論文備忘録) 螺旋卵割において重要な転写因子SPILEの分子進化
特定の遺伝子の分子進化について，解析の流れなど参考になる文献のメモ．
筑波大学の守野先生の論文ですね．
リファレンス
Dynamic evolutionary history of spiralian‐specific TALE homeobox genes in mollusks. Development.
Morino, Y. (2022). Growth & Differentiation, 64(4), 198-209.
https://doi.org/10.1111/dgd.12779
要旨
Homeobox genes play essential roles in the early development of many animals. Although the repertoire of most homeobox genes, including three amino acid loop extension (TALE)-type homeobox genes, is conserved in animals, spiralian-TALE (SPILE) genes are a notable exception. In this study, SPILE genes were extracted from the genomic data of 22 mollusk species and classified into four clades (-A/C, -B, -D, and -E) to determine which SPILE genes exhibit dynamic repertoire changes. While SPILE-D and -E duplications were rarely observed, SPILE-B duplication was observed in the bivalve lineage and SPILE-A/C duplication was observed in multiple clades. Conversely, most or all SPILE genes were lost in cephalopods and in some gastropod lineages. SPILE gene expression patterns were also analyzed in multiple mollusk species using publicly available RNA-seq data. The majority of SPILE genes examined, particularly those in the A/C- and B-clades, were specifically expressed during early development, suggesting that most SPILE genes exert specific roles in early development. This comprehensive cataloging and characterization revealed a dynamic evolutionary history, including SPILE-A/C and -B gene duplications and the loss of SPILE genes in several lineages. Furthermore, this study provides a useful resource for studying the molecular mechanism of spiralian early development and the evolution of young and lineage-specific transcription factors.
コメント
SPILE (spiralian-TALE) は螺旋卵割動物において急速に進化し，割球の運命決定に関与すると考えられる重要な転写因子である．SPILE遺伝子の重複や消失，機能の変化は頻繁に起こってきたと考えられるが，その研究は一部の種に限られていた．またSPILE遺伝子は4つのクレードに分けられるが，各クレードで重複がどの程度起こっているのか，進化のパターンはどのクレードでも同じなのかといった進化過程はわかっていなかった．
この論文では，公開されている軟体動物のゲノムやトランスクリプトームのデータを利用して様々な種でSPILE遺伝子の各クレードでどの程度重複が見られるのかなどを包括的に調べている．結果としては，SPILE-Bは二枚貝類で特異的に重複している，頭足類と腹足類の一部の系統ではSPILE遺伝子が失われているなど，非常に興味深い傾向が見られている．初期発生で重要な遺伝子なのに，螺旋卵割をする腹足類の一部でもロストしているのはかなり不思議．また多板綱などの情報はまだ少ないので，今後さらに情報が増えるとさらに知見も増えるのかなぁというところ．
分子進化の研究の流れとしてはとてもわかりやすく，ストーリーもシンプル．とても参考になる論文で，勉強になった．
論文備忘録) 頭足類ゲノム関連
頭足類のゲノム関連の論文が近年連続して出ているので，メモ．
今後ますます発展していく分野になると思うので，しっかり追っていきたい．
リファレンス-1
Genome and transcriptome mechanisms driving cephalopod evolution.
Albertin, C. B., Medina-Ruiz, S., Mitros, T., Schmidbaur, H., Sanchez, G., Wang, Z. Y., ... & Rokhsar, D. S. (2022). Nature communications, 13(1), 2427.
https://doi.org/10.1038/s41467-022-29748-w
要旨-1
Cephalopods are known for their large nervous systems, complex behaviors and morphological innovations. To investigate the genomic underpinnings of these features, we assembled the chromosomes of the Boston market squid, Doryteuthis (Loligo) pealeii, and the California two-spot octopus, Octopus bimaculoides, and compared them with those of the Hawaiian bobtail squid, Euprymna scolopes. The genomes of the soft-bodied (coleoid) cephalopods are highly rearranged relative to other extant molluscs, indicating an intense, early burst of genome restructuring. The coleoid genomes feature multi-megabase, tandem arrays of genes associated with brain development and cephalopod-specific innovations. We find that a known coleoid hallmark, extensive A-to-I mRNA editing, displays two fundamentally distinct patterns: one exclusive to the nervous system and concentrated in genic sequences, the other widespread and directed toward repetitive elements. We conclude that coleoid novelty is mediated in part by substantial genome reorganization, gene family expansion, and tissue-dependent mRNA editing.
