Biology Reference
In-Depth Information
the nematode
Caenorhabditis elegans
, became available in 2000 (
Adams
et al. 2000, Celniker 2000
). The
Drosophila
genome is
≈
180 megabases
(Mb) and contains
≈
13,600 genes and
≈
60Mb of noncoding DNA (
Celniker
2000
). Because much of the noncoding DNA cannot be cloned, the genome
sequences obtained primarily are from the euchromatin regions that contain
protein-coding genes. The function of the noncoding, heterochromatic DNA is
being deciphered now. Much of this so-called “junk” DNA is actually involved
in gene and developmental regulation.
3.3 C-Value Paradox: Is it Real?
There seemed to be a great deal more DNA in eukaryotic organisms than is actu-
ally needed to code for the number of genes estimated to occur in the nuclear
genome. This discrepancy is known as the
C-value paradox
. Genome size varies
widely
among
insect species, with up to 250-fold differences in C values known
(
Petrov et al. 2000
). For example, the locust
Schistocerca gregaria
has a C value
of 9,300,000 kilobases (kb), 52-fold more DNA than
Drosophila melanogaster
,
but is unlikely to have 52 times as many genes (
Wagner et al. 1993
). Among 37
species of tenebrionid beetles, nuclear DNA content varies by five-fold (
Juan and
Petitpierre 1991
).
Genome size also can vary
within
species. For example, diploid cells in the
mosquito
Aedes albopictus
contain 0.18-6 picograms (pg) of DNA, and C values
vary by three-fold (0.62-1.6pg) among different populations of
Ae. albopictus
(
Kumar and Rai 1990
). The amount of DNA in insect cells may vary because many
tissues are polyploid, with different tissues having different degrees of ploidy.
Polyploidy
occurs when the amount of DNA in an organism increases over
the usual diploid (2n) amount, usually by duplicating the
number of chromo-
somes
, perhaps to 3n or 4n or more. Polyploidy can occur throughout an organ-
ism's cells or in just some tissues. A few insects are polyploid in all tissues (
Otto
and Whitton 2000
), but many insects only have polyploid tissues within a dip-
loid body. For example, the diploid blood cells of the silkworm,
Bombyx mori
,
contain 1 pg of DNA/blood cell, but a polyploid silk-gland cell in the same insect
contains 170,000 pg of DNA. DNA content within cells also varies with develop-
mental stage. At metamorphosis, the amount of DNA in
B. mori
declines by 81%
after adults emerge from the pupal stage, which is probably due to histolysis of
the larval silk glands and other polyploid cells.
Noncoding DNA can constitute 30% to
>
90% of the insect genome. This non-
coding DNA has been called “
junk
” or “
parasitic
,” or “
“selfish,”
,” and the role of
this DNA has been the subject of intense study in the past 15 years and is still in
