Genome Size Variation and Evolution Driven by Transposable Elements in the Genus Oryza

Comparative analysis of simple sequence repeats and synteny across ten Oryza species: Implications for stress response and genetic diversity

Rice is a pivotal food source for most of the global population, necessitating a strategic focus on maximizing its production under diverse conditions through various methods. As molecular markers, simple sequence repeats (SSRs) emerge as instrumental tools in product enhancement and molecular research. This study employs in silico methods to predict the presence of molecular markers across distinct genomic and genic regions within ten Oryza species. Subsequently, we conducted a comprehensive comparison and synteny analysis of common molecular markers shared among most species, particularly those implicated in stress responses, utilizing McscanX. Beyond identifying common SSRs across the ten species under investigation, we delved into the functional analysis of these markers, specifically pinpointing those associated with stress. Additionally, our investigation illustrated the uniform distribution of SSRs along chromosomes and created a physical map showcasing their prevalence. Notably, chromosomes 1, 2, and 3 exhibited a higher density of molecular markers compared to their counterparts. Furthermore, our study highlighted that Oryza glumipatula, Oryza brachyantha, Oryza meridionalis, and Oryza longistaminata species manifested more pronounced differences in SSR markers compared to other Oryza species. The implications of these findings extend to applications in genetic diversity assessment, genetic mapping, and molecular marker-assisted selection breeding, providing valuable insights for future research and development in the field.

The expanded Bostrychia moritziana genome unveils evolution in the most diverse and complex order of red algae

Red algae are an ancient eukaryotic lineage that were among the first to evolve multicellularity. Although they share a common origin with modern-day plants and display complex multicellular development, comprehensive genome data from the most highly evolved red algal groups remain scarce. Here, we present a chromosome-level genome assembly of Bostrychia moritziana, a complex red seaweed in the Rhodomelaceae family of the Ceramiales-the largest and most diverse order of red algae. Contrary to the view that red algal genomes are typically small, we report significant genome size expansion in Bostrychia and other Ceramiales, which represents one of at least three independent expansion events in red algal evolution. Our analyses suggest that these expansions do not involve polyploidy or ancient whole-genome duplications, but in Bostrychia rather stem from the proliferation of a single lineage of giant Plavaka DNA transposons. Consistent with its enlarged genome, Bostrychia has an increased gene content shaped by de novo gene emergence and amplified gene families in common with other Ceramiales, providing insight into the genetic adaptations underpinning this successful and species-rich order. Finally, our sex-specific assemblies resolve the UV sex chromosomes in Bostrychia, which feature expanded gene-rich sex-linked regions. Notably, each sex chromosome harbors a three amino acid loop extension homeodomain (TALE-HD) transcription factor orthologous to ancient regulators of haploid-diploid transitions in other multicellular lineages. Together, our findings offer a unique perspective of the genomic adaptations driving red algal diversity and demonstrate how this red seaweed lineage can provide insight into the evolutionary origins and universal principles underpinning complex multicellularity.

Karyotype and genome size analyses for two spiders of the lycosidae family

Background

Karyotype and genome size are critical genetic characteristics with significant value for cytogenetics, taxonomy, phylogenetics, evolution, and molecular biology. The Lycosidae family, known for its diverse spiders with varying ecological habits and behavioral traits, has seen limited exploration of its karyotype and genome size.

Methods

We utilized an improved tissue drop technique to prepare chromosome slides and compare the features of male and female karyotypes for two wolf spiders with different habits of Lycosidae. Furthermore, we predicted their genome sizes using flow cytometry (FCM) and K-mer analysis.