ヤリイカの仲間である Doryteuthis (Loligo) pealeiiのゲノムを読み，染色体レベルでアセンブリを行った論文．ゲノムの構造に注目し， Octopus bimaculoides (カリフォルニアツースポットダコ) や Euprymna scolopes (ハワイミミイカ) についても染色体レベルで解析し直して比較を行っている．RNA edittingの機構にも注目しているグループなので，各組織のトランスクリプトーム解析も並行して行い，RNA edittingが各組織でどの程度見られるのか、にも着目して解析を行っている．
リファレンス-2
Emergence of novel cephalopod gene regulation and expression through large-scale genome reorganization.
Schmidbaur, H., Kawaguchi, A., Clarence, T., Fu, X., Hoang, O. P., Zimmermann, B., ... & Simakov, O. (2022). Nature communications, 13(1), 2172.
https://doi.org/10.1038/s41467-022-29694-7
要旨-2
Coleoid cephalopods (squid, cuttlefish, octopus) have the largest nervous system among invertebrates that together with many lineage-specific morphological traits enables complex behaviors. The genomic basis underlying these innovations remains unknown. Using comparative and functional genomics in the model squid Euprymna scolopes, we reveal the unique genomic, topological, and regulatory organization of cephalopod genomes. We show that coleoid cephalopod genomes have been extensively restructured compared to other animals, leading to the emergence of hundreds of tightly linked and evolutionary unique gene clusters (microsyntenies). Such novel microsyntenies correspond to topological compartments with a distinct regulatory structure and contribute to complex expression patterns. In particular, we identify a set of microsyntenies associated with cephalopod innovations (MACIs) broadly enriched in cephalopod nervous system expression. We posit that the emergence of MACIs was instrumental to cephalopod nervous system evolution and propose that microsyntenic profiling will be central to understanding cephalopod innovations.
こちらはEuprymna scolopesについて，染色体レベルのアセンブリとATAC-seqによるオープンクロマチン領域の解析を行った論文．頭足類に特異的に見られるmicrosyntenyについて、転写の制御と三次元的な構造との関連や，神経系や体内器官における発現動態なども調べられている．
リファレンス-3
A single-cell atlas of bobtail squid visual and nervous system highlights molecular principles of convergent evolution.
Gavriouchkina, D., Tan, Y., Künzli-Ziadi, F., Hasegawa, Y., Piovani, L., Zhang, L., ... & Rokhsar, D. S. (2022). bioRxiv, 2022-05.
https://doi.org/10.1101/2022.05.26.490366
要旨-3
Although the camera-type eyes of cephalopods and vertebrates are a canonical example of convergent morphological evolution, the cellular and molecular mechanisms underlying this convergence remain obscure. We used genomics and single cell transcriptomics to study these mechanisms in the visual system of the bobtail squid Euprymna berryi, an emerging cephalopod model. Analysis of 98,537 cellular transcriptomes from the squid visual and nervous system identified dozens of cell types that cannot be placed in simple correspondence with those of vertebrate or fly visual systems, as proposed by Ramón y Cajal and J.Z. Young. Instead, we find an unexpected diversity of neural types, dominated by dopamine, and previously uncharacterized glial cells. Surprisingly, we observe changes in cell populations and neurotransmitter usage during maturation and growth of the visual systems from hatchling to adult. Together these genomic and cellular findings shed new light on the parallel evolution of visual system complexity in cephalopods and vertebrates.
Euprymna berryiの眼について，single-cell transcriptomeを行い，cell type の構成について脊椎動物や節足動物 (ショウジョウバエ) との比較を行った論文．リファレンスのため，Euprymna berryi自体のゲノムアセンブリも行っている．cell type レベルでは必ずしもdeep homology が見られるわけでも無いというのは興味深い．分子メカニズムレベルで異なるものが収斂して似たような機能・形態を示しているということか．また，発生に興味がある身としては，孵化個体と成長した幼体でoptic lobeの形態とともにcell typeも明らかに変化しているというのがとても面白い．細胞レベルで再構成されている様子を発現遺伝子のレベルから解明できるのは素晴らしいな．そこからまた形態の描写まで落とし込めると良いのだろう．
ゲノム関連ではないが，上の論文でも扱われているように，モデル頭足類として整備されつつあるEuprymna berryiとEuprymna morseiというミミイカ属の2種の飼育系確立についての論文も出ている．
リファレンス-4
Lifecycle, culture, and maintenance of the emerging cephalopod models Euprymna berryi and Euprymna morsei.
Jolly, J., Hasegawa, Y., Sugimoto, C., Zhang, L., Kawaura, R., Sanchez, G., ... & Rokhsar, D. (2022). Frontiers in Marine Science, 9.
https://doi.org/10.3389/fmars.2022.1039775
論文備忘録) オウムガイ触手は感覚器官としても機能する
リファレンス
The sensory epithelium of the tentacles and the rhinophore of Nautilus pompilius L.(Cephalopoda, Nautiloidea).
Ruth, P., Schmidtberg, H., Westermann, B., & Schipp, R. (2002). Journal of Morphology, 251(3), 239-255.
https://onlinelibrary.wiley.com/doi/abs/10.1002/jmor.1086
コメント
オウムガイ Nautilus pompiliusの眼周辺にあるrhinophoreという感覚器官やocular tentacles，そして多数あるdigital tentaclesなどの触手にある感覚性の上皮細胞について，SEMや組織切片を用いて詳細に調べた論文．繊毛を持つ感覚性上皮細胞をタイプ分けし，その分布などの違いが器官や部位ごとに記述されている．
餌を採る際の行動などと合わせて機能的な面での考察もなされていた．以前より感覚器官とみなされていたrhinophoreやocular tentaclesに加え，digital tentaclesの一部も感覚器官として機能しているのではないかということが明らかにされている．また，digital tentaclesの中でも外側にあるlong digital tentaclesと，口周辺のbuccal tentaclesを取り囲むmedial digital tentaclesでは形態的，機能的に大きく違いがみられるとのことである．このような違いは異なる種のオウムガイ (オオベソオウムガイ Nautilus macromphalus) では見られないようであり，種特異的にdigital tentaclesの機能分化が起きているかもしれないというのはとても興味深い．この論文ではある程度成長した幼若個体を観察しているので，さらに発生過程ごとに形態にどのような変化がみられ，どのように二つのタイプに分化していくのか，ocular tentales との違いがどのように生じるのかというのも非常に気になるところ．
要旨
Nine intraepithelial ciliated cell types that are presumed to be sensory cells were identified in the epithelium of the pre- and postocular tentacles, the digital tentacles, and the rhinophore of the juvenile tetrabranchiate cephalopod Nautilus pompilius L. The morphological diversity and specialization in distribution of the different ciliated cell types analyzed by SEM methods suggest that these cells include receptors of several sensory functions. Ciliated cell types in different organs that show similar surface features were combined in named groups. The most striking cell, type I, is characterized by a tuft of long and numerous cilia. The highest density of this cell type occurs in ciliary fields in the epithelium of the lamellae of the pre- and postocular tentacles, in the olfactory pits of the rhinophores, and in the lamellae of four pairs of lateral digital tentacles, but not in the epithelium of the medial digital tentacles. The similar morphological data, together with behavioral observations on feeding habits, suggest that this cell type may serve in long-distance chemosensory function. The other ciliated cell types are solitary cells with specific spatial distributions in the various organs. Cell types with tufts of relatively short, stiff cilia (types III, IV, VIII), which are distributed in the lateral and aboral areas of the tentacles and at the base of the tentacle-like process of the rhinophore, are considered to be employed in mechanosensory transduction, while the solitary cells with bristle-like cilia at the margin of the ciliary fields (type II) and at the base of the rhinophore (type IX) may be involved in chemoreception. Histological investigation of the epithelium and the nerve structures of the different organs shows the proportion and distribution of the sensory pathways.
Two different types of digital tentacles can be distinguished according to their putative functions: lateral slender digital tentacles in four pairs, of which the lowermost are the so-called long digital tentacles, participate in distance chemoreception, and the medial digital tentacles, whose terminal axial nerve cord may represent a specialized neuromechanosensory structure, appear to have contact chemoreceptive abilities.
論文備忘録) 化石記録から見る頭足類吸盤・鉤爪の進化過程
最近、吸盤の化石記録についてのレビュー論文が出たのでメモしておく．古生物学は専門ではなくなかなか先行研究を追えていないので，とても参考になりそう．
リファレンス
Evolutionary development of the cephalopod arm armature: a review.
