Influence of the centrosome in cytokinesis of brown algae: polyspermic zygotes of Scytosiphon lomentaria (Scytosiphonales, Phaeophyceae)

Algal evolution: A touch of brown in a Paleozoic sea of greens and reds

Previous molecular clock studies indicated a Mesozoic origin for the brown algae (Phaeophyceae). New research based on phylogenetic evidence challenges this notion and provides novel insights into the origin and diversification of brown algae, which includes multiple transitions within the group from isogamy to oogamy (and back again!).

Sexual dimorphism and the evolution of sex-biased gene expression in the brown alga ectocarpus

Males and females often have marked phenotypic differences, and the expression of these dissimilarities invariably involves sex differences in gene expression. Sex-biased gene expression has been well characterized in animal species, where a high proportion of the genome may be differentially regulated in males and females during development. Male-biased genes tend to evolve more rapidly than female-biased genes, implying differences in the strength of the selective forces acting on the two sexes. Analyses of sex-biased gene expression have focused on organisms that exhibit separate sexes during the diploid phase of the life cycle (diploid sexual systems), but the genetic nature of the sexual system is expected to influence the evolutionary trajectories of sex-biased genes. We analyze here the patterns of sex-biased gene expression in Ectocarpus, a brown alga with haploid sex determination (dioicy) and a low level of phenotypic sexual dimorphism. In Ectocarpus, female-biased genes were found to be evolving as rapidly as male-biased genes. Moreover, genes expressed at fertility showed faster rates of evolution than genes expressed in immature gametophytes. Both male- and female-biased genes had a greater proportion of sites experiencing positive selection, suggesting that their accelerated evolution is at least partly driven by adaptive evolution. Gene duplication appears to have played a significant role in the generation of sex-biased genes in Ectocarpus, expanding previous models that propose this mechanism for the resolution of sexual antagonism in diploid systems. The patterns of sex-biased gene expression in Ectocarpus are consistent both with predicted characteristics of UV (haploid) sexual systems and with the distinctive aspects of this organism's reproductive biology.

Ultrastructural and structural characterization of zygotes and embryos during development in Sargassum cymosum (Phaeophyceae, Fucales)

This study investigates the pattern and performance of cellular structures during the early development of zygotes and embryos of Sargassum cymosum. The early development S. cymosum germlings has already been characterized and compared with the pattern of development established for all fucoid algae, in which the zygote remains attached to the receptacle by mucilage during the establishment of polarity and early cell division. As in the algae Fucus and Silvetia, the first division is transverse across the longer axis of the zygote of S. cymosum. However, the cell that will give rise to the rhizoids is not determined in the first division; rather, the formation of this cell occurs with the second division, forming a small cell in the embryo shaded site. Stabilizing polarity during the process of forming a multicellular embryo occurs rapidly. During development, significant cytoplasmic alterations take place. Initially, the cytoplasm shows large clusters of phenolic compounds located in specific parts, but later, in the course of development, these compounds are dispersed in the cytoplasm, although a significant amount remains confined to the nucleus. Moreover, to produce more zygotes and higher growth rates for the germlings, the best conditions found for the species S. cymosum were 22 and 26 °C, respectively.

Electron tomographic analysis of cytokinesis in the brown alga Silvetia babingtonii (Fucales, Phaeophyceae)

In brown algae, membrane resources for the new cell partition during cytokinesis are mainly flat cisternae (FCs) and Golgi-derived vesicles. We used electron tomography coupled with rapid freezing/freeze substitution of zygotes to clarify the structure of transient membrane compartments during cytokinesis in Silvetia zygotes. After mitosis, an amorphous membranous structure, considered to be an FC intermediate was observed near endoplasmic reticulum clusters, lying between two daughter nuclei. FCs were arrayed at the cytokinetic plane, and a tubular membranous network was formed around them. This network might be formed by the consecutive fusion of spherical vesicles that are linked to the edges of FCs to form a membranous network (MN). At the initial stage of the formation of a membranous sac (MS) from the MN, the MS had flat and swollen parts, with the latter showing membranous tunnels. Coated pits were detected with high frequency at the swollen parts of the MS. This observation indicated that membranous tunnels disappeared by recycling of excess membrane via endocytosis, and the swollen part became flat. The MN appeared at the edges of the growing MS. MN and the MN-MS complex were observed along the cytokinetic plane in several spaces. The MS expanded by the incorporation of MN or other MS in its neighborhood. With the maturation of the new cell partition membrane, the thickness of the MS became constant and the membrane cavity disappeared. The changes in the surface area and volume of the transient membrane compartment during cytokinesis were analyzed from the tomographic data.

