日本動物学会 第88回 富山大会 2017
 The 88th Annual Meeting of the Zoological Society of Japan
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▶ 本部企画特別講演(ナリシゲシンポジウム)

脊椎動物への道 The road to vertebrates

Zoological Letters 編集主幹 倉谷滋
(Evolutionary Morphology Laboratory, RIKEN)

Shigeru Kuratani 倉谷滋

Andreas Hejnol
(Sars International Centre for Marine Molecular Biology, University of Bergen):
Before the gill slits―an outgroup perspective on vertebrate origins

Christopher J. Lowe
(Hopkins Marine Station, Department of Biology, Stanford University.):
Patterning contrasting body plans with deeply conserved developmental programs.

Noriyuki Satoh 佐藤矩行
(Marine Genomics Unit, Okinawa Institute of Science and Technology Graduate University, Onna, Okinawa 904-0495, Japan)
Chordate origins and evolution:Suggestion from urochordates

公益社団法人日本動物学会は、1888 年以来の長い歴史を持つZoological Science を刊行してきたが、さらに、日本の動物学が世界に打って出るための動物学フラッグシップ誌とすることを目標としたオープン・アクセス誌Zoological Letters(ZL)を2015 年に創設し、BioMed Central 社から出版を開始した。今回のシンポジウムは、このZL を学会員の皆様にもよく知っていただき、どしどし投稿していただくため、ZL 編集主幹である倉谷滋博士にこのシンポジウムをオーガナイズしていただき、「脊椎動物への道」というトピックに関して現在世界をリードする3 人のスピーカーを招待し、本部企画のナリシゲシンポジウムとして開催するものである。

  19 世紀末に始まる比較発生学の歴史は、脊椎動物の起源を問いかける歴史であったといってよく、そのスピリットは、21 世紀の進化発生学にもそのまま引き継がれている。様々な動物の個体発生過程が細胞レベルで明らかにされ、ボディプランの成立に深く関わると見られる発生制御遺伝子も単離、発現解析され、さらに多くの動物についてゲノムが記述されたいま、この問題をあらためて俯瞰することは有意義であろう。ゲノムと体の構造について特異な脊椎動物への進化の道筋がどのような変化の歴史であったのか、無脊椎動物を扱う著名な研究者を招いて考えてみたい。
Before the gill slits―an outgroup perspective on vertebrate origins
 Andreas Hejnol et al
Sars International Centre for Marine Molecular Biology, University of Bergen

  Melissos’ Ex nihilo nihil fit emphasizes the need to gain a picture about the substance out of which novel things arose. Since more than 200 years scientists developed narratives that try to explain the origin of the vertebrates. These scenarios also include depictions of ancestors and their transformations  into chordates. More recently, the development and application of molecular tools has led to a much better understanding of animal relationships and the mechanisms that shape animal morphology. These insights refuted many of the old ideas and also allow a thorough testing of the remaining ones. But what do we know about the last common ancestor of the deuterostomes from which the ambulacrarians, chordates and vertebrates evolved? Here we report new data from xenacoelomorphs and informative protostome groups(rotifers, annelids, brachiopods, priapulids, nemerteans)that may allow to conclude about the morphology of the digestive tract and centralized nervous system of the last common deuterostome ancestor and the underlying molecular patterning systems. Together with the available data from other species it ultimately delivers a clearer picture about the deuterostome stem species, the starting point of the road to the vertebrates.

Patterning contrasting body plans with deeply conserved developmental programs.
Paul Minor1 ,Stephen Green3 ,Ariel Pani1 ,Yao Yao2 ,Douglas Epstein2Christopher J. Lowe1
1 Hopkins Marine Station, Department of Biology, Stanford University
2 Department of Genetics, University of Pennsylvania
3 Department of Biology, Caltech

   The origin of the vertebrate head has been a topic of debate for over a century. Much of what we understand about the origins of our own complex body plan has been based on comparative studies between the body plan of vertebrates and the simpler basal chordate lineages. Our work adds a new perspective to the origins of vertebrates:Hemichordates are a phylum closely related to chordates, but with a contrasting body plan. Despite this organizational and morphological disparity, our detailed studies using both descriptive and functional approaches reveal that hemichordate and vertebrate anterior developmental programs share some exquisite similarities. Surprisingly, recent transgenic approaches have revealed some of this conservation is a result of deep conservation of the underlying regulatory logic, not shared with basal chordates, despite their much closer morphological affinities with vertebrates. I will discuss the implications of our findings for early vertebrate origins, but also what our data suggests about the rather loose connection between gene regulatory network and morphological evolution.

Chordate origins and evolution:Suggestion from urochordates
Noriyuki Satoh
Marine Genomics Unit, Okinawa Institute of Science and Technology Graduate University, Onna, Okinawa 904-0495, Japan

  The origin and evolution of chordates is one of the most mysterious and intriguing phenomena in evolutionary developmental biology. The chordates consist of three taxa, cephalochordates, urochordates(or tunicates), and vertebrates. They are characterized by possession of a notochord, a dorsal hollow neural tube, and somites. Chordates belong to a super-phyletic group of deuterostomes, together with ambulacrarians(echinoderms and hemichordates), and thus chordates originated from a common ancestor of deuterostomes. Despite extensive studies and discussion of this evo-devo issue, many questions still remain, especially how chordates originated from the deuterostome ancestor. Here, I wish to discuss this issue by starting from our studies mainly done on urochordate ascidians. Soon after the beginning of our studies, I noticed a uniqueness of ascidian embryogenesis, which is sometimes called“precocious”. But what does it mean? The entire adult body of urochordates is invested with a thick covering, the tunic, and they are the only animals that can synthesize cellulose, a component of tunic. The tunic  may function as an outer protective structure, like a mollusk shell, and has undoubtedly influenced the evolution of lifestyles of this group, including the precocious mode of embryogenesis. In addition, given morphological and embryological features that characterize a taxon(phylum)of metazoans, urochordates should be recognized as an animal group that belong to a discrete phylum, not one of the three subphyla of the Phylum Chordata. Along with this line of interpretation, it is likely that(1)the main streamline of the evolutionary road to vertebrates is hemichordes-cephalochordes-vertebrates, while echinoderms and urochordates are more derived, and(2)the origin of chordates is profoundly associated with the modification of developmental mode to form larvae or the occurrence of fish-like larvae.

Satoh, N.(2016)Chordate Origins and Evolution:The Molecular Evolutionary Road to Vertebrates . Academic Press.


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