Chromosome-level genome assembly of Paralithodes platypus provides insights into evolution and adaptation of king crabs

Some molecular aspects of larval development in Paralithodes camtschaticus

The transcriptome of the red king crab, Paralithodes camtschaticus, was sequenced at four developmental stages: zoea I, zoea IV, glaucothoe, and juveniles. Based on our RNA-seq data and paired-end reads from 112 libraries obtained by other researchers earlier, the transcriptome assembly for P. camtschaticus that we obtained has proven to be the most complete of those reported to date. An analysis of enriched processes at different stages has shown, that some of adaptations, e.g., to elevated temperature and hypoxia, do not appear in early larvae. Thus, it is important to maintain optimal conditions for normal larval development and reduce mortality rates. According to the results of the expression profile clustering and transcription factor (TF) search, most TFs are associated with the development of various organs, metamorphosis, and immune responses. The data obtained provide an additional basis for deeper investigation into the mechanisms of the biphasic life cycle in decapods and can be helpful in commercial red king crab stock enhancement programs.

The chromosome-level genome provides insights into the adaptive evolution of the visual system in Oratosquilla oratoria

BACKGROUND

The marine crustacean Oratosquilla oratoria is economically significant in seafood and aquaculture industries. However, the lack of high-quality genome assembly has hindered our understanding of O. oratoria, particularly the mechanisms underlying its developed visual system.

RESULTS

We generated a chromosome-level genome assembly for O. oratoria (2.97 Gb, 44 pseudo-chromosomes) using combination sequencing strategies. Our analysis revealed that more than half of the genome was covered by repeat sequences, and LINE elements showed significant expansion. Furthermore, phylogenetic analysis revealed a close relationship of O. oratoria with Dendrobranchiata and Pleocyemata. In addition, the evolutionary rate of O. oratoria was slightly faster than that of Dendrobranchiata and slower than that of Pleocyemata. Interestingly, we observed the significant expansion of middle-wavelength-sensitive (MWS) opsins in the O. oratoria genome by tandem duplication, partially contributing to their unique visual capabilities. Compared with other crustaceans, O. oratoria has evolved a thicker cornea that was possibly driven by visual adaptations and ecological requirements. Employing comparative transcriptome analysis, we identified a tandemly duplicated cuticle protein (CP) cluster that was specifically expanded and expressed in the ocular tissues of O. oratoria, potentially contributing to the thick cornea of O. oratoria.

CONCLUSIONS

Our study established the first chromosome-level genome for Stomatopoda species, providing a valuable genomic resource for studying the molecular mechanisms underlying the developed visual system of O. oratoria.

High-resolution chromosome-level genome of Scylla paramamosain provides molecular insights into adaptive evolution in crabs

BACKGROUND

Evolutionary adaptation drives organismal adjustments to environmental pressures, exemplified in the diverse morphological and ecological adaptations seen in Decapoda crustaceans, particularly brachyuran crabs. Crabs thrive in diverse ecosystems, from coral reefs to hydrothermal vents and terrestrial habitats. Despite their ecological importance, the genetic mechanisms underpinning their developmental processes, reproductive strategies, and nutrient acquisition remain poorly understood.

RESULTS

Here, we report a comprehensive genomic analysis of the green mud crab Scylla paramamosain using ultralong sequencing technologies, achieving a high-quality chromosome-level assembly. The refined 1.21 Gb genome, with an impressive contig N50 of 11.45 Mb, offers a valuable genomic resource. The genome exhibits 33,662 protein-coding genes, enriched in various pathways related to development and environmental adaptation. Gene family analysis shows expansion in development-related pathways and contraction in metabolic pathways, indicating niche adaptations. Notably, investigation into Hox gene regulation sheds light on their role in pleopod development, with the Abd-A gene identified as a linchpin. Post-transcriptional regulation involving novel-miR1317 negatively regulates Abd-A levels. Furthermore, the potential role of fru gene in ovarian development and the identification of novel-miR35 as a regulator of Spfru2 add complexity to gene regulatory networks. Comparative functional analysis across Decapoda species reveals neo-functionalization of the elovl6 gene in the synthesis of long-chain polyunsaturated fatty acids (LC-PUFA), suggesting its importance in environmental adaptation.

