Sexual reproduction in plagiogrammacean diatoms: First insights into the early pennates

RESEARCH ARTICLE Sexual reproduction in plagiogrammacean diatoms: First insights into the early pennates Irena Kaczmarska1☯*, Benjamin S. Gray, Jr.1☯, James M. Ehrman2☯, Mary Thaler3☯ 1 Department of Biology, Mount Allison University, Sackville, New Brunswick, Canada, 2 Digital Microscopy Facility, Mount Allison University, Sackville, New Brunswick, Canada, 3 Institut de biologie intégrative et des systèmes, Université Laval, Québec, Québec, Canada ☯ These authors contributed equally to this work. * a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 OPEN ACCESS Citation: Kaczmarska I, Gray BS, Jr., Ehrman JM, Thaler M (2017) Sexual reproduction in plagiogrammacean diatoms: First insights into the early pennates. PLoS ONE 12(8): e0181413. https://doi.org/10.1371/journal.pone.0181413 Editor: Senjie Lin, University of Connecticut, UNITED STATES Received: February 10, 2017 Accepted: June 16, 2017 Published: August 16, 2017 Copyright: © 2017 Kaczmarska et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All sequence files and voucher specimens are available from either GenBank (https://www.ncbi.nlm.nih.gov/genbank/) or BOLDSystems (http://www.boldsystems.org/) as indicated in Table 1. Holotype material for new species including SEM stubs and fixed, noncleaned culture material have been deposited at Botanischer Garten und Botanisches Museum Berlin-Dahlem (https://www.bgbm.org/). Live cultures are freely available from the corresponding author (IK) from the laboratory culture collection as long as they remain alive. Abstract 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. Introduction Modern research interest in diatom auxospore structure and development began as soon as electron microscopes became commercially available; first using transmission electron PLOS ONE | https://doi.org/10.1371/journal.pone.0181413 August 16, 2017 1 / 44 Sexual reproduction in plagiogrammacean diatoms Funding: This work was supported by a Natural Sciences and Engineering Research Council of Canada (http://www.nserc-crsng.gc.ca/) Discovery Grant and the Mount Allison University Professional Development Fund awarded to IK. Neither NSERC Discovery Grants nor Professional Development Funds are associated with a grant number. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist. microscopy (TEM) in the early 1970s [1, 2], and then scanning electron microscopy (SEM) about a decade later [3, 4]. This work delivered many unanticipated results, some of which culminated in novel ideas about the origin of diatoms [5, 6] and relationships between various lineages within diatoms [2, 7, 8]. Most of that research, however, focused on centrics and raphid pennates. Marine araphid diatoms, on the other hand, are one of the least understood groups of diatoms in terms of their reproductive strategies, structure of the cells involved, and their behaviour. Because their occurrence, mostly in benthic habitats, is unpredictable and seldom at high abundances, these species are infrequently investigated using combined morphological and molecular approaches. Even fewer targeted studies used combined morphological and molecular methods to examine sexually compatible clones. Compatibility studies will likely be necessary to disentangle some of the most notoriously difficult to identify (and delineate) fragillarioid and synedrid species, cases of likely similar complexity to cryptic and semi-cryptic members of the genera Pseudo-nitzschia or Asterionellopsis [9, 10]. To date, sexual reproduction has been studied for species in only about a dozen araphid genera, e.g., [11–16]. From these and earlier works [2, 17–19], it has become evident that araphid pennates evolved some very unusual types of sex cell and auxospore structures that are not shared with centric or pennate diatoms. For example, a unique form of oogamy is known in species of the genus Rhabdonema [18], and non-flagellated but vigorously motile male gametes in Tabularia, Pseudostaurosira, and Ulnaria [20–23], among other characters. This leads to the question of whether any of these unusual characters were present in the earliest of the araphid pennates, or if they are restricted to the more recently diverged members. Our understanding of the evolution of pennates from among the polar centric pool, and their own subsequent diversification, depends on the answers to such questions. There is a long-standing consensus that araphid pennates emerged from among the polar centrics, and that raphid diatoms diverged from araphid ancestors [24–26]. However, which araphid diatom lineages share the most recent common ancestry with which polar centrics and which of them with the raphid pennates rema (...truncated)