Biology Reference
In-Depth Information
surrounding DNA sequences. Insulators regulate gene activity by blocking
enhancer-promoter interactions when positioned between the enhancer and
promoter (
Cai and Shen 2001
). Insulators also have the ability to protect against
“position effects.” When genes are moved from their normal site in the chromo-
some, they may be expressed differently in the new location. This differential
expression is often the case with
transgenes
(foreign genes artificially intro-
duced into an organism) (
Geyer 1997
). This variability in expression may be due
to the nearness of an enhancer or silencer or the presence of nearby inactive
heterochromatin.
2.17 Chromosome or Gene Imprinting in Insects
Imprinting
is a
reversible
, differential marking of genes or chromosomes that is
determined by the sex of the parent from whom the genetic material is inher-
ited. Imprinting is one type of a broader phenomenon called
epigenetics
, a term
that indicates there are changes in the phenotype and gene expression patterns
that cannot be explained by changes at the DNA sequence level or by classical
genetics (Lyco and Maleszka 2011). “Epi” means above, so epigenetics means
that nongenetic changes that affect development and phenotype can be herita-
ble, self-perpetuating, and reversible (
Allis et al. 2007
). Epigenetics may involve
DNA methylation, histone modification, nucleosome location, or noncoding
RNA. Imprinting is one type of epigenetic change to genomes (
Boxes 2.3, 2.4
).
Imprinting was first observed in the study of insect chromosomes. In the cit-
rus mealybug,
Planococcus citri
(Hemiptera: Pseudococcidae), for example, both
males and females develop from fertilized eggs, and there are no sex chro-
mosomes. In females, all of the chromosomes remain functional, but in male
embryos one haploid set of chromosomes becomes inactivated and the set
that is inactivated is derived from the father (
Brown and Nelson-Rees 1961
). As
a result, the males are functionally haploids. In these mealybugs, the chromo-
somes derived from the male parent are
undermethylated
compared with the
chromosomes derived from female parents (
Bongiorni et al. 1999, Buglia et al.
1999
). Thus, one method of imprinting DNA involves methylation of DNA.
Methylation
of cytosines at the carbon 5 positions of CpG dinucleotides is
common in prokaryotes and eukaryotes (
Colot and Rossignol 1999, Ng and
Bird 1999
). In prokaryotes, methylation is apparently part of a defense system
against invading DNA parasites. However, in eukaryotes, methylation can be
associated with inhibiting transcription initiation, arresting transcript elonga-
tion, serving as a signal for imprinting, and suppressing homologous recom-
bination (
Colot and Rossignol 1999
). Usually, methylated DNA is inactive or
