[Protista] Macronuclear regeneration

[paul earl]

Discovered in 1940 in Paramecium aurelia by Sonneborn, macronuclear 
regeneration (MR) in ciliates is now well established. Senescence, 
immaturity (Haga & Hiwatashi, 1981) and rejuvenation have entered the 
panorama of conjugation and autogamy even since Maupas in 1889. See 
Smith-Sonnenborn (1979) on aging.

A week ago, I made the mistake of writing that MR was not convincing, 
corrected now after reading Kloetzel on Euplotes aediculatus and Kimura 
& Mikami on Paramecium caudatum. If the initiating macronucleus (MA) is 
damaged or missing,, a MA fragment of the old MA can replace it as a 
form of life insurance. Kloetzel used a UV microbeam and Kimura & Mikami 
used microsurgery to inactivate the anlagen and induce MR from fragments.

Ciliates are multinucleate cells that contain macronuclei and 
micronuclei. The nuclear contents often change after meiosis. Crossing 
over, various segregations and hybridizations often involving 
allopolyploidy are still poorly understood. Exconjugant cells have 
maternal MA fragments, newly initiated MA and micronuclei (MI). The 
maternal macronucleus of P. caudatum is retained for long periods such 
as four cell cycles in some species and can continue to influence 
development.

However after a few cycles, the presence of the new MA anlage suppresses 
the synthesis of DNA by the maternal fragments that then become pycnotic.

Then MA fragmentation might begin with fertilization or later followed 
by MR followed by DNA inhibition in fragments and then their digestion. 
See Kimura & Mikami (2003). Without a new MA, the old one is 
regenerated, at least in both P. caudatum and Euplotes aediculatus. 
Sending a genetic signal through the cytoplasm from an anlage to a 
fragment is a miracle indeed. Is this one a somatic segregation ? Such a 
signal was assumed to originate in the MI-controlled synkaryon as MIs 
and MAs so segregate from it. That is the MIs from the synkaryon 
segregate into MIs and MA anlagen. See also Cole & Siegel (1969) leading 
to Chen on allopolyploidy in P. bursaria.

When ciliates say Make 4 MI, they might only need one. If the processes 
of meiosis and allopolyploidy are accidentally mixed, one result can be 
a new species in one jump though the common result leads to death. 
Ciliates are, by the precautions they take, trying to hold on to the 
status quo including of course their ploidy status. It is indicated by 
the volume of the MI. Segregations in the sense of being only related to 
crossingover in classic meiosis cannot explain all of the events during 
the Maupasian rejuvenation allowed by conjugation--promoted by sexual 
exchange.

Other necessary references follow.

Allen, Genetics (1967) 55: 797-822

Berger, Chromosome (1973) 44: 33-48, J. Protozool (1974) 21: 145-152

Cole & Siegel, (1969) Genetics 63: 361-368

Diller, J Morph (1965) 116: 51-64

Fidler, Jayaraman & Kloetzel, J Protozool (1985) 32: 429-438

Haga & Hiwatashi, Nature (1981) 289: 177–179.

Katashima, J Sci Hiroshima Univ Ser B Div 1 (1959) 18: 99-107; 1971, 23: 
59-93

Kimura & Mikami, Differentiation (2003) 71: 337–345

Kloetzel, Develop Biol (1981) 83: 20-32

Lipps, Arch Protistenkd 116: 106-131

Mikami, Chromosoma (1979) 73: 131-142, Dev Biol (1987) 123: 161–168, J 
Protozool (1975) 22: 536-540

Raikov, In Research in Protozoology, 1968, Ed. TT Chen, Vol 3, Pergamon, 
New York

Smith-Sonneborn, Science (1979) 203: 1115-1117.

Sonneborn, Anat Rec (1940) 78: 53–54.

Dr Paul R Earl

Facultad de Ciencias Biológicas

Universidad de Nuevo León

San Nicolás, NL, 66451 Mexico