Ectopic expression of a Chlamydomonas mt+-specific homeodomain protein in mt- gametes initiates zygote development without gamete fusion

Life cycle and morphogenetic differentiation in heteromorphic cell types of a cosmopolitan marine microalga

Gephyrocapsa huxleyi is a prevalent, bloom-forming phytoplankton species in the oceans. It exhibits a complex haplodiplontic life cycle, featuring a diploid-calcified phase, a haploid phase and a third 'decoupled' phase produced during viral infection. Decoupled cells display a haploid-like phenotype, but are diploid. Here, we investigated the fate of decoupled cells during culture observations and we compared the transcriptome profiles and the cellular ultrastructure of the three life cycle cell types. We found that decoupled cells can revert to the calcified form in the absence of viral pressure, revealing the ability of G. huxleyi to modulate cell differentiation as a function of external conditions. Ultrastructural analyses showed distinct nuclear organization with variations in chromatin volume. Transcriptomic analyses revealed gene expression patterns specific to each life phase. These included multiple regulatory functions in chromatin remodeling, broader epigenetic mechanisms and life cycling, likely contributing to cell differentiation. Finally, analyses of available host-virus transcriptomes support life cycle transition during viral infection. This study provides cellular and molecular foundations for nuclear remodeling and cell differentiation in coccolithophores and the identification of gene markers for studying coccolithophore life cycles in natural populations.

Control of sporophyte secondary cell wall development in Marchantia by a Class II KNOX gene

Land plants evolved from an ancestral alga around 470 mya, evolving complex multicellularity in both haploid gametophyte and diploid sporophyte generations. The evolution of water-conducting tissues in the sporophyte generation was crucial for the success of land plants, paving the way for the colonization of a variety of terrestrial habitats. Class II KNOX (KNOX2) genes are major regulators of secondary cell wall formation and seed mucilage (pectin) deposition in flowering plants. Here, we show that, in the liverwort Marchantia polymorpha, loss-of-function alleles of the KNOX2 ortholog, MpKNOX2, or its dimerization partner, MpBELL1, have defects in capsule wall secondary cell wall and spore pectin biosynthesis. Both genes are expressed in the gametophytic calyptra surrounding the sporophyte and exert maternal effects, suggesting intergenerational regulation from the maternal gametophyte to the sporophytic embryo. These findings also suggest the presence of a secondary wall genetic program in the non-vascular liverwort capsule wall, with attributes of secondary walls in vascular tissues.

Conserved and non-conserved RNA-target modules in plants: lessons for a better understanding of Marchantia development

A wide variety of functional regulatory non-coding RNAs (ncRNAs) have been identified as essential regulators of plant growth and development. Depending on their category, ncRNAs are not only involved in modulating target gene expression at the transcriptional and post-transcriptional levels but also are involved in processes like RNA splicing and RNA-directed DNA methylation. To fulfill their molecular roles properly, ncRNAs must be precisely processed by multiprotein complexes. In the case of small RNAs, DICER-LIKE (DCL) proteins play critical roles in the production of mature molecules. Land plant genomes contain at least four distinct classes of DCL family proteins (DCL1-DCL4), of which DCL1, DCL3 and DCL4 are also present in the genomes of bryophytes, indicating the early divergence of these genes. The liverwort Marchantia polymorpha has become an attractive model species for investigating the evolutionary history of regulatory ncRNAs and proteins that are responsible for ncRNA biogenesis. Recent studies on Marchantia have started to uncover the similarities and differences in ncRNA production and function between the basal lineage of bryophytes and other land plants. In this review, we summarize findings on the essential role of regulatory ncRNAs in Marchantia development. We provide a comprehensive overview of conserved ncRNA-target modules among M. polymorpha, the moss Physcomitrium patens and the dicot Arabidopsis thaliana, as well as Marchantia-specific modules. Based on functional studies and data from the literature, we propose new connections between regulatory pathways involved in Marchantia's vegetative and reproductive development and emphasize the need for further functional studies to understand the molecular mechanisms that control ncRNA-directed developmental processes.

