GREMA · Genetic REcombination and MAting in Mamiellophyceae

Genetic REcombination and MAting in Mamiellophyceae

Phytoplankton, often referred to as the "grass" of aquatic ecosystems, are the primary producers that capture CO2 and energy from the sun, sustaining all other living beings throughout the aquatic trophic chain. Despite accounting for only an estimated 1% of the global photosynthetic biomass due to their rapid turnover, phytoplankton are responsible for approximately 45% of Earth's net primary production. Their role in capturing CO2 is critical, as they fuel and regulate the global carbon biogeochemical cycle. Phytoplankton encompass a wide variety of microscopic species, ranging from cyanobacteria to large eukaryotic algae such as diatoms. Among eukaryotic phytoplankton, a group of tiny green algae, the Mamiellophyceae (class Chlorophyta), is particularly noteworthy. These algae are ubiquitous in the sunlit ocean and can comprise up to 80% of phytoplankton abundance in coastal environments. Mamiellophyceae are a diverse group, with the most abundant marine species often being very small in size (0.8 to 3 µm) and primarily represented by the lineages Mantoniella, Micromonas, Ostreococcus, and Bathycoccus. However, early-branching lineages such as Monomastix, Dolichomastix, and Crustomastix, along with the sister lineage Pyramomonas, are less studied. These lineages are typically larger, occupy different ecological niches, and remain underexplored, particularly with respect to their genomic sequences. Despite their ecological importance, Mamiellophyceae remain poorly studied. This project aims to expand our understanding of Mamiellophyceae diversity by obtaining genomic sequences from early-branching lineages and exploring the genetic basis of recombination and sexual reproduction. This will be achieved through genome sequencing of newly identified species and genetic experiments focusing on well-known genera such as Ostreococcus. State-of-the-Art Recombination, the exchange of DNA between homologous chromosomes, is a hallmark of sexual reproduction and has been observed in the vast majority of eukaryotic species. However, the molecular mechanisms underlying partner recognition, such as the determination of mating types, are largely clade-specific and remain unknown for many eukaryotic groups, including unicellular protists like algae. In the phylum Chlorophyta, much of the knowledge about sexual reproduction comes from the freshwater alga Chlamydomonas reinhardtii (Chlorophyceae). In C. reinhardtii, fertilization is regulated by a mating locus that exists in two genetically divergent forms, "+" and "-". Haploid cells of opposite mating types fuse, undergo meiosis, and engage in recombination, the reciprocal and non-reciprocal exchange of genetic material between chromatids. In Mamiellophyceae, indirect evidence of recombination has been derived from population genomics studies of Ostreococcus tauri. Patterns of single nucleotide polymorphisms (SNPs) in these studies reveal recombination events and allow the estimation of recombination rates and the ratio of meiotic to mitotic events. A mating-type locus with two alleles (MT+ and MT-) has been recently identified in O. tauri, enabling the description of mating types in other species such as O. lucimarinus, O. mediterraneus, and Micromonas commoda. However, it remains unclear whether this mating-type locus structure is conserved in earlier diverging Mamiellophyceae lineages or whether these groups have entirely distinct mechanisms of sexual reproduction. Additionally, the relative frequency of mating types in natural populations and the environmental conditions that trigger mating and recombination are poorly understood. These gaps are particularly urgent to address in the context of the Anthropocene and climate change. If environmental changes disproportionately affect one mating type, it could disrupt sexual reproduction in Mamiellophyceae, compromising their genetic diversity and adaptive potential. This project seeks to address these knowledge gaps by elucidating the evolution of mating types and the prevalence of recombination across natural populations of ecologically relevant Mamiellophyceae species. It also aims to investigate how environmental factors influence mating. To achieve these objectives, we employed a multidisciplinary approach combining bioinformatics and experimental methods, structured around the following specific goals: Objective. 1: Describe new early branching Mamiellophycean species. We first, will expand the known diversity of Mamiellophycean species by describe new isolated Mamiellophycean species, or describing properly some species already present in culture collections that awaited for a formal description and come from overlooked early branching lineages that are sister to the most common known species that populate marine ecosystems. Objective. 2: Capture the ecology and evolution of mating types accross Mamiellophyceae 1) by using a comparative genomics analysis of 10 new different species across Mamiellophyceae and its sister lineage Pyramimonas. In order to to identify the loci of the mating types how it is conserved throught the evolution 2) to estimate the frequency of the two mating types present in Mamiellophyceae natural populations using metagenomic data. Objective. 3: Detect the abiotic and biotic factors that influence mating in O. tauri by experimental crossing of haploid strains by taking advantage of cell transformation to select for recombinant cells (WP3).

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз