Summary: Akiyoshi, 2025 presented a hypothesis with implications for the early evolution of eukaryotes and eukaryotic cell division machinery. In this Correspondence, the authors conclude that this hypothesis leads to a highly implausible scenario.

Asgard archaea played a key role in the origin of the eukaryotic cell, with extant genomes encoding relatives of diverse eukaryotic signature proteins (ESPs) involved in cellular organization. However, their often punctuated distribution and the absence of detectable homologues for many eukaryotic proteins limit our ability to reconstruct the cellular complexity of the Asgard archaeal ancestor of eukaryotes. Here we used de novo protein structure modelling and sequence similarity detection across an expanded Asgard archaeal genomic dataset to build a structural catalogue of the Asgard archaeal pangenome. We identified 908 'isomorphic' ESPs-Asgard archaeal proteins with statistically enriched structural matches to eukaryotic proteins, often bridging deep sequence divergence. These isomorphic ESPs are enriched in information storage and processing roles and contain key components of the eukaryotic Vault (MVP) and Commander (COMMD) complexes, with potential roles in cellular compartmentalization and endosomal processing. These findings expand the repertoire of eukaryotic-like proteins in Asgard archaea and suggest a higher degree of eukaryote-like cellular complexity in the archaeal ancestor of eukaryotes.

The genomes of organisms across the tree of life are structurally and functionally organized into chromatin. In eukaryotes, within an organelle called the nucleus, chromatin is shaped by histones and structural maintenance of chromosomes (SMC) complexes, among other factors. The closest prokaryotic relatives of eukaryotes, the Asgard archaea, lack a nucleus, but their genomes encode multiple histones and SMC complexes. Understanding chromatin organization in Asgard archaea is key to understanding how eukaryotic chromatin evolved. However, to date, experimental information on the mechanisms of action of these proteins is largely lacking, particularly in vivo. In this review, we discuss the remarkable diversity of Asgard archaeal histones, the characteristics of SMC complexes, and their potential structural and regulatory roles in Asgard archaea.

Across life, structural maintenance of chromosomes (SMC) complexes organize chromosomes. While most prokaryotes have one, eukaryotes usually possess four (condensin I, condensin II, cohesin, and SMC5/6), shaping their considerably larger genomes. Although essential, SMC complexes differ among model eukaryotes, suggesting underexplored diversity. Here, we reconstruct eukaryotic SMC complex evolution, revealing that the last eukaryotic common ancestor (LECA) had all four complexes, supporting a sophisticated LECA. Subsequently, condensin II was lost at least 30 times, making it one of the most frequently lost eukaryotic machineries. Moreover, multiple SMC complex components are more ancient and widespread than previously appreciated. Tracing prokaryotic origins, we propose that the SMC complex was already duplicated in the Thaumarchaeota-Aigarchaeota-Crenarchaeota-Korarchaeota (TACK) and Asgard archaeal ancestor, suggesting sophisticated chromosome organization in eukaryotes' archaeal ancestor. Gene duplications further expanded the eukaryotic SMC complex inventory, highlighting their significance in establishing eukaryotic complexity. Altogether, our work suggests major shifts in genome organization throughout eukaryote history.

Lateral gene transfer (LGT) is a fundamental process that has contributed to the genetic makeup of various eukaryotic lineages. Yet our comprehension of its prevalence and significance across the eukaryotic domain remains incomplete, particularly when it comes to eukaryote-to-eukaryote transfers. The Rhizaria forms an expansive, ancient, and morphologically diverse clade of mostly free-living, single-celled phagotrophs, whose genetic diversity reflects their adaptability and evolutionary success across a range of habitats. Here, we undertake a comprehensive investigation of LGT within Rhizaria, tracing its role from the clade's emergence to the present day, employing advanced phylogenetic analyses and machine learning techniques. Contrary to previous assertions that LGT in eukaryotes is rare, our findings suggest that at least 8% of genes in contemporary rhizarian genomes were acquired through LGT at various points during their billion-year history. Our analysis reveals that although gene duplications exceed the number of LGT events, the duplicated genes that were originally acquired through LGT have a visible effect on the evolutionary trajectory of the recipient organisms. Our results also indicate that LGTs originating from other eukaryotes are more common than those from prokaryotes and exhibit unique patterns. By offering both quantitative and qualitative insights into the role of LGT in shaping the evolution of a major eukaryotic lineage, this work further demystifies lateral gene transfer in eukaryotes.