CONCLUSIONS

Our findings shed light on various aspects of crab biology, including genome sequencing, assembly, and annotation, as well as gene family expansion, contraction, and regulatory mechanisms governing crucial developmental processes such as metamorphosis, reproductive strategies, and fatty acid metabolism.

Chromosome-level assembly for the complex genome of land hermit crab Coenobita brevimanus

Land hermit crabs are a group of shell-carrying crabs that have evolved remarkable terrestrial adaptations in behavior, morphology, physiology, and biochemistry. However, the genetic mechanisms underlying these adaptations remain unclear. In addition, usually it is very difficult to get good genome assemblies for crustaceans. In this study, we managed to assemble the first chromosome-level genome for a land hermit crab (Coenobita brevimanus) with careful manual curation. The final assembly spans 4.74 Gb, with the contig N50 of 1.75 Mb and scaffold N50 of 42.95 Mb, encompassing 117 chromosomes that account for 96.54% of the genome. The evaluations including genome BUSCO (95.26%), Merqury qv (35.88) and the mapping ratio of pair-end short reads (99.48%) showed the high-continuity of C. brevimanus genome assembly, making it the genome with the highest quality in crustaceans with genome size bigger than 3 Gb. The availability of this chromosome-scale genome of crustaceans represents a valuable resource for the land hermit crab, which represents an independent water-to-land transition evolutionary event in the animal kingdom.

Genome variations in sea cucumbers: Insights from genome survey sequencing and comparative analysis of mitochondrial genomes

Sea cucumbers, marine benthic invertebrates, play crucial roles in maintaining the stability of marine ecosystems and hold key evolutionary positions. However, information regarding their genomes remains limited. Here, we conducted genome survey analyses on seven species from four orders. Results indicated that Colochirus anceps, Colochirus quadrangularis, and Pseudocolochirus violaceus within the order Dendrochirotida have significantly larger genomes (2238-3754 Mbp) compared to conventional sea cucumber genomes, accompanied by a very high proportion of repeat sequences (69.39-72.52 %). While Holothuria edulis and Holothuria atra exhibited similar genome sizes comparable to those of other species within the order Holothuriida, heterozygosity and repeat content varied among all the six species in this order. The representative species Apostichopus californicus of the order Synallactida possesses the smallest genome size (573.45Mbp) within its order, but its heterozygosity (2.24 %) is significantly higher than that of other species. The representative species Synapta maculata of the order Apodida exhibited a normal genome size (900.97 Mbp), lower proportion of repeat sequences (42.19 %), and lower heterozygosity (0.84 %), making it the species with the least challenges for genome sequencing and assembly in the future among all surveyed species. Subsequently, we compiled genomic information from a total of 19 sea cucumber genomes, both newly sequenced and previously reported, revealing a significant linear relationship (P = 0.0001) between genome size and the proportion of repeat sequences in sea cucumbers. Additionally, phylogenetic and comparative analysis of mitochondrial genomes among them indicated extensive rearrangements within the order Apodida, leading to significant discrepancies between mitochondrial and nuclear genome phylogenies.

Genome assemblies of two species of porcelain crab, Petrolisthes cinctipes and Petrolisthes manimaculis (Anomura: Porcellanidae)

Crabs are a large subtaxon of the Arthropoda, the most diverse and species-rich metazoan group. Several outstanding questions remain regarding crab diversification, including about the genomic capacitors of physiological and morphological adaptation, that cannot be answered with available genomic resources. Physiologically and ecologically diverse Anomuran porcelain crabs offer a valuable model for investigating these questions and hence genomic resources of these crabs would be particularly useful. Here, we present the first two genome assemblies of congeneric and sympatric Anomuran porcelain crabs, Petrolisthes cinctipes and Petrolisthes manimaculis from different microhabitats. Pacific Biosciences high-fidelity sequencing led to genome assemblies of 1.5 and 0.9 Gb, with N50s of 706.7 and 218.9 Kb, respectively. Their assembly length difference can largely be attributed to the different levels of interspersed repeats in their assemblies: The larger genome of P. cinctipes has more repeats (1.12 Gb) than the smaller genome of P. manimaculis (0.54 Gb). For obtaining high-quality annotations of 44,543 and 40,315 protein-coding genes in P. cinctipes and P. manimaculis, respectively, we used RNA-seq as part of a larger annotation pipeline. Contrarily to the large-scale differences in repeat content, divergence levels between the two species as estimated from orthologous protein-coding genes are moderate. These two high-quality genome assemblies allow future studies to examine the role of environmental regulation of gene expression in the two focal species to better understand physiological response to climate change, and provide the foundation for studies in fine-scale genome evolution and diversification of crabs.

