Department of Biology, Mount Allison University, Sackville, New Brunswick, Canada.
Digital Microscopy Facility, Mount Allison University, Sackville, New Brunswick, Canada.
PLoS One. 2022 Sep 6;17(9):e0272778. doi: 10.1371/journal.pone.0272778. eCollection 2022.
Phylogenetic relationships among mediophycean diatoms with elliptical valve outline and elevated apices have long been a subject of interest and debate, particularly with respect to their relationship to pennates. However, results remain inconclusive, whether based on vegetative valve morphology, reproduction, or molecular phylogenetic data. Searching for phylogenetically informative features, we re-examined sexual reproduction, auxospore structure and development in the diatom Biddulphia biddulphiana. Several unique or unusual features and processes characterized its sexual reproduction. A unique spermatogenesis occurs with premeiotic separation of an anucleate protoplast containing all chloroplasts and likely other organelles. Additionally, their auxospore walls are some of the most complex documented, retaining earlier deposited layers that obscure layers formed during later stages of development. The oldest layer consists of thick, mostly organic incunabulum, underlain by outer and inner epizonia and finally transverse (TP) and longitudinal (LP) perizonia. The complexity of the fine structure of these layers is unprecedented. The orientation of some TP bands is also unique in mediophytes, with some perpendicular to the auxospore apical axis, parallel to each other, and open with aligned ends, as typically seen in pennates. The TP also contains rings slanting toward the apices, as in some other mediophytes, e.g., eupodiscaceans. However, both eupodiscaceans and biddulphiaceans show perizonial band structure derived from anastomosing radial scales, thus termed "scaly bands". Pinnate TP bands, common among pennate auxospores, were not found. Thus B. biddulphiana auxospore wall structure contains a mixture of characters specific to this clone but also known from mediophytes and araphid pennates. However, these features do not provide unequivocal evidence that this or the other Biddulphia species examined to date are the closest extant relatives of basal araphid pennates.
具有椭圆形壳面和高顶角的中带形硅藻的系统发育关系一直是人们关注和争论的焦点，特别是它们与羽纹硅藻的关系。然而，无论是基于营养细胞壳面形态、繁殖方式还是分子系统发育数据，结果都没有定论。为了寻找具有系统发育意义的特征，我们重新研究了硅藻 Biddulphia biddulphiana 的有性繁殖、休眠孢子结构和发育。其有性繁殖具有一些独特或不寻常的特征和过程。独特的精发生殖发生在前期减数分裂之前，此时一个无核原生质体分离出来，其中包含所有的叶绿体和可能的其他细胞器。此外，它们的休眠孢子壁是有记载的最复杂的壁之一，保留了早期沉积的层，这些层掩盖了在发育后期形成的层。最古老的层由厚的、主要是有机的始基组成，下面是外、内珠带，最后是横向（TP）和纵向（LP）珠带。这些层的精细结构的复杂性是前所未有的。这些层的某些 TP 带的取向在中带形硅藻中也是独特的，有些与休眠孢子的顶端轴垂直，有些彼此平行，有些开口，其末端对齐，这通常见于羽纹硅藻。TP 还包含朝向顶端倾斜的环，如在其他一些中带形硅藻中，例如盘形藻目硅藻。然而，盘形藻目硅藻和 Biddulphiaceae 都显示出由吻合的放射状鳞片衍生的珠带结构，因此被称为“鳞片状带”。没有发现羽纹硅藻休眠孢子中常见的羽状 TP 带。因此，B. biddulphiana 休眠孢子壁结构包含了一些特定于该克隆的特征，但也存在于中带形硅藻和无壳羽纹硅藻中。然而，这些特征并不能提供确凿的证据表明，该种或迄今研究的其他 Biddulphia 种是最接近原始无壳羽纹硅藻的现存近亲。
