Chromosomal-Level Assembly of Antarctic Scaly Rockcod, Trematomus loennbergii Genome Using Long-Read Sequencing and Chromosome Conformation Capture (Hi-C) Technologies

Generating single-copy nuclear gene data for a recent adaptive radiation

Molecular taxonomy and identification within the Antarctic genus Trematomus (Notothenioidei, Teleostei): How valuable is barcoding with COI?

Background“Explosive” adaptive radiations on islands remain one of the most puzzling evolutionary phenomena. The rate of phenotypic and ecological adaptations is extremely fast during such events, suggesting that many genes may be under fairly strong selection. However, no evidence for adaptation at the level of protein coding genes was found, so it has been suggested that selection may work mainly on regulatory elements. Here we report the first evidence that positive selection does operate at the level of protein coding genes during rapid adaptive radiations. We studied molecular adaptation in Hawaiian endemic plant genus Schiedea (Caryophyllaceae), which includes closely related species with a striking range of morphological and ecological forms, varying from rainforest vines to woody shrubs growing in desert-like conditions on cliffs. Given the remarkable difference in photosynthetic performance between Schiedea species from different habitats, we focused on the “photosynthetic” Rubisco enzyme, the efficiency of which is known to be a limiting step in plant photosynthesis.ResultsWe demonstrate that the chloroplast rbcL gene, encoding the large subunit of Rubisco enzyme, evolved under strong positive selection in Schiedea. Adaptive amino acid changes occurred in functionally important regions of Rubisco that interact with Rubisco activase, a chaperone which promotes and maintains the catalytic activity of Rubisco. Interestingly, positive selection acting on the rbcL might have caused favorable cytotypes to spread across several Schiedea species.SignificanceWe report the first evidence for adaptive changes at the DNA and protein sequence level that may have been associated with the evolution of photosynthetic performance and colonization of new habitats during a recent adaptive radiation in an island plant genus. This illustrates how small changes at the molecular level may change ecological species performance and helps us to understand the molecular bases of extremely fast rate of adaptation during island adaptive radiations.

Variation in blood serum antifreeze activity of Antarctic Trematomus fishes across habitat temperature and depth

BackgroundThe importance of transposable elements (TEs) in the genomic remodeling and chromosomal rearrangements that accompany lineage diversification in vertebrates remains the subject of debate. The major impediment to understanding the roles of TEs in genome evolution is the lack of comparative and integrative analyses on complete taxonomic groups. To help overcome this problem, we have focused on the Antarctic teleost genus Trematomus (Notothenioidei: Nototheniidae), as they experienced rapid speciation accompanied by dramatic chromosomal diversity. Here we apply a multi-strategy approach to determine the role of large-scale TE mobilization in chromosomal diversification within Trematomus species.ResultsDespite the extensive chromosomal rearrangements observed in Trematomus species, our measurements revealed strong interspecific genome size conservation. After identifying the DIRS1, Gypsy and Copia retrotransposon superfamilies in genomes of 13 nototheniid species, we evaluated their diversity, abundance (copy numbers) and chromosomal distribution. Four families of DIRS1, nine of Gypsy, and two of Copia were highly conserved in these genomes; DIRS1 being the most represented within Trematomus genomes. Fluorescence in situ hybridization mapping showed preferential accumulation of DIRS1 in centromeric and pericentromeric regions, both in Trematomus and other nototheniid species, but not in outgroups: species of the Sub-Antarctic notothenioid families Bovichtidae and Eleginopsidae, and the non-notothenioid family Percidae.ConclusionsIn contrast to the outgroups, High-Antarctic notothenioid species, including the genus Trematomus, were subjected to strong environmental stresses involving repeated bouts of warming above the freezing point of seawater and cooling to sub-zero temperatures on the Antarctic continental shelf during the past 40 millions of years (My). As a consequence of these repetitive environmental changes, including thermal shocks; a breakdown of epigenetic regulation that normally represses TE activity may have led to sequential waves of TE activation within their genomes. The predominance of DIRS1 in Trematomus species, their transposition mechanism, and their strategic location in “hot spots” of insertion on chromosomes are likely to have facilitated nonhomologous recombination, thereby increasing genomic rearrangements. The resulting centric and tandem fusions and fissions would favor the rapid lineage diversification, characteristic of the nototheniid adaptive radiation.

