Auxosporulation in Biddulphia tridens (Ehrenberg) Ehrenberg (Mediophyceae, Bacillariophyta)

We documented male and female gametogenesis, auxospore structure and development of the biddulphioid diatom species Biddulphia tridens (Ehrenberg) Ehrenberg, a close relative of the generitype. Our mating experiments showed that the B. tridens clones were homothallic, self-fertile and produced two eggs per oogonium. During spermatogenesis, spermatogonia developed by depauperating mitosis from spermatogonangium cells and four hologenous uniflagellate male gametes were produced per spermatocyte. In the early stage of auxospore development, cells were globular to sub-globular in shape and covered with a siliceous envelope. The incunabular scales on these auxospores showed a great diversity in size and shape. In the later stages of development, auxospores grew anisodiametrically, facilitated by transverse perizonial bands. Initial epivalve formation was preceded by the retraction of the protoplast from the auxospore wall. The structural features of sex cells such as incunabular scales found in B. tridens are distinct from other broadly defined biddulphioid diatoms thus far examined. This is consistent with the distant relationship between true Biddulphia species and the Biddulphia-like diatoms indicated by molecular phylogenies. Morphological and molecular examination of B. tridens clones were performed here only to confirm species identity. This complete reproductive dataset from a true Biddulphia species will provide valuable insights for future studies of the relationships between Biddulphia-like diatoms and true Biddulphia species.本研究记录了双菱藻类硅藻（biddulphioid diatom）三齿双菱藻[Biddulphia tridens (Ehrenberg) Ehrenberg]——该种是其所在属模式种（generitype）的近缘类群——的雌雄配子发生、复大孢子（auxospore）结构与发育过程。交配实验结果表明，三齿双菱藻的无性繁殖系为同宗配合（homothallic）且自育型，每个卵囊（oogonium）可产生两枚卵子。在精子发生（spermatogenesis）过程中，精原细胞（spermatogonia）由精囊（spermatogonangium）细胞经退化有丝分裂发育而来，每个精母细胞（spermatocyte）可产生4枚全生型单鞭毛雄配子。复大孢子发育早期，细胞呈球状至近球状，外被硅质包膜（siliceous envelope）。此类复大孢子表面的初生鳞片（incunabular scales）在尺寸与形态上呈现出丰富的多样性。发育至后期时，复大孢子在横向围带（transverse perizonial bands）的辅助下进行异向生长。初上壳（initial epivalve）形成前，原生质体（protoplast）会从复大孢子壁中回缩。三齿双菱藻中发现的初生鳞片等生殖细胞结构特征，与迄今已研究的广义双菱藻类硅藻均存在显著差异。这与分子系统发育（molecular phylogenies）所揭示的真正双菱藻属物种与类双菱藻硅藻之间亲缘关系较远的结论一致。本研究仅通过形态学与分子生物学手段对三齿双菱藻的无性繁殖系进行了物种鉴定（species identity）确认。这份源自真正双菱藻属物种的完整生殖数据集，将为后续开展类双菱藻硅藻与真正双菱藻属物种间亲缘关系的研究提供极具价值的参考视角。