Results and Problems in Cell Differentiation

Results and Problems in Cell Differentiation A Series of Topical Volumes in Developmental Biology 13 Editors W. Hennig, Nijmegen and 1. Reinert, Berlin

Germ Line - Soma Differentiation Edited by W. Hennig With 34 Figures Springer-Verlag Berlin Heidelberg GmbH

Professor Dr. WOLFGANG HENNIG University of Nijmegen, Department of Genetics Faculty of Sciences Toernooiveld, 6525 ED Nijmegen The Netherlands ISBN 978-3-662-21958-4 DOI 10.1007/978-3-540-39838-7 ISBN 978-3-540-39838-7 (ebook) Library of Congress Cataloging in Publication Data. Germ line- soma differentiation. (Results and problems in cell differentiation; 13). Includes bibliographies and index. 1. Cell differentiation. 2. Germ cells. 3. Somatic cells. I. Hennig, Wolfgang, 1941-. II. Series. [DNLM: 1. Cell Differentiation. 2. Germ Cells- cytology. 3. Hybrid Cells- cytology. W1 RE248X v. 13/ WQ 205 G3724] QH607.R4 vol. 13 574.87'612s [574.87'612] 86-10145 [QH607] This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically those of translation, reprinting, re-use of illustrations, broadcasting, reproduction by photocopying machine or similar means, and storage in data banks. Under 54 of the German Copyright Law, where copies are made for other than private use, a fee is payable to the publisher, the amount of the fee to be determined by agreement with the publisher. by Springer-Verlag Berlin Heidelberg 1986 Originally published by Springer-Verlag Berlin Heidelberg New York in 1986 Softcover reprint of the hardcover 1st edition 1986 The use of registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. 2131/3130-543210

The authors and the editor wish to dedicate this volume to the memory of SIGRID HEERMANN in appreciation of her pioneering cytological studies of chromatin elimination in Copepods

Preface One of the oldest problems in developmental biology is the differentiation between germ line and somatic cells. The continuity of germ line cells between subsequent generations of multicellular organisms was first suggested by Owen, and later elaborated by A. Weismann to his famous germ line theory. His additional assumption that cellular differentiation was based on a differential representation of the genetic material in somatic cells was soon disproved. In some, apparently exceptional, cases, however, such differences in the genetic material between germ line and somatic cells were discovered. The best-known example is the nematod Ascaris. Boveri discovered and studied the fundamental differences in the karyotypes of germ line and soma of Parascaris equorum. Later, similar situations were found in some other organisms. However, in particular the work ofspemann demonstrated that cellular differentiation in general is not accompanied by fundamental changes of the genetic material. Subsequently, the relatively few examples of germ line-soma differences achieved by chromatin elimination processes have been considered as a curiosity. Experimental studies have been essentially restricted to Ascaris species and to the pioneering cytological studies of chromatin elimination by S. Beermann. Despite the large proportions of the genome involved in chromatin elimination, our knowledge of this process is still very restricted. In particular the biological meaning of this differentiation process is entirely obscure. In this context one must, however, consider that also for the majority of DNA sequences in eukaryotic genomes the biological relevance is unclear. It is therefore not unlikely that an understanding of the biological role of chromatin elimination might be indicative for the biological role of those DNA sequences not belonging to the protein coding DNA fraction. In this volume germ line-restricted parts of the genome are considered from different points of view. It is attempted to give a representative account of the known biological features of this portion of the genome. In a final chapter I have tried to provide some ideas which might correlate germ line-restricted DNA sequences to a more universal component of eukaryotic genomes, heterochromatin, and to introduce some ideas on the possible biological role of this part of the genome. I should like to thank the authors of this volume for their commitment and collaboration. Spring 1986 WOLFGANG HENNIG