Results

The karyotypes of female and male Hippasa lycosina were 2n♀ = 26 = 14 m + 12 sm and 2n♂ = 24 = 10 m + 14 sm, respectively, and were composed of metacentric (m) and submetacentric (sm) chromosomes. In contrast, the karyotypes of Lycosa grahami consisted of telocentric (t) and subtelocentric (st) chromosomes (2n♀ = 20 = 20th and 2n♂ = 18 = 12th + 6t, for females and males). The sex chromosomes were both X1X2O. The estimated sizes of the H. lycosina and L. grahami genomes were 1966.54-2099.89 Mb and 3692.81-4012.56 Mb, respectively. Flow cytometry yielded slightly smaller estimates for genome size compared to k-mer analysis. K-mer analysis revealed a genome heterozygosity of 0.42% for H. lycosina and 0.80% for L. grahami, along with duplication ratios of 21.39% and 54.91%, respectively.

Conclusion

This study describes the first analysis of the genome sizes and karyotypes of two spiders from the Lycosidae that exhibit differential habits and provides essential data for future phylogenetic, cytogenetic, and genomic studies.

Revealing Genomic Traits and Evolutionary Insights of Oryza officinalis from Southern China Through Genome Assembly and Transcriptome Analysis

Wild rice, as the ancestor of cultivated rice, has accumulated a wide range of beneficial traits through prolonged natural selection and evolution. Oryza officinalis, belonging to the CC genome, differs significantly from the AA genome. In this study, we utilized second- and third-generation sequencing, along with Hi-C technology, to assemble the genome of MT10 (O. officinalis). The assembled genome is 552.58 Mb, with contigs and scaffold N50 values of 40.04 and 44.48 Mb, respectively, and 96.73% of the sequences anchored to 12 chromosomes. A total of 33,813 genes were annotated, and repetitive sequences account for 54.24% of the MT10 genome. The number of unique genes in MT10 exceeds that in the O. officinalis genome from Thailand, and their divergence time is estimated at 1.6 million years ago. The MT10 genome exhibits fewer expanded gene families compared to contracted ones, with the expanded families predominantly associated with disease and pest resistance. Comparative genomic analysis of MT10 and Nipponbare reveals sequence variations in biotic and abiotic resistance-related genes. In particular, the presence of R genes and cystatin gene families in MT10 may contribute to its unique insect resistance. Transcriptome analyses indicate that flavonoid biosynthesis and MAPK-related genes are expressed in response to brown planthopper infestation. This study represents the first chromosome-level genome assembly of MT10, providing a reference sequence for the efficient cloning of beneficial genes from O. officinalis, which holds significant potential for the genetic improvement of cultivated rice.

Genome-based discovery of pachysiphine synthases in Tabernaemontana elegans

Plant-specialized metabolism represents an inexhaustible source of active molecules, some of which have been used in human health for decades. Among these, monoterpene indole alkaloids (MIAs) include a wide range of valuable compounds with anticancer, antihypertensive, or neuroactive properties. This is particularly the case for the pachysiphine derivatives which show interesting antitumor and anti-Alzheimer activities but accumulate at very low levels in several Tabernaemontana species. Unfortunately, genome data in Tabernaemontanaceae are lacking and knowledge on the biogenesis of pachysiphine-related MIAs in planta remains scarce, limiting the prospects for the biotechnological supply of many pachysiphine-derived biopharmaceuticals. Here, we report a raw version of the toad tree (Tabernaemontana elegans) genome sequence. These new genomic resources led to the identification and characterization of a couple of genes encoding cytochrome P450 with pachysiphine synthase activity. Our phylogenomic and docking analyses highlight the different evolutionary processes that have been recruited to epoxidize the pachysiphine precursor tabersonine at a specific position and in a dedicated orientation, thus enriching our understanding of the diversification and speciation of the MIA metabolism in plants. These gene discoveries also allowed us to engineer the synthesis of MIAs in yeast through the combinatorial association of metabolic enzymes resulting in the tailor-made synthesis of non-natural MIAs. Overall, this work represents a step forward for the future supply of pachysiphine-derived drugs by microbial cell factories.

The evolutionary dynamics of genome sizes and repetitive elements in Ensifera (Insecta: Orthoptera)

BACKGROUND

In evolutionary biology, identifying and quantifying inter-lineage genome size variation and elucidating the underlying causes of that variation have long been goals. Repetitive elements (REs) have been proposed and confirmed as being among the most important contributors to genome size variation. However, the evolutionary implications of genome size variation and RE dynamics are not well understood.