Fuchs, D., Hoffmann, R. & Klug, C. 2021. Swiss J Palaeontol 140, 27.
https://doi.org/10.1186/s13358-021-00241-z
要旨
The cephalopod arm armature is certainly one of the most important morphological innovations responsible for the evolutionary success of the Cephalopoda. New palaeontological discoveries in the recent past afford to review and reassess origin and homology of suckers, sucker rings, hooks, and cirri. Since a priori character state reconstructions are still ambiguous, we suggest and discuss three different evolutionary scenarios. Each of them is based on the following assumptions: (1) Neocoleoidea uniting extant Decabrachia and Octobrachia is monophyletic (= proostracum-bearing coleoids); (2) extinct Belemnitida and Diplobelida are stem decabrachians; (3) proostracum-less coleoids (Hematitida, Donovaniconida, Aulacoceratida) represent stem-neocoleoids; (4) Ammonoidea and Bactritoidea are stem coleoids. We consider a scenario where belemnoid hooks derived from primitive suckers as well-supported. Regarding belemnoid hooks and suckers as homologues implies that belemnoid, oegopsid, and probably ammonoid arm hooks arose through parallel evolution. Our conclusions challenge the widespread opinion, whereupon belemnoid hooks evolved de novo, and instead support earlier ideas formulated by Sigurd von Boletzky.
このレビューの中で引用されている論文で、ベレムナイトの軟体部の化石に関する報告で見落としていた重要なものがあったのでこちらも控えておく．鉤爪だけでなく吸盤らしきものもしっかり見えている．この化石はすごい．こういうの生で見てみたいなぁ．
リファレンス
New evidence of functional suckers in belemnoid coleoids (Cephalopoda) weakens support for the ‘Neocoleoidea’ concept.
Fuchs, D., von Boletzky, S., & Tischlinger, H. 2010. Journal of Molluscan Studies, 76(4), 404-406.
https://doi.org/10.1093/mollus/eyq032
論文備忘録) 二枚貝の眼
二枚貝の外套膜に現れる眼について，指導教官と話すことがあり少し調べてみることにした．改めてみるととてもきれいで面白い形質だが，意外ときちんとした発生過程の記載はすぐに見つからなかった．もっと古い文献も探してみるべきだろうが，見つかった形態のレビュー論文を中心に文献を挙げておいた．要旨が長いので先にコメントをまとめておく．
コメント
文献一つ目が二枚貝の外套膜にできる眼の多様性について解説したもの．二つ目は孵化後の幼若個体と成体について、眼の後胚発生過程を詳しく調べた論文．三つ目は眼に着目したものではないが，二枚貝の幼生から幼若個体まで発生過程を記載した論文で，眼の形成が始まるタイミングや形成初期の形態についても述べられている．
そもそも二枚貝の眼の形態が系統ごとにかなり多様化しているのが面白い．発生過程なども見ているとどうも外套膜の辺縁にできるrentacleと相同な器官のようだ．二枚貝の外套膜は2，3重に重なっており，そのうち外側の部分にできるのか，内側の部分にできるのか等形成される場所がかなりバラバラなのは興味深い．tentacleから眼が進化したというのもとても面白いのだが，その形成過程の差が生じ始めるところの記載がまだ見つけられず，非常に気になるところ．今自分が行っている研究にもヒントが得られそうなんやけど…．自分で観察してみようか．
リファレンス-1
The evolution of eyes in the Bivalvia: new insights.
Morton, B. (2008). American Malacological Bulletin, 26(1/2), 35-45.
