Ardissonea crystallina has a type of sexual reproduction that is unusual for centric diatoms

Transcriptional chronology reveals conserved genes involved in pennate diatom sexual reproduction

Sexual reproduction is a major driver of adaptation and speciation in eukaryotes. In diatoms, siliceous microalgae with a unique cell size reduction-restitution life cycle and among the world's most prolific primary producers, sex also acts as the main mechanism for cell size restoration through the formation of an expanding auxospore. However, the molecular regulators of the different stages of sexual reproduction and size restoration are poorly explored. Here, we combined RNA sequencing with the assembly of a 55 Mbp reference genome for Cylindrotheca closterium to identify patterns of gene expression during different stages of sexual reproduction. These were compared with a corresponding transcriptomic time series of Seminavis robusta to assess the degree of expression conservation. Integrative orthology analysis revealed 138 one-to-one orthologues that are upregulated during sex in both species, among which 56 genes consistently upregulated during cell pairing and gametogenesis, and 11 genes induced when auxospores are present. Several early, sex-specific transcription factors and B-type cyclins were also upregulated during sex in other pennate and centric diatoms, pointing towards a conserved core regulatory machinery for meiosis and gametogenesis across diatoms. Furthermore, we find molecular evidence that the pheromone-induced cell cycle arrest is short-lived in benthic diatoms, which may be linked to their active mode of mate finding through gliding. Finally, we exploit the temporal resolution of our comparative analysis to report the first marker genes for auxospore identity called AAE1-3 ("Auxospore-Associated Expression"). Altogether, we introduce a multi-species model of the transcriptional dynamics during size restoration in diatoms and highlight conserved gene expression dynamics during different stages of sexual reproduction.

Craspedostaurosnazmii sp. nov., a new diatom species (Bacillariophyta) from the Turkish Coast of the Black Sea

Craspedostauros E.J. Cox is a diatom genus comprising 17 taxa reported from various regions of the world. While many species of Craspedostauros are epibiontic, the taxa have variable ecological preferences. In this study we formally describe Craspedostaurosnazmiisp. nov., an epilithic species discovered along the Turkish Black Sea Coast, based on light and scanning electron microscopy. Craspedostaurosnazmiisp. nov. is characterized by valves that are lanceolate to narrowly lanceolate, slightly constricted near the apices with uniseriate, parallel throughout the whole valve, transapical striae and and the presence of an apical silica flap. The areolae are distributed over the valve face and the mantle. The differences and similarities between C.nazmiisp. nov. and established species of Craspedostauros are discussed. Based on shape and morphometrics, the most similar species is Craspedostauroscapensis, but it is easily distinguished from C.nazmiisp. nov. by its lack of an apical silica flap.

Revision of Ardissoneaceae (Bacillariophyta, Mediophyceae) from Micronesian populations, with descriptions of two new genera, Ardissoneopsis and Grunowago, and new species in Ardissonea, Synedrosphenia and Climacosphenia

Ardissonea was resurrected from Synedra in 1986 and was included as a genus by Round, Crawford and Mann ("The Diatoms") in its own Family and Order. They commented that there might be several genera involved since the type species of the genus possesses a double-walled structure and other taxa placed in Ardissonea have only a single-walled structure. Two other genera of "big sticks," Toxarium and Climacosphenia, were placed in their own Families and Orders but share many characters with Ardissoneaceae, especially growth from a bifacial annulus. Eighteen taxa (11 new species) from Micronesia were compared with the literature and remnant material from Grunow's Honduras Sargassum sample to address the concepts of Ardissonea and Ardissoneaceae. Phylogenetic and morphological analyses showed three clades within Ardissonea sensu lato: Ardissonea emend. for the double-walled taxa, Synedrosphenia emend. and Ardissoneopsis gen. nov. for single-walled taxa. New species include Ardissoneadensistriata sp. nov.; Synedrospheniabikarensis sp. nov., S.licmophoropsis sp. nov., S.parva sp. nov., and S.recta sp. nov.; Ardissoneopsisfulgicans sp. nov., A.appressata sp. nov., and A.gracilis sp. nov. Transfers include Synedrospheniacrystallina comb. nov. and S.fulgens comb. nov. Synedraundosa, seen for the first time in SEM in Grunow's material, is transferred to Ardissoneopsisundosa comb. nov. Three more genera have similar structure: Toxarium, Climacosphenia and Grunowago gen. nov., erected for Synedrabacillaris and a lanceolate species, G.pacifica sp. nov. Morphological characters of Toxarium in our region support separation of Toxariumhennedyanum and T.undulatum and suggest additional species here and elsewhere. Climacospheniamoniligera was not found but we clarify its characters based on the literature and distinguish C.soulonalis sp. nov. from it. Climacospheniaelongata and a very long, slender C.elegantissima sp. nov., previously identified as C.elongata, were present along with C.scimiter. Morphological and molecular phylogenetics strongly suggested that all these genera belong in one family and we propose to include them in the Ardissoneacae and to reinstate the Order Ardissoneales Round.