Three-Dimensional Morphological Analysis Revealed the Cell Patterning Bases for the Sexual Dimorphism Development in the Liverwort Marchantia polymorpha

In land plants, sexual dimorphism can develop in both diploid sporophytes and haploid gametophytes. While developmental processes of sexual dimorphism have been extensively studied in the sporophytic reproductive organs of model flowering plants such as stamens and carpels of Arabidopsis thaliana, those occurring in gametophyte generation are less well characterized due to the lack of amenable model systems. In this study, we performed three-dimensional morphological analyses of gametophytic sexual branch differentiation in the liverwort Marchantia polymorpha, using high-depth confocal imaging and a computational cell segmentation technique. Our analysis revealed that the specification of germline precursors initiates in a very early stage of sexual branch development, where incipient branch primordia are barely recognizable in the apical notch region. Moreover, spatial distribution patterns of germline precursors differ between males and females from the initial stage of primordium development in a manner dependent on the master sexual differentiation regulator MpFGMYB. At later stages, distribution patterns of germline precursors predict the sex-specific gametangia arrangement and receptacle morphologies seen in mature sexual branches. Taken together, our data suggest a tightly coupled progression of germline segregation and sexual dimorphism development in M. polymorpha.

Characteristics of N6-methyladenosine Modification During Sexual Reproduction of Chlamydomonas reinhardtii

The unicellular green alga Chlamydomonas reinhardtii (hereafter Chlamydomonas) possesses both plant and animal attributes, and it is an ideal model organism for studying fundamental processes such as photosynthesis, sexual reproduction, and life cycle. N6-methyladenosine (m6A) is the most prevalent mRNA modification, and it plays important roles during sexual reproduction in animals and plants. However, the pattern and function of m6A modification during the sexual reproduction of Chlamydomonas remain unknown. Here, we performed transcriptome and methylated RNA immunoprecipitation sequencing (MeRIP-seq) analyses on six samples from different stages during sexual reproduction of the Chlamydomonas life cycle. The results show that m6A modification frequently occurs at the main motif of DRAC (D = G/A/U, R = A/G) in Chlamydomonas mRNAs. Moreover, m6A peaks in Chlamydomonas mRNAs are mainly enriched in the 3' untranslated regions (3'UTRs) and negatively correlated with the abundance of transcripts at each stage. In particular, there is a significant negative correlation between the expression levels and the m6A levels of genes involved in the microtubule-associated pathway, indicating that m6A modification influences the sexual reproduction and the life cycle of Chlamydomonas by regulating microtubule-based movement. In summary, our findings are the first to demonstrate the distribution and the functions of m6A modification in Chlamydomonas mRNAs and provide new evolutionary insights into m6A modification in the process of sexual reproduction in other plant organisms.

Uncovering an ancestral green ménage à trois: Contributions of Chlamydomonas to the discovery of a broadly conserved triad of plant fertilization proteins

During sexual reproduction in the unicellular green alga Chlamydomonas, gametes undergo the conserved cellular events that define fertilization across the tree of life. After initial ciliary adhesion, plus and minus gametes attach to each other at plasma membrane sites specialized for fusion, their bilayers merge, and cell coalescence into a quadri-ciliated cell signals for nuclear fusion. Recent findings show that these conserved cellular events are driven by 3 conserved protein families, FUS1/GEX2, HAP2/GCS1, and KAR5/GEX1. New results also show that species-specific recognition in Chlamydomonas activates the ancestral, viral-like fusogen HAP2 to drive fusion; that the conserved nuclear envelope fusion protein KAR5/GEX1 is also essential for nuclear fusion in Arabidopsis; and that heterodimerization of BELL-KNOX proteins signals for nuclear fusion in Chlamydomonas through early diverging land plants. This review outlines how Chlamydomonas's Janus-like position in evolution along with the ease of working with its gametes have revealed broadly conserved mechanisms.

Gamete expression of TALE class HD genes activates the diploid sporophyte program in Marchantia polymorpha

Eukaryotic life cycles alternate between haploid and diploid phases and in phylogenetically diverse unicellular eukaryotes, expression of paralogous homeodomain genes in gametes primes the haploid-to-diploid transition. In the unicellular chlorophyte alga Chlamydomonas, KNOX and BELL TALE-homeodomain genes mediate this transition. We demonstrate that in the liverwort Marchantia polymorpha, paternal (sperm) expression of three of five phylogenetically diverse BELL genes, MpBELL234, and maternal (egg) expression of both MpKNOX1 and MpBELL34 mediate the haploid-to-diploid transition. Loss-of-function alleles of MpKNOX1 result in zygotic arrest, whereas a loss of either maternal or paternal MpBELL234 results in variable zygotic and early embryonic arrest. Expression of MpKNOX1 and MpBELL34 during diploid sporophyte development is consistent with a later role for these genes in patterning the sporophyte. These results indicate that the ancestral mechanism to activate diploid gene expression was retained in early diverging land plants and subsequently co-opted during evolution of the diploid sporophyte body.

TALE homeobox heterodimer GSM1/GSP1 is a molecular switch that prevents unwarranted genetic recombination in Chlamydomonas

Eukaryotic sexual life cycles alternate between haploid and diploid stages, the transitions between which are delineated by cell fusion and meiotic division. Transcription factors in the TALE-class homeobox family, GSM1 and GSP1, predominantly control gene expression for the haploid-to-diploid transition during sexual reproduction in the unicellular green alga, Chlamydomonas reinhardtii. To understand the roles that GSM1 and GSP1 play in zygote development, we used gsm1 and gsp1 mutants and examined fused gametes that normally undergo the multiple organellar fusions required for the genetic unity of the zygotes. In gsm1 and gsp1 zygotes, no fusion was observed for the nucleus and chloroplast. Surprisingly, mitochondria and endoplasmic reticulum, which undergo dynamic autologous fusion/fission, did not undergo heterologous fusions in gsm1 or gsp1 zygotes. Furthermore, the mutants failed to resorb their flagella, an event that normally renders the zygotes immotile. When gsm1 and gsp1 zygotes resumed the mitotic cycle, their two nuclei fused prior to mitosis, but neither chloroplastic nor mitochondrial fusion took place, suggesting that these fusions are specifically turned on by GSM1/GSP1. Taken together, this study shows that organellar restructuring during zygotic diploidization does not occur by default but is triggered by a combinatorial switch, the GSM1/GSP1 dyad. This switch may represent an ancient mechanism that evolved to restrict genetic recombination during sexual development.

Gene Regulatory Networks for the Haploid-to-Diploid Transition of Chlamydomonas reinhardtii

The sexual cycle of the unicellular Chlamydomonas reinhardtii culminates in the formation of diploid zygotes that differentiate into dormant spores that eventually undergo meiosis. Mating between gametes induces rapid cell wall shedding via the enzyme g-lysin; cell fusion is followed by heterodimerization of sex-specific homeobox transcription factors, GSM1 and GSP1, and initiation of zygote-specific gene expression. To investigate the genetic underpinnings of the zygote developmental pathway, we performed comparative transcriptome analysis of both pre- and post-fertilization samples. We identified 253 transcripts specifically enriched in early zygotes, 82% of which were not up-regulated in gsp1 null zygotes. We also found that the GSM1/GSP1 heterodimer negatively regulates the vegetative wall program at the posttranscriptional level, enabling prompt transition from vegetative wall to zygotic wall assembly. Annotation of the g-lysin-induced and early zygote genes reveals distinct vegetative and zygotic wall programs, supported by concerted up-regulation of genes encoding cell wall-modifying enzymes and proteins involved in nucleotide-sugar metabolism. The haploid-to-diploid transition in Chlamydomonas is masterfully controlled by the GSM1/GSP1 heterodimer, translating fertilization and gamete coalescence into a bona fide differentiation program. The fertilization-triggered integration of genes required to make related, but structurally and functionally distinct organelles-the vegetative versus zygote cell wall-presents a likely scenario for the evolution of complex developmental gene regulatory networks.

