Reproduction and Development in Echinodermata and Prochordata

Description

Echinoderms and prochordates occupy a key position in vertebrate evolution. The genomes of sea urchin share 70% homology with humans. Researches on cell cycle in sea urchin and phagocytosis in asteroids have fetched Nobel Prizes. In this context, this book assumes immense importance. Echinoderms are unique, as their symmetry is bilateral in larvae but pentamerous radial in adults. The latter has eliminated the development of an anterior head and bilateral appendages. Further, the obligate need to face the substratum for locomotion and acquisition of food has eliminated their planktonic and nektonic existence. Egg size, a decisive factor in recruitment, increases with decreasing depths up to 2,000-5,000 m in lecithotrophic asteroids and ophiuroids but remains constant in their planktotrophics. Smaller ( 110 mm) asteroids generate planktotrophic eggs only. Publications on sex ratio of echinoderms indicate the genetic determination of sex at fertilization but those on hybridization, karyotype and ploidy induction do not provide evidence for heterogametism. But the herbivorous echinoids and larvacea with their gonads harboring both germ cells and Nutritive Phagocytes (NPs) have economized the transportation and hormonal costs on gonadal function. Despite the amazing potential just 2 and 3% of echinoderms undergo clonal reproduction and regeneration, respectively. Fission is triggered, when adequate reserve nutrients are accumulated. It is the most prevalent mode of clonal reproduction in holothuroids, asteroids and ophiuroids. However, budding is a more prevalent mode of clonal reproduction in colonial hemichordates and urochordates. In echinoderms, fission and budding eliminate each other. Similarly, autoregulation of early development eliminates clonal reproduction in echinoids and solitary urochordates. In pterobranchs, thaliaceans and ascidians, the repeated and rapid budding leads to colonial formation. Coloniality imposes reductions in species number and body size, generation time and life span, gonad number and fecundity as well as switching from gonochorism to simultaneous hermaphorditism and oviparity to ovoviviparity/viviparity. Content: Cover Half title Title Copyrights Preface to the Series Preface Acknowledgements Contents Section I Non-Chordate-Deuterostomia Section Ia Echinodermata 1. Introduction 1.1 Taxonomy and Structural Diversity 1.2 Distribution, Locomotion and Dispersal 1.3 Population Density and Microhabitats 1.4 Energy Budget and Reproduction 1.5 Gonad Index and Fecundity 1.5.1 Gonad Index 1.5.2 Fecundity 1.6 Egg Size and its Implications 1.6.1 Maternal Investment 1.6.2 Egg Size and Fertilizability 1.6.3 Endogenous and Exogenous Nutrients 1.6.4 Egg Size Manipulations 1.6.5 Ontogenetic Pathways1.7 Larval Developments and Thyroid Hormones 1.8 Brooding and Viviparity 1.9 Size and Life Span 2. Fisheries and Aquaculture Introduction 2.1 Capture Fisheries 2.2 Aquaculture 2.2.1 Sea Urchins 2.2.2 Sea Cucumbers 3. Sexual Reproduction Introduction 3.1 Sexuality 3.2 Gonochorism 3.2.1 Sex Ratio 3.2.2 Reproductive Systems 3.2.3 Gametogenesis 3.3 Parthenogenesis 3.4 Hermaphroditism 3.5 Reproductive Cycle 4. Asexual Reproduction Introduction 4.1 Types and Characteristics 4.2 Fission and Reproduction 4.3 Clonal and Sexual Reproduction 4.4 Induced Fission4.5 The Trigger 4.6 Clonal Autotomy 4.7 Larval Cloning 4.8 Eggs and Embryos 4.9 Searching Stem Cells 5. Regeneration Introduction 5.1 Incidence and Prevalence 5.2 Induction of Autotomy 5.3 Causes and Consequences 5.4 Growth and Differentiation 5.5 Growth Factors and Genes 6. Sex Determination Introduction 6.1 Species and Fidelity 6.2 Karyotypes and Heteromorphism 6.3 Ploidy Induction 6.4 Genome and Sequencing 7. Sex Differentiation Introduction 7.1 Asteroid Model 7.2 Echinoid Model 7.3 Induction of Spawning 7.4 Endocrine Disruption 7.5 Parasitic DisruptionSection Ib Hemichordata 8. Reproductive Biology Introduction 8.1 Life Cycles 8.2 Gonads and Consequences 8.3 Regeneration in Enteropneusts Section II Chordate Deuterostomia 9. Cephalochordata Introduction 9.1 Reproductive Biology 10. Urochordata Introduction 10.1 Pelagic Tunicates 10.2 Benthic Tunicates 10.3 Germline Lineage 11. New Findings and Highlights Introduction 11.1 Structure and Distribution 11.2 Fecundity, Size and Depth 11.3 Aquaculture: Sea Urchins and Cucumbers 11.4 Intromittent Organ 11.5 Gonad and Hormonal Economy 11.6 Regenerative Potential11.7 Gonads of Cloners 11.8 Clonal Reproduction 11.9 Autoregulation and Stem Cells 11.10 Cloning and Coloniality 12. References Author Index Species Index Subject Index