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Table 1 Comparison of inversion in V. globator and V. carteri.

From: There is more than one way to turn a spherical cellular monolayer inside out: type B embryo inversion in Volvox globator

Characteristic

V. carteri

V. globator

Type of inversion

A

B

Duration of inversion

Approximately 45 min [20, 22]

Approximately 50 min

Sequence of processes

The phialopore opens;

the anterior hemisphere inverts;

the posterior hemisphere contracts;

the posterior hemisphere inverts;

the phialopore closes [20, 22, 26]

The posterior hemisphere contracts;

the posterior hemisphere inverts;

the phialopore opens;

the anterior hemisphere inverts;

the phialopore closes

[25, 36, 37]

Site of the initiation of inversion

Point-shaped initiation site at the anterior pole of the spheroid [22]

Ring-shaped initiation site at the equator of the spheroid

Direction of cell shape changes

From the anterior pole to the posterior pole [22]

Initially, from the equator to the posterior pole; then, from the equator to the anterior pole

Diameter of embryo

Immediately before inversion: approximately 65 μm [22]

immediately after inversion: approximately 60 μm [22]

Immediately before inversion: approximately 80 μm

Immediately after inversion: approximately 60 μm

Relative size (volume) of the cells of the embryo during inversion

Reproductive cells are roughly 100 to 200 times larger than somatic cells [22]

Reproductive cells are roughly one to five times larger than somatic cells

Cell shape immediately before inversion

All cells are spindle shaped [20, 22]

All cells are teardrop shaped

Cell shape in bend region during inversion

Flask-shaped cells

(radially symmetrical at the chloroplast ends) [20, 22, 24]

Paddle-shaped cells (wedge shaped at the chloroplast ends)

Cell shape of cells that have passed the bend region

Column-shaped cells [20, 22]

Pencil-shaped cells (the column shape does not appear until inversion is completed)

Order of cell shape changes

All cells undergo the same consecutive cell shape changes: spindle → flask → column [20, 22, 24]

Posterior and anterior hemispheres invert with different cell shapes changes;

cells in the posterior hemisphere: teardrop → spindle → paddle (except for the cells close to the posterior pole) → pencil → column;

cells in the anterior hemisphere: teardrop → disc → pencil → column

Shape of nuclei throughout inversion

Minor changes [20]

Major changes

Cytoplasmic bridges

Only between cells of the embryo [20, 22, 24, 29]

Between cells of the embryo and between embryonic and parent cells

Phialopore

Swastika-shaped opening; four lips of the cell monolayer curl outward and backward [20, 22]

Circular opening; while the opening widens, the cells and cytoplasmic bridges around the phialopore become passively stretched

Denting

Only before inversion [20, 26, 39]

Before and after inversion

Flagella

Only flagellar stubs approximately 2 μm in length that do not grow during inversion; flagella grow asymmetrically after inversion [76]

Flagella grow symmetrically throughout inversion