Adachi Noritaka, Pascual-Anaya Juan, Hirai Tamami, Higuchi Shinnosuke, Kuratani Shigeru
1Evolutionary Morphology Laboratory, RIKEN center for Developmental Biology, 2-2-3 Minatojima-minami, Chuo-ku, Kobe, 650-0047 Japan.
2Department of Biology, Graduate School of Science, Kobe University, Kobe, 657-8501 Japan.
Zoological Lett. 2018 Feb 18;4:5. doi: 10.1186/s40851-018-0087-x. eCollection 2018.
The extant vertebrates include cyclostomes (lamprey and hagfish) and crown gnathostomes (jawed vertebrates), but there are various anatomical disparities between these two groups. Conspicuous in the gnathostomes is the neck, which occupies the interfacial domain between the head and trunk, including the occipital part of the cranium, the shoulder girdle, and the cucullaris and hypobranchial muscles (HBMs). Of these, HBMs originate from occipital somites to form the ventral pharyngeal and neck musculature in gnathostomes. Cyclostomes also have HBMs on the ventral pharynx, but lack the other neck elements, including the occipital region, the pectoral girdle, and cucullaris muscles. These anatomical differences raise questions about the evolution of the neck in vertebrates.
In this study, we observed developing HBMs as a basis for comparison between the two groups and show that the arrangement of the head-trunk interface in gnathostomes is distinct from that of lampreys. Our comparative analyses reveal that, although HBM precursors initially pass through the lateral side of the pericardium in both groups, the relative positions of the pericardium withrespect to the pharyngeal arches differ between the two, resulting in diverse trajectories of HBMs in gnathostomes and lampreys.
We suggest that a heterotopic rearrangement of early embryonic components, including the pericardium and pharyngeal arches, may have played a fundamental role in establishing the gnathostome HBMs, which would also have served as the basis for neck formation in the jawed vertebrate lineage.
现存的脊椎动物包括圆口纲动物（七鳃鳗和盲鳗）和硬骨鱼纲动物（有颌脊椎动物），但这两类动物之间存在各种解剖学差异。硬骨鱼纲动物中显著的部分是颈部，它占据头部和躯干之间的界面区域，包括颅骨的枕部、肩带以及颈头肌和鳃下肌（HBMs）。其中，鳃下肌起源于枕部体节，在硬骨鱼纲动物中形成腹侧咽部和颈部肌肉组织。圆口纲动物在腹侧咽部也有鳃下肌，但缺少其他颈部结构，包括枕部区域、胸带和颈头肌。这些解剖学差异引发了关于脊椎动物颈部进化的问题。
在本研究中，我们观察了发育中的鳃下肌，以此作为两组动物比较的基础，并表明硬骨鱼纲动物中头-躯干界面的排列与七鳃鳗不同。我们的比较分析表明，尽管两组动物中鳃下肌前体最初都穿过心包的侧面，但心包相对于咽弓的相对位置在两者之间有所不同，导致硬骨鱼纲动物和七鳃鳗中鳃下肌的轨迹不同。
我们认为，早期胚胎成分（包括心包和咽弓）的异位重排可能在建立硬骨鱼纲动物的鳃下肌中起到了基础性作用，而鳃下肌也可能是有颌脊椎动物谱系中颈部形成的基础。