Homeodomain-like DNA binding proteins control the haploid-to-diploid transition in Dictyostelium

Homeodomain proteins control the developmental transition between the haploid and diploid phases in several eukaryotic lineages, but it is not known whether this regulatory mechanism reflects the ancestral condition or, instead, convergent evolution. We have characterized the mating-type locus of the amoebozoan Dictyostelium discoideum, which encodes two pairs of small proteins that determine the three mating types of this species; none of these proteins display recognizable homology to known families. We report that the nuclear magnetic resonance structures of two of them, MatA and MatB, contain helix-turn-helix folds flanked by largely disordered amino- and carboxyl-terminal tails. This fold closely resembles that of homeodomain transcription factors, and, like those proteins, MatA and MatB each bind DNA characteristically using the third helix of their folded domains. By constructing chimeric versions containing parts of MatA and MatB, we demonstrate that the carboxyl-terminal tail, not the central DNA binding motif, confers mating specificity, providing mechanistic insight into how a third mating type might have originated. Finally, we show that these homeodomain-like proteins specify zygote function: Hemizygous diploids, formed in crosses between a wild-type strain and a mat null mutant, grow and differentiate identically to haploids. We propose that Dictyostelium MatA and MatB are divergent homeodomain proteins with a conserved function in triggering the haploid-to-diploid transition that can be traced back to the last common ancestor of eukaryotes.

Early plant embryogenesis-dark ages or dark matter?

In nearly all flowering plants, the basic body plan is laid down during embryogenesis. In Arabidopsis, the crucial cell types are established extremely early as reflected in the stereotypic sequence of oriented cell divisions in the developing young embryo. Research into early embryogenesis was especially focused on the role of the infamous tryptophan derivative auxin in establishing embryo polarity and generating the main body axis. However, it is becoming obvious that the mere link to auxin does not provide any mechanistic understanding of early embryo patterning. Taking recent research into account, we discuss mechanisms underlying early embryonic patterning from an evolutionary perspective.

Evolution in the Cycles of Life

The life cycles of eukaryotes alternate between haploid and diploid phases, which are initiated by meiosis and gamete fusion, respectively. In both ascomycete and basidiomycete fungi and chlorophyte algae, the haploid-to-diploid transition is regulated by a pair of paralogous homeodomain protein encoding genes. That a common genetic program controls the haploid-to-diploid transition in phylogenetically disparate eukaryotic lineages suggests this may be the ancestral function for homeodomain proteins. Multicellularity has evolved independently in many eukaryotic lineages in either one or both phases of the life cycle. Organisms, such as land plants, exhibiting a life cycle whereby multicellular bodies develop in both the haploid and diploid phases are often referred to as possessing an alternation of generations. We review recent progress on understanding the genetic basis for the land plant alternation of generations and highlight the roles that homeodomain-encoding genes may have played in the evolution of complex multicellularity in this lineage.

Identification and Characterization of a cis-Regulatory Element for Zygotic Gene Expression in Chlamydomonas reinhardtii

Upon fertilization Chlamydomonas reinhardtii zygotes undergo a program of differentiation into a diploid zygospore that is accompanied by transcription of hundreds of zygote-specific genes. We identified a distinct sequence motif we term a zygotic response element (ZYRE) that is highly enriched in promoter regions of C reinhardtii early zygotic genes. A luciferase reporter assay was used to show that native ZYRE motifs within the promoter of zygotic gene ZYS3 or intron of zygotic gene DMT4 are necessary for zygotic induction. A synthetic luciferase reporter with a minimal promoter was used to show that ZYRE motifs introduced upstream are sufficient to confer zygotic upregulation, and that ZYRE-controlled zygotic transcription is dependent on the homeodomain transcription factor GSP1. We predict that ZYRE motifs will correspond to binding sites for the homeodomain proteins GSP1-GSM1 that heterodimerize and activate zygotic gene expression in early zygotes.