Contents The Differentiation of Germ and Somatic Cell Lines in Nematodes By H. TOBLER (With 14 Figures) 1 Introduction......... 1 2 The Germ Line Theory..... 2 2.1 Weismann's Germ Plasm Theory. 2 2.2 The Significance of Boveri's Discovery of Chromatin Diminution. 3 2.3 Phylogenetic Aspects of the Establishment of the Germ Line. 5 3 Segregation of Germ Line and Soma Cells.......... 7 3.1 Definitions........................ 7 3.2 Origin of the Germ Line and Experimental Confirmation of Germ Line Determination............... 8 3.3 Ontogenetic Aspects of Germ Line-Soma Segregation. 9 3.4 Possible Mechanisms for Germ Line Determination. 11 4 Mosaic Versus Regulative Development....... 14 4.1 Definitions................... 14 4.2 Experimental Analysis to Define the Type of Determination and Examples for Mosaic and Regulative Development.. 15 5 Chromatin Diminution and Chromosome Elimination 16 5.1 Historical Review and Definitions......... 16 5.2 Occurrence of Chromatin and Chromosome Elimination 18 5.3 Cytological Observations of the Chromatin Elimination Process in Nematodes........................ 23 5.4 Cytoplasmic Factors and Cytochemistry........... 28 6 Functional Aspects of Chromatin Diminution and the DNA Sequences Restricted to the Germ Line................. 29 6.1 Experimental Analysis of Chromatin Diminution in Parascaris.. 29 6.2 Hypothesis Concerning the Induction or Prevention of Chromatin Diminution in Parascaris................. 31 6.3 Functional Aspects of Chromatin and Chromosome Elimination 32 6.4 Possible Function of the Germ Line-Specific Genetic Material 33 7 Amount of Eliminated DNA............ 35 7.1 Evaluation of the Different Methods for the Quantitative Determination of Eliminated DNA.......... 35 7.2 Different Amounts of Eliminated DNA in Ascaris lumbricoides var. suum and Parascaris equorum................. 36

X Contents 7.3 Comparison of the Quantity of Eliminated DNA in Different Taxonomic Groups.................. 8 Molecular Characterization of the Eliminated DNA Sequences. 8.1 Density Gradient Analysis........... 8.2 DNA Renaturation Kinetics........... 8.3 Elimination of rrna and/or trna Coding Genes? 8.4 Elimination of Satellite DNA Sequences..... 8.5 Elimination of Nonsatellite DNA Sequences.... 8.6 Interspersion of Repetitive and Unique DNA Sequences in the Germ Line and Somatic Genome......... 48 9 The Mechanism of Chromatin Elimination......... 49 9.1 Cytological and Electron Microscopical Observations..... 49 9.2 Molecular Approach to Analyze the Process of Chromatin Diminution 50 10 Chromosome and Chromatin Elimination Compared to Other Phenomena that Lead to Genome Alterations.... 10.1 The Dogma of Genetic Identity of Different Cell Types 10.2 Cell Type-Specific Differences in DNA Content 10.3 Gene Rearrangements.. 10.4 DNA Modification........ 10.5 Chromosome Inactivation..... 11 Summary and Concluding Remarks References.............. 37 38 38 40 42 43 46 51 51 53 55 56 56 57 58 Unusual Chromosome Movements in Sciarid Flies By S.A. GERBI (With 12 Figures) 1 Introduction........ 71 1.1 Background........ 71 1.2 The Chromosome Complem~nt 71 1.2.1 The "Ordinary" Chromosomes 71 1.2.2 The Germ Line Limited "L" Chromosomes. 73 1.3 Sex Determination.......... 73 1.3.1 The X' Chromosome......... 73 1.3.2 Role of the Sperm in Sex Determination 74 1.4 Overview of the Chromosome Cycle 76 2 Oogenesis......... 78 3 Spermatogenesis......... 78 3.1 First Meiotic Division in Males.. 78 3.1.1 Cytological Description of Meiosis I 78 3.1.2 Genetic Evidence for Segregation of the Paternal Chromosome Set. 80 3.1.3 Mechanism of Chromosome "Imprinting"........... 81 3.1.4 Mechanism for Chromosome Movement on the Monopolar Spindle 82 3.2 Second Meiotic Division in Males.............. 84 3.2.1 Cytological Description of Meiosis II............ 84 3.2.2 Identification of the Control Locus for Precocious Chromosome Behavior......................... 86

