Unravelling epigenetic mechanisms in Cerastoderma edule genome: a comparison of healthy and neoplastic cockles.

Cancer is a multifaceted genetic disease characterized by the acquisition of several essential hallmarks. Notably, certain cancers exhibit horizontal transmissibility, observed across mammalian species and diverse bivalves, the latter referred to as hemic neoplasia. Within this complex landscape, epigenetic mechanisms such as histone modifications and cytosine methylation emerge as fundamental contributors to the pathogenesis of these transmissible cancers. Our study delves into the epigenetic landscape of Cerastoderma edule, focusing on whole-genome methylation and hydroxymethylation profiles in heathy specimens and transmissible neoplasias by means of Nanopore long-read sequencing. Our results unveiled a global hypomethylation in the neoplastic specimens compared to their healthy counterparts, emphasizing the role of DNA methylation in these tumorigenic processes. Furthermore, we verified that intragenic CpG methylation positively correlated with gene expression, emphasizing its role in modulating transcription in healthy and neoplastic cockles, as also highlighted by some up-methylated oncogenic genes. Hydroxymethylation levels were significantly more elevated in the neoplastic samples, particularly within satellites and complex repeats, likely related to structural functions. Additionally, our analysis also revealed distinct methylation and activity patterns in retrotransposons, providing additional insights into bivalve neoplastic processes. Altogether, these findings contribute to understanding the epigenetic dynamics of bivalve neoplasias and shed light on the roles of DNA methylation and hydroxymethylation in tumorigenesis. Understanding these epigenetic alterations holds promise for advancing our broader understanding of cancer epigenetics.

Dispersal history of SARS-CoV-2 in Galicia, Spain.

The dynamics of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission are influenced by a variety of factors, including social restrictions and the emergence of distinct variants. In this study, we delve into the origins and dissemination of the Alpha, Delta, and Omicron-BA.1 variants of concern in Galicia, northwest Spain. For this, we leveraged genomic data collected by the EPICOVIGAL Consortium and from the GISAID database, along with mobility information from other Spanish regions and foreign countries. Our analysis indicates that initial introductions during the Alpha phase were predominantly from other Spanish regions and France. However, as the pandemic progressed, introductions from Portugal and the United States became increasingly significant. The number of detected introductions varied from 96 and 101 for Alpha and Delta to 39 for Omicron-BA.1. Most of these introductions left a low number of descendants (<10), suggesting a limited impact on the evolution of the pandemic in Galicia. Notably, Galicia's major coastal cities emerged as critical hubs for viral transmission, highlighting their role in sustaining and spreading the virus. This research emphasizes the critical role of regional connectivity in the spread of SARS-CoV-2 and offers essential insights for enhancing public health strategies and surveillance measures.

Hiding from heat: The transcriptomic response of two clam species is modulated by behaviour and habitat.

Rising occurrence of extreme warming events are profoundly impacting ecosystems, altering their functioning and services with significant socio-economic consequences. Particularly susceptible to heatwaves are intertidal shellfish beds, located in estuarine areas already stressed by factors such as rainfall events, red tides, eutrophication, and pollution. In Galicia, Northwestern Spain, these beds support vital shellfisheries, featuring the native clam Ruditapes decussatus and the non-indigenous R. philippinarum. Over recent decades, these populations have experienced notable abundance shifts due to various anthropogenic impacts, including climate change. In this habitat, patches of the seagrass Zostera noltei that coexist with bare sand can act as thermal refuges for benthic organisms such as clams. To assess the impact of heatwaves on these ecosystems, a mesocosm experiment was conducted. Juveniles of both clam species in two habitat types-bare sand and sand with Z. noltei-were exposed to simulated atmospheric heatwaves during diurnal low tide for four consecutive days. Subsequent transcriptomic analysis revealed that high temperatures had a more pronounced impact on the transcriptome of R. philippinarum compared to R. decussatus. The habitat type played a crucial role in mitigating heat stress in R. philippinarum, with the presence of Z. noltei notably ameliorating the transcriptomic response. These findings have direct applications in shellfishery management, emphasizing the importance of preserving undisturbed patches of Z. noltei as thermal refuges, contributing to the mitigation of heatwave effects on shellfish populations.

Microglial angiotensin type 2 receptors mediate sex-specific expression of inflammatory cytokines independently of circulating estrogen.