The 16 species of Molossus (Chiroptera: Molossidae) are distributed throughout the Neotropical region and can be classified into two groups: one consisting of morphologically similar yet phylogenetically divergent species, and another of morphologically distinct but closely related species. This dynamic has led to frequent revisions in the systematics and taxonomy of this genus. This study aimed to analyze patterns of diversification in cranial shape and size within Molossus species using geometric morphometrics (GM), integrating genetic and morphological data. A total of 299 specimens from ten Molossus species widely distributed across the Neotropics were examined, focusing on cranial size, shape diversity, and evolution, and correlating these findings with mitochondrial DNA-based phylogenetic data. Integrated morphometric and phylogenetic analyses revealed a complex evolutionary history within Molossus, with most speciation events occurring during the Pleistocene, suggesting a recent rapid adaptive radiation. GM analyses demonstrated patterns of divergence in cranial size with shape conservatism, and these traits were not significantly related to phylogeny. The data indicate that phylogenetic relationships have limited influence on cranial morphology due to the lack of a strong phylogenetic signal, suggesting that ecological factors, such as diet and habitat, have played central roles in the diversification of Molossus.

Hybridization is increasingly being identified in the genomes of species across the tree of life. This has led to a general recognition that hybridization plays an important role in the generation of species diversity. While hybridization may increase species diversity directly via the formation of new taxa through hybrid speciation, it may also act indirectly via the exchange of phenotypic and genetic variance between species, which may in turn stimulate future speciation events. Using high-throughput sequence data, we resolve phylogenetic relationships and investigate the role of hybridization as a diversification mechanism in the shrubby beardtongues (Penstemon subgenus Dasanthera), a group of North American wildflowers that has undergone a recent and rapid adaptive radiation. Specifically, we test four hypotheses of hybrid taxon formation resulting from hybridization between P. davidsonii and P. fruticosus. Species tree inference supports the monophyly of subgenus Dasanthera and elucidates relationships between taxa distributed in the Cascades and Sierra Nevada Mountains. Results also provide evidence of gene flow between P. davidsonii and P. fruticosus and support at least one hybrid origin hypothesis (P. davidsonii var. menziesii) in a region of contemporary distributional overlap. Hybridization may have also been facilitated by historical overlap in geographic distribution caused by species' responses to climatic changes during the Pleistocene. Our results support a history of hybridization between focal taxa in a rapidly radiating clade of plants and more broadly contribute to our growing understanding of the role of hybridization as a diversification mechanism in plants. This article is protected by copyright. All rights reserved.

The iterative nature of ecomorphological diversification is observed in various groups of animals. However, studies explicitly testing the consistency of morphological variation across and within species are scarce. Antarctic notothenioids represent a textbook example of adaptive radiation in marine fishes. Within Nototheniidae, the endemic Antarctic genus Trematomus consists of 15 extant species, some with documented large intraspecific variability. Here, we quantify head shape disparity in 11 species of Trematomus by landmark-based geometric morphometrics, and we illustrate repeated events of divergence and convergence of their head morphology. Taking advantage of the polymorphism observed in some species of Trematomus, we also show that two closely related species or clades (e.g., Trematomus bernacchii and T. hansoni) are characterised by the same level of morphological disparity as observed at the level of the entire genus. Interestingly, the same main axes of shape variation are shared between and within species, indicating repeated morphological diversification. Overall, we illustrate a similarity of intra- and interspecific patterns of phenotypic diversity providing new insights into the mechanisms that underlie the diversification of Antarctic fishes.

Trichosanthes truncata C. B. Clarke, an important medicinal plant, is a dioecious plant belonging to the Cucurbitaceae family. This study presents a chromosomal-level reference genome assembly for T. truncata. Through the integration of PacBio high-fidelity sequencing and high-throughput chromosome conformation capture technology, a final genome sequence of 637.41 Mb was assembled, with an N50 of 57.24 Mb and consisting of 11 pseudochromosomes. Additionally, 97.21 Mb of repetitive sequences and 36,172 protein-coding genes were annotated. This high-quality genome assembly is of utmost significance for studying the molecular mechanisms underlying the biosynthesis of bioactive compounds. Furthermore, this study provided valuable insights into plant comparative genomics research.