Evolution and regulation of complex life cycles: a brown algal perspective

The life cycle of an organism is one of its fundamental features, influencing many aspects of its biology. The brown algae exhibit a diverse range of life cycles indicating that transitions between life cycle types may have been key adaptive events in the evolution of this group. Life cycle mutants, identified in the model organism Ectocarpus, are providing information about how life cycle progression is regulated at the molecular level in brown algae. We explore some of the implications of the phenotypes of the life cycle mutants described to date and draw comparisons with recent insights into life cycle regulation in the green lineage. Given the importance of coordinating growth and development with life cycle progression, we suggest that the co-option of ancient life cycle regulators to control key developmental events may be a common feature in diverse groups of multicellular eukaryotes.

KNOX2 genes regulate the haploid-to-diploid morphological transition in land plants

Unlike animals, land plants undergo an alternation of generations, producing multicellular bodies in both haploid (1n: gametophyte) and diploid (2n: sporophyte) generations. Plant body plans in each generation are regulated by distinct developmental programs initiated at either meiosis or fertilization, respectively. In mosses, the haploid gametophyte generation is dominant, whereas in vascular plants-including ferns, gymnosperms, and angiosperms-the diploid sporophyte generation is dominant. Deletion of the class 2 KNOTTED1-LIKE HOMEOBOX (KNOX2) transcription factors in the moss Physcomitrella patens results in the development of gametophyte bodies from diploid embryos without meiosis. Thus, KNOX2 acts to prevent the haploid-specific body plan from developing in the diploid plant body, indicating a critical role for the evolution of KNOX2 in establishing an alternation of generations in land plants.

Gsp1 triggers the sexual developmental program including inheritance of chloroplast DNA and mitochondrial DNA in Chlamydomonas reinhardtii

The isogamous green alga Chlamydomonas reinhardtii has emerged as a premier model for studying the genetic regulation of fertilization and sexual development. A key regulator is known to be a homeoprotein gene, GAMETE-SPECIFIC PLUS1 (GSP1), which triggers the zygotic program. In this study, we isolated a mutant, biparental31 (bp31), which lacks GSP1. bp31 mt+ gametes fuse normally to form zygotes, but the sexual development of the resulting diploid cell is arrested and pellicle/zygospore/tetrad formation is abolished. The uniparental inheritance of chloroplast (cp) and mitochondrial (mt) DNA (cytoplasmic inheritance) was also impaired. bp31 has a deletion of ∼60 kb on chromosome 2, including GSP1. The mutant phenotype was not rescued by transformation with GSP1 alone but could be rescued by the cotransformation with GSP1 and another gene, INOSITOL MONOPHOSPHATASE-LIKE1, which is involved in various cellular processes, including the phosphatidylinositol signaling pathway. This study confirms the importance of Gsp1 in mediating the zygotic program, including the uniparental inheritance of cp/mtDNA. Moreover, the results also suggest a role for inositol metabolism in the sexual developmental program.

What uses are mating types? The "developmental switch" model

Why mating types exist at all is subject to much debate. Among hypotheses, mating types evolved to control organelle transmission during sexual reproduction, or to prevent inbreeding or same-clone mating. Here I review data from a diversity of taxa (including ciliates, algae, slime molds, ascomycetes, and basidiomycetes) to show that the structure and function of mating types run counter the above hypotheses. I argue instead for a key role in triggering developmental switches. Genomes must fulfill a diversity of alternative programs along the sexual cycle. As a haploid gametophyte, an individual may grow vegetatively (through haploid mitoses), or initiate gametogenesis and mating. As a diploid sporophyte, similarly, it may grow vegetatively (through diploid mitoses) or initiate meiosis and sporulation. Only diploid sporophytes (and not haploid gametophytes) should switch on the meiotic program. Similarly, only haploid gametophytes (not sporophytes) should switch on gametogenesis and mating. And they should only do so when other gametophytes are ready to do the same in the neighborhood. As argued here, mating types have evolved primarily to switch on the right program at the right moment.

