Department of Microbiology, School of Life Dentistry at Niigata, The Nippon Dental University, 1-8 Hamaura-cho, Chuo-ku, Niigata-city, Niigata 951-8580, Japan.
Aquaculture Research Institute, Shingu Station, Kindai University, 1330 Takata, Shingu-city, Wakayama 647-1101, Japan.
Gene. 2022 Feb 15;811:146091. doi: 10.1016/j.gene.2021.146091. Epub 2021 Dec 2.
Various secretory calcium-binding phosphoprotein (SCPP) genes are expressed in the skin and jaw during the formation of bone, teeth, and scales in osteichthyans (bony vertebrates). Among these mineralized skeletal units is the ganoid scale, found in many fossil actinopterygians (ray-finned fish) but confirmed only in Polypteriformes (bichirs, reedfish) and Lepisosteiformes (gars) among extant clades. Here, we examined SCPP genes in the genome of seven non-teleost actinopterygian species that possess or do not possess ganoid scales. As a result, 39-43 SCPP genes were identified in Polypteriformes and Lepisosteiformes, whereas 22-24 SCPP genes were found in Acipenseriformes (sturgeons, paddlefish) and Amiiformes (bowfin). Most of these genes form two clusters in the genome of Polypteriformes, Lepisosteiformes, and Amiiformes, and these two clusters are duplicated in Acipenseriformes. Despite their distant phylogenetic relationship, Polypteriformes and Lepisosteiformes retain many orthologous SCPP genes. These results imply that common ancestors of extant actinopterygians possessed a large repertoire of SCPP genes, and that many SCPP genes were lost independently in Acipenseriformes and Amiiformes. Notably, most SCPP genes originally located in one of the two SCPP gene clusters are retained in Polypteriformes and Lepisosteiformes but were secondarily lost in Acipenseriformes and Amiiformes. In Lepisosteiformes, orthologs of these lost genes show high or detectable expression levels in the skin but not in the jaw. We thus hypothesize that many SCPP genes located in this cluster are involved in the formation of ganoid scales in Polypteriformes and Lepisosteiformes, and that their orthologs and ganoid scales were convergently lost in Acipenseriformes and Amiiformes.
各种分泌型钙结合磷蛋白 (SCPP) 基因在硬骨鱼（有骨脊椎动物）的骨骼、牙齿和鳞片形成过程中在皮肤和下颚中表达。在这些矿化的骨骼单元中，有一种被称为硬鳞的鳞片，存在于许多化石肉鳍鱼类（有鳍鱼类）中，但仅在现存的类群中被确认存在于多鳍鱼目（多鳍鱼、弓鳍鱼）和雀鳝目（雀鳝）中。在这里，我们研究了 7 种非硬骨鱼的硬骨鱼基因组中的 SCPP 基因，这些鱼类具有或不具有硬鳞。结果，在多鳍鱼目和雀鳝目中鉴定出 39-43 个 SCPP 基因，而在鲟形目（鲟鱼、匙吻鲟）和雀鳝目中发现了 22-24 个 SCPP 基因。这些基因中的大多数在多鳍鱼目、雀鳝目和雀鳝目中的基因组中形成两个簇，而这两个簇在鲟形目中是重复的。尽管它们在系统发育上的关系很远，但多鳍鱼目和雀鳝目保留了许多直系同源的 SCPP 基因。这些结果表明，现生硬骨鱼的共同祖先拥有大量的 SCPP 基因，而许多 SCPP 基因在鲟形目和雀鳝目中是独立丢失的。值得注意的是，原本位于两个 SCPP 基因簇之一的大多数 SCPP 基因在多鳍鱼目和雀鳝目中被保留，但在鲟形目和雀鳝目中是次生丢失的。在雀鳝目中，这些丢失基因的直系同源物在皮肤中表现出高或可检测的表达水平，但在下颚中没有表达。因此，我们假设位于该簇中的许多 SCPP 基因参与了多鳍鱼目和雀鳝目的硬鳞形成，而它们的直系同源物和硬鳞在鲟形目和雀鳝目中是趋同丢失的。
