Worm Breeder's Gazette 11(4): 33

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

Genome Complexity of Panagrellus redivivus shrunk by the Presence of Abundant Satellite DNA

Yves de Chastonay, Fritz Mller and Heinz Tobler

The genome of Panagrellus redivivus contains two distinct satellites,
dubbed E155 and E167.  They have a reiteration frequency of nearly 30,
000 copies per haploid genome for the former and of about 40,000 for 
E167.  There is no homology between the two satellites and neither of 
them cross-hybridizes to C.  elegans or A.  Lumbricoides DNA.  As 
deduced from genomic Southern blots, the repeats are arranged in long 
tandem arrays and the two repeat classes are not intermingled.  
Northern blot analysis turned out to be negative for both satellites 
at the pg level using 10 g total Panagrellus redivivus RNA per slot, 
corresponding to the majority of transcription studies done on 
satellite DNAs.
P.  redivivus and C.  elegans have nearly identical genome sizes.  
However, whereas the latter nematode has very little satellite DNA, 
such sequences represent at least 17% of the P.  redivivus genome.  
This proportion is quite high considering the C-value of merely 70 Mb 
in P.  redivivus, thought to be the lower limit for metazoans.  
Consequently, maximal genome complexity is of 58 Mb which is 
approximately equivalent to the genome size of the slime mold 
Dictyostelium discoideum.  Although the P.  redivivus genome remains 
complex enough to englobe the predicted 35,000 kb coding sequences of 
the closely related nematode C.  elegans, the low complexity does set 
a milestone in terms of the C-value paradox.  Moreover, the genome of C. 
elegans contains 17% of moderately repeated sequences, as found by 
reassociation kinetics.  Some of these sequences are genes which must 
be equally represented in P.  redivivus.  Another fraction is made up 
of transposable elements, an example of which is the PAT element, 
recently isolated from P.  redivivus and present in about 10 to 50 
copies per haploid genome.  The majority of middle repetitive elements,
however, have been poorly studied, but a possible function for these 
remains the control of gene expression.  Such regulatory elements 
would surely also be required in P.  redivivus and if so, would yet 
decrease the genome complexity to a further extent, perhaps implying 
the necessity to re-evaluate minimal gene numbers required in simple 
nematodes.