http://www.bio.net/hypermail/MOLECULAR-EVOLUTION/lost of genes in primate

Keith Robison robison at mito.harvard.edu
Wed Mar 6 14:59:55 EST 1996


Jeffrey Mattox (jeff at cher.heurikon.com) wrote:
: In article <ralph.1176045731C at 128.196.137.12>,
: R M Bernstein <ralph at ccit.arizona.edu> wrote:
: >OK, i'll bite: What is an example of an organism that has more _coding_
: >sequences but is considered less complex than another comperable organism
: >with _less coding_ sequences.

: Other than some examples that you could fairly classify as exceptions,
: I cannot point to overwhelming evidence that the number of GENES (not
: genome size) is uncorrelated with complexity -- so I'll yield to you.
: The point of my original comment a few posts back was that the total DNA
: content in a genome is unrelated to complexity, and that this phenomenon
: is not explained by conventional evolution theory.

: From: "Fundamentals of Molecular Evolution," by W-H Li and D Graur, 1991,
: Sinauer Associates, Sunderland, MA.

:   "The C values in eukaryotes vary from 8.8 x 10^6 bp to 6.9 x 10^11 bp,
:    approx. 80,000-fold range.  Unicellular protists, particularly sarcodine
:    amoebae, show the greatest variation in C values. The three amniote
:    classes (mammals, birds, and reptiles), on the contrary, are exceptional
:    among eukaryotes in their small variation in genome size (up to four-fold).
:    Other classes, for which a substantial body of C-value data exists, show
:    variation of at least 100-fold.

:    Interestingly, the huge interspecfic variation in genome sizes among
:    eukaryotes seems to bear no relationship to either organismic complexity
:    or the likely number of genes encoded by the organism.  For example,
:    several unicellular protozoans possess much more DNA than mammals, which
:    are presumably more complex.  Moreover, organisms that seem similar in
:    complexity (e.g., flies and locusts, onion and lily, paramecium aurelia
:    and P. caudatum) exhibit vastly different C values.  This lack of
:    correspondence between C values and the presumed amount of genetic
:    information contained within genomes has become known in the literature
:    as the C-value paradox."

: And from "The Cell" (Alberts, Bray, Lewis, Raff, Roberts, & Watson; 3rd
: edition, page340):

:    Figure 8-6 shows the "Lack of relationship between amount of DNA and
:    organism complexity.  The amount of DNA in a haploid genome varies over
:    a 100,000-fold range from the smallest procaryotic cell -- the mycoplasma
:    -- to the large cells f some plants and amphibia.  Note that the genome
:    size of humans is much smaller than that of many organisms that appear
:    to be simpler."

: Sorry to put up so much -- I guess nobody here is arguing that this
: situation does not exist.

No, there is no argument that the "C-value" paradox exists
(genome size not related to genetic complexity or number of
genetic loci).  As Dr. Felsenstein pointed out, it was much more
of a paradox before we understood the detailed nature of genomes -- that
many genomes contain various sorts of repetitive DNA. The remaining 
paradox is why in some genomes the repetitive DNA runs amok, 
whilst in others it is apparently kept under control.

These repeated sequences are not stable either; as others have pointed out, 
some are independently mobile (transposons), some are non-independently
mobile (defective transposons, Alu's), some appear to be extraneous
junk acquired from the environment (defective retroviruses), some
expand by recombination or replication errors (slippage).  The genome
is a dynamic world, and any explanation of the C-value paradox
must take this into account.

Keith Robison
Harvard University
Department of Molecular & Cellular Biology
Department of Genetics 

robison at mito.harvard.edu 










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