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amphibians in general, is unclear. Several studies have been done comparing
localized RNAs in
Rana
(
Lithobates
)
pipiens
and
Eleutherodactylus coqui
, addi-
tional members of the order Anura. Although a more comprehensive anal-
ysis has yet to be done, representative early- and late pathway mRNAs,
vegt
and
dazl
, localize to the vegetal cortex in
Rana
(
Nath et al., 2005
). Since
Xenopus
and
Rana
are in distinct phylogenetic families, with a time of diver-
gence of over 200 MYA, these patterns of localization are likely a basal con-
dition present in the last common ancestor of anurans (
Elinson and del Pino,
2011
). Germ plasm is present in many anuran species, although
Rana
lacks a
conspicuous mitochondrial cloud in early oocytes (
Nath et al., 2005
).
RNA localization has also been examined in
E. coqui
, a direct developing
frog with an egg much larger than
Xenopus
, rapid development and the
absence of a tadpole stage (
Callery et al., 2001
). Interestingly, transcripts
for
dazl
and
ddx25
are vegetally localized to the germ plasm in
E. coqui
,
suggesting a conservation of this pathway throughout anurans (
Elinson
et al., 2011
). By contrast, late pathway RNAs,
vegt
and
gdf1
, are not vegetal
in
E. coqui
but enriched diffusely in the animal pole (
Beckham et al., 2003
).
This loss of late pathway localization is thus a derived condition, since
Eleutherodactylus
diverged from
Rana
within the Neobatrachia clade, about
150 MYA (
Elinson and del Pino, 2011
).
In the other order of amphibians that are extensively studied, the
Caudata (urodeles), RNA localization appears to be absent from oocytes.
Urodeles have been generally known to lack germ plasm and maternal spec-
ification of the germline (
Nieuwkoop and Sutasurya, 1976
). Instead, PGCs
are induced by somatic signals, similar to the process in mammals. Corre-
spondingly,
dazl
is expressed but not localized in axolotl (
Ambystoma
mexicanum
) oocytes (
Johnson et al., 2001
). Similarly,
vegt
is not localized
in axolotls (
Nath and Elinson, 2007
).
Despite these differences in germline formation and germ layer induc-
tion, specification of the dorsal axis is likely to be similar across the amphib-
ians. A dorsal gray crescent forms in anurans and urodeles (
Clavert, 1962
),
and parallel microtubule arrays have been described in
Rana
,
Xenopus
,
Eleutherodactylus
(anurans;
Elinson and Ninomiya, 2003; Elinson and
Rowning, 1988
), and
Cynops pyrrhogaster
(a urodele;
Iwao et al., 1997
). It
will thus be interesting to determine whether localized RNAs associated
with axis formation, such as
wnt11b
, and, in particular, those involved in
microtubule assembly,
trim36
and
plin2
, are localized in other amphibian
species. Additionally, it will be important to learn the evolutionary bases
for differences in RNA localizations in amphibians.
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