Reconstruction of the Doradinae (Siluriformes-Doradidae) ancestral diploid number and NOR pattern reveals new insights about the karyotypic diversification of the Neotropical thorny catfishes

Abstract Doradinae (Siluriformes: Doradidae) is the most species-rich subfamily among thorny catfishes, encompassing over 77 valid species, found mainly in Amazon and Platina hydrographic basins. Here, we analyzed seven Doradinae species using combined methods (e.g., cytogenetic tools and Mesquite ancestral reconstruction software) in order to scrutinize the processes that mediated the karyotype diversification in this subfamily. Our ancestral reconstruction recovered that 2n=58 chromosomes and simple nucleolar organizer regions (NOR) are ancestral features only for Wertheimerinae and the most clades of Doradinae. Some exceptions were found in Trachydoras paraguayensis (2n=56), Trachydoras steindachneri (2n=60), Ossancora punctata (2n=66) and Platydoras hancockii whose karyotypes showed a multiple NOR system. The large thorny catfishes, such as Pterodoras granulosus, Oxydoras niger and Centrodoras brachiatus share several karyotype features, with subtle variations only regarding their heterochromatin distribution. On the other hand, a remarkable karyotypic variability has been reported in the fimbriate barbells thorny catfishes. These two contrasting karyoevolution trajectories emerged from a complex interaction between chromosome rearrangements (e.g., inversions and Robertsonian translocations) and mechanisms of heterochromatin dispersion. Moreover, we believe that biological features, such as microhabitats preferences, populational size, low vagility and migratory behavior played a key role during the origin and maintenance of chromosome diversity in Doradinae subfamily.

摘要：棘甲鲶科（Doradidae，鲇形目Siluriformes）下的棘甲鲶亚科（Doradinae）是棘鲶类中物种最丰富的亚科，涵盖77个以上有效定名物种，主要分布于亚马孙（Amazon）与拉普拉塔（Platina）水文流域。本研究采用细胞遗传学工具（cytogenetic tools）与Mesquite祖先重建软件等联合分析方法，对7种棘甲鲶亚科物种进行研究，以厘清驱动该亚科核型多样化的演化进程。本研究的祖先重建结果显示，染色体数目2n=58与简单核仁组织者区（nucleolar organizer regions，简称NOR）仅为韦氏棘甲鲶亚科（Wertheimerinae）以及多数棘甲鲶亚科演化支的祖征性状。巴拉圭滑口鲶（Trachydoras paraguayensis，2n=56）、斯氏滑口鲶（Trachydoras steindachneri，2n=60）、斑点奥森鲶（Ossancora punctata，2n=66）与汉考克扁甲鲶（Platydoras hancockii）的核型呈现多套核仁组织者系统，属于该规律的例外情况。大型棘鲶类物种，如粒翅甲鲶（Pterodoras granulosus）、黑真棘鲶（Oxydoras niger）以及臂棘甲鲶（Centrodoras brachiatus），共享多项核心核型特征，仅在异染色质（heterochromatin）分布上存在细微差异。另一方面，具缨状触须的棘鲶类则表现出显著的核型变异性。这两种截然相反的核型演化轨迹，源自染色体重排（如倒位（inversions）与罗伯逊易位（Robertsonian translocations））与异染色质扩散机制之间的复杂相互作用。此外，我们认为微生境偏好、种群规模、移动能力低下以及洄游行为等生物学特征，在棘甲鲶亚科染色体多样性的起源与维持过程中发挥了关键作用。