The developmental basis for the evolution of new morphologies in Spiralia

Abstract

Spiralia is a clade of bilaterally symmetrical animals, which display an extreme diversity of divergent body plans accompanied by the presence of multiple subclades- specific morphological and developmental characters. This vast diversity makes them an interesting model clade for the investigation of the evolution of new morphological features. In my work, I combined evolutionary developmental biology and comparative morphology to study several aspects of the morphological evolution in Spiralia. In the first part of the project I studied morphology of the nervous system in females and dwarf males of the monogonont rotifer Epiphanes senta in order to test what changes in the ancestral development contributed to the emergence of the monogonont-specific dwarf form. The obtained results suggest that monogonont dwarf males evolved by heterochronic modification of the ancestral development, either through process of progenesis or proportional dwarfism. The second part of the project was focused on the expression of Hox genes, the well- studied family of homeodomain transcription factors, during development of two lophophorates – the brachiopod Terebratalia transversa and the phoronid Phoronopsis harmeri. Both of those species lack a classical collinear expression of Hox genes and despite being closely related, they vary greatly in the Hox expression patterns. In the brachiopod, several of the Hox genes are expressed in apomorphic structures (such as shell forming epithelium or chaetal sacs), while the phoronid larva develops without expressing any of the Hox genes, which instead are expressed only in the rudiment of the adult worm. These observations show that Hox genes might play and important role in the emergence of evolutionary novelties in lophophorates and suggest that the new larval type was likely intercalated into the ancestral life cycle of phoronids. In the last part of the project, I focused on the evolution of excretory organs present in Spiralia. By comparing the morphology and gene expression profiles in the developing nephridia of the two aforementioned lophophorates and the annelid Owenia fusiformis, as well as other non-spiralian invertebrates (onychophoran, priapulid and hemichordate), I demonstrated the presence of conserved set of transcription factors and structural genes expressed in the excretory organs with very diverse morphologies. Although the gross morphology of investigated nephridia differs markedly, they all share basic physiology (ultrafiltration) and certain ultrastructural properties. These similarities, combined with gene expression results and distribution of nephridial types on phylogenetic tree, strongly support homology of all ultrafiltration-based excretory organs. By combining comparative morphology, gene expression studies and phylogenetic framework, I provided new insight into evolution of various morphological and developmental characters in the clade Spiralia, showing that consolidation of those fields is crucial for a better understanding of the animal evolution.