Development of the ctenophore Beroe

Below are pictures of early embryos of the ctenophore Beroe sp. Three adults were collected around Charleston docks near OIMB in mid-May and shed strings of ~ 220 µm eggs in the lab. 

Ctenophore embryos are beautifully clear, and possess a unique type of development. Like cnidarians, ctenophores have unilateral cleavage, which means that the cleavage furrow cuts in from one pole of the egg and gradually proceeds to the other side (see a video of cleaveage in another species of Beroe by George von Dassow). Ctenophore cleavage is stereotypical (all embryos exhibit the same series of highly ordered divisions), and the same cells consistently give rise to certain structures in the adult. Ctenophores exemplify mosaic development, which means that cell fates are determined early in development by inheritance of different regions of the egg’s cytoplasm which possess different properties. As a result, with few exceptions, when specific blastomeres are damaged, they are not replaced; and the embryo lacks the structures these blastomeres normally give rise to (Martindale & Henry 1999). 

This is a gastrula of the same species of Beroe. Ctenophores undergo gastrulation by a variety of mechanisms simultaneously, including epiboly, delamination, invagination, and involution. At the end of gastrulation large cells, called macromeres, end up surrounded by small cells called micromeres (see another video by George von Dassow). 

These embryos developed into little juvenile Beroe, pictured here, with beautiful red pigment granules in the epidermis. Another curious fact about the development of Beroe is that apparently, at least one species, Beroe ovata, can tolerate naturally occurring polyspermy (Carré & Sardet 1984). Polyspermy is a condition in which more than one sperm enter the egg. In most animals polyspermic embryos develop abnormally. However, Carre & Sardet (1984) demonstrated that in Beroe ovata the female pronucleus appears to preferentially select a sperm pronucleus with which to bind. The female pronucleus visits each sperm pronucleus, returning to the site of formation of polar bodies in between visits, and finally fuses with one of them (see video on Christian Sardet's website). The other male pronuclei are destroyed and do not participate in development which proceeds normally. 

Carré, D., Sardet, C. (1984). Fertilization and Early Development in Beroe ovata. Developmental Biology 105: 188-195. 

Martindale, M.Q., Henry, J.Q. (1999). Intracellular fate mapping in a basal metazoan, the ctenophore Mnemiopsis leidyi, reveals the origins of mesoderm and existence of indeterminate cell lineages. Developmental Biology 214: 243-257.