Genomics and transcriptomics of the Chinese mitten crabs (Eriocheir sinensis)

To gain a deeper understanding of the genetic factors influencing the growth and development of Eriocheir sinensis, a well-known species of hairy crab found in Yangcheng Lake, this study focused on the de novo genome and full-length transcriptome information of the selected subjects. Specifically, Yangcheng Lake hairy crabs were chosen as the experimental samples. Initially, a genome analysis was performed, resulting in the identification of gene fragments with a combined length of 1266,092,319 bp. Subsequently, a transcriptome analysis was conducted on a mixture of tissues from four different sites, namely muscle, brain, eye, and heart, to further investigate the genetic characteristics at the transcriptome level. The Pacific Biosciences (Pacio) single-molecule real-time sequencing system generated a total of 36.93 G sub-fragments and 175,90041 effective inserts. This research contributes to the indirect comprehension of genetic variations underlying individual traits. Furthermore, a comparison of the obtained data with relevant literature emphasizes the advantages of this study and establishes a basis for further investigations on the Chinese mitten crab.

Abundance and Diversification of Repetitive Elements in Decapoda Genomes

Repetitive elements are a major component of DNA sequences due to their ability to propagate through the genome. Characterization of Metazoan repetitive profiles is improving; however, current pipelines fail to identify a significant proportion of divergent repeats in non-model organisms. The Decapoda order, for which repeat content analyses are largely lacking, is characterized by extremely variable genome sizes that suggest an important presence of repetitive elements. Here, we developed a new standardized pipeline to annotate repetitive elements in non-model organisms, which we applied to twenty Decapoda and six other Crustacea genomes. Using this new tool, we identified 10% more repetitive elements than standard pipelines. Repetitive elements were more abundant in Decapoda species than in other Crustacea, with a very large number of highly repeated satellite DNA families. Moreover, we demonstrated a high correlation between assembly size and transposable elements and different repeat dynamics between Dendrobranchiata and Reptantia. The patterns of repetitive elements largely reflect the phylogenetic relationships of Decapoda and the distinct evolutionary trajectories within Crustacea. In summary, our results highlight the impact of repetitive elements on genome evolution in Decapoda and the value of our novel annotation pipeline, which will provide a baseline for future comparative analyses.

Whole genome evaluation analysis and preliminary Assembly of Oratosquilla oratoria (Stomatopoda: Squillidae)

BACKGROUND

As the dominant species of Stomatopoda, Oratosquilla oratoria has not been fully cultivated artificially, and the fishery production mainly depends on marine fishing. Due to the lack of stomatopod genome, the development of molecular breeding of mantis shrimps still lags behind.

METHODS AND RESULTS

A survey analysis was performed to obtain the genome size, GC content and heterozygosity ratio in order to provide a fundation for subsequent whole-genome sequencing. The results showed that the estimated genome size of the O. oratoria was about 2.56 G, and the heterozygosity ratio was 1.81%, indicating that it is a complex genome. Then the sequencing data was preliminarily assembled with k-mer = 51 by SOAPdenovo software to obtain a genome size of 3.01G and GC content of 40.37%. According to ReapeatMasker and RepeatModerler analysis, the percentage of repeats in O. oratoria was 45.23% in the total genome, similar to 44% in Survey analysis. The MISA tool was used to analyze the simple sequence repeat (SSR) characteristics of genome sequences including Oratosquilla oratoria, Macrobrachium nipponense, Fenneropenaeus chinensis, Eriocheir japonica sinensis, Scylla paramamosain and Paralithodes platypus. All crustacean genomes showed similar SSRs characteristics, with the highest proportion of di-nucleotide repeat sequences. And AC/GT and AGG/CCT repeats were the main types of di-nucleotide and tri-nucleotide repeats in O. oratoria.

CONCLUSION

This study provided a reference for the genome assembly and annotation of the O. oratoria, and also provided a theoretical basis for the development of molecular markers of O. oratoria.

