We previously identified a mutation, smt15-1, that partially suppresses the cell cycle defects caused by loss of the retinoblastoma tumor suppressor related protein (RBR) encoded by the MAT3 gene in Chlamydomonas reinhardtii. smt15-1 single mutants were also found to have a cell cycle defect leading to a small-cell phenotype. SMT15 belongs to a previously uncharacterized sub-family of putative membrane localized sulfate/anion transporters that contain a Sulfate_transp domain and are found in a widely distributed subset of eukaryotes and bacteria. Although we observed that smt15-1 has a defect in acclimation to sulfur-limited growth conditions, sac mutants that are more severely defective for acclimation to sulfur limitation do not have cell cycle defects and cannot suppress mat3. Moreover, we found that smt15-1, but not sac mutants, over-accumulates glutathione. In wild-type cells glutathione fluctuated during the cell cycle with highest levels in mid-G1 phase and lower levels during S and M phases, while in smt15-1 glutathione levels remained elevated during S and M. In addition to increased total glutathione levels, smt15-1 cells had increased GSH/GSSH redox ratio throughout the cell cycle. These data suggest a role for SMT15 in maintaining glutathione homeostasis that impacts the cell cycle and sulfur acclimation responses.
The green lineage of chlorophyte algae and streptophytes form a large and diverse clade with multiple independent transitions to produce multicellular and/or macroscopically complex organization. In this review, I focus on two of the best-studied multicellular groups of green algae: charophytes and volvocines. Charophyte algae are the closest relatives of land plants and encompass the transition from unicellularity to simple multicellularity. Many of the innovations present in land plants have their roots in the cell and developmental biology of charophyte algae. Volvocine algae evolved an independent route to multicellularity that is captured by a graded series of increasing cell-type specialization and developmental complexity. The study of volvocine algae has provided unprecedented insights into the innovations required to achieve multicellularity.
Male and female sexes have evolved repeatedly in eukaryotes but the origins of dimorphic sexes and their relationship to mating types in unicellular species are not understood. Volvocine algae include isogamous species such as Chlamydomonas reinhardtii, with two equal-sized mating types, and oogamous multicellular species such as Volvox carteri with sperm-producing males and egg-producing females. Theoretical work predicts genetic linkage of a gamete cell-size regulatory gene(s) to an ancestral mating-type locus as a possible step in the evolution of dimorphic gametes, but this idea has not been tested. Here we show that, contrary to predictions, a single conserved mating locus (MT) gene in volvocine algae-MID, which encodes a RWP-RK domain transcription factor-evolved from its ancestral role in C. reinhardtii as a mating-type specifier, to become a determinant of sperm and egg development in V. carteri. Transgenic female V. carteri expressing male MID produced functional sperm packets during sexual development. Transgenic male V. carteri with RNA interference (RNAi)-mediated knockdowns of VcMID produced functional eggs, or self-fertile hermaphrodites. Post-transcriptional controls were found to regulate cell-type-limited expression and nuclear localization of VcMid protein that restricted its activity to nuclei of developing male germ cells and sperm. Crosses with sex-reversed strains uncoupled sex determination from sex chromosome identity and revealed gender-specific roles for male and female mating locus genes in sexual development, gamete fitness and reproductive success. Our data show genetic continuity between the mating-type specification and sex determination pathways of volvocine algae, and reveal evidence for gender-specific adaptations in the male and female mating locus haplotypes of Volvox. These findings will enable a deeper understanding of how a master regulator of mating-type determination in an ancestral unicellular species was reprogrammed to control sexually dimorphic gamete development in a multicellular descendant.
The green alga Chlamydomonas reinhardtii is a popular unicellular organism for studying photosynthesis, cilia biogenesis, and micronutrient homeostasis. Ten years since its genome project was initiated an iterative process of improvements to the genome and gene predictions has propelled this organism to the forefront of the omics era. Housed at Phytozome, the plant genomics portal of the Joint Genome Institute (JGI), the most up-to-date genomic data include a genome arranged on chromosomes and high-quality gene models with alternative splice forms supported by an abundance of whole transcriptome sequencing (RNA-Seq) data. We present here the past, present, and future of Chlamydomonas genomics. Specifically, we detail progress on genome assembly and gene model refinement, discuss resources for gene annotations, functional predictions, and locus ID mapping between versions and, importantly, outline a standardized framework for naming genes.