Meiosis in flowering plants and other green organisms

Sexual eukaryotes generate gametes using a specialized cell division called meiosis that serves both to halve the number of chromosomes and to reshuffle genetic variation present in the parent. The nature and mechanism of the meiotic cell division in plants and its effect on genetic variation are reviewed here. As flowers are the site of meiosis and fertilization in angiosperms, meiotic control will be considered within this developmental context. Finally, we review what is known about the control of meiosis in green algae and non-flowering land plants and discuss evolutionary transitions relating to meiosis that have occurred in the lineages giving rise to the angiosperms.

Membrane fusion triggers rapid degradation of two gamete-specific, fusion-essential proteins in a membrane block to polygamy in Chlamydomonas

The plasma membranes of gametes are specialized for fusion, yet, once fusion occurs, in many organisms the new zygote becomes incapable of further membrane fusion reactions. The molecular mechanisms that underlie this loss of fusion capacity (block to polygamy) remain unknown. During fertilization in the green alga Chlamydomonas, the plus gamete-specific membrane protein FUS1 is required for adhesion between the apically localized sites on the plasma membranes of plus and minus gametes that are specialized for fusion, and the minus-specific membrane protein HAP2 is essential for completion of the membrane fusion reaction. HAP2 (GCS1) family members are also required for fertilization in Arabidopsis, and for the membrane fusion reaction in the malaria organism Plasmodium berghei. Here, we tested whether Chlamydomonas gamete fusion triggers alterations in FUS1 and HAP2 and renders the plasma membranes of the cells incapable of subsequent fusion. We find that, even though the fusogenic sites support multi-cell adhesions, triploid zygotes are rare, indicating a fusion-triggered block to the membrane fusion reaction. Consistent with the extinction of fusogenic capacity, both FUS1 and HAP2 are degraded upon fusion. The rapid, fusion-triggered cleavage of HAP2 in zygotes is distinct from degradation occurring during constitutive turnover in gametes. Thus, gamete fusion triggers specific degradation of fusion-essential proteins and renders the zygote incapable of fusion. Our results provide the first molecular explanation for a membrane block to polygamy in any organism.

Body building on land: morphological evolution of land plants

Land plants are derived from green algal ancestors and made their first appearance on land 460 million years ago. The life cycle of the land plant body comprises two multicellular stages -- one haploid (gametophyte) and the other diploid (sporophyte). Recent discoveries suggest that the genes controlling diploid development in ancestral green algal zygotes diversified in the land plant lineage where they control the development of the diploid body plan. There are also numerous examples of the independent recruitment of sets of genes to control the development of structures that are morphologically and functionally similar. These discoveries are giving insights into the mechanism by which land plant morphologies changed over the past 460 million years.

Early sexual origins of homeoprotein heterodimerization and evolution of the plant KNOX/BELL family

Developmental mechanisms that yield multicellular diversity are proving to be well conserved within lineages, generating interest in their origins in unicellular ancestors. We report that molecular regulation of the haploid-diploid transition in Chlamydomonas, a unicellular green soil alga, shares common ancestry with differentiation pathways in land plants. Two homeoproteins, Gsp1 and Gsm1, contributed by gametes of plus and minus mating types respectively, physically interact and translocate from the cytosol to the nucleus upon gametic fusion, initiating zygote development. Their ectopic expression activates zygote development in vegetative cells and, in a diploid background, the resulting zygotes undergo a normal meiosis. Gsm1/Gsp1 dyads share sequence homology with and are functionally related to KNOX/BELL dyads regulating stem-cell (meristem) specification in land plants. We propose that combinatorial homeoprotein-based transcriptional control, a core feature of the fungal/animal radiation, may have originated in a sexual context and enabled the evolution of land-plant body plans.

Sex determination in Chlamydomonas

The sex-determination system of the unicellular green alga, Chlamydomonas reinhardtii, is governed by genes in the mating-type (MT) locus and entails additional genes located in autosomes. Gene expression is initiated by nitrogen starvation, and cells differentiate into plus or minus gametes within 6h. Reviewed is our current understanding of gametic differentiation and fertilization, initiation of zygote development, and the uniparental inheritance of organelle genomes.

