Trends Cell Biol
December 1, 1997;
Expeditions to the pole: RNA localization in Xenopus and Drosophila.
In Xenopus and Drosophila oocytes, a number of maternally synthesized RNAs encoding molecules that act in formation and patterning of embryonic tissues are localized to the vegetal and posterior
poles, respectively. In Drosophila, and probably in Xenopus, localization of their RNAs within the oocyte
generates the regionalized distributions of these molecules in the early embryo
that are required for proper development. Studies described here have begun to reveal components of the cellular machinery that effects RNA localization. While specific aspects of localization differ among RNAs, similarities between pathways used by Xenopus and Drosophila suggest that common themes have been conserved among localization mechanisms.
Trends Cell Biol
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Late pathway (b) Stage 2, 3
Nurse cells ~ Follicle cells
Oocyte Stage 8
. OSKAR mRNA
Localization pathways. (a) Vegetal localization pathways in Xenopus oocytes. Xenopus oocytes are shown at successive stages of
development. Xlsirts, Xcat-2 and Xwnt-11 RNAs, which use the'early pathway', are localized to mitochondrial aggregates in the earliest
oocytes (coincident localization of all three indicated in black) 2°. During stages 1-2, Xlsirts (yellow), Xcat-2 (pink) and Xwnt-11 (blue)
become localized to a distinct region of the mitochondrial cloud z3. Xlsirts RNA is present throughout this region, Xcat-2 is distributed
in a ring around the outside and Xwnt-11 is centrally located. The organization of these RNAs within the cloud and the putative role of
the cloud in their vegetal cortical localization have led to the designation of this region as the messenger transport organizer (METRO) 23.
The RNAs show a layered distribution at the vegetal cortex initially (stages 2-3), with Xcat-2 nearest the cortex and Xlsirts farthest
from it. By stage 4, this layering is no longer apparent and the RNAs appear coincident (black). Vgl RNA (green), which uses the 'tare
pathway', is present throughout the cytoplasm in early oocytes but is excluded from the mitochondriai aggregates and cloud 6,2°,23.
During localization (stages 2-3), Vgl shows a punctate, wedge-shaped pattern (stippling). Vgl RNA accumulates at the vegetal pole in
the region occupied by Xlsirts, Xcat-2 and Xwnt-11 (stages 2-3) before becoming distributed along the entire vegetal cortex (stage 4) 23
(b) OSKAR (OSK) RNA localization pathway. The Drosophila ovary is composed of a set of ovarioles, each containing a series of egg
chambers in successive stages of development. Each egg chamber contains an oocyte-nurse cell complex surrounded by a layer of
somatic follicle cells. Egg chambers from an ovariole are shown at representative stages of development (anterior towards the left).
Throughout the first half of oogenesis, OSK RNA (red) accumulates uniformly within the oocyte, which assumes the posterior-most
position of the oocyte-nurse cell complex. During stages 8-9, OSK accumulates at the anterior margin of the oocyte and at the
posterior pole. Posterior localization of OSK RNA is maintained through the remainder of oogenesis and in the early embryo 15,16
Patterns of vegetal localization in Xenopus oocytes. (a) Distribution of Xlsirts RNA in a disc-shaped region at the vegetal pole of a
stage 2 oocyte revealed by whole-mount in situ hybridization. (b) Distribution of Xcot-2 RNA in a stage 2 oocyl:e. (c) Distribution of
Vgl RNA throughout the vegetal hemisphere of stage 4 oocytes. Oocytes used in these experiments are albino and therefore lack
animal hemisphere pigmentation. (Images courtesy of M. Kioc and L. Etkin, Houston, Texas, USA.)