Biology Reference
In-Depth Information
Box 2.4 Genetics and Epigenetics of DNA
Genetics
Epigenetics
Mutations in the DNA are
inherited in the soma and germ
line.
Changes in chromatin structure such as histone modifications or
effects of small noncoding RNAs are potentially reversible. Some
modifications are limited to the soma, whereas others may be
transmitted through the germ line. These modifications regulate
access to or processivity of transcription of the DNA.
Nuclear inheritance that is not based on differences in DNA
sequences.
expressed at a very low level. However, DNA methylation is not always the cause
of gene inactivity.
Methylation of DNA in insects seems to occur much less often than in mam-
mals (
Lyko and Maleszka 2011
). DNA methylation has been reported in hemip-
terans, including
Megoura viciae
,
Planococcus lilacius
,
Pseudococcus calceolariae
,
Pseudococcus obscurus
, and
Myzus persicae
; orthopterans, including
Locusta
migratoria
,
Eyprepocnemis ploras
,
Pyrgomorpha conica
,
Gryllotalpa fossor
, and
Baetica ustalata
; and the lepidopteran
B. mori
(
Manicardi et al. 1994, Regev
et al. 1998
). Differential expression of alleles due to imprinting has been observed
in
D. melanogaster
(
Golic et al. 1998, Regev et al. 1998
), but the subject of DNA
methylation in
D. melanogaster
has been controversial (
Gowher et al. 2000,
Lyko et al. 2000
).
Gowher et al. (2000)
estimated that
D. melanogaster
DNA has
≈
50 times less methylcytosine than DNA of mammals, thereby explains why it
is difficult to detect methylation in
Drosophila
. DNA methylation seems to be
lacking in some Diptera (
Drosophila virilis
,
Sciara coprophila
,
Musca domestica
,
and
Sarcophaga bullata
) or is present at only low levels (
Culex bitaeniorhynchus
,
Chironomus plumosus
,
Anopheles maculipennis
, and
Aedes albopictus
) (
Regev
et al. 1998, Tweedie et al. 1999
).
The sequencing of whole genomes of insects has resulted in the recognition
that a conserved gene family, consisting of three subfamilies (DNA methyl-
transferases), involved in methylation is found in some insects. The standard
DNA methyltransferase set consists of one copy of subfamilies 1 and 2 and
three versions of subfamily 3. However, some insects have lost one or more of
the subfamilies. For example, the silkworm has the first two subfamilies, but
it has lost the third subfamily, whereas other insects have expanded the num-
ber of their copies in subfamily 1 (
Lyko and Maleszka 2011
). Species within the
same order may vary in the number and type of the three subfamilies of DNA
