Mustafa
The huge disparity in genome size among organisms of roughly similar
structural complexity was once called "the c-value paradox." (c value
refers to the size of a haploid genome.) The paradox has since been
resolved, as Andrew Roger explained, by the discovery that many
eukarytoic genomes are composed partly or largely of DNA that does not
code for proteins. This leaves, however, the very interesting question of
why so much non-coding DNA exists in the genome. This has proven to
be a difficult question to resolve.
Robin from Cal Tech is right. Cavalier-Smith's book on the evolution of
genome size, although certainly one you should check out, is heavily
slanted toward the view that non-coding DNA is adaptive. In this view,
increases in non-coding DNA occur because this DNA provides
physiological advantages to the cell that result from its mere presence in
the nucleus. (You'll quickly discover as you read what these putative
advantages are.)
However, many workers in this area view increases in genome size as
non-adaptive or even maladaptive. In this view, non-coding DNA
increases in the genome due to intrinsic genetic processes until it reaches
an upper limit tolerated by selection . Some processes, like unequal
crossing-over, increase (or decrease) DNA passively; but there also are
DNA sequences called transposons that can actively spread copies of
themselves throughout the gemone. This DNA is called selfish DNA
because it expands at the expense of the organism that houses it.
Although there is a moderatley large literature on this subject, few studies,
in my view, have done a good job of testing these competing (although
not mutually exclusive) hypotheses. Most papers have simply surveyed
DNA contents in a group and compared the DNA data with presumptive
physiological correlates. A conclusive test of the adapative vs.
nonadaptive hypotheses has proven difficult to devise. One serious
attempt is this direction is described by Stanely Sessions and Alan Larson
on plethodontid salamanders in Evolution in 1987. (I can't give you the
exact citation because I'm stuck at home under a foot of snow.) To
balance one partisan point of view (Cavalier-Smith's) with an equally
partisan opponent, I recommend The Eukaryotic Genome in Evolution
and Development by Bernard John and M. Miklos (1989?). The term
selfish DNA was first coined by Doolittle, W.F. and C. Sapienza. 1980.
Nature 284:601-3 and Orgel, L.E. and F.H.C. Crick. 1980. Nature
284:604-607. A Biosys search using the keywords "evolution " and
"DNA content" or "genome size" will net you around a hundred papers.
Good luck
Robert Dawley
Ursinus College
Pennsylvania, currently within the Arctic Circle
rdawley at acad.ursinus.edu
