Biology Reference
In-Depth Information
Species vary in the number of repeated elements in their genome. For exam-
ple,
Drosophila melanogaster
has
≈
30% of its genome as heterochromatic
(repetitive) DNA, but
≈
60% of the genome of
Drosophila nasutoides
is repeti-
tive DNA. More than 90% of a genome can be repetitive DNA. Some insects,
such as aphids, have small amounts of repetitive DNA, and this DNA could be
associated with a faster developmental time (
Ma et al. 1992
).
Repetitive and single-copy DNA are present in two different patterns in
insect genomes. The
short-period interspersion pattern
has single-copy DNA,
with a length of 1000-2000 base pairs (bp) long, alternating with short (200-
600bp) and moderately long (1000-4000bp) repetitive sequences. The house
ly
Musca domestica
, the Australian sheep blowfly
Lucilia cuprina
, and the wild
silk moth
Antheraea pernyi
have this pattern.
Long-period interspersion pat-
terns
have long (
>
5600bp) repeats alternating with very long (
>
12 kilobases
[kb]) uninterrupted stretches of unique DNA sequences. This pattern is often
found in species with small genomes (0.1-0.5pg of DNA/haploid genome),
including
D. melanogaster
, as well as in the aphid
Schizaphis graminum
(
Ma et al. 1992
), the midge
Chironomus tentans
(
Wells et al. 1976
); the flesh-
ly
Sarcophaga bullata
(
Samols and Swift 1979
), the honeybee
Apis mellifera
(
Crain et al. 1976
) and the red flour beetle
Tribolium castaneum
(
Brown et al.
1990
).
Even within an insect family, genome organization can vary. Total DNA in the
genome of four mosquito species varies from 0.186 to 0.899pg, and the amount
of repetitive elements varies from 0.009 to 0.150pg of foldback DNA (
Black and
Rai 1988
). The mosquito
Anopheles quadrimaculatus
has a long-period intersper-
sion type of genome organization, but
Culex pipiens
,
Ae. albopictus
, and
Aedes
triseriatus
have the short-period interspersion type. Generally, the amounts of
fold-back, highly repetitive, and middle-repetitive DNA increase linearly with
genome size in these mosquitoes. Intraspecific variation in the amount of highly
repetitive DNA was found in
Ae. albopictus
colonies and is due to differences in
the number or type of transposable elements. The amounts of repetitive DNA
in mosquitoes varies from 20% in
An. quadrimaculatus
to 84% in
Ae. triseriatus
(
Besansky and Collins 1992
).
Satellite DNA is a type of highly repetitive DNA that differs sufficiently in its
base composition from the majority of DNA in a eukaryotic species that it sepa-
rates out as one or more distinct bands when DNA is isolated by centrifugation
with cesium chloride. Satellite DNA is rich in either adenine
+
thymine (A
+
T) or
in guanine
+
cytosine (G
+
C) sequences, and it is found in long tandem arrays
within the heterochromatic regions of chromosomes.
