Biology Reference
In-Depth Information
confirmed that both
miR
-
iab-8
and, to a lesser extent,
miR-iab-4
can repress
the
D. melanogaster Ubx
3
0
UTR (
Ronshaugen
et al
., 2005
;
Stark
et al
., 2008
;
Tyler
et al
., 2008
).
miR-iab-4
and
miR-iab-8
are also predicted to target
Antp
,
abd-A
, and
Abd-B
(
Table 2.1
). Between 1 and 3 binding sites for each have been
predicted in the
Antp
3
0
UTRs of most
Drosophilids
. In addition, six
perfectly conserved
miR-iab-8
binding sites are present in the
abd-A
3
0
UTR (
Stark
et al
., 2008
;
Tyler
et al
., 2008
)
.
Hox regulation by these
miRNAs appears to be a common feature of arthropods, although the
number and degree of Hox mRNAs targeted is labile; varying numbers of
sites in
Antp
,
Ubx
,
abd-A
,
Abd-B
, and
Scr
are predicted in non-
Drosophilid
insects and in
Daphnia
(
Miura
et al
., 2011
).
In vivo
reporter assays show that
miR
-
iab-8
, but not
miR-iab-4
, can repress the
Drosophila abd-A
3
0
UTR
(
Stark
et al
., 2008
;
Tyler
et al
., 2008
). Similar assays have not yet been
reported for
Antp
. Finally, although
miR-iab-4
and
miR-iab-8
sites have also
been predicted in the
Drosophila Abd-B 3
0
UTR
(the only predicted
miR-iab-
8
target that is coexpressed with it), regulation by
miR-iab-8
was not
detected in a reporter assay in cultured cells (
Miura
et al
., 2011
;
Stark
et al
., 2008
;
Thomsen
et al
., 2010
).
5.3. Function
miR-iab-4
or
miR-iab-8
misexpression in haltere discs induces a classic
Ubx
loss-of-function phenotype: partial transformation of halteres to wings. The
miR-iab-8
-induced homeotic transformation is more complete than that
induced by
miR-iab-4
, in keeping with its stronger regulation of the
Ubx
3
0
UTR in reporter assays (
Ronshaugen
et al
., 2005
;
Stark
et al
., 2008
;
Tyler
et al
., 2008
).
Given these interactions, the
miR
-
iab-4/
8 loss-of-function phenotype is
surprisingly subtle (
Bender, 2008
). Flies homozygous for a targeted deletion
of both pre-miRNAs (which leaves other
iab
elements intact) are viable and
lack patterning phenotypes in adult cuticles. Homozygotes are male and
female sterile, but both sexes produce mature gametes; sterility seems to be
due to a behavioral or locomotor defect. Interestingly, subtle upregulation
of Ubx protein is observed in the neural ectoderm of late gastrula abdominal
segments and is most dramatic within the normal
miR-iab-8
expression
domain posterior to A8. Whether this upregulation causes the observed
phenotypic defects remains to be determined. However, complementation
tests show that sterility is associated specifically with loss of
miR-iab-8
; alleles
that block transcription of only
miR
-
iab-4
fully complement the miRNA
deletion (
Bender, 2008
). No changes in Abd-A or Abd-B protein were
observed by immunohistochemical analysis. Together, these results show
that miRNA interaction cannot be the primary mode for Hox mRNA
repression in posterior segments. Dissecting the redundancy with other