Faithful transmission of genetic material is crucial for the survival of all organisms. In many eukaryotes, a feedback control mechanism called the spindle checkpoint ensures chromosome segregation fidelity by delaying cell cycle progression until all chromosomes achieve proper attachment to the mitotic spindle. Kinetochores are the macromolecular complexes that act as the interface between chromosomes and spindle microtubules. While most eukaryotes have canonical kinetochore proteins that are widely conserved, kinetoplastids such as Trypanosoma brucei have a seemingly unique set of kinetochore proteins including KKT1–25. It remains poorly understood how kinetoplastids regulate cell cycle progression or ensure chromosome segregation fidelity. Here, we report a crystal structure of the C-terminal domain of KKT14 from Apiculatamorpha spiralis and uncover that it is a pseudokinase. Its structure is most similar to the kinase domain of a spindle checkpoint protein Bub1. In addition, KKT14 has a putative ABBA motif that is present in Bub1 and its paralogue BubR1. We also find that the N-terminal part of KKT14 interacts with KKT15, whose WD40 repeat beta-propeller is phylogenetically closely related to a direct interactor of Bub1/BubR1 called Bub3. Our findings indicate that KKT14–KKT15 are divergent orthologues of Bub1/BubR1–Bub3, which promote accurate chromosome segregation in trypanosomes.

Understanding the origin of eukaryotic cells is one of the most difficult problems in all of biology. A key challenge relevant to the question of eukaryogenesis is reconstructing the gene repertoire of the last eukaryotic common ancestor (LECA). As data sets grow, sketching an accurate genomics-informed picture of early eukaryotic cellular complexity requires provision of analytical resources and a commitment to data sharing. Here, we summarise progress towards understanding the biology of LECA and outline a community approach to inferring its wider gene repertoire. Once assembled, a robust LECA gene set will be a useful tool for evaluating alternative hypotheses about the origin of eukaryotes and understanding the evolution of traits in all descendant lineages, with relevance in diverse fields such as cell biology, microbial ecology, biotechnology, agriculture, and medicine. In this Consensus View, we put forth the status quo and an agreed path forward to reconstruct LECA’s gene content.

Asgard archaea played a key role in the origin of the eukaryotic cell. While previous studies found that Asgard genomes encode diverse eukaryotic signature proteins (ESPs), representing homologs of proteins that play important roles in the complex organization of eukaryotic cells, the cellular characteristics and complexity of the Asgard archaeal ancestor of eukaryotes remain unclear. Here, we used de novo protein structure modeling and sensitive sequence similarity detection algorithms within an expanded Asgard archaeal genomic dataset to build a structural catalogue of the Asgard archaeal pangenome and identify 908 new ‘isomorphic’ ESPs (iESPs), representing clusters of protein structures most similar to eukaryotic proteins and that likely underwent extensive sequence divergence. While most previously identified ESPs were involved in cellular processes and signaling, iESPs are enriched in information storage and processing functions, with several being potentially implicated in facilitating cellular complexity. By expanding the complement of eukaryotic proteins in Asgard archaea, this study indicates that the archaeal ancestor of eukaryotes was more complex than previously assumed. ### Competing Interest Statement The authors have declared no competing interest.

The origin of the eukaryotic cell, with its compartmentalized nature and generally large size compared with bacterial and archaeal cells, represents a cornerstone event in the evolution of complex life on Earth. In a process referred to as eukaryogenesis, the eukaryotic cell is believed to have evolved between approximately 1.8 and 2.7 billion years ago from its archaeal ancestors, with a symbiosis with a bacterial (proto-mitochondrial) partner being a key event. In the tree of life, the branch separating the first from the last common ancestor of all eukaryotes is long and lacks evolutionary intermediates. As a result, the timing and driving forces of the emergence of complex eukaryotic features remain poorly understood. During the past decade, environmental and comparative genomic studies have revealed vital details about the identity and nature of the host cell and the proto-mitochondrial endosymbiont, enabling a critical reappraisal of hypotheses underlying the symbiotic origin of the eukaryotic cell. Here we outline our current understanding of the key players and events underlying the emergence of cellular complexity during the prokaryote-to-eukaryote transition and discuss potential avenues of future research that might provide new insights into the enigmatic origin of the eukaryotic cell.

