Schmidtea mediterranea reproduction
Sexually reproducing Smed are cross-fertilizing hermaphrodites that contain a pair of ovaries, situated immediately posterior to the brain and adjacent to the ventral nerve cords, and numerous testes located along the dorsolateral flanks of the animal. Oocytes are fertilized internally by sperm from a partner as they enter the oviducts. Smed embryos are ectolecithal: yolk is not contained within oocytes, but rather is produced by somatic vitellaria (yolk glands) arrayed ventrolaterally beneath the testes (Chong et al., 2011; Steiner et al., 2016; Stevens, 1904). One or more zygotes are packaged, along with yolk cells, into an egg capsule in the genital atrium (Chong et al., 2011; Hyman, 1951; Newmark et al., 2008; Stevens, 1904). Egg capsules are laid through the gonopore. Smed embryos gestate in egg capsules for approximately two weeks at 20˚C prior to hatching.
For more information on Smed hermaphrodite anatomy and gametogenesis, please visit the Newmark and Rouhana lab websites.
Smedflatworms are direct developers: newborn hatchlings grow and mature into adult worms without an intervening larval stage (Sánchez Alvarado, 2003). At hatching, juveniles are sexually immature but otherwise possess a body plan grossly similar to the adult hermaphrodite (Sánchez Alvarado, 2003; Wang et al., 2007).
Smedembryos undergo an evolutionarily divergent mode of development that bears little resemblance to the ancestral Spiralian cleavage programs utilized by many Lophotrochozoans. In contrast to the synchronous, oriented blastomere cleavage patterns of Spiralian embryos (Lambert, 2010), blastomeres in freshwater planarian embryos undergo dispersed cleavage among yolk cells: they divide asynchronously and are not in direct contact with one another (Bardeen, 1902; Cardona et al., 2005; Hallez, 1887; Ijima, 1884; Le Moigne, 1963; Metschnikoff, 1883; Vara et al., 2008).
Sphere formation and Embryo Architecture
During sphere formation, some blastomeres differentiate into temporary embryonic cell types that provide form and function to the embryo, including the primitive ectoderm, temporary embryonic pharynx and primitive gut (Cardona et al., 2005; Hallez, 1887).
Primitive ectoderm cells are the first to differentiate, forming a single cell layer bounding the sphere (Hallez, 1887; Ijima, 1884; Le Moigne, 1963; Metschnikoff, 1883). The temporary embryonic pharynx is an innervated pump containing neurons, radial muscle and associated epithelial cells that ingests yolk into an inner gut cavity. The primitive gut consists of the inner gut cavity and phagocytic cells associated with the temporary embryonic pharynx. Temporary embryonic tissues are not thought to contribute to the juvenile body plan; they are thought to degenerate as the definitive organs form and morphogenesis proceeds (Cardona et al., 2005; Le Moigne, 1963; Vara et al., 2008).
A population of undifferentiated blastomeres and yolk cells remain in the embryonic wall, the parenchymal space between the primitive ectoderm and endoderm, in nascent spheres (Hyman, 1951; Sánchez Alvarado, 2003). These undifferentiated blastomeres are thought to give rise to all definitive tissues found in juvenile worms (Hallez, 1887; Hyman, 1951; Le Moigne, 1963; Sánchez Alvarado, 2003; Stevens, 1904). Definitive organ formation and morphogenesis occur during the second week of embryogenesis.
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Steiner, J.K., Tasaki, J., and Rouhana, L. (2016). Germline Defects Caused by Smed-boule RNA-Interference Reveal That Egg Capsule Deposition Occurs Independently of Fertilization, Ovulation, Mating, or the Presence of Gametes in Planarian Flatworms. PLoS Genet 12, e1006030.
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