All vertebrates have a definitive thyroid gland,or thyroid for short.As a critical organ for growth,development and metabolism,its origin and evolution have long received attention.On the basis of anatomical position,endodermal origination and histological features,the endostyle of amphioxus has been proposed as a homologue to the thyroid of vertebrates.This homology is further supported by the findings that the amphioxus endostyle abounds in thyroid hormones,possesses several thyroid-specific proteins such as thyroperoxidase,nicotinamide adenine dinucleotide phosphate(NADPH)oxidase and thyroglobulin,and expresses the thyroid-related transcription factors involved in the regulation of development of the vertebrate thyroid,including Nkx2.1,FoxE4 and Pax2/5/8.Importantly,our study on functionality,together with others,indicates significant similarities between the amphioxus endostyle and the vertebrate thyroid gland.Moreover,we show that the production of thyroid hormones by the amphioxus endostyle is mediated in a fashion similar to that of the vertebrate thyroid.These provide solid evidences that the amphioxus endostyle is the homologue of the vertebrate thyroid.From a phylogenetic viewpoint,we propose that the hypobranchial ridge,or endostyle-like structure,of hemichordates is the most primitive forerunner of the thyroid,from which the vertebrate thyroid is formed through the transformation of non-follicular endostyle of amphioxus to follicular endocrine organ of vertebrates.We also raise a couple of questions that demand further study.
Dongshan Bay,along with Xiamen Bay,serves as a significant distribution area for amphioxus in Fujian Province.In this survey,we observed that the sedimentary environment of Dongshan Bay has been seriously silted due to rapid economic development and the thriving aquaculture industry.Consequently,there has been a significant reduction in suitable habitats for amphioxus and a sharp decline in its resource abundance.Totally 9 amphioxuses were collected from its eastern region in 2022 with a population density of 18.75 ind m^(-2) and a body length from 34.15 to 48.02 mm.The amphioxuses acquired in this investigation were classified as Branchiostoma japonicum based on morphological traits,such as a cuspate rostral fin,measurements of angles between the dorsal and super-caudal fins,angles between the pre-anal and sub-caudal fins,along with an accurate count of pre-anal fin chambers which matched those observed in B.japonicum.Moreover,a molecular barcoding analysis was conducted using a fragment of the mitochondrial COI gene.The results indicated that Branchiostoma sp.from Dongshan Bay in this study belongs to B.japonicum,providing additional support for the morphological identification results.Considering amphioxuses are still surviving in Dongshan Bay despite the destruction of their habitat,it is recommended to implement measures to safeguard their resources and habitats.The measures may include designating protection areas,strengthening law enforcement management to prohibit sand mining,regulating the scale of cage aquaculture,and augmenting wild resources through artificial breeding and release.
Liver is characteristic of all vertebrates.As a critical hub for many physiological processes including metabolism,innate immunity,protein synthesis and detoxification,its evolutionary origin was largely underappreciated in history,and only received due attention in recent decades.It has been suggested by morphological,ultrastructural and immunohistochemical studies that the hepatic caecum of amphioxus is homologous to the liver of vertebrate species.Molecular biology studies demonstrated that amphioxus hepatic caecum expresses plenty of vertebrate liver-specific genes.Our functional studies revealed significant similarities between amphioxus hepatic caecum and vertebrate liver.We also found that the functions of hepatic caecum are subjected to the regulation of pituitary hormones just as the liver does.These provide solid evidences supporting the notion that the hepatic caecum is the homologue of liver,which may represent the first stage in chordate evolution,laying a foundation for the subsequent formation of the liver as we know it in vertebrates.Further studies on the specification and morphogenesis of hepatic caecum in amphioxus will shed more lights on the origin and evolution of vertebrate liver.
为了在头索动物文昌鱼中对zglp-1(Zinc finger, GATA-like protein 1)基因进行鉴定,并检测其在青岛文昌鱼(Branchiostomajaponicum)中的组织定位,本文对文昌鱼zglp-1基因的进化及表达进行了研究。同线性分析表明,文昌鱼zglp-1基因是斑马鱼zglp-1和人ZGLP-1基因的同源基因。文昌鱼Zglp-1蛋白的锌指结构域在进化上保守性较高,其三维结构与脊椎动物Zglp-1蛋白的三维结构也高度相似。系统进化分析显示,文昌鱼Zglp-1处于脊椎动物Zglp-1进化的基部,可能代表了脊椎动物Zglp-1进化的原始形式。利用RACE技术克隆得到青岛文昌鱼zglp-1基因完整的ORF序列,其编码389个氨基酸。实时定量PCR技术显示zglp-1基因在文昌鱼性腺中特异高表达,其中在卵巢表达最高;再利用切片原位杂交技术发现zglp-1基因在卵巢、精巢、肝盲囊和鳃等组织中有阳性信号,其中在卵巢中信号最强,与实时定量PCR检测结果基本一致。综上,本研究表明文昌鱼zglp-1为脊椎动物Zglp-1的同源基因,其在文昌鱼性腺高表达,这与该基因在脊椎动物性腺中的表达模式相类似。
The Gli transcription factors are the primary mediators of Hedgehog(Hh)signaling.Vertebrate genomes contain multiple Gli paralogues with different functions downstream of Hh signal receipt,in part explaining the complexity of cellular responses to Hh that allow concentration-dependent target gene activation.Amphioxus is a chordate that split from the vertebrate lineage early in the evolution of chordates,before the genome duplications that occurred in early vertebrate evolution.It has a single Gli gene whose transcripts can be alternately spliced to yield two protein isoforms called GliS and GliL.We generated two knockout mutations in amphioxus Gli,one that affects the whole gene and a second that only affects GliL.Both knockouts showed major morphological and molecular defects in the development of left–right asymmetry,a phenotype that is similar but not identical to that previously found in Hh mutants.Hh signaling also patterns the amphioxus neural tube.Here,however,knockout of GliL showed no identifiable phenotype,while knockout of the full gene showed only small changes to the expression of one gene family,Olig.Other genes that were prominently affected by Hh knockout were not altered in expression in either knockout.Reasons for the differences between Hh and Gli knockouts in the pharynx and neural tube are discussed in the context of the likely different functions of amphioxus Gli isoforms.
Xin HuangQiongqiong RenYiquan WangSebastian M.ShimeldGuang Li
