http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=120345 5&dopt=Abstract</a>

Biosystems 1975 Nov;7(3-4):298-301

Primitive mitotic mechanisms.

Ris H.

Unorthodox mitotic mechanisms are reviewed and their contribution to the understanding of evolution of the orthodox mitotic apparatus is considered. Dinoflagellates and hypermastigote flagellates are of particular significance because the microtubular mitotic apparatus is entirely extranuclear with the nuclear membrane persisting through mitosis. Chromosomes are attached to the nuclear membrane. In hypermastigole flagellates early kinetochore separation is on the nuclear membrane without any contribution from microtubules. In dinoflagellates the chromosomes are also attached to the nuclear membrane, but at least in some species cytoplasmic microtubules connect to the attachment site. In Syndinium the attachment site resembles a typical kinetochore, but is inserted in the nuclear membrane. A similar kinetochore is found in certain Radiolaria, but with an intranuclear spindle apparatus the association with the nuclear membrane is no longer necessary and has been lost. Mitosis in the yeast Saccharomyces is essentially orthodox, though chromosomes do not condense. No kinetochores are seen, but a single microtubule makes direct contact with the 20 nm chromatin fiber of each chromosome and shortens during anaphase. About 5-10 microtubules are continuous between the spindle pole bodies and form the elongating central spindle.

nic

DNAunion · 03-22-2002, 08:20 PM

DNAunion: Hey Nic, thanks for all the info.

One sure gets a lopsided view of things like mitosis when consulting only general cell biology and genetics texts (the "orthodox" form is all that is explained, leading one to believe it is the only form of mitosis that exists). It seemed that there were basically only two very different forms of asexual nuclear division (one type used by all prokaryotes and the other, invariant, type used by all eukaryotes), and the differences between the two were immense and not easily bridged conceptually (mitotic spindle in eukaryotes - including tubulin, MTOC, etc. - none in prokaryotes; linear chromosomes in eukaryotes, circular in prokaryotes; condensation of chromatin into chromosomes in eukaryotes, not in prokaryotes; dismantling of the nuclear envelope in eukaryotes, obviously not present in prokaryotes; etc.). But all of the various variations on eukaryotic mitosis that you pointed out seem to make an evolutionary pathway much more plausible by offering potential intermediates. Kind of reminds me of Darwin's explanation for the evolution of the vertebrate eye: look at all of the various kinds of eyes out there and the range of complexity they exhibit (ocelli to "camera-type"), and then you can imagine a possible pathway from the simplest to the most complex using those variants (of course many do not accept that kind of reasoning as being conclusive or convincing).

[ March 22, 2002: Message edited by: DNAunion ]</p>

DNAunion · 03-24-2002, 04:54 PM

DNAunion: I am shocked. I realized after making my post that I misstated something, but haven't had the time to get back online and correct it. I expected to have had at least one person call me an idiot (thinking I didn't know that what I said would be incorrect if taken as written). But no one jumped on it??????? I won't try to correct the statement, just point out the slip.

Quote:

DNAunion: It seemed that there were basically only two very different forms of asexual nuclear division (one type used by all prokaryotes and the other, invariant, type used by all eukaryotes...

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03-17-2002, 08:14 PM	#42
Nic Tamzek Senior Member Join Date: Jan 2002 Location: California Posts: 646	Yeah, microsporidia being basal is out-of-date. They are widely agreed to group within fungi on the basis of protein trees. Which reinforces several points I was making with perhaps limited effectiveness previously: 1) The rRNA tree is not the last word on phylogeny, it it subject to long-branch attraction artefacts ("long-branch attraction" is where rapid sequence divergence, for whatever reason), results in a "long branch" on a group of sequence trees. All of the long branches will, for statistical reasons, tend to branch down near the base of the tree (the "attraction"), and screw up the rooting of the rest of the tree (the root will be "dragged" towards the longest branches). 2) Microspordia is a classic case of #1, and as it is a parasite it falls nicely into one of the 'rapid evolution more likely' categories discussed in a previous post. There are ways of detecting long-branch artefacts via statistical tests, and testing it via comparison of many different sequences etc., but now we're reaching the point where discussion on UBB boards by amateurs like me is ridiculous and those wishing to know more are encouraged to join their local molecular phylogenetics graduate program. Giardia is the new favorite basal eukaryote, but personally I'm afraid that similar problems may apply there. I agree that it's annoying that most of the molecular work on mitosis is being done on a few model organisms -- but of course this is a problem right across molecular biology. There are good reasons for it (e.g. human disease) but it will be misleading for evolution-related thinking unless this data bias is clearly recognized. Most of the work on alternative mitosis is via old-fashioned microscopy and electron microscopy, and those obscure journals & researchers are the ones where this stuff has been studied, most of it before the internet revolution so a lot of it won't even come up on search engines. This is where people like Cavalier-Smith (and Lynn Margulis, and other 'evolutionary protistologists') come from originally, although they make active use of the molecular data that has become common (again, biased towards model and disease organisms) in the 1990s. Nic

03-22-2002, 08:20 PM	#45
DNAunion Veteran Member Join Date: Jan 2001 Location: USA Posts: 1,072	DNAunion: Hey Nic, thanks for all the info. One sure gets a lopsided view of things like mitosis when consulting only general cell biology and genetics texts (the "orthodox" form is all that is explained, leading one to believe it is the only form of mitosis that exists). It seemed that there were basically only two very different forms of asexual nuclear division (one type used by all prokaryotes and the other, invariant, type used by all eukaryotes), and the differences between the two were immense and not easily bridged conceptually (mitotic spindle in eukaryotes - including tubulin, MTOC, etc. - none in prokaryotes; linear chromosomes in eukaryotes, circular in prokaryotes; condensation of chromatin into chromosomes in eukaryotes, not in prokaryotes; dismantling of the nuclear envelope in eukaryotes, obviously not present in prokaryotes; etc.). But all of the various variations on eukaryotic mitosis that you pointed out seem to make an evolutionary pathway much more plausible by offering potential intermediates. Kind of reminds me of Darwin's explanation for the evolution of the vertebrate eye: look at all of the various kinds of eyes out there and the range of complexity they exhibit (ocelli to "camera-type"), and then you can imagine a possible pathway from the simplest to the most complex using those variants (of course many do not accept that kind of reasoning as being conclusive or convincing). [ March 22, 2002: Message edited by: DNAunion ]</p>

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