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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 (< 18 mm)
ophiuroids do not produce a lecithotrophic
eggs but larger (> 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.
■ در این کتاب چه
میخوانیم:
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
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