https://doi.org/10.4003/006.026.0205
要旨-1
Two types of multi-cellular eyes have been identified in the Bivalvia. Paired cephalic eyes occurring internally above the anterior end of the ctenidia are seen only in representatives of the Arcoidea, Limopsoidea, Mytiloidea, Anomioidea, Ostreoidea, and Limoidea. These eyes, comprising a pit of photo-sensory cells and a simple lens, are thought to represent the earliest method of photoreception. Many shallow-water marine, estuarine, and freshwater bivalves also possess simple photoreceptive cells in the mantle that enable them to respond to shadows. In some other marine, shallow-water taxa, however, a second type of more complex photoreceptors has evolved. These comprise ectopic pallial eyes that can be divided into three broad categories, in terms of their locations on the (i) outer mantle fold in representatives of the Arcoidea, Limopsoidea, Pterioidea, and Anomioidea, (ii) middle fold in the Pectinoidea and Limoidea, and (iii) inner fold in the Cardioidea, Tridacnoidea, and Laternulidae (Anomalodesmata). Eyes do not occur in deep-sea bivalve taxa. Where ectopic pallial eyes occur, they measure amounts of light and integrate intensities from different directions, thereby supplying information to the individual possessing them about the distribution of light in its immediate environment. This does not mean, however, despite broad, phylogenetically related advances in pallial eye complexity, that any bivalve can perceive an image. A revised picture of the independent evolution of ectopic pallial eyes in the Bivalvia is provided. In bivalves, pallial fold duplication has resulted in improvements to the peripheral visual senses, albeit at different times in different phylogenies and on different components of the mantle margin. This has been achieved, it is herein argued, through: (i) selective gene-induced ectopism; (ii) pigment cup evagination in Category 1 eyes; (iii) invagination in Categories 2 and 3; and (iv) natural selection. The invaginated distal retina in representatives of the Pectinidae and Laternulidae provides the potential for image formation and the detection of movement. In the absence of optic lobes capable of synthesizing such information, however, these complex eyes must await matching cerebral sophistication.
リファレンス-2
Development of the pallial eye in Nodipecten nodosus (Mollusca: Bivalvia): insights into early visual performance in scallops.
Audino, J. A., Marian, J. E. A., Wanninger, A., & Lopes, S. G. (2015). Zoomorphology, 134(3), 403-415.
https://doi.org/10.1007/s00435-015-0265-8
要旨-2
Scallop pallial eyes have been the most studied optical system in bivalve mollusks. Despite recent advances in our understanding of the function and evolution of scallop eyes, little attention has been focused on eye development and early visual performance. Here, the anatomy and development of pallial eyes were investigated in the scallop Nodipecten nodosus (Linnaeus, 1758) by means of integrative microscopy techniques (i.e., light, electron, and confocal microscopy). After metamorphosis, juvenile scallops bear small papillae that rapidly transform into minute ocular organs on the middle mantle fold. The distal epithelium gradually becomes pigmented, except for the cornea formed at the distal center of the eye. Internally, the optic vesicle comprises undifferentiated cells in the distal region, while mirror plates are secreted at the base of the eye, next to pigmented cells. Within the undifferentiated cell mass, the proximal retina is the first to be formed, followed by the distal retina and then by the lens. In this respect, the late development of the scallop lens from retina precursor cells may represent a unique condition among animal eyes. Adult eyes are characterized by large pigment distribution in the epithelium, tall columnar cornea, and lens above a slightly curved double retina. Whereas the pallial eyes from adult scallops are a complex visual system based on a mirror mechanism to form a focused image on the retina, early eye condition suggests a simple degree of directional photoreception, with no spatial vision.
リファレンス-3
The development and external morphology of pelagic larval and post-larval stages of the bay scallop, Aequipecten irradians concentricus Say, reared in the laboratory.
Sastry, A. N. (1965). Bulletin of Marine Science, 15(2), 417-435.
https://www.ingentaconnect.com/content/umrsmas/bullmar/1965/00000015/00000002/art00006
要旨-3
Bay scallops, Aequipecten irradians concentricus Say, are reared from fertilized eggs to preadults in the laboratory at 24.0 ± 1.0°C. The external morphology of veliger larval and post-larval stages are described. After ten days of pelagic life the larvae settle and crawl on the bottom before metamorphosis and attachment on the thirteenth day. The metamorphosis of scallop larvae involves: complete loss of velum; movement of mouth through 90° to an anterior and dorsal position; loss of anterior adductor muscle and the development of posterior adductor muscle; and development of gills. The larvae after metamorphosis attach to the substratum with byssus threads.
After attachment, the post-larval (dissoconch) shell rapidly grows to reach the adult form by the 29th day. During post-larval development, the gill filaments increase in number and size. The long tentacles and the eyes are developed on the mantle by the 23rd day and the 27th day respectively. The posterior adductor muscle and the heart enlarge by the 25th day. The post-larval scallops can free and reattach to the substratum and also show crawling movements when free on the bottom. The plicacations (ribs) appear on the 29th day and extend over the entire shell by the 35th day. The plicated scallops crawl on the vertical sides and attach at the level of water surface. The swimming ability in adult manner is developed in the plicated scallops. The young scallops also float on the water surface with extended foot and tentacles held by surface tension. The sequence of developmental changes and the functional morphology of larval and post-larval organs appear to have preadapted to the habits of young scallops.