The tea plant (Camellia sinensis) is an important economic crop, which is becoming increasingly popular worldwide, and is now planted in more than 50 countries. Tea green leafhopper is one of the major pests in tea plantations, which can significantly reduce the yield and quality of tea during the growth of plant. In this study, we report a genome assembly for DuyunMaojian tea plants using a combination of Oxford Nanopore Technology PromethION™ with high-throughput chromosome conformation capture technology and used multi-omics to study how the tea plant responds to infestation with tea green leafhoppers. The final genome was 3.08 Gb. A total of 2.97 Gb of the genome was mapped to 15 pseudo-chromosomes, and 2.79 Gb of them could confirm the order and direction. The contig N50, scaffold N50 and GC content were 723.7 kb, 207.72 Mb and 38.54%, respectively. There were 2.67 Gb (86.77%) repetitive sequences, 34,896 protein-coding genes, 104 miRNAs, 261 rRNA, 669 tRNA, and 6,502 pseudogenes. A comparative genomics analysis showed that DuyunMaojian was the most closely related to Shuchazao and Yunkang 10, followed by DASZ and tea-oil tree. The multi-omics results indicated that phenylpropanoid biosynthesis, α-linolenic acid metabolism, flavonoid biosynthesis and 50 differentially expressed genes, particularly peroxidase, played important roles in response to infestation with tea green leafhoppers (Empoasca vitis Göthe). This study on the tea tree is highly significant for its role in illustrating the evolution of its genome and discovering how the tea plant responds to infestation with tea green leafhoppers will contribute to a theoretical foundation to breed tea plants resistant to insects that will ultimately result in an increase in the yield and quality of tea.

The sea toad genus Chaunax is a group of small benthic fishes that predominantly inhabiting the deep seas of the Atlantic, Indian, and Pacific Oceans. Although they have the potential to make excellent systems for studies of evolutionary adaptation to deep-sea environments, genomic research on Chaunax has been hindered by a scarcity of high-quality genomic resources. We present a chromosome-scale genome assembly of a Chaunax specimen generated using PacBio long-read sequencing and high-throughput chromosome conformation capture technology. The size of the assembled genome was 706.94 Mb, with a contig N50 of 15.24 Mb and scaffold N50 of 29.42 Mb. Approximately 96.11% of assembled sequences were anchored and oriented onto 24 pseudo-chromosomes. The genome contained 213.47 Mb repetitive sequences, 25,280 protein-coding genes, and 5,090 non-coding RNAs. The high ratio of complete BUSCO genes (97.20%) indicates high quality of genome assembly. The chromosomal-level reference genome of Chaunax sp. provides a preliminary molecular basis for understanding deep-sea adaptation and phenotypic evolution as well as an important reference for whole-genome sequencing of related species.

Background Papilio bianor Cramer, 1777 (commonly known as the Chinese peacock butterfly) (Insecta, Lepidoptera, Papilionidae) is a widely distributed swallowtail butterfly with a wide number of geographic populations ranging from the southeast of Russia to China, Japan, India, Vietnam, Myanmar, and Thailand. Its wing color consists of both pigmentary colored scales (black, reddish) and structural colored scales (iridescent blue or green dust). A high-quality reference genome of P. bianor is an important foundation for investigating iridescent color evolution, phylogeography, and the evolution of swallowtail butterflies.FindingsWe obtained a chromosome-level de novo genome assembly of the highly heterozygous P. bianor using long Pacific Biosciences sequencing reads and high-throughput chromosome conformation capture technology. The final assembly is 421.52 Mb on 30 chromosomes (29 autosomes and 1 Z sex chromosome) with 13.12 Mb scaffold N50. In total, 15,375 protein-coding genes and 233.09 Mb of repetitive sequences were identified. Phylogenetic analyses indicated that P. bianor separated from a common ancestor of swallowtails ∼23.69–36.04 million years ago. Demographic history suggested that the population expansion of this species from the last interglacial period to the last glacial maximum possibly resulted from its decreased natural enemies and its adaptation to climate change during the glacial period.ConclusionsWe present a high-quality chromosome-level reference genome of P. bianor using long-read single-molecule sequencing and Hi-C–based chromatin interaction maps. Our results lay the foundation for exploring the genetic basis of special biological features of P. bianor and also provide a useful data source for comparative genomics and phylogenomics among butterflies and moths.