There are sex differences in microglia, which can maintain sex-related gene expression and functional differences in the absence of circulating sex steroids. The angiotensin type 2 (AT2) receptors mediate anti-inflammatory actions in different tissues, including brain. In mice, we performed RT-PCR analysis of microglia isolated from adult brains and RNA scope in situ hybridization from males, females, ovariectomized females, orchiectomized males and brain masculinized females. We also compared wild type and AT2 knockout mice. The expression of AT2 receptors in microglial cells showed sex differences with much higher AT2 mRNA expression in females than in males, and this was not dependent on circulating gonadal hormones, as observed using ovariectomized females, brain masculinized females and orchiectomized males. These results suggest genomic reasons, possibly related to sex chromosome complement, for sex differences in AT2 expression in microglia, as the AT2 receptor gene is located in the X chromosome. Furthermore, sex differences in expression of AT2 receptors were associated to sex differences in microglial expression of key anti-inflammatory cytokines such as interleukin-10 and pro-inflammatory cytokines such as interleukin-1ß and interleukin-6. In conclusion, sex differences in microglial AT2 receptor expression appear as a major factor contributing to sex differences in the neuroinflammatory responses beyond the effects of circulating steroids.

Satellitome Analysis of the Pacific Oyster Crassostrea gigas Reveals New Pattern of Satellite DNA Organization, Highly Scattered across the Genome.

Several features already qualified the invasive bivalve species Crassostrea gigas as a valuable non-standard model organism in genome research. C. gigas is characterized by the low contribution of satellite DNAs (satDNAs) vs. mobile elements and has an extremely low amount of heterochromatin, predominantly built of DNA transposons. In this work, we have identified 52 satDNAs composing the satellitome of C. gigas and constituting about 6.33% of the genome. Satellitome analysis reveals unusual, highly scattered organization of relatively short satDNA arrays across the whole genome. However, peculiar chromosomal distribution and densities are specific for each satDNA. The inspection of the organizational forms of the 11 most abundant satDNAs shows association with constitutive parts of Helitron mobile elements. Nine of the inspected satDNAs are dominantly found in mobile element-associated form, two mostly appear standalone, and only one is present exclusively as Helitron-associated sequence. The Helitron-related satDNAs appear in more chromosomes than other satDNAs, indicating that these mobile elements could be leading satDNA propagation in C. gigas. No significant accumulation of satDNAs on certain chromosomal positions was detected in C. gigas, thus establishing a novel pattern of satDNA organization on the genome level.

Karyotype differentiation in tellin shells (Bivalvia: Tellinidae).

BACKGROUND: Although Tellinidae is one of the largest and most diverse families of bivalves, its taxonomy is utterly chaotic. This is mainly due to the morphological diversity and homoplasy displayed by their shells and to the scarcity of the molecular phylogenetic studies performed on them. A molecular cytogenetic analysis of four tellin shell species, Bosemprella incarnata, Macomangulus tenuis, Moerella donacina and Serratina serrata, was performed. To molecularly characterize the analyzed specimens, the sequence of a fragment of the mitochondrial cytochrome c oxidase subunit I (COI) was also studied. RESULTS: The karyotypes of the four species were composed of different amounts of bi-armed and telocentric chromosomes. The chromosomal mapping of 45S and 5S rDNA and H3 histone gene clusters by fluorescent in situ hybridization also revealed conspicuous differences on the distribution of these DNA sequences on their karyotypes. Vertebrate type telomeric sequences were located solely on both ends of each chromosome in all four tellin shells. CONCLUSION: We present clear evidence of the valuable information provided by FISH signals in both analyzing chromosome evolution in Tellinidae and as a further tool in identifying tellin shell specimens for molecular phylogenies.

Molecular Cytogenetics in Digenean Parasites: Linked and Unlinked Major and 5S rDNAs, B Chromosomes and Karyotype Diversification.

Digenetic trematodes are the largest group of internal metazoan parasites, but their chromosomes are poorly studied. Although chromosome numbers and/or karyotypes are known for about 300 of the 18,000 described species, molecular cytogenetic knowledge is mostly limited to the mapping of telomeric sequences and/or of major rDNA clusters in 9 species. In this work we mapped major and 5S rDNA clusters and telomeric sequences in chromosomes of Bucephalus minimus, B. australis, Prosorhynchoides carvajali (Bucephaloidea), Monascus filiformis (Gymnophalloidea), Parorchis acanthus (Echinostomatoidea), Cryptocotyle lingua (Opisthorchioidea), Cercaria longicaudata, Monorchis parvus (Monorchioidea), Diphterostomum brusinae, and Bacciger bacciger (Microphalloidea). Whilst single major and minor rDNA clusters were mapped to different chromosome pairs in B. minimus and P. acanthus, overlapping signals were detected on a single chromosome pair in the remaining taxa. FISH experiments using major rDNA and telomeric probes clearly demonstrated the presence of highly stretched NORs in most of the digenean taxa analyzed. B chromosomes were detected in the B. bacciger samples hosted by Ruditapes decussatus. Although the cercariae specimens obtained from Donax trunculus, Tellina tenuis, and R. decussatus were in agreement with B. bacciger, their karyotypes showed striking morphological differences in agreement with the proposed assignation of these cercariae to different species of the genus Bacciger. Results are discussed in comparison with previous data on digenean chromosomes.

