Phylogenetics of Aplanulata (Cnidaria: Hydrozoa) and the evolution and development of Ectopleura larynx

Abstract

The model organism Hydra belongs to the hydrozoan clade Aplanulata. Despite being a popular model system for diverse fields of biological research, the morphology and development of Hydra are atypical of most hydrozoans. For example, most hydrozoans develop gonophores (structures housing gametes) on the body of the polyp, or release free-swimming medusae that spawn in the water column. In contrast, Hydra produce no gonophores or medusae and instead form gametes directly in the epithelia of the body column. Additionally, Hydra embryos are difficult to isolate for developmental studies (embryos encyst and are thus difficult to study), so there is currently no model species in Aplanulata for examining gene expression in developing polyps. In this dissertation, I examine the phylogenetic relationships of Aplanulata and the clade Capitata sensu stricto, originally thought to group with Aplanulata, and examine the evolution and development of the Aplanulata species Ectopleura larynx. This close relative of Hydra is ideally suited for evolutionary developmental studies because it develops directly in brooding structures, and produces attached gonophores. Because Ectopleura larynx broods on the body of the polyp, its juveniles and gonophores are easily procured for gene expression and developmental studies. My examination of Ectopleura larynx development reveals a unique type of colony formation that has never before been described in Hydrozoa in that Ectopleura larynx colonies form through sexual reproduction followed by epithelial fusion of offspring polyps to adult colonies. I characterize the expression of the paired-like homeobox gene manacle to determine polyp-colony boundaries, and suggest that stalks beneath the neck of Ectopleura larynx polyps do not have polyp identity and instead are specialized structures that interconnect polyps (stolons). Lastly, I characterize the canonical Wnt pathway in Ectopleura larynx, and examine its role in axial patterning of polyp and gonophore structures. My results are consistent with the Wnt pathway playing a role in patterning oral structures of the polyp and gonophore, and suggest that changes in expression patterns of Wnt pathway genes could explain the sexually-dimorphic morphologies of male and female gonophores of Ectopleura larynx, and the truncation of medusa development in this species.