Contents XI 3.2.3 Mechanism for Precocious Chromosome Behavior 88 3.3 Spermiogenesis........ 89 3.3.1 Formation of the Giant Centriole....... 90 3.3.2 Mitochondrial Fusion............ 90 4 Elimination of Chromosomes from the Soma of the Embryo 91 4.1 Fertilization and Early Cleavage Divisions..... 91 4.2 Elimination of the L Chromosomes from the Soma.... 91 4.3 Elimination of the X Chromosome(s) from the Soma.... 92 5 X Chromosome Elimination from the Germ Line of the Embryo. 94 6 Germ Line Limited "L" Chromosomes..... 96 6.1 L Chromosome Elimination from the Germ Line 96 6.2 L Chromosomes and the Sex Ratio 97 6.3 L Chromosome Condensation Cycle...... 98 7 Sex Determination.............. 98 7.1 Exceptional Offspring Among Unisexual Progeny 98 7.2 Bisexual Strains Derived from Monogenic Species 99 7.3 Sex Determination of the Gonad 100 References.................... 101 Molecular Reorganization During Nuclear Differentiation in Ciliates By G. STEINBRUCK (With 8 Figures) 1 Introduction...................... 105 2 A General Description of Nuclear Dualism in Ciliates.... 108 3 Macronuclear Development in "Primitive" Ciliates of the Order Karyorelictida.................... 111 4 Molecular Changes During Macronuclear Development in Hymenostomatid Ciliates 114 4.1 Tetrahymena.................... 114 4.1.1 The Species Problem................. 114 4.1.2 Gross Differences Between Macro- and Micronuclear Genomes 115 4.1.3 Differences in Chromatin Structure and Histone Composition Between Macro- and Micronuclei............ 117 4.1.4 Organization of Macronuclear Genomes......... 118 4.1.5 Amplification, Elimination, and Rearrangement of Individual Sequences....... 120 4.1.5.1 Ribosomal RNA Genes. 121 4.1.5.2 Protein Coding Genes. 123 4.1.5.3 Other Sequences.... 123 4.1.6 Putative Somatic Functions of Micronucleus-Specific Sequences 124 4.1.7 Concluding Remarks. 125 4.2 Glaucoma..... 126 4.3 Paramecium..... 126 4.3.1 The Species Problem. 127 4.3.2 Indications of Molecular Reorganization During Macronuclear Development.................... 127

XII Contents 40303 403.4 5 5.1 502 503 5o3.1 50302 50303 5o3.4 5.4 505 6 6.1 602 603 6.4 7 7.1 702 703 8 801 801.1 801.2 8o1.3 801.4 802 803 804 805 806 9 Putative Somatic Functions of Micronuclei 129 Conclusions 0 0 0 0 0 0 0 0 0 0 0 0 0 0 129 Nuclear Differentiation in Hypotrichous Ciliates 130 The Species Problem 0 0 0 0 0 0 0 0 0 0 0 0 130 Peculiarities of the Macronuclear Development in Hypotrichs 131 Molecular Differences Between Macro- and Micronuclear Genomes 134 DNA Content, Base Composition, and Molecular Weight of DNAs 134 Terminal Repeats 0 0 0 0 0 0 0 0 138 Comparison of Individual Sequences 0 0 0 0 0 0 0 0 0 0 0 0 0 139 Chromatin Structure and Histones 0 0 0 0 0 0 0 0 0 0 0 0 0 0 140 Putative Somatic Functions of the Germ Line Nuclei ofhypotrichs 141 Concluding Remarks 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 142 Indications for Molecular Differences Between Macro- and Micronuclear Genomes in Other Groups of Ciliates Bursariomorphida Cyrtophorida 0 Suctorida 0 0 0 0 Heterotrichida 0 0 Origin and Evolutionary Trends of Nuclear Differentiation Origin of Nuclear Dualism 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Trends in the Evolution of Nuclear Differentiation in Ciliates Did the Complex Features of Nuclear Differentiation Arise Several Times Independently Among Ciliates? 0 0 0 0 0 0 0 0 0 0 0 0 Putative Functions of Molecular Reorganization Processes During Macronuclear Development 0 0 0 0 0 0 0 o, 0 0 0 0 0 0 0 0 The C-Value Paradox 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Did Ciliates Receive a Specific Solution of the Problems Raised by the Micronuclear C-Value Paradox? The Skeletal DNA Hypothesis 0 0 0 0 0 The Selfish DNA Hypothesis 0 0 0 0 0 Elimination of Pseudogenes and Introns Nuclear Differentiation and Gene Activation Differential Regulation of Replication 0 0 Other Putative Regulatory Functions 0 0 0 Putative Functions of Giant Chromosomes Protection Against Genetic Damages 0 0 0 Are Differences Between Germ Line and Soma Genomes in Ciliates of General Importance for Comparable Phenomena in Multicellular Eukaryotes? 0 References 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 142 142 143 144 144 145 146 147 149 150 151 151 152 153 155 156 157 159 160 161 162 165 Heterochromatin and Germ Line-Restricted DNA By Wo HENNIG 1 What is Heterochromatin 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 176 2 Heterochromatic Appearance Reflects Functional Inactivity 0 177

Contents XIII 3 Heterochromatin Is Not Exclusively Composed of Highly Repetitive DNA....................... 180 4 TheY Chromosome as Model for Heterochromatin Structure.... 181 5 Are There Possible Germ Line-Related Functions of Heterochromatin? 182 5.1 Protein Coding and Protein Binding............... 182 5.2 Has Heterochromatin a Target Functions for Transposable Elements? 186 Concluding Remarks 188 References.. 189 Subject Index 193