A High-Quality Genome Assembly of Striped Catfish (Pangasianodon hypophthalmus) Based on Highly Accurate Long-Read HiFi Sequencing Data

The HiFi sequencing technology yields highly accurate long-read data with accuracies greater than 99.9% that can be used to improve results for complex applications such as genome assembly. Our study presents a high-quality chromosome-scale genome assembly of striped catfish (Pangasianodon hypophthalmus), a commercially important species cultured mainly in Vietnam, integrating HiFi reads and Hi-C data. A 788.4 Mb genome containing 381 scaffolds with an N50 length of 21.8 Mb has been obtained from HiFi reads. These scaffolds have been further ordered and clustered into 30 chromosome groups, ranging from 1.4 to 57.6 Mb, based on Hi-C data. The present updated assembly has a contig N50 of 14.7 Mb, representing a 245-fold and 4.2-fold improvement over the previous Illumina and Illumina-Nanopore-Hi-C based version, respectively. In addition, the proportion of repeat elements and BUSCO genes identified in our genome is remarkably higher than in the two previously released striped catfish genomes. These results highlight the power of using HiFi reads to assemble the highly repetitive regions and to improve the quality of genome assembly. The updated, high-quality genome assembled in this work will provide a valuable genomic resource for future population genetics, conservation biology and selective breeding studies of striped catfish.

Chromosome-level genome assembly, annotation, and phylogenomics of the gooseneck barnacle Pollicipes pollicipes

BACKGROUND

The barnacles are a group of >2,000 species that have fascinated biologists, including Darwin, for centuries. Their lifestyles are extremely diverse, from free-swimming larvae to sessile adults, and even root-like endoparasites. Barnacles also cause hundreds of millions of dollars of losses annually due to biofouling. However, genomic resources for crustaceans, and barnacles in particular, are lacking.

RESULTS

Using 62× Pacific Biosciences coverage, 189× Illumina whole-genome sequencing coverage, 203× HiC coverage, and 69× CHi-C coverage, we produced a chromosome-level genome assembly of the gooseneck barnacle Pollicipes pollicipes. The P. pollicipes genome is 770 Mb long and its assembly is one of the most contiguous and complete crustacean genomes available, with a scaffold N50 of 47 Mb and 90.5% of the BUSCO Arthropoda gene set. Using the genome annotation produced here along with transcriptomes of 13 other barnacle species, we completed phylogenomic analyses on a nearly 2 million amino acid alignment. Contrary to previous studies, our phylogenies suggest that the Pollicipedomorpha is monophyletic and sister to the Balanomorpha, which alters our understanding of barnacle larval evolution and suggests homoplasy in a number of naupliar characters. We also compared transcriptomes of P. pollicipes nauplius larvae and adults and found that nearly one-half of the genes in the genome are differentially expressed, highlighting the vastly different transcriptomes of larvae and adult gooseneck barnacles. Annotation of the genes with KEGG and GO terms reveals that these stages exhibit many differences including cuticle binding, chitin binding, microtubule motor activity, and membrane adhesion.

CONCLUSION

This study provides high-quality genomic resources for a key group of crustaceans. This is especially valuable given the roles P. pollicipes plays in European fisheries, as a sentinel species for coastal ecosystems, and as a model for studying barnacle adhesion as well as its key position in the barnacle tree of life. A combination of genomic, phylogenetic, and transcriptomic analyses here provides valuable insights into the evolution and development of barnacles.

Genome Sequencing and Assembly Strategies and a Comparative Analysis of the Genomic Characteristics in Penaeid Shrimp Species

Penaeid shrimp (family Penaeidae) represents one of the most economically and ecologically important groups of crustaceans. However, their genome sequencing and assembly have encountered extreme difficulties during the last 20 years. In this study, based on our previous genomic data, we investigated the genomic characteristics of four penaeid shrimp species and identified potential factors that result in their poor genome assembly, including heterozygosity, polyploidization, and repeats. Genome sequencing and comparison of somatic cells (diploid) of the four shrimp species and a single sperm cell (haploid) of Litopenaeus vannamei identified a common bimodal distribution of K-mer depths, suggesting either high heterozygosity or abundant homo-duplicated sequences present in their genomes. However, penaeids have not undergone whole-genome duplication as indicated by a series of approaches. Besides, the remarkable expansion of simple sequence repeats was another outstanding character of penaeid genomes, which also made the genome assembly highly fragmented. Due to this situation, we tried to assemble the genome of penaeid shrimp using various genome sequencing and assembly strategies and compared the quality. Therefore, this study provides new insights about the genomic characteristics of penaeid shrimps while improving their genome assemblies.