Mitotic recombination between homologous chromosomes drives genomic diversity in diatoms

Diatoms, an evolutionarily successful group of microalgae, display high levels of intraspecific genetic variability in natural populations. However, the contribution of various mechanisms generating such diversity is unknown. Here we estimated the genetic micro-diversity within a natural diatom population and mapped the genomic changes arising within clonally propagated diatom cell cultures. Through quantification of haplotype diversity by next-generation sequencing and amplicon re-sequencing of selected loci, we documented a rapid accumulation of multiple haplotypes accompanied by the appearance of novel protein variants in cell cultures initiated from a single founder cell. Comparison of the genomic changes between mother and daughter cells revealed copy number variation and copy-neutral loss of heterozygosity leading to the fixation of alleles within individual daughter cells. The loss of heterozygosity can be accomplished by recombination between homologous chromosomes. To test this hypothesis, we established an endogenous readout system and estimated that the frequency of interhomolog mitotic recombination was under standard growth conditions 4.2 events per 100 cell divisions. This frequency is increased under environmental stress conditions, including treatment with hydrogen peroxide and cadmium. These data demonstrate that copy number variation and mitotic recombination between homologous chromosomes underlie clonal variability in diatom populations. We discuss the potential adaptive evolutionary benefits of the plastic response in the interhomolog mitotic recombination rate, and we propose that this may have contributed to the ecological success of diatoms.

Structure and Development of the Auxospore in Ardissonea crystallina (C. Agardh) Grunow Demonstrates Another Way for a Centric to Look Like a Pennate

Reproductive development in Ardissonea crystallina revealed a unique mode of enlargement involving a combination of novel and known structures. In light microscopy, auxospores of this elongated polar centric diatom were superficially similar to the auxospores of pennates. With SEM we found three different components in the auxospore wall. In the youngest, nearly spherical cell-stage, the wall consisted only of a delicate veil containing minute siliceous spherules. Incunabular elements developed underneath this layer. Second, a previously unknown form of specifically modified incunabular scales shaped the subsequent ellipsoidal-capsule auxospore stage. Third, there was a clear contribution of scales to the development of scaly transverse perizonial bands (or scaly bands, for brevity). Such bands, although noted by previous researchers, have not been fully appreciated for the evolutionary information they may convey: possibly common among polar centrics but not pennates. Finally, we propose maintaining the term transverse perizonium to refer to these bands in polar diatoms, but to introduce the differentiation of scaly bands described here from pinnate bands (currently known as typical of pennates). Further research into band types among polar centrics may provide new insights into the relationship between the groups within polar centrics that are currently unresolved by molecular methods.

Accelerated diversification is related to life history and locomotion in a hyperdiverse lineage of microbial eukaryotes (Diatoms, Bacillariophyta)

Patterns of species richness are commonly linked to life history strategies. In diatoms, an exceptionally diverse lineage of photosynthetic heterokonts important for global photosynthesis and burial of atmospheric carbon, lineages with different locomotory and reproductive traits differ dramatically in species richness, but any potential association between life history strategy and diversification has not been tested in a phylogenetic framework. We constructed a time-calibrated, 11-gene, 1151-taxon phylogeny of diatoms - the most inclusive diatom species tree to date. We used this phylogeny, together with a comprehensive inventory of first-last occurrences of Cenozoic fossil diatoms, to estimate ranges of expected species richness, diversification and its variation through time and across lineages. Diversification rates varied with life history traits. Although anisogamous lineages diversified faster than oogamous ones, this increase was restricted to a nested clade with active motility in the vegetative cells. We propose that the evolution of motility in vegetative cells, following an earlier transition from oogamy to anisogamy, facilitated outcrossing and improved utilization of habitat complexity, ultimately leading to enhanced opportunity for adaptive divergence across a variety of novel habitats. Together, these contributed to a species radiation that gave rise to the majority of present-day diatom diversity.