Across the tree of life, SMC complexes organize, segregate and regulate DNA. In contrast to prokaryotes, which often possess one SMC complex, eukaryotes usually have four such complexes. This expanded set is involved in managing the considerably larger nuclear genomes of eukaryotes, which are distributed across multiple chromosomes. Despite their essential functions, SMC complexes exhibit variations across model eukaryotes, suggesting an even greater variety of these complexes that remains unexplored. Here, we aimed to uncover the diversity and evolution of SMC complexes across eukaryotes, and their deeper, prokaryotic evolutionary origins. For this, we conducted in-depth comparative genomic and phylogenetic analyses of SMC complexes. We show that the last eukaryotic common ancestor (LECA) likely had fully-fledged versions of all four complexes and that the condensin II complex was later lost at least 30 times in eukaryotes. We report evidence that proteins previously designated as functional analogs in various model organisms (e.g., Sororin, Securin, Nse5 and Nse6) are in fact genuine orthologs. Finally, we traced the prokaryotic origins of these complexes and propose that a single SMC complex duplicated in an early archaeon. Altogether, we provide a comprehensive overview of eukaryotic SMC complex diversity and evolution, both addressing and generating questions about their functioning in ancestral and contemporary organisms.### Competing Interest StatementThe authors have declared no competing interest.

With 3D genome mapping maturing over the past decade, studies exposed the differences between eukaryotic and prokaryotic genome organization. This raises the question of how the complex eukaryotic genome organization originated. Here, I explore potential pathways to answering this question, guided by our changing understanding of the origins of eukaryotes.

Kinetochores connect chromosomes to spindle microtubules to ensure their correct segregation during cell division. Kinetochores of human and yeasts are largely homologous, their ability to track depolymerizing microtubules, however, is carried out by the nonhomologous complexes Ska1-C and Dam1-C, respectively. We previously reported the unique anti-correlating phylogenetic profiles of Dam1-C and Ska-C found among a wide variety of eukaryotes. Based on these profiles and the limited presence of Dam1-C, we speculated that horizontal gene transfer could have played a role in the evolutionary history of Dam1-C. Here, we present an expanded analysis of Dam1-C evolution, using additional genome as well as transcriptome sequences and recently published 3D structures. This analysis revealed a wider and more complete presence of Dam1-C in Cryptista, Rhizaria, Ichthyosporea, CRuMs, and Colponemidia. The fungal Dam1-C cryo-EM structure supports earlier hypothesized intracomplex homologies, which enables the reconstruction of rooted and unrooted phylogenies. The rooted tree of concatenated Dam1-C subunits is statistically consistent with the species tree of eukaryotes, suggesting that Dam1-C is ancient, and that the present-day phylogenetic distribution is best explained by multiple, independent losses and no horizontal gene transfer was involved. Furthermore, we investigated the ancient origin of Dam1-C via profile-versus-profile searches. Homology among 8 out of the 10 Dam1-C subunits suggests that the complex largely evolved from a single multimerizing subunit that diversified into a hetero-octameric core via stepwise subunit duplication and subfunctionalization of the subunits before the origin of the last eukaryotic common ancestor.

Aneuploidy is the leading cause of miscarriage and congenital birth defects, and a hallmark of cancer. Despite this strong association with human disease, the genetic causes of aneuploidy remain largely unknown. Through exome sequencing of patients with constitutional mosaic aneuploidy, we identified biallelic truncating mutations in CENATAC (CCDC84). We show that CENATAC is a novel component of the minor (U12‐dependent) spliceosome that promotes splicing of a specific, rare minor intron subtype. This subtype is characterized by AT‐AN splice sites and relatively high basal levels of intron retention. CENATAC depletion or expression of disease mutants resulted in excessive retention of AT‐AN minor introns in ˜ 100 genes enriched for nucleocytoplasmic transport and cell cycle regulators, and caused chromosome segregation errors. Our findings reveal selectivity in minor intron splicing and suggest a link between minor spliceosome defects and constitutional aneuploidy in humans. Genetic causes of aneuploidy in humans remain largely unknown. Here, patient exome sequencing reveals pathogenic patient mutations in CENATAC/CCDC84, encoding a novel component of the minor spliceosome, and downstream effects on chromosome segregation in mitosis. Biallelic CCDC84/CENATAC mutations identified through patient exome sequencing link altered minor intron splicing to constitutional mosaic aneuploidy in humans.

