Segment Development in Owenia

Detailed developmental descriptions in two species of the genus OweniaO. fusi- formis and O. collaris—are currently available (D.P. Wilson 1932a; Smart and Von Dassow 2009; Helm et al. 2016). Owenia spp. lay oligolecithal eggs of between 70 and 120 pm that cleave following the usual spiralian pattern. Embryos develop into a bell-shaped planktotrophic larva known as mitraria (Figure 4.4A). Mitraria larvae have a U-shaped larval gut whose ends are separated by a small region of ventral ectoderm. The gut is covered by a conical episphere with an apical organ on top. They harbor a pair of larval chaetal sacs that secrete two bundles of characteristic needle-like bristles. During larval life, this ventral region invaginates, forming a pocket that comes to surround the hindgut and becomes the trunk rudiment. As this rudiment grows, it protrudes from under the episphere (Figure 4.4B). Metamorphosis of larvae into juveniles is drastic and takes places over few minutes. The larval blastocoelic cavity deflates, the epithelium is orally invaginated and incorporated to the head of the juvenile, and the ventral pocket is evaginated, so that the anterior end narrows to match the width of the trunk. Ciliary bands and chaetal sacs disappear and larval bristles are shed.

Early studies of larval development in Owenia fusiformis mitraria (D.P. Wilson 1932a) state that trunk mesoderm derives from a single pair of M cells located adjacent to the larval anus, below the angle formed between the endoderm and the posterior ectoderm (Figure 4.5A). These cells “give rise in the manner of teloblasts” to paired mesodermal blocks with an increasing number of cells (Figure 4.5B). As each block grows in cell number, an internal cavity forms inside, giving rise to the mesodermal coelom (Figure 4.5C).[1] Walls of consecutive blocks fuse to form intersegmental septa. At the same time, the ring of ectoteloblasts generates a layer of ectoderm over the developing mesoderm. In contrast to the mesoderm, this ectoderm is not initially obviously divided in segmental units; segmental demarcation appears a bit later, as mesodermal segments develop (Figure 4.5C). While this suggests that modulation of ectodermal segment formation by underlying mesoderm (as described for clitellates, see Chapter 7) might be a general annelid feature, there are many examples of groups where ectodermal segmental boundaries form before mesodermal segments become evident (Anderson 1973). Since during lineage-driven segmentation mesoderm cells are born with segmental identity, it is still possible that they can modulate the overlying ectoderm before they become obviously segmented.

  • [1] In a more recent study of O. collaris (Smart and Von Dassow 2009), the segmentally iterated structuresdescribed as mesodermal somites by Wilson (1932a) are interpreted as nephridial precursors, “derivedfrom the ventral epidermis of the larva.” While this interpretation clearly contradicts Wilson’s (1932a)and Anderson’s (1973) descriptions of teloblastic segmental mesoderm formation in O. fusiformis, noadditional evidence is presented, in contrast to the detailed series of histological snapshots providedby Wilson. However, given that no cell-tracing experiments were conducted in either study, it is notpossible at this time to reject either interpretation.
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