The Importance of Gene Duplication and Domain Repeat Expansion for the Function and Evolution of Fertilization Proteins

Conservation and divergence of the molecular regulators of the vertebrate fertilization synapse

Fertilization - the process during which sperm and egg find each other, bind and eventually fuse - marks the beginning of a new individual. Research over the past years in vertebrates has shed new light on conserved and divergent molecular regulators that mediate the formation of the fertilization synapse, the close apposition of the two plasma membranes before fusion. Here, we review the known proteins that are required for sperm-egg interaction in mammals and fish from a phylogenetic perspective. While some sperm factors are only conserved in vertebrates and share phylogenetic and structural features, others have a longer evolutionary history. In contrast, the egg factors have changed even within vertebrates despite recognizing the preserved sperm machinery. Future functional work on these factors will be essential to understand the fusion mechanism of vertebrate sperm and egg.

Mechanistic understanding of bacterial FAALs and the role of their homologs in eukaryotes

Fatty acids are used in fundamental cellular processes, such as membrane biogenesis, energy generation, post-translational modification of proteins, and so forth. These processes require the activation of fatty acids by adenosine triphosphate (ATP), followed by condensation with coenzyme-A (CoA), catalyzed by the omnipresent enzyme called Fatty acyl-CoA ligases (FACLs). However, Fatty acyl-AMP ligases (FAALs), the structural homologs of FACLs, operate in an unprecedented CoA-independent manner. FAALs transfer fatty acids to the acyl carrier protein (ACP) domain of polyketide synthases (PKS) and non-ribosomal peptide synthetases (NRPS) for the biosynthesis of various antibiotics, lipopeptides, virulent complex lipids, and so forth in bacteria. Recent structural and biochemical insights from our group provide a detailed understanding of the mode of CoA rejection and ACP acceptance by FAALs. In this review, we have discussed advances in the mechanistic, evolutionary, and functional understanding of FAALs and FAAL-like domains across life forms. Here, we are proposing a "Five-tier" mechanistic model to explain the specificity of FAALs. We further demonstrate how FAAL-like domains have been repurposed into a new family of proteins in eukaryotes with a novel function in lipid metabolism.

A conserved fertilization complex bridges sperm and egg in vertebrates

Fertilization, the basis for sexual reproduction, culminates in the binding and fusion of sperm and egg. Although several proteins are known to be crucial for this process in vertebrates, the molecular mechanisms remain poorly understood. Using an AlphaFold-Multimer screen, we identified the protein Tmem81 as part of a conserved trimeric sperm complex with the essential fertilization factors Izumo1 and Spaca6. We demonstrate that Tmem81 is essential for male fertility in zebrafish and mice. In line with trimer formation, we show that Izumo1, Spaca6, and Tmem81 interact in zebrafish sperm and that the human orthologs interact in vitro. Notably, complex formation creates the binding site for the egg fertilization factor Bouncer in zebrafish. Together, our work presents a comprehensive model for fertilization across vertebrates, where a conserved sperm complex binds to divergent egg proteins-Bouncer in fish and JUNO in mammals-to mediate sperm-egg interaction.

Comprehensive Genome-Wide Identification and Expression Profiling of bHLH Transcription Factors in Areca catechu Under Abiotic Stress

The basic helix-loop-helix (bHLH) transcription factor (TF) family, the second-largest among eukaryotes, is known for its evolutionary and functional diversity across plant species. However, bHLH genes have not yet been characterized in Areca catechu. In this study, we identified 76 AcbHLH genes, which exhibit a variety of physicochemical properties. Phylogenetic analysis revealed evolutionary relationships between Arabidopsis thaliana bHLH genes (AtbHLH) and their counterparts in A. catechu (AcbHLH). These analyses also highlighted conserved amino acid motifs (S, R, K, P, L, A, G, and D), conserved domains, and evolutionary changes, such as insertions, deletions, and exon gains or losses. Promoter analysis of AcbHLH genes revealed 76 cis-elements related to growth, phytohormones, light, and stress. Gene duplication analysis revealed four tandem duplications and twenty-three segmental duplications, while AcbHLH63 in the Areca genome exhibited significant synteny with bHLH genes from A. thaliana, Vitis vinifera, Solanum lycopersicum, Brachypodium distachyon, Oryza sativa, and Zea mays. Furthermore, relative expression analysis showed that under drought stress (DS), AcbHLH22, AcbHLH39, AcbHLH45, and AcbHLH58 showed distinct upregulation in leaves at specific time points, while all nine AcbHLH genes were upregulated in roots. Under salt stress (SS), AcbHLH22, AcbHLH39, AcbHLH45, and AcbHLH58 were upregulated in leaves, and AcbHLH22, AcbHLH34, and AcbHLH39 exhibited differential expression in roots at various time points. This study provides valuable insights into the bHLH superfamily in A. catechu, offering a solid foundation for further investigation into its role in responding to abiotic stresses.