The apical complex is the instrument of invasion used by apicomplexan parasites, and the conoid is a conspicuous feature of this apparatus found throughout this phylum. The conoid, however, is believed to be heavily reduced or missing from Plasmodium species and other members of the class Aconoidasida. Relatively few conoid proteins have previously been identified, making it difficult to address how conserved this feature is throughout the phylum, and whether it is genuinely missing from some major groups. Moreover, parasites such as Plasmodium species cycle through 3 invasive forms, and there is the possibility of differential presence of the conoid between these stages. We have applied spatial proteomics and high-resolution microscopy to develop a more complete molecular inventory and understanding of the organisation of conoid-associated proteins in the model apicomplexan Toxoplasma gondii. These data revealed molecular conservation of all conoid substructures throughout Apicomplexa, including Plasmodium, and even in allied Myzozoa such as Chromera and dinoflagellates. We reporter-tagged and observed the expression and location of several conoid complex proteins in the malaria model P. berghei and revealed equivalent structures in all of its zoite forms, as well as evidence of molecular differentiation between blood-stage merozoites and the ookinetes and sporozoites of the mosquito vector. Collectively, we show that the conoid is a conserved apicomplexan element at the heart of the invasion mechanisms of these highly successful and often devastating parasites.

Fasta files, phylogenetic trees and their annotations.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Aneuploidy is the leading cause of miscarriage and congenital birth defects, and a hallmark of cancer. Despite this strong association with human disease, the genetic causes of aneuploidy remain largely unknown. Through exome sequencing of patients with constitutional mosaic aneuploidy, we identified biallelic truncating mutations in CENATAC ( CCDC84 ). We show that CENATAC is a novel component of the minor (U12-dependent) spliceosome that promotes splicing of a specific, rare minor intron subtype. This subtype is characterized by AT-AN splice sites and relatively high basal levels of intron retention. CENATAC depletion or expression of disease mutants resulted in excessive retention of AT-AN minor introns in ~100 genes enriched for nucleocytoplasmic transport and cell cycle regulators, and caused chromosome segregation errors. Our findings reveal selectivity in minor intron splicing with a specific impact on the chromosome segregation process, and show how defects herein can cause constitutional aneuploidy.### Competing Interest StatementNazneen Rahman is a Non-Executive Director of AstraZeneca. The other authors declare no competing interests.

Eukaryogenesis is one of the most enigmatic evolutionary transitions, during which simple prokaryotic cells gave rise to complex eukaryotic cells. While evolutionary intermediates are lacking, gene duplications provide information on the order of events by which eukaryotes originated. Here we use a phylogenomics approach to reconstruct successive steps during eukaryogenesis. We find that gene duplications roughly doubled the proto-eukaryotic gene repertoire, with families inherited from the Asgard archaea-related host being duplicated most. By relatively timing events using phylogenetic distances, we inferred that duplications in cytoskeletal and membrane-trafficking families were among the earliest events, whereas most other families expanded predominantly after mitochondrial endosymbiosis. Altogether, we infer that the host that engulfed the proto-mitochondrion had some eukaryote-like complexity, which drastically increased upon mitochondrial acquisition. This scenario bridges the signs of complexity observed in Asgard archaeal genomes to the proposed role of mitochondria in triggering eukaryogenesis.

The apical complex is the instrument of invasion used by apicomplexan parasites, and the conoid is a conspicuous feature of this apparatus found throughout this phylum. The conoid, however, is believed to be heavily reduced or missing from Plasmodium species and other members of the class Aconoidasida. Relatively few conoid proteins have previously been identified, making it difficult to address how conserved this feature is throughout the phylum, and whether it is genuinely missing from some major groups. Moreover, parasites such as Plasmodium species cycle through three invasive forms and there is the possibility of differential presence of the conoid between these stages. We have applied spatial proteomics and high-resolution microscopy to develop a more complete molecular inventory and understanding of the organisation of conoid-associated proteins in the model apicomplexan Toxoplasma gondii . These data revealed molecular conservation of all conoid substructures throughout Apicomplexa, including Plasmodium , and even in allied Myzozoa such as Chromera and dinoflagellates. We reporter-tagged and observed the expression and location of several conoid complex proteins in the malaria model P. berghei and revealed equivalent structures in all of its zoite forms, as well as evidence of molecular differentiation between blood-stage merozoites and the ookinetes and sporozoites of the mosquito vector. Collectively we show that the conoid is a conserved apicomplexan element at the heart of the invasion mechanisms of these highly successful and often devastating parasites. ### Competing Interest Statement The authors have declared no competing interest.