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An important feature that has played a large role in the quest to place annelids within the system of the Metazoa is their development. They undergo spiral cleavage, a presumed highly-conserved form of development that is also found in molluscs, sipunculids, echiurans, nemerteans, myzostomids, ectoprocts, some flatworms and maybe gnathostomulids, and some have used spiral cleavage as an autapomorphy to unite these taxa under a single monophyletic Spiralia taxon (first proposed by Schleip in his 1929 book “Die Determination der Primitiventwicklung”). We will look at this in more detail in the next post.
I will not attempt to describe spiralian cleavage in my own words, so this paragraph may violate copyright laws – it’s mostly derived from the caption of that picture. Please don’t sue me. The first two cleavges (cell divisions) occur at right angles to the animal-vegetal axis (animal = anterior; vegetal = posterior) and divide the emryo into four quadrants: A, B, C and D, whereby D is most of the time larger than the rest (but not always). The picture starts at the third cleavage. At the animal pole, each quadrant cleaves off one micromere (a1, b1, c1 and d1). These micromeres then cleave off their own micromeres (a2, b2, c2, d2), and so on. In annelids (and molluscs), bilateral symmetry is established at the d4 stage, when d4 divides into two mesoteloblasts, one on the left and another on the right. The designation ‘spiralian cleavage’ comes from the fact that each micromere quarter is rotated in respect to the parent blastomere, as indicated by the arrows in (A). (B) shows a fate map for a nemertean; (C) and (D) for two different annelids, and they basically show that later on, significant differences in the morphology of the embryo arise, even though they all come from the same original type of cleavage.
One of the key characteristics of spiralian cleavage is the formation of coelomic cavities derived from the mesoderm – this is called schizocoely and is what underlies segmentation in annelids (after all, these paired mesodermal sacs, or coeloms, are repeated serially along the anterior-posterior axis).
One thing that must be noted, for those of you who are interested in further reading about annelid development, is that the majority of the research has traditionally focused on the development of a few model Clitellata. This is unfortunate, as the clitellates are derived and don’t represent the ancestral annelidan state. They don’t even have a trochophore larva. The situation is being rectified with studies on polychaete development – keeping in mind, of course, that polychaetes are most likely paraphyletic; nevertheless, they provide a much closer picture of what the ancestral annelid looked like.
In planktonic annelidan trochophore larvae, muscles develop before segments, starting with those of the digestive tract, followed closely by those controlling the cillia – the two most basic needs of a trochophore larva are, after all, moving and feeding. In the direct-developing clitellates, the body wall musculature is the first to differentiate. In the planktonic larvae, those begin to form when the hyposphere begins to elongate. It must be mentioned that the musculature of polychaetes is significantly more complex than the clitellates’, but as mentioned already, there is a lack of research.
Spiral cleavage is not as highly-conserved as is always emphasised. If we accept the Spiralia taxon, then it has been completely lost in the cephalopods and parasitic flatworms. There are also many variations of it within the Spiralia. The first cell division can be either equal or unequal; in the latter case, C and D are considerably larger than A and B. This is significant, as the D quadrant is what later gives rise to the mesoderm (although in at least some annelids, it only has a minor contribution there), and the size of the quadrants affects specification patterns. In those that cleave equally, the D quadrant has to be induced (chemical signals allow it to differentiate); in unequal cleavers, the D quadrant is already induced by the cell division, hinting that something within the cell is already at work.
Shankland, M. & Seaver, E. C. 2000. Evolution of the bilaterian body plan: What have we learned from annelids? PNAS 97, 4434-4437.