The roles of gene duplications in the dynamics of evolutionary conflicts

Evolutionary conflicts occur when there is antagonistic selection between different individuals of the same or different species, life stages or between levels of biological organization. Remarkably, conflicts can occur within species or within genomes. In the dynamics of evolutionary conflicts, gene duplications can play a major role because they can bring very specific changes to the genome: changes in protein dose, the generation of novel paralogues with different functions or expression patterns or the evolution of small antisense RNAs. As we describe here, by having those effects, gene duplication might spark evolutionary conflict or fuel arms race dynamics that takes place during conflicts. Interestingly, gene duplication can also contribute to the resolution of a within-locus evolutionary conflict by partitioning the functions of the gene that is under an evolutionary trade-off. In this review, we focus on intraspecific conflicts, including sexual conflict and illustrate the various roles of gene duplications with a compilation of examples. These examples reveal the level of complexity and the differences in the patterns of gene duplications within genomes under different conflicts. These examples also reveal the gene ontologies involved in conflict and the genomic location of the elements of the conflict. The examples provide a blueprint for the direct study of these conflicts or the exploration of the presence of similar conflicts in other lineages.

Systematic Analysis of Diverse Polynucleotide Kinase Clp1 Family Proteins in Eukaryotes: Three Unique Clp1 Proteins of Trypanosoma brucei

The Clp1 family proteins, consisting of the Clp1 and Nol9/Grc3 groups, have polynucleotide kinase (PNK) activity at the 5' end of RNA strands and are important enzymes in the processing of some precursor RNAs. However, it remains unclear how this enzyme family diversified in the eukaryotes. We performed a large-scale molecular evolutionary analysis of the full-length genomes of 358 eukaryotic species to classify the diverse Clp1 family proteins. The average number of Clp1 family proteins in eukaryotes was 2.3 ± 1.0, and most representative species had both Clp1 and Nol9/Grc3 proteins, suggesting that the Clp1 and Nol9/Grc3 groups were already formed in the eukaryotic ancestor by gene duplication. We also detected an average of 4.1 ± 0.4 Clp1 family proteins in members of the protist phylum Euglenozoa. For example, in Trypanosoma brucei, there are three genes of the Clp1 group and one gene of the Nol9/Grc3 group. In the Clp1 group proteins encoded by these three genes, the C-terminal domains have been replaced by unique characteristics domains, so we designated these proteins Tb-Clp1-t1, Tb-Clp1-t2, and Tb-Clp1-t3. Experimental validation showed that only Tb-Clp1-t2 has PNK activity against RNA strands. As in this example, N-terminal and C-terminal domain replacement also contributed to the diversification of the Clp1 family proteins in other eukaryotic species. Our analysis also revealed that the Clp1 family proteins in humans and plants diversified through isoforms created by alternative splicing.

Divergent molecular signatures in fish Bouncer proteins define cross-fertilization boundaries

Molecular compatibility between gametes is a prerequisite for successful fertilization. As long as a sperm and egg can recognize and bind each other via their surface proteins, gamete fusion may occur even between members of separate species, resulting in hybrids that can impact speciation. The egg membrane protein Bouncer confers species specificity to gamete interactions between medaka and zebrafish, preventing their cross-fertilization. Here, we leverage this specificity to uncover distinct amino acid residues and N-glycosylation patterns that differentially influence the function of medaka and zebrafish Bouncer and contribute to cross-species incompatibility. Curiously, in contrast to the specificity observed for medaka and zebrafish Bouncer, seahorse and fugu Bouncer are compatible with both zebrafish and medaka sperm, in line with the pervasive purifying selection that dominates Bouncer's evolution. The Bouncer-sperm interaction is therefore the product of seemingly opposing evolutionary forces that, for some species, restrict fertilization to closely related fish, and for others, allow broad gamete compatibility that enables hybridization.

Domain Expansion and Functional Diversification in Vertebrate Reproductive Proteins

The rapid evolution of fertilization proteins has generated remarkable diversity in molecular structure and function. Glycoproteins of vertebrate egg coats contain multiple zona pellucida (ZP)-N domains (1-6 copies) that facilitate multiple reproductive functions, including species-specific sperm recognition. In this report, we integrate phylogenetics and machine learning to investigate how ZP-N domains diversify in structure and function. The most C-terminal ZP-N domain of each paralog is associated with another domain type (ZP-C), which together form a "ZP module." All modular ZP-N domains are phylogenetically distinct from nonmodular or free ZP-N domains. Machine learning-based classification identifies eight residues that form a stabilizing network in modular ZP-N domains that is absent in free domains. Positive selection is identified in some free ZP-N domains. Our findings support that strong purifying selection has conserved an essential structural core in modular ZP-N domains, with the relaxation of this structural constraint allowing free N-terminal domains to functionally diversify.