BackgroundThe hard-shelled mussel (Mytilus coruscus) is widely distributed in the temperate seas of East Asia and is an important commercial bivalve in China. Chromosome-level genome information of this species will contribute not only to the development of hard-shelled mussel genetic breeding but also to studies on larval ecology, climate change biology, marine biology, aquaculture, biofouling, and antifouling.FindingsWe applied a combination of Illumina sequencing, Oxford Nanopore Technologies sequencing, and high-throughput chromosome conformation capture technologies to construct a chromosome-level genome of the hard-shelled mussel, with a total length of 1.57 Gb and a median contig length of 1.49 Mb. Approximately 90.9% of the assemblies were anchored to 14 linkage groups. We assayed the genome completeness using BUSCO. In the metazoan dataset, the present assemblies have 89.4% complete, 1.9% incomplete, and 8.7% missing BUSCOs. Gene modeling enabled the annotation of 37,478 protein-coding genes and 26,917 non-coding RNA loci. Phylogenetic analysis showed that M. coruscus is the sister taxon to the clade including Modiolus philippinarum and Bathymodiolus platifrons. Conserved chromosome synteny was observed between hard-shelled mussel and king scallop, suggesting that this is shared ancestrally. Transcriptomic profiling indicated that the pathways of catecholamine biosynthesis and adrenergic signaling in cardiomyocytes might be involved in metamorphosis.ConclusionsThe chromosome-level assembly of the hard-shelled mussel genome will provide novel insights into mussel genome evolution and serve as a fundamental platform for studies regarding the planktonic-sessile transition, genetic diversity, and genomic breeding of this bivalve.

Alone among piscine taxa, the antarctic icefishes (family Channichthyidae, suborder Notothenioidei) have evolved compensatory adaptations that maintain normal metabolic functions in the absence of erythrocytes and the respiratory oxygen transporter hemoglobin. Although the uniquely "colorless" or "white" condition of the blood of icefishes has been recognized since the early 20th century, the status of globin genes in the icefish genomes has, surprisingly, remained unexplored. Using alpha- and beta-globin cDNAs from the antarctic rockcod Notothenia coriiceps (family Nototheniidae, suborder Notothenioidei), we have probed the genomes of three white-blooded icefishes and four red-blooded notothenioid relatives (three antarctic, one temperate) for globin-related DNA sequences. We detect specific, high-stringency hybridization of the alpha-globin probe to genomic DNAs of both white- and red-blooded species, whereas the beta-globin cDNA hybridizes only to the genomes of the red-blooded fishes. Our results suggest that icefishes retain inactive genomic remnants of alpha-globin genes but have lost, either through deletion or through rapid mutation, the gene that encodes beta-globin. We propose that the hemoglobinless phenotype of extant icefishes is the result of deletion of the single adult beta-globin locus prior to the diversification of the clade.

Rapid adaptive radiation poses a distinct question apart from speciation and adaptation: what happens after one speciation event? That is, how are some lineages able to continue speciating through a rapid burst? This question connects global macroevolutionary patterns to microevolutionary processes. Here we review major features of rapid radiations in nature and their mismatch with theoretical models and what is currently known about speciation mechanisms. Rapid radiations occur on three major diversification axes - species richness, phenotypic disparity, and ecological diversity - with exceptional outliers on each axis. The paradox is that the hallmark early stage of adaptive radiation, a rapid burst of speciation and niche diversification, is contradicted by most existing speciation models which instead predict continuously decelerating speciation rates and niche subdivision through time. Furthermore, while speciation mechanisms such as magic traits, phenotype matching, and physical linkage of co-adapted alleles promote speciation, it is often not discussed how these mechanisms could promote multiple speciation events in rapid succession. Additional mechanisms beyond ecological opportunity are needed to understand how rapid radiations occur. We review the evidence for five emerging theories: 1) the 'transporter' hypothesis: introgression and the ancient origins of adaptive alleles, 2) the 'signal complexity' hypothesis: the dimensionality of sexual traits, 3) the connectivity of fitness landscapes, 4) 'diversity begets diversity', and 5) flexible stem/'plasticity first'. We propose new questions and predictions to guide future work on the mechanisms underlying the rare origins of rapid radiation.