Kaczmarska Irena, Gray Benjamin S, Ehrman James M, Thaler Mary
Department of Biology, Mount Allison University, Sackville, New Brunswick, Canada.
Digital Microscopy Facility, Mount Allison University, Sackville, New Brunswick, Canada.
PLoS One. 2017 Aug 16;12(8):e0181413. doi: 10.1371/journal.pone.0181413. eCollection 2017.
The genera Plagiogramma and Dimeregramma are members of a small, but evolutionarily important group of diatoms, the "basal" araphids. They are sister to all other pennates, both araphid and raphid taxa. Thus, their phylogenetic position carries the potential for providing insights into the earliest pennates. We documented sexual reproduction, mating system and sex cell development in the first members of the "basal" araphid clade ever investigated. The mating system in all these species involved heterothally. It was, however, more complex in P. tsawwassen, where in addition to heterothallic clones, intraclonal and polysexual clones also exist. Auxospore development and wall structure was similar in all three species and demonstrated several characters also reported from "core" araphids. Of these, vigorous, pseudopodial motility of male secondary spermatocytes and gametes was most notable because it indicates that this character was likely present in the last common ancestor of all the pennates. Pseudopodial motility of the male sex cells might have afforded sufficient compensation and/or benefits to the emerging pennates for replacing flagellated sperm, present in centrics. The characters thus far uniquely present among our plagiogrammaceans but not reported from other pennates were: the "gametic" fusion between sex-compatible secondary spermatocytes, in some cases before completion of Meiosis II in males, transverse perizonial bands produced all together or in quick succession rather than being added to the auxospore apex one at a time, and expanding auxospores with 3-4 nuclei. An initial epivalve, similar in morphology to what in some diatoms had been interpreted as a "longitudinal" perizonium, may be more widespread among pennates than thus far appreciated. In addition, we discovered two species new to science (D. acutumontgo, P. tsawwassen), and refined delineation of P. staurophorum by including metric data from the original material.
斜纹藻属（Plagiogramma）和双纹藻属（Dimeregramma）是硅藻中一个小型但在进化上具有重要意义的类群——“基部”无沟藻的成员。它们是所有其他羽纹藻（包括无沟和有沟类群）的姐妹群。因此，它们的系统发育位置有可能为了解最早的羽纹藻提供见解。我们记录了首个被研究的“基部”无沟藻进化枝成员的有性生殖、交配系统和性细胞发育情况。所有这些物种的交配系统都涉及异宗配合。然而，在察沃森斜纹藻（P. tsawwassen）中更为复杂，除了异宗配合的克隆外，还存在克隆内和多性别的克隆。在所有三个物种中，复大孢子发育和壁结构相似，并表现出一些也在“核心”无沟藻中报道过的特征。其中，雄性次生精母细胞和配子强烈的伪足运动最为显著，因为这表明该特征可能存在于所有羽纹藻的最后一个共同祖先中。雄性性细胞的伪足运动可能为新兴的羽纹藻提供了足够的补偿和/或益处，以取代中心硅藻中存在的具鞭毛精子。到目前为止，在我们的斜纹藻科中独特存在但未在其他羽纹藻中报道的特征有：在某些情况下，雄性减数分裂II完成之前，性兼容的次生精母细胞之间的“配子”融合；横向周壁带一起或快速连续产生，而不是一次添加到复大孢子顶端一个；以及具有3 - 4个细胞核的膨胀复大孢子。一种初始的上壳瓣，其形态与在一些硅藻中被解释为“纵向”周壁的结构相似，可能在羽纹藻中比目前所认识的更为普遍。此外，我们发现了两个新的科学物种（尖峰双纹藻（D. acutumontgo）、察沃森斜纹藻（P. tsawwassen）），并通过纳入原始材料的度量数据，对十字斜纹藻（P. staurophorum）进行了更精确的描述。
