Date of Award

2001

Level of Access

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Zoology

Advisor

Seth Tyler

Second Committee Member

Mary S. Tyler

Third Committee Member

Irving Kornfield

Abstract

The systematics of turbellarian platyhelminths (also known as free-living flatworms) has proven difficult since few taxonomically useful characters can be discerned in them. Generally, features of the reproductive tract, observed through conventional light microscopy, provide key taxonomic characters. Through the newer techniques of electron microscopy and fluorescence microscopy, new characters are emerging that provide better clues to the phylogenetic relationships of these animals. We have applied both of these microscopies to representatives of two groups of turbellarians whose phylogenetic positions are uncertain and controversial, the Acoela and the Genostomatidae. Because fluorescence microscopy of phalloidin-labeled acoel turbellarians has provided new taxonomically relevant characters in the arrangement of muscles, we studied another feature that has emerged in such phalloidin-labeled preparations, namely sensory receptors. By correlating electron micrographic images and fluorescence images of Convoluta pulchra, we determined that these structures are sensory receptors with a central cilium surrounded by a collar of microvilli. The collared sensory receptors were inserted between epidermal cells, and each bore a central cilium surrounded by a collar of 6-18 microvilli and an additional centrally positioned 2-7 microvilli of which 2 or 3 were associated with a modified rootlet called the swallow's nest (Bedini et al. 1973). Confocal scanning laser microscopy resolved the core of actin filaments within the microvilli of the collar and their rootlet-like connections to the base of the sensory cell. Many other acoels have similar receptors in various patterns of distribution across the epidermis, and so further study of these organs in these other species may provide meaningful characters for deciphering phylogenetic relationships. Whereas assignment of Convoluta pulchra to the Acoela is straightforward, another species, Genostoma kozlofSi cannot be readily assigned to any higher taxon within the Rhabditophora. Some have proposed that the family in which it is classified, the Genostomatidae, is part of the sister group to the Neodermata, the major group of parasitic flatworms, and so stands as a representative of the link between turbellarians and parasitic flatworms. Characters of spermiogenesis and spermatozoa have been used by others to distinguish between turbellarians and the Neodermata. By applying electron microscopy to elucidate the process of spermiogenesis in Genostoma kozlofl, we found that its sperm are more like those of the turbellarians than those of the Neodermata. Sperrniogenesis in Genostoma kozlofSi occurs in a distal-proximal fashion, just as in those free-living turbellarians in which the axoneme is incorporated into the body of the sperm. Mature spermatozoa are filiform, possess an elongate rod-like nucleus and one short single, fully incorporated, axoneme. A rod of multiple, fused mitochondria accompanies the nucleus and axoneme and an array of cortical microtubules with thickened walls are present. In this study more evidence in sperm and spermiogenesis, linking Genostoma kozloffi to Kalyptorhynchia, more specifically to Schizorhynchia, was found.

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