Dominance of transposable element-related satDNAs results in great complexity of "satDNA library" and invokes the extension towards "repetitive DNA library".

Research on bivalves is fast-growing, including genome-wide analyses and genome sequencing. Several characteristics qualify oysters as a valuable model to explore repetitive DNA sequences and their genome organization. Here we characterize the satellitomes of five species in the family Ostreidae (Crassostrea angulata, C. virginica, C. hongkongensis, C. ariakensis, Ostrea edulis), revealing a substantial number of satellite DNAs (satDNAs) per genome (ranging between 33 and 61) and peculiarities in the composition of their satellitomes. Numerous satDNAs were either associated to or derived from transposable elements, displaying a scarcity of transposable element-unrelated satDNAs in these genomes. Due to the non-conventional satellitome constitution and dominance of Helitron-associated satDNAs, comparative satellitomics demanded more in-depth analyses than standardly employed. Comparative analyses (including C. gigas, the first bivalve species with a defined satellitome) revealed that 13 satDNAs occur in all six oyster genomes, with Cg170/HindIII satDNA being the most abundant in all of them. Evaluating the "satDNA library model" highlighted the necessity to adjust this term when studying tandem repeat evolution in organisms with such satellitomes. When repetitive sequences with potential variation in the organizational form and repeat-type affiliation are examined across related species, the introduction of the terms "TE library" and "repetitive DNA library" becomes essential. Supplementary Information: The online version contains supplementary material available at 10.1007/s42995-024-00218-0.

Dispersal history of SARS-CoV-2 in Galicia, Spain.

The dynamics of SARS-CoV-2 transmission are influenced by a variety of factors, including social restrictions and the emergence of distinct variants. In this study, we delve into the origins and dissemination of the Alpha, Delta, and Omicron variants of concern in Galicia, northwest Spain. For this, we leveraged genomic data collected by the EPICOVIGAL Consortium and from the GISAID database, along with mobility information from other Spanish regions and foreign countries. Our analysis indicates that initial introductions during the Alpha phase were predominantly from other Spanish regions and France. However, as the pandemic progressed, introductions from Portugal and the USA became increasingly significant. Notably, Galicia's major coastal cities emerged as critical hubs for viral transmission, highlighting their role in sustaining and spreading the virus. This research emphasizes the critical role of regional connectivity in the spread of SARS-CoV-2 and offers essential insights for enhancing public health strategies and surveillance measures.

Somatic evolution of marine transmissible leukemias in the common cockle, Cerastoderma edule.

Transmissible cancers are malignant cell lineages that spread clonally between individuals. Several such cancers, termed bivalve transmissible neoplasia (BTN), induce leukemia-like disease in marine bivalves. This is the case of BTN lineages affecting the common cockle, Cerastoderma edule, which inhabits the Atlantic coasts of Europe and northwest Africa. To investigate the evolution of cockle BTN, we collected 6,854 cockles, diagnosed 390 BTN tumors, generated a reference genome and assessed genomic variation across 61 tumors. Our analyses confirmed the existence of two BTN lineages with hemocytic origins. Mitochondrial variation revealed mitochondrial capture and host co-infection events. Mutational analyses identified lineage-specific signatures, one of which likely reflects DNA alkylation. Cytogenetic and copy number analyses uncovered pervasive genomic instability, with whole-genome duplication, oncogene amplification and alkylation-repair suppression as likely drivers. Satellite DNA distributions suggested ancient clonal origins. Our study illuminates long-term cancer evolution under the sea and reveals tolerance of extreme instability in neoplastic genomes.

Mitochondrial genome sequencing of marine leukaemias reveals cancer contagion between clam species in the Seas of Southern Europe.