The role of knox genes in plant development

knox genes encode homeodomain-containing transcription factors that are required for meristem maintenance and proper patterning of organ initiation. In plants with simple leaves, knox genes are expressed exclusively in the meristem and stem, but in dissected leaves, they are also expressed in leaf primordia, suggesting that they may play a role in the diversity of leaf form. This hypothesis is supported by the intriguing phenotypes found in gain-of-function mutations where knox gene misexpression affects leaf and petal shape. Similar phenotypes are also found in recessive mutations of genes that function to negatively regulate knox genes. KNOX proteins function as heterodimers with other homeodomains in the TALE superclass. The gibberellin and lignin biosynthetic pathways are known to be negatively regulated by KNOX proteins, which results in indeterminate cell fates.

Evolution of fungal sex chromosomes

Sexual reproduction enables organisms to shuffle two parental genomes to produce recombinant progeny, and to purge the genome of deleterious mutations. Sex is conserved in virtually all organisms, from bacteria and fungi to plants and animals, and yet the mechanisms by which sexual identity are established share both conserved general features and are remarkably diverse. In animals, sexual identity is established by dimorphic sex chromosomes, whereas in fungi a specialized region of the genome, known as the mating-type locus, governs the establishment of cell type identity and differs in DNA sequence between cells of different mating-types. Recent studies on the mating-type loci of fungi and algae reveal features shared with the mammalian X and Y chromosomes, suggesting that these represent early steps in the evolution of sex chromosomes.

Flagellar adhesion between mating type plus and mating type minus gametes activates a flagellar protein-tyrosine kinase during fertilization in Chlamydomonas

When Chlamydomonas gametes of opposite mating type are mixed together, flagellar adhesion through sex-specific adhesion molecules triggers a transient elevation of intracellular cAMP, leading to gamete activation in preparation for cell-cell fusion and zygote formation. Here, we have identified a protein-tyrosine kinase (PTK) activity that is stimulated by flagellar adhesion. We determined that the protein-tyrosine kinase inhibitor genistein inhibited fertilization, and that fertilization was rescued by dibutyryl cAMP, indicating that the genistein-sensitive step was upstream of the increase in cAMP. Incubation with ATP of flagella isolated from non-adhering and adhering gametes followed by SDS-PAGE and immunoblotting with anti-phosphotyrosine antibodies showed that adhesion activated a flagellar PTK that phosphorylated a 105-kDa flagellar protein. Assays using an exogenous protein-tyrosine kinase substrate confirmed that the activated PTK could be detected only in flagella isolated from adhering gametes. Our results indicate that stimulation of the PTK is a very early event during fertilization. Activation of the PTK was blocked when gametes underwent flagellar adhesion in the presence of the protein kinase inhibitor staurosporine, but not in the presence of the cyclic nucleotide-dependent protein kinase inhibitor, H8, which (unlike staurosporine) does not block the increases in cAMP. In addition, incubation of gametes of a single mating type in dibutyryl cAMP failed to activate the PTK. Finally, flagella adhesion between plus and minus fla10-1 gametes, which have a temperature-sensitive lesion in the microtubule motor protein kinesin-II, failed to activate the PTK at elevated temperatures. Our results show that kinesin-II is essential for coupling flagellar adhesion to activation of a flagellar PTK and cAMP generation during fertilization in Chlamydomonas.

Protein transport and signal transduction during fertilization in chlamydomonas

Fertilization in Chlamydomonas begins with flagellar adhesion between mating type plus and mating type minus gametes and is consummated within minutes by zygote formation. Once fusion occurs, the newly merged gametes cease existence as distinct entities, and the diploid zygote immediately initiates transcription of zygote-specific genes. Accomplishing fertilization within such a short time requires the rapid and signaled movement of pre-existing membrane and cytoplasmic proteins between and within several cellular compartments. Generation within the adhering flagella of the initial signals for protein movement, as well as movement itself of at least one cytoplasmic protein from the cell body to the flagella, depend on the microtubule motor, kinesin-II and presumably on intraflagellar transport (IFT). Adhesion and fusion of the two gametes depend on a second translocation event, the movement of an adhesion/fusion protein onto the surface of a rapidly elongating, microvillous-like fusion organelle. Finally, the merging of the two separate gametes, each containing sex-specific proteins, into a single cell allows the formerly separate proteins to form new interactions that regulate zygote development. Two proteins - a nuclease and a homeodomain protein - which were present only in the plus gamete, are 'delivered' to the cytoplasm of the zygote during gamete fusion. The nuclease is selectively imported into the minus chloroplast, where it degrades the chloroplast DNA, thereby ensuring uniparental inheritance of plus chloroplast traits. The homeodomain protein binds with an as yet unidentified protein delivered by the minus gamete, and the new complex activates transcription of zygote-specific genes.