Complete mitochondrial genome of the brown alga Scytosiphon lomentaria (Scytosiphonaceae, Phaeophyceae)

We determined the complete mitochondrial genome of Scytosiphon lomentaria (Lyngbye) Link, which is the first representative of the genus Scytosiphon C. Agardh. The circular mitogenome of S. lomentaria is 36,918 bp in length, with the overall A+T content of 65.86%. The genome contains 67 genes, including 3 ribosomal RNA genes (rRNA), 25 transfer RNA genes (tRNA), 35 protein-coding genes and 4 unidentified open reading frames (ORFs). The gene order of S. lomentaria mitogenome conforms to that of Ectocarpales mitogenomes (not including Pylaiella littoralis), i.e. Petalonia fascia, and Ectocarpus siliculosus, but differs from Laminariales, Desmarestiales, Fucales and Dictyotales with position variation of several genes. The S. lomentaria mitogenome has an overall nucleotide sequence identity of 80.4% with P. fascia, and 74.9% with E. siliculosus. The present data is of value to phylogenetic analyses of such a diverse Scytosiphonaceae family as well as to understanding of mitogenome evolution in brown algae.

Proteomics analysis of heterogeneous flagella in brown algae (stramenopiles)

Flagella are conserved organelles among eukaryotes and they are composed of many proteins, which are necessary for flagellar assembly, maintenance and function. Stramenopiles, which include brown algae, diatoms and oomycetes, possess two laterally inserted flagella. The anterior flagellum (AF) extends forward and bears tripartite mastigonemes, whilst the smooth posterior flagellum (PF) often has a paraflagellar body structure. These heterogeneous flagella have served as crucial structures in algal studies especially from a viewpoint of phylogeny. However, the protein compositions of the flagella are still largely unknown. Here we report a LC-MS/MS based proteomics analysis of brown algal flagella. In total, 495 flagellar proteins were identified. Functional annotation of the proteome data revealed that brown algal flagellar proteins were associated with cell motility, signal transduction and various metabolic activities. We separately isolated AF and PF and analyzed their protein compositions. This analysis led to the identification of several AF- and PF-specific proteins. Among the PF-specific proteins, we found a candidate novel blue light receptor protein involved in phototaxis, and named it HELMCHROME because of the steering function of PF. Immunological analysis revealed that this protein was localized along the whole length of the PF and concentrated in the paraflagellar body.

Ultrastructural analysis of flagellar development in plurilocular sporangia of Ectocarpus siliculosus (Phaeophyceae)

Flagellar development in the plurilocular zoidangia of sporophytes of the brown alga Ectocarpus siliculosus was analyzed in detail using transmission electron microscopy and electron tomography. A series of cell divisions in the plurilocular zoidangia produced the spore-mother cells. In these cells, the centrioles differentiated into flagellar basal bodies with basal plates at their distal ends and attached to the plasma membrane. The plasma membrane formed a depression (flagellar pocket) into where the flagella elongated and in which variously sized vesicles and cytoplasmic fragments accumulated. The anterior and posterior flagella started elongating simultaneously, and the vesicles and cytoplasmic fragments in the flagellar pocket fused to the flagellar membranes. The two flagella (anterior and posterior) could be clearly distinguished from each other at the initial stage of their development by differences in length, diameter and the appendage flagellar rootlets. Flagella continued to elongate in the flagellar pocket and maintained their mutually parallel arrangement as the flagellar pocket gradually changed position. In mature zoids, the basal part of the posterior flagellum (paraflagellar body) characteristically became swollen and faced the eyespot region. Electron dense materials accumulated between the axoneme and the flagellar membrane, and crystallized materials could also be observed in the swollen region. Before liberation of the zoospores from the plurilocular zoidangia, mastigoneme attachment was restricted to the distal region of the anterior flagellum. Structures just below the flagellar membrane that connected to the mastigonemes were clearly visible by electron tomography.

