Polarized Dishevelled dissolution and reassembly drives embryonic axis specification in sea star oocytes

Lineage tracing shows that cell size asymmetries predict the dorsoventral axis in the sea star embryo

“…Interestingly, Maruyama and Shinoda find that all isolated blastomeres at the 2-and 4-cell stages, and all of the vegetal blastomeres isolated at the 8-cell stage, form bipinnaria, although they do not discuss the timing of these events. Our results also align with recent experiments showing that the vegetal-most portion of cytoplasm in the oocyte is necessary for gut induction in P. miniata [12]. Therefore, P. miniata is similar to most other echinoderms analyzed so far in that differential allocation of maternal determinants is involved in the determination of the AP axis, but not strictly necessary for the determination of the DV axis.…”

“…The fact that both cell volumes and cell fate determinants are asymmetrically partitioned during the 4th cell division makes the micromeres the first morphological hallmark of the antero-posterior (AP) axis, with the micromeres positioned at the future posterior side of the embryo [2,9,10]. While cell fate determinants responsible for AP axis formation are also localized in the vegetal region of other echinoderm embryos, such as sea stars [11,12] and brittle stars [13], cleavage is approximately equal in those species [14]. However, more subtle cell size asymmetries have been observed in the early embryos of asteroid sea stars [15], sea cucumbers [16] and feather stars [17], raising the questions of i) how consistent and stereotypical cell size asymmetries are and ii) if cell size asymmetries are relevant to the establishment of embryonic axes in echinoderm embryos other than the sea urchin.…”

“…The animal-vegetal axis is established during oogenesis: the germinal vesicle is asymmetrically positioned in the immature oocyte, predicting both the site of polar body extrusion and the anterior side of the embryo [15]. Therefore, the AP axis of the sea star embryo can be identified already in the oocytes and its establishment is thought to depend on the asymmetric localization of maternal determinants [12,15,20,21]. The first morphological hallmark of DV axis formation, instead, can be detected only at 3 days post fertilization (dpf ), when the archenteron joins the anterior ectoderm to form the mouth on the ventral side of the embryo [15,20].…”

Cell size asymmetries in the sea star embryo

“…The animal-vegetal axis is established during oogenesis: the germinal vesicle is asymmetrically positioned in the immature oocyte, predicting both the site of polar body extrusion and the anterior side of the embryo (Kominami, 1983). Therefore, the AP axis of the sea star embryo can be identified already in the oocytes and its establishment is thought to depend on asymmetric localization of maternal determinants (Kominami, 1983; Kuraishi and Osanai, 1992;Swartz et al, 2021; Wanninger, 2015). The first morphological hallmark of DV axis formation, instead, can be detected only at 3 days post fertilization (dpf), when the archenteron joins the anterior ectoderm to form the mouth on the ventral side of the embryo (Kominami, 1983;Wanninger, 2015).…”

“…The fact that both cell volumes and cell fate determinants are asymmetrically partitioned during the 4th cell division makes the micromeres the first morphological hallmark of the antero-posterior (AP) axis, with the micromeres positioned at the future posterior side of the embryo (Cameron et al, 1987; Hörstadius and Horstadius, 1973; Ruffins and Ettensohn, 1996). While cell fate determinants responsible for AP axis formation are also localized in the vegetal region of other echinoderm embryos, such as asteroid sea stars (Maruyama and Shinoda, 1990;Swartz et al, 2021) and brittle stars (Primus, 2005), cleavage is approximately equal in those species (Arnone et al, 2015). However, more subtle cell size asymmetries have been observed in the early embryos of asteroid sea stars (Kominami, 1983), holothuroids (Holland, 1981) and crinoids (Mladenov, 1983), raising the questions of i) how consistent and stereotypical cell size asymmetries are and ii) if cell size asymmetries are relevant to the establishment of embryonic axes in echinoderm embryos other than the sea urchin.…”