RESULTS

A total of 35 Ensifera insects were collected from different areas in China, including nine species of crickets and 26 species of katydids. The genome sizes of seven species were then determined using flow cytometry. The RepeatExplorer2 pipeline was employed to retrieve the repeated sequences for each species, based on low-coverage (0.1 X) high-throughput Illumina unassembled short reads. The genome sizes of the 35 Ensifera insects exhibited a considerable degree of variation, ranging from 1.00 to 18.34 pg. This variation was more than 18-fold. Similarly, the RE abundances exhibited considerable variation, ranging from 13.66 to 61.16%. In addition, the Tettigonioidea had larger genomes and contained significantly more REs than did the Grylloidea genomes. Analysis of the correlation between RE abundance and the genome size of 35 Ensifera insects revealed that the abundance of REs, transposable elements (TEs), long terminal repeats (LTRs), and long interspersed nuclear elements (LINEs) are significantly correlated with genome size. Notably, there is an inflection point in this correlation, where species with increasingly large genomes (e.g., > 5-10 pg) have repeats that contribute less to genome expansion than expected. Furthermore, this study revealed contrasting evolutionary directions between the Tettigonioidea and Grylloidea clades in terms of the expansion of REs. Tettigonioidea species exhibit a gradual increase in ancestral genome size and RE abundance as they diverge, while Grylloidea species experience sustained genome contraction.

CONCLUSIONS

This study reveals extensive variation in genome size and RE abundance in Ensifera insects, with distinct evolutionary patterns across two major groups, Tettigonioidea and Grylloidea. This provides valuable insights into the variation in genome size and RE abundance in Ensifera insects, offering a comprehensive understanding of their evolutionary history.

A draft reference genome assembly of California Pipevine, Aristolochia californica Torr

The California Pipevine, Aristolochia californica Torr., is the only endemic California species within the cosmopolitan birthwort family Aristolochiaceae. It occurs as an understory vine in riparian and chaparral areas and in forest edges and windrows. The geographic range of this plant species almost entirely overlaps with that of its major specialized herbivore, the California Pipevine Swallowtail Butterfly Battus philenor hirsuta. While this species pair is a useful, ecologically well-understood system to study co-evolution, until recently, genomic resources for both have been lacking. Here, we report a new, chromosome-level assembly of A. californica as part of the California Conservation Genomics Project (CCGP). Following the sequencing and assembly strategy of the CCGP, we used Pacific Biosciences HiFi long reads and Hi-C chromatin proximity sequencing technology to produce a de novo assembled genome. Our genome assembly, the first for any species in the genus, contains 531 scaffolds spanning 661 megabase (Mb) pairs, with a contig N50 of 6.53 Mb, a scaffold N50 of 42.2 Mb, and BUSCO complete score of 98%. In combination with the recently published B. philenor hirsuta reference genome assembly, the A. californica reference genome assembly will be a powerful tool for studying co-evolution in a rapidly changing California landscape.

Reclassification of Botryococcus braunii chemical races into separate species based on a comparative genomics analysis

The colonial green microalga Botryococcus braunii is well known for producing liquid hydrocarbons that can be utilized as biofuel feedstocks. B. braunii is taxonomically classified as a single species made up of three chemical races, A, B, and L, that are mainly distinguished by the hydrocarbons produced. We previously reported a B race draft nuclear genome, and here we report the draft nuclear genomes for the A and L races. A comparative genomic study of the three B. braunii races and 14 other algal species within Chlorophyta revealed significant differences in the genomes of each race of B. braunii. Phylogenomically, there was a clear divergence of the three races with the A race diverging earlier than both the B and L races, and the B and L races diverging from a later common ancestor not shared by the A race. DNA repeat content analysis suggested the B race had more repeat content than the A or L races. Orthogroup analysis revealed the B. braunii races displayed more gene orthogroup diversity than three closely related Chlamydomonas species, with nearly 24-36% of all genes in each B. braunii race being specific to each race. This analysis suggests the three races are distinct species based on sufficient differences in their respective genomes. We propose reclassification of the three chemical races to the following species names: Botryococcus alkenealis (A race), Botryococcus braunii (B race), and Botryococcus lycopadienor (L race).