Brown algae as a model for plant organogenesis

Brown algae are an extremely interesting, but surprisingly poorly explored, group of organisms. They are one of only five eukaryotic lineages to have independently evolved complex multicellularity, which they express through a wide variety of morphologies ranging from uniseriate branched filaments to complex parenchymatous thalli with multiple cell types. Despite their very distinct evolutionary history, brown algae and land plants share a striking amount of developmental features. This has led to an interest in several aspects of brown algal development, including embryogenesis, polarity, cell cycle, asymmetric cell division and a putative role for plant hormone signalling. This review describes how investigations using brown algal models have helped to increase our understanding of the processes controlling early embryo development, in particular polarization, axis formation and asymmetric cell division. Additionally, the diversity of life cycles in the brown lineage and the emergence of Ectocarpus as a powerful model organism, are affording interesting insights on the molecular mechanisms underlying haploid-diploid life cycles. The use of these and other emerging brown algal models will undoubtedly add to our knowledge on the mechanisms that regulate development in multicellular photosynthetic organisms.

Membrane fusion process and assembly of cell wall during cytokinesis in the brown alga, Silvetia babingtonii (Fucales, Phaeophyceae)

During cytokinesis in brown algal cells, Golgi-derived vesicles (GVs) and flat cisternae (FCs) are involved in building the new cell partition membrane. In this study, we followed the membrane fusion process in Silvetia babingtonii zygotes using electron microscopy together with rapid freezing and freeze substitution. After mitosis, many FCs were formed around endoplasmic reticulum clusters and these then spread toward the future cytokinetic plane. Actin depolymerization using latrunculin B prevented the appearance of the FCs. Fusion of GVs to FCs resulted in structures that were thicker and more elongated (EFCs; expanded flat cisternae). Some complicated membranous structures (MN; membranous network) were formed by interconnection of EFCs and following the arrival of additional GVs. The MN grew into membranous sacs (MSs) as gaps between the MNs disappeared. The MSs were observed in patches along the cytokinetic plane. Neighboring MSs were united to form the new cell partition membrane. An immunocytochemical analysis indicated that fucoidan was synthesized in Golgi bodies and transported by vesicles to the future cytokinetic plane, where the vesicles fused with the FCs. Alginate was not detected until the MS phase. Incubation of sections with cellulase-gold showed that the cellulose content of the new cross wall was not comparable to that of the parent cell wall.

Cytoplasmic inheritance of organelles in brown algae

Brown algae, together with diatoms and chrysophytes, are a member of the heterokonts. They have either a characteristic life cycle of diplohaplontic alternation of gametophytic and sporophytic generations that are isomorphic or heteromorphic, or a diplontic life cycle. Isogamy, anisogamy and oogamy have been recognized as the mode of sexual reproduction. Brown algae are the characteristic group having elaborated multicellular organization within the heterokonts. In this study, cytoplasmic inheritance of chloroplasts, mitochondria and centrioles was examined, with special focus on sexual reproduction and subsequent zygote development. In oogamy, chloroplasts and mitochondria are inherited maternally. In isogamy, chloroplasts in sporophyte cells are inherited biparentally (maternal or paternal); however, mitochondria (or mitochondrial DNA) derived from the female gamete only remained during zygote development after fertilization. Centrioles in zygotes are definitely derived from the male gamete, irrespective of the sexual reproduction pattern. Female centrioles in zygotes are selectively broken down within 1-2 h after fertilization. The remaining male centrioles play a crucial role as a part of the centrosome for microtubule organization, mitosis, determination of the cytokinetic plane and cytokinesis, as well as for maintaining multicellularity and regular morphogenesis in brown algae.

