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Your Unpublished Thesis, Submitted for a Degree at Williams College and Administered by the Williams College Libraries, Will Be Made Available for Research Use WILLIAMS COLLEGE LIBRARIES COPYRIGHT ASSIGNMENT AND INSTRUCTIO S FOR A STUDENT THESIS Your unpublished thesis, submitted for a degree at Williams College and administered by the Williams College Libraries, will be made available for research use. You may, through this form, provide instructions regarding copyright, access, dissemination and reproduction ofyour thesis. 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In addition, I1we grant permission to Williams College to provide access to (and therefore copying of) the thesis in electronic format via the Internet or other means ofelectronic transmission after a period of years. Selecting this option allows the Libraries to transmit the thesis in electronic format via the Internet after a period of years. Once the restriction period has ended, this option permits worldwide access to the thesis, and copying of the electronic and hardcopy versions. _ I/we grant permission to Williams College to maintain, provide access to, and provide copies ofthe thesis in hardcopy format only, for as long as I/we retain copyright. Selecting this option allows access to your work only from the hardcopy you submit fix as long as you retain cupyright in the work. Such access pertains to the entirety of your work. including any mcdia that it incorporates. Selecting this option allows the Libraries to provide cupies of the thesis to rt~searchers in hardcopy form only, not in electronic format. _ I/we grant permission to Williams College to maintain and to provide access to the thesis in hardcopy format only, for as long as I1we retain copyright. Selecting this option allows access to your work only from the hardcopy you sllblnil for as long as you retain copyright in the \York. Such access pertains to the entirety of your work, including any media that it incorporates. This option cloes NOT permit the Libraries to provide copies of the thesis to researchers. Signatures removed A Paraxis Homolog in Leech by Elizabeth Bailey Brickley Robert M. Savage, Advisor A thesis submitted in partial fulfillment ofthe requirements for the Degree of Bachelor ofArts with Honors in Biology WILLIAMS COLLEGE Williamstown, Massachusetts May 21,2010 2 "AND IT ISA STRANGE THING THAT MOSTOFTHE FEELING WE CALL RELIGIOUS, MOST OF THE MYSTICAL OUTCRYING WHICH IS ONE OF THE MOST PRIZED AND USED AND DESIRED REACTIONS OF OUR SPECIES, IS REALLY THE UNDERSTANDING AND THE ATTEMPT TO SAY THAT MAN IS RELATED TO THE WHOLE THING, RELATED INEXTRICABLY TO ALL REALITY, KNOWN AND UNKNOWABLE. THIS IS A SIMPLE THING TO SAY, BUT THE PROFOUND FEELING OF IT MADE A JESUS, A ST. AUGUSTINE, A ROGER BACON, A CHARLES DARWIN, AND AN EINSTEIN. EACH OF THEM IN HIS OWN TEMPO AND WITH HIS OWN VOICE DISCOVERED AND REAFFIRMED WITH ASTONISHMENT THE KNOWLEDGE THAT ALL THINGS ARE ONE THING AND THAT ONE THING IS ALL THINGS - PLANKTON,A SHIMMERING PHOSPHORESCENCE ON THE SEA AND THE SPINNING PLANETS AND AN EXPANDING UNIVERSE, ALL BOUND TOGETHER BY THE ELASTIC STRING OF TIME. IT IS ADVISABLE TO LOOK FROM THE TIDE POOL TO THE STARS AND THEN BACK TO THE TIDE POOL AGAIN." - ED RICKETTS, THE LOG FROM THE SEA OF CORTEZ Many thanks to Professor Savage, Kris Anderson, and Sophia Sequeira for your guidance and support throughout this academic endeavor. Love and thanks to my inspiring and incredible family. And always, much love to the Penthouse. Writing this thesis challenged me to focus on the infinitesimal in order to understand that which is universal, inspired me to discover for myself the complexity that underlies the simplest functions of life, and ultimately taught me to find joy in the journey that leads to understanding. Thank you all for supporting me on the way. 3 ABSTRACT A fundamental question in evolutionary developmental biology is whether molecular mechanisms of segmentation are conserved across the three segmented phyla: chordates, arthropods, and annelids. Conducting an EST screen with mRNA transcripts enriched during segmentation, Greenberger identified a bHLH gene in the leech, Helobdella robusta, with a high sequence homology to vertebrate Paraxis and Scleraxis genes (2008). In vertebrates, Paraxis and Scleraxis genes are critical for segmentation and mesodermal tissue formation during development. To confirm the identity ofthe candidate Paraxis-like gene in the annelid Helobdella robusta, I constructed a molecular phylogeny of putative Twist-family genes using Bayesian analysis for the evolution ofthe bHLH domains. Then, using whole mount in situ hybridization to characterize the spatial and temporal expression pattern ofthe Paraxis-like gene in the leech Helobdella robusta, I found Paraxis-like is initially present as a maternal gene product and, with the onset of blast cell formation, is zygotically expressed in waves of expression progressing posteriorly along first the mesodermal and then the neuroectodermal lineages. Paraxis-like in Helobdella robusta is the first reported expression of a Paraxis homolog in a Protostome. While Paraxis was previously believed to be restricted to chordates, these data demonstrate that Paraxis is expressed in the mesodermal tissue of Protostome annelids during segmentation and organogenesis. This finding suggests that the mesodermal function of Paraxis originated in the last common ancestor of Bilateria and provides support for a common origin for segmental pattern formation in Bilateria. 4 TABLE OF CONTENTS INTRODUCTION 5 The Model Organism, HelobdelJa robusta 8 The Evolution ofSegmentation 11 A Candidate Gene 12 TWist-family ofTranscription Factors 15 The Role ofParaxis in Somitogenesis 16 The Role ofScleraxis in Somitogenesis 19 Characterization of a Paraxis-like Gene in Leech 20 MATERIALS AND METHODS 21 Animals 21 Phylogenetic Reconstruction 21 in situ Hybridization 22 Imaging 23 RESULTS 24 Molecular Phylogeny 24 Selecting Sequences 24 Sequence Alignment and Phylogeny 37 Spatial and Temporal Expression of Paraxis-like in HelobdelJa robusta 40 Paraxis-like Expression in Early Development 40 Paraxis-like Expression during Gastrulation 42 Paraxis-like Expression Post-gastrulation 43 Paraxis-like Expression during Organogenesis 46 DISCUSSION 48 Evolution of the Paraxis subfamily 48 Paraxis Is Expressed throughout H. robusta Embryonic Development 49 Maternal RNA. 50 Segmental Precursorfor Neuroectodermal and Mesodermal Tissue 51 Implications of Leech Paraxis for Bilateria 54 Comparative Expression 54 Nervous System 56 Hau-Pax3/7 59 A Paraxis Homolog in Leech 61 WORKS CITED 63 5 INTRODUCTION With approximately thirty ofthe thirty-five animal phyla and over one million species, Bilaterians represent the most diverse and abundant classification of animals (Burton, 2008). Critical to the origin of Bilaterians was the emergence of a third germ layer, the mesoderm, in early Metazoans. Germ layers are primary tissue layers that become apparent during the gastrulation of embryogenesis and give rise to all adult tissues and organs (Technau and Scholz, 2003). The development ofthe mesoderm is linked to the origin of a second body axis; this profound innovation led to the transition from radial symmetry to the unique bilateral symmetry for which Bilateria are named (Technau and Scholz, 2003).
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