Comparative genomics of the coconut crab and other decapod crustaceans: exploring the molecular basis of terrestrial adaptation

Background

The complex life cycle of the coconut crab, Birgus latro, begins when an obligate terrestrial adult female visits the intertidal to hatch zoea larvae into the surf. After drifting for several weeks in the ocean, the post-larval glaucothoes settle in the shallow subtidal zone, undergo metamorphosis, and the early juveniles then subsequently make their way to land where they undergo further physiological changes that prevent them from ever entering the sea again. Here, we sequenced, assembled and analyzed the coconut crab genome to shed light on its adaptation to terrestrial life. For comparison, we also assembled the genomes of the long-tailed marine-living ornate spiny lobster, Panulirus ornatus, and the short-tailed marine-living red king crab, Paralithodes camtschaticus. Our selection of the latter two organisms furthermore allowed us to explore parallel evolution of the crab-like form in anomurans.

Results

All three assembled genomes are large, repeat-rich and AT-rich. Functional analysis reveals that the coconut crab has undergone proliferation of genes involved in the visual, respiratory, olfactory and cytoskeletal systems. Given that the coconut crab has atypical mitochondrial DNA compared to other anomurans, we argue that an abundance of kif22 and other significantly proliferated genes annotated with mitochondrial and microtubule functions, point to unique mechanisms involved in providing cellular energy via nuclear protein-coding genes supplementing mitochondrial and microtubule function. We furthermore detected in the coconut crab a significantly proliferated HOX gene, caudal, that has been associated with posterior development in Drosophila, but we could not definitively associate this gene with carcinization in the Anomura since it is also significantly proliferated in the ornate spiny lobster. However, a cuticle-associated coatomer gene, gammacop, that is significantly proliferated in the coconut crab, may play a role in hardening of the adult coconut crab abdomen in order to mitigate desiccation in terrestrial environments.

Conclusion

The abundance of genomic features in the three assembled genomes serve as a source of hypotheses for future studies of anomuran environmental adaptations such as shell-utilization, perception of visual and olfactory cues in terrestrial environments, and cuticle sclerotization. We hypothesize that the coconut crab exhibits gene proliferation in lieu of alternative splicing as a terrestrial adaptation mechanism and propose life-stage transcriptomic assays to test this hypothesis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07636-9.

How to become a crab: Phenotypic constraints on a recurring body plan

A fundamental question in biology is whether phenotypes can be predicted by ecological or genomic rules. At least five cases of convergent evolution of the crab-like body plan (with a wide and flattened shape, and a bent abdomen) are known in decapod crustaceans, and have, for over 140 years, been known as "carcinization." The repeated loss of this body plan has been identified as "decarcinization." In reviewing the field, we offer phylogenetic strategies to include poorly known groups, and direct evidence from fossils, that will resolve the history of crab evolution and the degree of phenotypic variation within crabs. Proposed ecological advantages of the crab body are summarized into a hypothesis of phenotypic integration suggesting correlated evolution of the carapace shape and abdomen. Our premise provides fertile ground for future studies of the genomic and developmental basis, and the predictability, of the crab-like body form.

Chromosome-level genome assembly of Paralithodes platypus provides insights into evolution and adaptation of king crabs

The blue king crab, Paralithodes platypus, which belongs to the family Lithodidae, is a commercially and ecologically important species. However, a high-quality reference genome for the king crab has not yet been reported. Here, we assembled the first chromosome-level blue king crab genome, which contains 104 chromosomes and an N50 length of 51.15 Mb. Furthermore, we determined that the large genome size can be attributed to the insertion of long interspersed nuclear elements and long tandem repeats. Genome assembly assessment showed that 96.54% of the assembled transcripts could be aligned to the assembled genome. Phylogenetic analysis showed the blue king crab to have a close relationship with the Eubrachyura crabs, from which it diverged 272.5 million years ago. Population history analyses indicated that the effective population of the blue king crab declined sharply and then gradually increased from the Cretaceous and Neogene periods, respectively. Furthermore, gene families related to developmental pathways, steroid and thyroid hormone synthesis, and inflammatory regulation were expanded in the genome, suggesting that these genes contributed substantially to the environmental adaptation and unique body plan evolution of the blue king crab. The high-quality reference genome reported here provides a solid molecular basis for further study of the blue king crab's development and environmental adaptation.