EFFECT OF LATRUNCULIN B AND BREFELDIN A ON CYTOKINESIS IN THE BROWN ALGA SCYTOSIPHON LOMENTARIA (SCYTOSIPHONALES, PHAEOPHYCEAE)(1)

In zygotes of the brown alga Scytosiphon lomentaria (Lyngb.) Link, cytokinesis proceeds by growth of membranous sacs, which are formed by fusion of Golgi vesicles and flat cisternae accumulated at the future cytokinetic plane. It has been reported that depolymerization of actin filaments by latrunculin B does not inhibit mitosis. However, this molecule prevents the formation of the actin plate, which appears at the region of intermingled microtubules from each centrosome just before and during cytokinesis. In this study, zygotes treated with latrunculin B were observed using EM. Remarkably, this reagent inhibited the formation of flat cisternae. Golgi vesicles gathered around the midzone between the two daughter nuclei and fused with the plasma membrane there. As a result, the plasma membrane invaginated, in a complicated manner, into the cytoplasm. However, these invaginations of the plasma membrane never produced a continuous partition membrane. The ultrastructure of zygotes treated with brefeldin A, which prevents Golgi-mediated secretion, was also examined. Flat cisternae appeared at the future cytokinetic plane, and a new cell partition membrane was formed. However, the partition membrane became thick, because it was filled with amorphous material rather than the normal rigid fibrous material. These results suggested that actin is involved in the formation of flat cisternae, where it is necessary for completion of the new cell partition membrane, and that Golgi vesicles may play an important role in the deposition of cell wall material.

Homologs of the sexually induced gene 1 (sig1) product constitute the stramenopile mastigonemes

The tripartite tubular mastigoneme on the anterior flagellum is a morphological feature that characterizes the stramenopiles. Mastigonemes are significant and potentially informative structures not only from the viewpoint of systematics, but also of cell biology. Nevertheless, few biochemical studies have been reported on stramenopile mastigonemes. The flagella of Scytosiphon lomentaria (Phaeophyceae) were successfully isolated and analyzed using SDS-PAGE followed by protein sequencing. The partial amino acid sequence of one flagellar protein (115kDa) showed high similarity with the sexually induced gene 1 (sig1) product of centric diatoms. A polyclonal antibody against the 115-kDa protein reacted not only to the shaft of mastigonemes in Scytosiphon lomentaria, but also another distinctly different stramenopile flagellate, Sulcochrysis biplastida (Dictyochophyceae). Therefore, we propose that the 115-kDa protein (i.e. Sig1 homologs) is a constituent of the tubular shaft of the mastigoneme.

Enzymes of type II fatty acid synthesis and apicoplast differentiation and division in Eimeria tenella

Apicomplexan parasites, Eimeria tenella, Plasmodium spp. and Toxoplasma gondii, possess a homologous plastid-like organelle termed the apicoplast, derived from the endosymbiotic enslavement of a photosynthetic alga. However, currently no eimerian nuclear encoded apicoplast targeted proteins have been identified, unlike in Plasmodium spp. and T. gondii. In this study, we demonstrate that nuclear encoded enoyl reductase of E. tenella (EtENR) has a predicted N-terminal bipartite transit sequence, typical of apicoplast-targeted proteins. Using a combination of immunocytochemistry and EM we demonstrate that this fatty acid biosynthesis protein is located in the apicoplast of E. tenella. Using the EtENR as a tool to mark apicoplast development during the Eimeria lifecycle, we demonstrate that nuclear and apicoplast division appear to be independent events, both organelles dividing prior to daughter cell formation, with each daughter cell possessing one to four apicoplasts. We believe this is the first report of multiple apicoplasts present in the infectious stage of an apicomplexan parasite. Furthermore, the microgametes lacked an identifiable apicoplast consistent with maternal inheritance via the macrogamete. It was found that the size of the organelle and the abundance of EtENR varied with developmental stage of the E. tenella lifecycle. The high levels of EtENR protein observed during asexual development and macrogametogony is potentially associated with the increased synthesis of fatty acids required for the rapid formation of numerous merozoites and for the extracellular development and survival of the oocyst. Taken together the data demonstrate that the E. tenella apicoplast participates in type II fatty acid biosynthesis with increased expression of ENR during parasite growth. Apicoplast division results in the simultaneous formation of multiple fragments. The division mechanism is unknown, but is independent of nuclear division and occurs prior to daughter formation.