Comprehensive analysis of the Xya riparia genome uncovers the dominance of DNA transposons, LTR/Gypsy elements, and their evolutionary dynamics

Transposable elements (TEs) are DNA sequences that can move or replicate within a genome, and their study has become increasingly important in understanding genome evolution and function. The Tridactylidae family, including Xya riparia (pygmy mole cricket), harbors a variety of transposable elements (TEs) that have been insufficiently investigated. Further research is required to fully understand their diversity and evolutionary characteristics. Hence, we conducted a comprehensive repeatome analysis of X. riparia species using the chromosome-level assembled genome. The study aimed to comprehensively analyze the abundance, distribution, and age of transposable elements (TEs) in the genome. The results indicated that the genome was 1.67 Gb, with 731.63 Mb of repetitive sequences, comprising 27% of Class II (443.25 Mb), 16% of Class I (268.45 Mb), and 1% of unknown TEs (19.92 Mb). The study found that DNA transposons dominate the genome, accounting for approximately 60% of the total repeat size, with retrotransposons and unknown elements accounting for 37% and 3% of the genome, respectively. The members of the Gypsy superfamily were the most abundant amongst retrotransposons, accounting for 63% of them. The transposable superfamilies (LTR/Gypsy, DNA/nMITE, DNA/hAT, and DNA/Helitron) collectively constituted almost 70% of the total repeat size of all six chromosomes. The study further unveiled a significant linear correlation (Pearson correlation: r = 0.99, p-value = 0.00003) between the size of the chromosomes and the repetitive sequences. The average age of DNA transposon and retrotransposon insertions ranges from 25 My (million years) to 5 My. The satellitome analysis discovered 13 satellite DNA families that comprise about 0.15% of the entire genome. In addition, the transcriptional analysis of TEs found that DNA transposons were more transcriptionally active than retrotransposons. Overall, the study suggests that the genome of X. riparia is complex, characterized by a substantial portion of repetitive elements. These findings not only enhance our understanding of TE evolution within the Tridactylidae family but also provide a foundation for future investigations into the genomic intricacies of related species.

LocoGSE, a sequence-based genome size estimator for plants

Extensive research has focused on exploring the range of genome sizes in eukaryotes, with a particular emphasis on land plants, where significant variability has been observed. Accurate estimation of genome size is essential for various research purposes, but existing sequence-based methods have limitations, particularly for low-coverage datasets. In this study, we introduce LocoGSE, a novel genome size estimator designed specifically for low-coverage datasets generated by genome skimming approaches. LocoGSE relies on mapping the reads on single copy consensus proteins without the need for a reference genome assembly. We calibrated LocoGSE using 430 low-coverage Angiosperm genome skimming datasets and compared its performance against other estimators. Our results demonstrate that LocoGSE accurately predicts monoploid genome size even at very low depth of coverage (<1X) and on highly heterozygous samples. Additionally, LocoGSE provides stable estimates across individuals with varying ploidy levels. LocoGSE fills a gap in sequence-based plant genome size estimation by offering a user-friendly and reliable tool that does not rely on high coverage or reference assemblies. We anticipate that LocoGSE will facilitate plant genome size analysis and contribute to evolutionary and ecological studies in the field. Furthermore, at the cost of an initial calibration, LocoGSE can be used in other lineages.

A Flow Cytometry-Based Assessment of the Genomic Size and Ploidy Level of Wild Musa Species in India

The genome size variation is an important attribute in evolutionary and species characterization. Musa L. is regarded as one of the taxonomically complicated genera within the order Zingiberales, with more than 75 species from wild seeded to seedless cultivars that may be diploid, triploid or tetraploid. The knowledge of total nuclear DNA content in terms of genome size and ploidy level in wild species of Musa is absolutely important in evolutionary and genomic studies.