論文備忘録) アカイカ科稚仔の触腕分裂 / ink glandの組織形態学的観察
Invertebrate Biology という雑誌の頭足類関連論文をWeb上でざっと見ていたときに見つけ，少し気になった論文をピックアップ.
リファレンス-1
Ontogeny of the fused tentacles in three species of ommastrephid squids (Cephalopoda, Ommastrephidae).
Shea, E. K. (2005). Invertebrate Biology, 124(1), 25-38.
https://doi.org/10.1111/j.1744-7410.2005.1241-04.x
要旨-1
The tentacles of ommastrephid squids fuse during embryonic development and remain fused as they grow through hatching, but eventually separate to become two fully functional adult tentacles. The external anatomy of individuals at several post-hatching ontogenetic stages of three species of ommastrephid squids (Ommastrephes bartramii, Sthenoteuthis oualaniensis, and Hyaloteuthis pelagica) was examined using scanning electron microscopy and morphometrics. The fusion of the transverse muscle mass of the tentacles was examined using light microscopy. Five ontogenetic stages of tentacle separation were defined based on landmark features such as the extent of the fusion and the presence of suckers or sucker buds at the distal tip. The total tentacle length and fused tentacle length reached a maximum when the dorsal mantle length (ML) equaled 3–4 mm (H. pelagica) or 4–6 mm (O. bartramii, S. oualaniensis), and then decreased with increasing ML. The average split length (measured from the base of the tentacles to the point of tentacle fusion) increased gradually with increasing ML, and the separate tentacle diameter was roughly half the diameter of the fused portion at all sizes. In all three species, separation of the fused tentacles began earlier in development (2–3-mm ML) and was more advanced at smaller sizes than previously reported. The sizes presented here are conservative because excess epithelium at the location of the split may disguise the actual site of separation. Post-separation tentacles were much shorter than the arms, and the carpal region appeared torn in 2 of the 4 specimens of S. oualaniensis examined. Finally, none of the original distal tip suckers were retained on the post-separation tentacles of S. oualaniensis. These observations are consistent with the hypothesis that the tentacles separate gradually then rupture at the “wrist” (presumptive carpus), and argue against the possibility of prey capture by the fused tentacles.
コメント-1
スルメイカなどを含むアカイカ科の稚仔の持つ触腕の特殊な形態について調べた論文．アカイカ科の稚仔は他のイカとは異なり，左右の触腕が融合した独特の形態を持つことが知られている．この触腕がどのように左右に分かれて成体に見られる機能的な2本の触腕になるのか調べるため，3種のイカについて触腕が分かれる様子をSEM等を用いて観察している．融合していた際に先端に見られる吸盤が分裂が進むにつれて失われ，新たに他の腕と同じような吸盤原基が作られる様子なども見られ，かなりダイナミックに形態の変化が起きているのは非常に興味深い．
リファレンス-2
Histology and ultrastructure of ink gland and melanogenesis in the cuttlefish Sepia pharaonis.
Jiang, M., Xue, R., Chen, Q., Zhan, P., Han, Q., Peng, R., & Jiang, X. (2020). Invertebrate Biology, 139(4), e12306.
https://doi.org/10.1111/ivb.12306
要旨-2
The ink gland in cephalopods is a modified part of the digestive tract that generates melanin used in defense activities against predators. In the present study, we document the histological structure of the ink gland of the cuttlefish Sepia pharaonis following light microscopy (LM) and transmission electron microscopy (TEM) examinations. Based on the observations, we infer the series of cellular events leading to the formation and secretion of melanin. LM observations revealed that the cuttlefish ink gland was cord-like, and could be separated into two distinct regions with different functions, based on markedly different cellular and biochemical characteristics. The region with the function of secreting ink was the epithelial cell region (i.e., mature ink gland cells) and was mainly composed of melanosomes, rough endoplasmic reticulum, Golgi apparatus, and mitochondria. Conversely, the connective tissue region was the site of formation of cells and energy supply. In addition, the most probable sequence of activities from melanogenesis to the release, based on TEM observations, is as follows: (a) formation of a matrix of opaque electronic substances (melanin precursors); (b) melanization in the precursor matrix to form melanin-like particles; (c) further melanin granule development and enlargement, and gradual intensifying of the hue; (d) production of melanin granules (melanosomes) in the precursor matrix; (e) binding of melanosomes to the cytoskeleton and migration to the cell surface; (f) fusion of melanosomes with the cell membrane at the apical pole, and discharge of melanin granules into intercellular spaces or lumen by exocytosis or cell fragmentation. We also investigated how the process of continuous ink release influences ink gland histology and ultrastructure. Inking stimulation induced the release of large amounts of ink by epithelial cells, and was associated with cell disorganization, and severe cellular vacuolization, in addition to severe organelle damage.