Cytokinesis in plant and animal cells: endosomes 'shut the door'

For many years, cytokinesis in eukaryotic cells was considered to be a process that took a variety of forms. This is rather surprising in the face of an apparently conservative mitosis. Animal cytokinesis was described as a process based on an actomyosin-based contractile ring, assembling, and acting at the cell periphery. In contrast, cytokinesis of plant cells was viewed as the centrifugal generation of a new cell wall by fusion of Golgi apparatus-derived vesicles. However, recent advances in animal and plant cell biology have revealed that many features formerly considered as plant-specific are, in fact, valid also for cytokinetic animal cells. For example, vesicular trafficking has turned out to be important not only for plant but also for animal cytokinesis. Moreover, the terminal phase of animal cytokinesis based on midbody microtubule activity resembles plant cytokinesis in that interdigitating microtubules play a decisive role in the recruitment of cytokinetic vesicles and directing them towards the cytokinetic spaces which need to be plugged by fusing endosomes. Presently, we are approaching another turning point which brings cytokinesis in plant and animal cells even closer. As an unexpected twist, new studies reveal that both plant and animal cytokinesis is driven not so much by Golgi-derived vesicles but rather by homotypically and heterotypically fusing endosomes. These are generated from cytokinetic cortical sites defined by preprophase microtubules and contractile actomyosin ring, which induce local endocytosis of both the plasma membrane and cell wall material. Finally, plant and animal cytokinesis meet together at the physical separation of daughter cells despite obvious differences in their preparatory events.

Behavior and function of paternally inherited centrioles in brown algal zygotes

In brown algal cells, the centrosome, consisting of a pair of centrioles and the pericentriolar material, is primarily involved in the organization of microtubules (MTs) throughout the cell cycle. In motile cells, the centrioles participate in the formation of flagellar axoneme as flagellar basal bodies, and in somatic cells they play a crucial role in many cellular activities as a part of the centrosome. With respect to the role of the centrosome as a microtubule organizing center (MTOC), brown algal cells resemble animal cells. In most animal fertilization processes, the sperm cell introduces centrioles, the core of the centrosome, into the egg cytoplasm. In this study, the behavior of centrioles from gametogenesis and fertilization to the first cell division of the zygote was examined in the three sexual reproduction patterns occurring in brown algae, i.e., oogamy, anisogamy and isogamy, by electron- and immunofluorescence-microscopy. The pair of centrioles contained in somatic cells was shown to be derived from the male gamete, irrespective of the sexual reproductive pattern. The paternally derived centrioles were duplicated before mitosis and were involved in spindle pole formation. Moreover, MTs from the centrosome play a crucial role in the process of cytokinesis, as the position of centrosomes accompanying daughter nuclei seems to determine the cytokinetic plane. A new approach to clarifying the mode of cytokinesis in brown algae is presented in this study.

Destruction of maternal centrioles during fertilization of the brown alga, Scytosiphon lomentaria (Scytosiphonales, Phaeophyceae)

In brown algal fertilization, a pair of centrioles is derived from the male gamete, irrespective of the sexual reproduction pattern, i.e., isogamy, anisogamy, or oogamy. In this study, the manner in which the maternal centriole structure is destroyed in early zygotes of the isogamous brown alga Scytosiphon lomentaria was examined by electron microscopy. At fertilization, the zygote had two pairs of centrioles (flagellar basal bodies) derived from motile male and female gametes, and there was no morphological difference between the two pairs. The flagellar basal plate and the axonemal microtubules were still connected with the distal end of centrioles. Ultrastructural observations showed that the integrity of maternal-derived centrioles began to degenerate even in the 1-h-old zygote. At that time, the cylinder of triplet microtubules of the maternal centrioles became shorter from the distal end, and a section passing through the centrioles indicated that a part of the nine triplets of microtubules changed into doublet or singlet microtubules by degeneration of B and/or C tubules. In 2-h-old zygote, there was no trace of maternal centrioles ultrastructurally, and only the paternal centrioles remained. Further, reduction of centrin accompanying destruction of the maternal centrioles was examined in immunofluorescence microscopy. Centrin localized at the paternal and the maternal centrioles had the same fluorescence intensity in the early zygotes. At 4-6 h after fertilization, two spots indicating centrin localization showed different fluorescence intensity. Later, the weaker spot disappeared completely. These results showed that there is a difference in time between the destruction of the centriolar cylinders and the reduction of centrin molecules around them.