METHODS

In this paper, chromosome spreading was performed via protoplast isolation and a fast air-dry dropping method and flow cytometry were used with Raphanus sativus L. (Brassicaceae) as a standard for ploidy and genome size estimation.

RESULTS

The results showed that genome size (2C) varied amongst Musa species, based on the ratio of G1 peak positions. The lowest genome size (2C) was found in M. balbisiana var. andamanica (1.051 ± 0.060 pg) and the highest genome size (2C) was recorded for Musa ABB.cv. Meitei-hei (1.812 ± 0.108 pg) for the section Eumusa. Among the species belonging to the section Rhodochlamys, M. rosae had the lowest 2C content of 1.194 ± 0.033 pg whereas the highest nuclear DNA content (2C) was observed in M. velutina (1.488 ± 0.203 pg). Cytogenetic analysis revealed that the chromosome number of 14 wild Musa species was 2n = 22, while 1 species-Ensete glaucum-showed a chromosome number of 2n = 18 (diploid), and for 3 species, the chromosome number was 2n = 33 (triploids). An association study based on the Pearson correlation coefficient showed 2C nuclear DNA content was significant and positively correlated with ploidy level (R = 0.9) and chromosome number (R = 0.84).

CONCLUSIONS

The present study provides reliable information on the genome size and ploidy level of wild Musa species from the Indian region through flow cytometric analysis, which could be further utilized in taxonomic and crop improvement programs. For the first time, the nuclear DNA content of eight wild diploid and three triploid Indian species were estimated and reported. Genome size could be an effective indicator in identification of species and evolutionary studies in Musa with varying ploidy levels and morphological similarities.

Genome sizes and repeatome evolution in zoantharians (Cnidaria: Hexacorallia: Zoantharia)

Across eukaryotes, large variations of genome sizes have been observed even between closely related species. Transposable elements as part of the repeated DNA have been proposed and confirmed as one of the most important contributors to genome size variation. However, the evolutionary implications of genome size variation and transposable element dynamics are not well understood. Together with phenotypic traits, they are commonly referred to as the "C-value enigma". The order Zoantharia are benthic cnidarians found from intertidal zones to the deep sea, and some species are particularly abundant in coral reefs. Despite their high ecological relevance, zoantharians have yet to be largely studied from the genomic point of view. This study aims at investigating the role of the repeatome (total content of repeated elements) in genome size variations across the order Zoantharia. To this end, whole-genomes of 32 zoantharian species representing five families were sequenced. Genome sizes were estimated and the abundances of different repeat classes were assessed. In addition, the repeat overlap between species was assessed by a sequence clustering method. The genome sizes in the dataset varied up to 2.4 fold magnitude. Significant correlations between genome size, repeated DNA content and transposable elements, respectively (Pearson's correlation test R2 = 0.47, p = 0.0016; R2 = 0.22, p = 0.05) were found, suggesting their involvement in the dynamics of genome expansion and reduction. In all species, long interspersed nuclear elements and DNA transposons were the most abundant identified elements. These transposable elements also appeared to have had a recent expansion event. This was in contrast to the comparative clustering analysis which revealed species-specific patterns of satellite elements' amplification. In summary, the genome sizes of zoantharians likely result from the complex dynamics of repeated elements. Finally, the majority of repeated elements (up to 70%) could not be annotated to a known repeat class, highlighting the need to further investigate non-model cnidarian genomes. More research is needed to understand how repeated DNA dynamics relate to zoantharian evolution and their biology.

