Non-Haeckelian Development

Some fun weirdness in the tangled “tree of life.”  Donald Williamson, a marine biologist who wondered why very different larval forms pop up willy-nilly in very different species, has a bold hypothesis.  Although his explanation is very heterodox (though it received serious support from Lynn Margulis) and is yet to be supported by any molecular data, what is much more fascinating is the extent of the problem his theory aims to solve, including the following oddities:

1. Very similar larval forms appear in phyla with dissimilar adult body plans.
2. Wholly different larval forms appear in the same phylum
3. Some echinoderms are develop as protostomes, some as deuterostomes. 
4. Some mollusks, annelids, sipunculids, and even some deuterostomes have rotifer-like trochophore larvae.

These larval variations not only challenge the Haeckelian expectations of neo-Darwinism, but they potentially put the whole protostome/ deuterostome division in question.  Here’s Williamson’s attempt to fit these larval epicycles into a Darwinistic history:

   … Brittle stars and sea urchins are very different as adults but share the unique pluteus form of larva. Why would similar larvae produce dissimilar adults [in different phyla]? Larval transfer proposes that the basic pluteus larva evolved only once, in a sea urchin, and was retained in this sea urchin’s descendants.  An ancestor of most existing brittle stars then acquired a pluteus larva by hybridizing with a sea urchin. 

    One member of this group, Kirk’s brittle star, develops directly from the fertilized egg, with no trace of a bilateral larva, and its blastopore becomes a mouth. It is, therefore, a protostome.  All echinoderm larvae are in a different developmental class, the deuterostomes, in which the mouth is a new opening; several echinoderms that have lost their larvae develop as deuterostomes. If Kirk’s brittle star lost its larva, it must have also adopted a fundamentally different pattern of cell division in the embryo. We believe, however, that Kirk’s brittle star has no larva because none of its ancestors hybridized with a sea urchin. The heart urchin Abatus cordatus and the three known species of sea daisies also have no larvae and develop as protostomes.  We suggest that this is the ancestral method of echinoderm development, and the pattern called deuterostomy came with the transferred larvae. 

  Two sea urchins, Lytechinus variegatus and Lytechinus verruculatus, are of the same genus, but the similar adults each develop from very different pluteus larvae. Such cases are difficult to explain if larvae and their corresponding adults evolved from one common ancestor.

 

It’s pretty curious stuff.