コメント-2
トラフコウイカ Sepia pharaonis を用いて墨を生成するink glandの形態を組織学的に調べた論文．構成する細胞の種類などから，ink glandはメラニン合成を行う上皮細胞層と，上皮細胞に分化する前の細胞増殖やエネルギーの供給を行う層に分かれていることが組織切片の観察により示され，またTEMにより詳細なメラニン合成の様子が記載されている．
ink glandは頭足類に独特の新規形質の一つであり，消化管が変化して獲得されたと考えられている．生理学的なメラニン合成のメカニズムなどは良く研究されているが，形態学的な情報は意外と少ないので，記載的とはいえこのような基礎的な研究はとても大事だし，興味深い．消化管からの進化を考える上でも非常に重要な知見かと思われる．
論文備忘録) タコの二足歩行
リファレンス
Bipedal locomotion in Octopus vulgaris: A complementary observation and some preliminary considerations.
Amodio, P., Josef, N., Shashar, N., & Fiorito, G. (2021). Ecology and evolution, 11(9), 3679-3684.
https://doi.org/10.1002/ece3.7328
要旨
Lacking an external shell and a rigid endoskeleton, octopuses exhibit a remarkable flexibility in their movements. Bipedal locomotion is perhaps the most iconic example in this regard. Until recently, this peculiar mode of locomotion had been observed only in two species of tropical octopuses: Amphioctopus marginatus and Abdopus aculeatus. Yet, recent evidence indicates that bipedal walking is also part of the behavioral repertoire of the common octopus, Octopus vulgaris. Here we report a further observation of a defense behavior that encompasses both postural and locomotory elements of bipedal locomotion in this cephalopod. By highlighting differences and similarities with the other recently published report, we provide preliminary considerations with regard to bipedal locomotion in the common octopus.
コメント
タコの仲間の示す2足歩行の移動様式についての論文．海底を移動する動物だと多数の足を使って”這う”ものが一般的で，タコに関しても8本の足をなめらかに動かして這うように移動するか，泳いで移動するものがほとんどなので，わざわざ足を2本だけ使って歩行するように移動する、というのは考えてみればとても珍しいこと．実際にこれまで2足歩行するのが確認されているのはタコの中でも2種のみだったらしい．
ではなぜわざわざこのような風変わりな移動方法を示すのかというと，この論文や先行研究では，歩くのに使う以外の足と体色のパターンの変化により，周辺の海藻のついた岩などに擬態しつつ移動する逃避行動なのではないか，と考えられているようである．
この考察は他の動物の擬態と比べて考えると非常に興味深い．他の動物に擬態するものなどは除くが，周囲の環境にある物体に擬態するものではじっとして動かないのが基本である．しかしおそらくこのタコは海底を海流によって動くものを真似，多数ある足を活かして擬態と移動の両方を可能にしている．やはり頭足類らしく賢い行動なのかなと感じるところ．ただ実際に観察された例が少ないことを考えると，論文内でも考察されていたが周囲に隠れるところがない環境であることや，体サイズが小さい幼体の時期等，かなり限定された環境で獲得される行動なのかもしれない．
また，実際に観察されている行動のパターンなどを読むと，2足歩行をし続けるというよりは漏斗から吐き出した水による遊泳やホッピング，2本以上の足による移動の中に混じって見られる行動でもあるようである．そう考えると若干インパクトは下がるような気もするが，むしろ頭足類の柔軟な判断力や複雑な行動パターンを可能にする知能の高さを示した事例でもあるように思われる．実際にどのような刺激に対しどの移動方法を選択するのかなど，まだまだ解析のしがいがある内容．体全体の動き・腕の動き・脳の働きなど複合的に解析できれば頭足類の思考パターンの理解などにも繋がりそう．