Chromosomal-Scale Genome Assemblies of Two Coastal Plant Species, Scaevola taccada and S. hainanensis-Insight into Adaptation Outside of the Common Range

While most of the species in Goodeniaceae family, excluding the Scaevola genus, are endemic to Australasia, S. taccada and S. hainanensis have expanded their distribution range to the tropical coastlines of the Atlantic and Indian Oceans. S. taccada appears to be highly adapted to coastal sandy lands and cliffs, and it has become invasive in places. S. hainanensis is found mainly in salt marshes near mangrove forests, and is at risk of extinction. These two species provide a good system to investigate adaptive evolution outside the common distribution range of this taxonomic group. Here, we report their chromosomal-scale genome assemblies with the objective of probing their genomic mechanisms related to divergent adaptation after leaving Australasia. The scaffolds were assembled into eight chromosome-scale pseudomolecules, which covered 90.12% and 89.46% of the whole genome assembly for S. taccada and S. hainanensis, respectively. Interestingly, unlike many mangroves, neither species has undergone whole-genome duplication. We show that private genes, specifically copy-number expanded genes are essential for stress response, photosynthesis, and carbon fixation. The gene families that are expanded in S. hainanensis and contracted in S. taccada might have facilitated adaptation to high salinity in S. hainanensis. Moreover, the genes under positive selection in S. hainanensis have contributed to its response to stress and its tolerance of flooding and anoxic environments. In contrast, compared with S. hainanensis, the more drastic copy number expansion of FAR1 genes in S. taccada might have facilitated its adaptation to the stronger light radiation present in sandy coastal lands. In conclusion, our study of the chromosomal-scale genomes of S. taccada and S. hainanensis provides novel insights into their genomic evolution after leaving Australasia.

Genome Size Variation across a Cypriot Fabeae Tribe Germplasm Collection

DNA content is an important trait linked to the evolutionary routes of taxa and often connected to speciation. In the present study, we studied C-values variation across the Cypriot Fabeae gene pool. Several hundred plants (Vicia spp., Lens spp., Pisum spp.) were sampled across Cyprus. Accurate estimates were established by flow cytometry and propidium iodine staining for 155 discrete populations/accessions. A ten-fold variation was detected across lineages with 1C DNA content varying from 1.584 pg for V. cretica (ARI02420) to 13.983 pg for V. faba (ARI00187). In general, flow cytometry was precise for the characterization of species, even though there were instances of genome overlapping across taxa. Most analyses in the current work refer to species that have not been characterized before by flow cytometry (or any other DNA content estimation method). Still, a correlation to C-values previously reported in Kew Plant DNA C-values database was attempted. A high degree of correlation except for V. dalmatica was established. The evaluation of genome size trait in relation with the Fabeae phylogeny, revealed that Pisum and Lens genera were rather homogenous, but an astonishing fluctuation was shown for Vicia spp. Moreover, it was established that genome up- or down-scaling was not directly linked to speciation drivers. The genomic size measurements presented here could deliver extra quality control for the identification and characterization of taxa in germplasm collections, particularly in cases where species share morphological characters.

Genome and pan-genome assembly of asparagus bean (Vigna unguiculata ssp. sesquipedialis) reveal the genetic basis of cold adaptation

Asparagus bean (Vigna unguiculata ssp. sesquipedialis) is an important cowpea subspecies. We assembled the genomes of Ningjiang 3 (NJ, 550.31 Mb) and Dubai bean (DB, 564.12 Mb) for comparative genomics analysis. The whole-genome duplication events of DB and NJ occurred at 64.55 and 64.81 Mya, respectively, while the divergence between soybean and Vigna occurred in the Paleogene period. NJ genes underwent positive selection and amplification in response to temperature and abiotic stress. In species-specific gene families, NJ is mainly enriched in response to abiotic stress, while DB is primarily enriched in respiration and photosynthesis. We established the pan-genomes of four accessions (NJ, DB, IT97K-499-35 and Xiabao II) and identified 20,336 (70.5%) core genes present in all the accessions, 6,507 (55.56%) variable genes in two individuals, and 2,004 (6.95%) unique genes. The final pan genome is 616.35 Mb, and the core genome is 399.78 Mb. The variable genes are manifested mainly in stress response functions, ABC transporters, seed storage, and dormancy control. In the pan-genome sequence variation analysis, genes affected by presence/absence variants were enriched in biological processes associated with defense responses, immune system processes, signal transduction, and agronomic traits. The results of the present study provide genetic data that could facilitate efficient asparagus bean genetic improvement, especially in producing cold-adapted asparagus bean.