When the sta6 (starch-null) strain of the green microalga Chlamydomonas reinhardtii is nitrogen starved in acetate and then "boosted" after 2 days with additional acetate, the cells become "obese" after 8 days, with triacylglyceride (TAG)-filled lipid bodies filling their cytoplasm and chloroplasts. To assess the transcriptional correlates of this response, the sta6 strain and the starch-forming cw15 strain were subjected to RNA-Seq analysis during the 2 days prior and 2 days after the boost, and the data were compared with published reports using other strains and growth conditions. During the 2 h after the boost, ?425 genes are upregulated ?2-fold and ?875 genes are downregulated ?2-fold in each strain. Expression of a small subset of "sensitive" genes, encoding enzymes involved in the glyoxylate and Calvin-Benson cycles, gluconeogenesis, and the pentose phosphate pathway, is responsive to culture conditions and genetic background as well as to boosting. Four genes-encoding a diacylglycerol acyltransferase (DGTT2), a glycerol-3-P dehydrogenase (GPD3), and two candidate lipases (Cre03.g155250 and Cre17.g735600)-are selectively upregulated in the sta6 strain. Although the bulk rate of acetate depletion from the medium is not boost enhanced, three candidate acetate permease-encoding genes in the GPR1/FUN34/YaaH superfamily are boost upregulated, and 13 of the "sensitive" genes are strongly responsive to the cell's acetate status. A cohort of 64 autophagy-related genes is downregulated by the boost. Our results indicate that the boost serves both to avert an autophagy program and to prolong the operation of key pathways that shuttle carbon from acetate into storage lipid, the combined outcome being enhanced TAG accumulation, notably in the sta6 strain.
Sex is pervasive throughout eukaryotes, yet many species have not been caught in the act. Now, a sexual cycle has been discovered in the choanoflagellate Salpingoeca rosetta, a pre-metazoan evolutionary model, opening a window on metazoan sexual evolution.
Heteromorphic sex-determining regions or mating-type loci can contain large regions of non-recombining sequence where selection operates under different constraints than in freely recombining autosomal regions. Detailed studies of these non-recombining regions can provide insights into how genes are gained and lost, and how genetic isolation is maintained between mating haplotypes or sex chromosomes. The Chlamydomonas reinhardtii mating-type locus (MT) is a complex polygenic region characterized by sequence rearrangements and suppressed recombination between its two haplotypes, MT+ and MT-. We used new sequence information to redefine the genetic contents of MT and found repeated translocations from autosomes as well as sexually controlled expression patterns for several newly identified genes. We examined sequence diversity of MT genes from wild isolates of C. reinhardtii to investigate the impacts of recombination suppression. Our population data revealed two previously unreported types of genetic exchange in Chlamydomonas MT--gene conversion in the rearranged domains, and crossover exchanges in flanking domains--both of which contribute to maintenance of genetic homogeneity between haplotypes. To investigate the cause of blocked recombination in MT we assessed recombination rates in crosses where the parents were homozygous at MT. While normal recombination was restored in MT+ ×MT+ crosses, it was still suppressed in MT- ×MT- crosses. These data revealed an underlying asymmetry in the two MT haplotypes and suggest that sequence rearrangements are insufficient to fully account for recombination suppression. Together our findings reveal new evolutionary dynamics for mating loci and have implications for the evolution of heteromorphic sex chromosomes and other non-recombining genomic regions.
The molecular bases for the evolution of male-female sexual dimorphism are possible to study in volvocine algae because they encompass the entire range of reproductive morphologies from isogamy to oogamy. In 1978, Charlesworth suggested the model of a gamete size gene becoming linked to the sex-determining or mating type locus (MT) as a mechanism for the evolution of anisogamy. Here, we carried out the first comprehensive study of a candidate MT-linked oogamy gene, MAT3/RB, across the volvocine lineage. We found that evolution of anisogamy/oogamy predates the extremely high male-female divergence of MAT3 that characterizes the Volvox carteri lineage. These data demonstrate very little sex-linked sequence divergence of MAT3 between the two sexes in other volvocine groups, though linkage between MAT3 and the mating locus appears to be conserved. These data implicate genetic determinants other than or in addition to MAT3 in the evolution of anisogamy in volvocine algae.
Sexual reproduction in Volvocine algae coevolved with the acquisition of multicellularity. Unicellular genera such as Chlamydomonas and small colonial genera from this group have classical mating types with equal-sized gametes, while larger multicellular genera such as Volvox have differentiated males and females that produce sperm and eggs respectively. Newly available sequence from the Volvox and Chlamydomonas genomes and mating loci open up the potential to investigate how sex-determining regions co-evolve with major changes in development and sexual reproduction. The expanded size and sequence divergence between the male and female haplotypes of the Volvox mating locus (MT) not only provide insights into how the colonial Volvocine algae might have evolved sexual dimorphism, but also raise questions about why the putative ancestral-like MT locus in Chlamydomonas shows less divergence between haplotypes than expected.
