Biology Reference
In-Depth Information
strands in eukaryotes if it were not for telomeres. Telomeres prevent the loss of
genetic information from the ends of chromosomes.
Telomeric DNA sequences and structure seem to be similar among many arthro-
pods, including Hymenoptera, Lepidoptera, Trichoptera, Mecoptera, Coleoptera,
Orthoptera, Isoptera, Blattodea, and Crustacea (
Okazaki et al. 1993, Sahara et al.
1999
). The telomeric sequence TTAGG was isolated from the silk moth,
B. mori
(
Okazaki et al. 1993
), where it is repeated over a 6- to 8-kb segment. TTAGG has
been found only in telomeres from arthropods and could be ancestral in this group.
Not all insects have the TTAGG sequences in their telomeres, including several
dipterans. For example,
Drosophila
species have non-long terminal repeat (LTR) ret-
rotransposons (called HeT-A, TART, and TAHRE) that function as telomeres (
Mason
and Biessmann 1995, Pardue and DeBaryshe 2011
). In
Drosophila
, chromosome
length is maintained, despite incomplete DNA replication, by the addition of ret-
roposon sequences through transposition, but the retrotransposons do not trans-
pose into the gene-rich euchromatic regions. It is not known whether
Drosophila
recruited existing retrotransposon elements to replace the “standard” telomeres or
whether the retrotransposons represent a more ancient arrangement used by the
earliest eukaryotes (
Pardue and DeBaryshe 1999
).
Eickbush (1997)
suggests that
non-LTR retrotransposons gave rise to telomerases and that a parasite was recruited
in early eukaryotes to supply this important function. Perhaps the
D. melanogaster
case can be viewed as a recent example of a similar recruitment event.
Two dipteran genera,
Chironomus
and
Anopheles
, and some dermapterans,
hemipterans, and coleopterans also lack the TTAGG repeat, suggesting that the
loss of the TTAGG sequences has occurred independently several times during
insect evolution (
Sasaki and Fujiwara 2000
). In chironomids, a third type of chro-
mosome termination occurs (
Kamnert et al. 1997, Rosen and Edstrom 2000
). In
Chironomus pallidivittatus
and
C. tentans
, the size of the repeats is unusually
large, consisting of 340 bp and 350 bp, respectively. In
C. pallidivittatus
, the telo-
meric repeat units are present in blocks up to 200 kb.
Telomere terminal transferase
, or
telomerase
, was discovered first in the pro-
tozoan
Tetrahymena
. Telomerase is a ribonucleoprotein, whose RNA component
is essential to its function. Telomerase recognizes single-stranded oligonucle-
otides ending in -GGG3
′
in a variety of eukaryotes and adds GGGG sequences to
the ends of chromosomes. Telomerase functions as a kind of reverse transcrip-
tase, because its own RNA codes for a DNA sequence.
Adjacent to the telomeres are subtelomeric repetitive sequences. In most
insects, recombination is relatively common in subtelomeric DNA. Subtelomeric