The Chlamydomonas Fus1 protein is present on the mating type plus fusion organelle and required for a critical membrane adhesion event during fusion with minus gametes

The molecular mechanisms of the defining event in fertilization, gamete fusion, remain poorly understood. The FUS1 gene in the unicellular, biflagellated green alga Chlamydomonas is one of the few sex-specific eukaryotic genes shown by genetic analysis to be essential for gamete fusion during fertilization. In Chlamydomonas, adhesion and fusion of the plasma membranes of activated mt+ and mt- gametes is accomplished via specialized fusion organelles called mating structures. Herein, we identify the endogenous Fus1 protein, test the idea that Fus1 is at the site of fusion, and identify the step in fusion that requires Fus1. Our results show that Fus1 is a approximately 95-kDa protein present on the external surface of both unactivated and activated mt+ gametes. Bioassays indicate that adhesion between mating type plus and mating type minus fusion organelles requires Fus1 and that Fus1 is functional only after gamete activation. Finally, immunofluorescence demonstrates that the Fus1 protein is present as an apical patch on unactivated gametes and redistributes during gamete activation over the entire surface of the microvillous-like activated plus mating structure, the fertilization tubule. Thus, Fus1 is present on mt+ gametes at the site of cell-cell fusion and essential for an early step in the fusion process.

Cell identity and sexual development in Cryptococcus neoformans are controlled by the mating-type-specific homeodomain protein Sxi1alpha

Virulence in the human fungal pathogen Cryptococcus neoformans is associated with the alpha mating type. Studies to identify the properties of alpha cells that enhance pathogenesis have led to the identification of a mating-type locus of unusually large size and distinct architecture. Here, we demonstrate that the previously identified MATalpha components are insufficient to regulate sexual differentiation, and we identify a novel alpha-specific regulator, SXI1alpha. Our data show that SXI1alpha establishes alpha cell identity and controls progression through the sexual cycle, and we discover that ectopic expression of SXI1alpha in a cells is sufficient to drive a/alpha sexual development. SXI1alpha is the first example of a key regulator of cell identity and sexual differentiation in C. neoformans, and its identification and characterization lead to a new model of how cell fate and the sexual cycle are controlled in C. neoformans.

An mt(+) gamete-specific nuclease that targets mt(-) chloroplasts during sexual reproduction in C. reinhardtii

Although the active digestion of mating-type minus (mt-) chloroplast DNA (cpDNA) in young zygotes is considered to be the basis for the uniparental inheritance of cpDNA in Chlamydomonas reinhardtii, little is known about the underlying molecular mechanism. One model of active digestion proposes that nucleases are either synthesized or activated to digest mt- cpDNA. We used a native-PAGE/in gelo assay to investigate nuclease activities in chloroplasts from young zygotes, and identified a novel Ca(2+)-dependent nuclease activity. The timing of activation (approximately 60-90 min after mating) and the localization of the nuclease activity (in mt- chloroplasts) coincided with the active digestion of mt- cpDNA. Furthermore, the activity of the nuclease was coregulated with the maturation of mating-type plus (mt+) gametes, which would enable the efficient digestion of mt- cpDNA. Based on these observations, we propose that the nuclease (designated as Mt(+)-specific DNase, MDN) is a developmentally controlled nuclease that is activated in mt+ gametes and participates in the destruction of mt- cpDNA in young zygotes, thereby ensuring uniparental inheritance of chloroplast traits.