The translocator protein 18 kDa (TSPO), previously known as the peripheral-type benzodiazepine receptor (PBR), is important for many cellular functions in mammals and bacteria, such as steroid biosynthesis, cellular respiration, cell proliferation, apoptosis, immunomodulation, transport of porphyrins and anions. Arabidopsis thaliana contains a single TSPO/PBR-related gene with a 40 amino acid N-terminal extension compared to its homologs in bacteria or mammals suggesting it might be chloroplast or mitochondrial localized.
We examined the cell cycle dynamics of the retinoblastoma (RB) protein complex in the unicellular alga Chlamydomonas reinhardtii that has single homologs for each subunit-RB, E2F, and DP. We found that Chlamydomonas RB (encoded by MAT3) is a cell cycle-regulated phosphoprotein, that E2F1-DP1 can bind to a consensus E2F site, and that all three proteins interact in vivo to form a complex that can be quantitatively immunopurified. Yeast two-hybrid assays revealed the formation of a ternary complex between MAT3, DP1, and E2F1 that requires a C-terminal motif in E2F1 analogous to the RB binding domain of plant and animal E2Fs. We examined the abundance of MAT3/RB and E2F1-DP1 in highly synchronous cultures and found that they are synthesized and remain stably associated throughout the cell cycle with no detectable fraction of free E2F1-DP1. Consistent with their stable association, MAT3/RB and DP1 are constitutively nuclear, and MAT3/RB does not require DP1-E2F1 for nuclear localization. In the nucleus, MAT3/RB remains bound to chromatin throughout the cell cycle, and its chromatin binding is mediated through E2F1-DP1. Together, our data show that E2F-DP complexes can regulate the cell cycle without dissociation of their RB-related subunit and that other changes may be sufficient to convert RB-E2F-DP from a cell cycle repressor to an activator.
The multicellular green alga Volvox carteri and its morphologically diverse close relatives (the volvocine algae) are well suited for the investigation of the evolution of multicellularity and development. We sequenced the 138-mega-base pair genome of V. carteri and compared its approximately 14,500 predicted proteins to those of its unicellular relative Chlamydomonas reinhardtii. Despite fundamental differences in organismal complexity and life history, the two species have similar protein-coding potentials and few species-specific protein-coding gene predictions. Volvox is enriched in volvocine-algal-specific proteins, including those associated with an expanded and highly compartmentalized extracellular matrix. Our analysis shows that increases in organismal complexity can be associated with modifications of lineage-specific proteins rather than large-scale invention of protein-coding capacity.
Although dimorphic sexes have evolved repeatedly in multicellular eukaryotes, their origins are unknown. The mating locus (MT) of the sexually dimorphic multicellular green alga Volvox carteri specifies the production of eggs and sperm and has undergone a remarkable expansion and divergence relative to MT from Chlamydomonas reinhardtii, which is a closely related unicellular species that has equal-sized gametes. Transcriptome analysis revealed a rewired gametic expression program for Volvox MT genes relative to Chlamydomonas and identified multiple gender-specific and sex-regulated transcripts. The retinoblastoma tumor suppressor homolog MAT3 is a Volvox MT gene that displays sexually regulated alternative splicing and evidence of gender-specific selection, both of which are indicative of cooption into the sexual cycle. Thus, sex-determining loci affect the evolution of both sex-related and non-sex-related genes.
Intraflagellar transport (IFT) proteins are well established as conserved mediators of flagellum/cilium assembly and disassembly. However, data has begun to accumulate in support of IFT protein involvement in other processes elsewhere in the cell. Here, we used synchronous cultures of Chlamydomonas to investigate the temporal patterns of accumulation and localization of IFT proteins during the cell cycle. Their mRNAs showed periodic expression that peaked during S and M phase (S/M). Unlike most proteins that are synthesized continuously during G1 phase, IFT27 and IFT46 levels were found to increase only during S/M phase. During cell division, IFT27, IFT46, IFT72, and IFT139 re-localized from the flagella and basal bodies to the cleavage furrow. IFT27 was further shown to be associated with membrane vesicles in this region. This localization pattern suggests a role for IFT in cell division.
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