Clonally transmissible cancers are tumour lineages that are transmitted between individuals via the transfer of living cancer cells. In marine bivalves, leukaemia-like transmissible cancers, called hemic neoplasia (HN), have demonstrated the ability to infect individuals from different species. We performed whole-genome sequencing in eight warty venus clams that were diagnosed with HN, from two sampling points located more than 1000 nautical miles away in the Atlantic Ocean and the Mediterranean Sea Coasts of Spain. Mitochondrial genome sequencing analysis from neoplastic animals revealed the coexistence of haplotypes from two different clam species. Phylogenies estimated from mitochondrial and nuclear markers confirmed this leukaemia originated in striped venus clams and later transmitted to clams of the species warty venus, in which it survives as a contagious cancer. The analysis of mitochondrial and nuclear gene sequences supports all studied tumours belong to a single neoplastic lineage that spreads in the Seas of Southern Europe.

In humans and other animals, cancer cells divide excessively, forming tumours or flooding the blood, but they rarely spread to other individuals. However, some animals, including dogs, Tasmanian devils and bivalve molluscs like clams, cockles and mussels, can develop cancers that are transmitted from one individual to another. Despite these cancers being contagious, each one originates in a single animal, meaning that even when the cancer has spread to many individuals, its origins can be traced through its DNA. Cancer contagion is rare, but transmissible cancers seem to be particularly common in the oceans. In fact, 7 types of contagious cancer have been described in bivalve species so far. These cancers are known as 'hemic neoplasias', and are characterized by the uncontrolled division of blood-like cells, which can be released by the host they developed in, and survive in ocean water. When these cells encounter individuals from the same species, they can infect them, causing them to develop hemic neoplasia too There are still many unanswered questions about contagious cancers in bivalves. For example, how many species do the cancers affect, and which species do the cancers originate in? To address these questions, Garcia-Souto, Bruzos, Díaz et al. gathered over 400 specimens of a species of clam called the warty venus clam from the coastlines of Europe and examined them for signs of cancer. Clams collected in two regions of Spain showed signs of hemic neoplasia: one of the populations was from the Balearic Islands in the Mediterranean Sea, while the other came from the Atlantic coast of northwestern Spain. Analyzing the genomes of the tumours from each population showed that the cancer cells from both regions had likely originated in the same animal, indicating that the cancer is contagious and had spread through different populations. The analysis also revealed that the cancer did not originally develop in warty venus clams: the cancer cells contained DNA from both warty venus clams and another species called striped venus clams. These two species live close together in the Mediterranean Sea, suggesting that the cancer started in a striped venus clam and then spread to a warty venus clam. To determine whether the cancer still affected both species, Garcia-Souto, Bruzos, Díaz et al. screened 200 striped venus clams from the same areas, but no signs of cancer were found in these clams. This suggests that currently the cancer only affects the warty venus clam. These findings confirm that contagious cancers can jump between clam species, which could be threat to the marine environment. The fact that the cancer was so similar in clams from the Atlantic coast and from the Mediterranean Sea, however, suggests that it may have emerged very recently, or that human activity helped it to spread from one place to another. If the latter is the case, it may be possible to prevent further spread of these sea-borne cancers through human intervention.

Aberrant integration of Hepatitis B virus DNA promotes major restructuring of human hepatocellular carcinoma genome architecture.

Most cancers are characterized by the somatic acquisition of genomic rearrangements during tumour evolution that eventually drive the oncogenesis. Here, using multiplatform sequencing technologies, we identify and characterize a remarkable mutational mechanism in human hepatocellular carcinoma caused by Hepatitis B virus, by which DNA molecules from the virus are inserted into the tumour genome causing dramatic changes in its configuration, including non-homologous chromosomal fusions, dicentric chromosomes and megabase-size telomeric deletions. This aberrant mutational mechanism, present in at least 8% of all HCC tumours, can provide the driver rearrangements that a cancer clone requires to survive and grow, including loss of relevant tumour suppressor genes. Most of these events are clonal and occur early during liver cancer evolution. Real-time timing estimation reveals some HBV-mediated rearrangements occur as early as two decades before cancer diagnosis. Overall, these data underscore the importance of characterising liver cancer genomes for patterns of HBV integration.

Sequence Composition Underlying Centromeric and Heterochromatic Genome Compartments of the Pacific Oyster Crassostrea gigas.

Segments of the genome enriched in repetitive sequences still present a challenge and are omitted in genome assemblies. For that reason, the exact composition of DNA sequences underlying the heterochromatic regions and the active centromeres are still unexplored for many organisms. The centromere is a crucial region of eukaryotic chromosomes responsible for the accurate segregation of genetic material. The typical landmark of centromere chromatin is the rapidly-evolving variant of the histone H3, CenH3, while DNA sequences packed in constitutive heterochromatin are associated with H3K9me3-modified histones. In the Pacific oyster Crassostrea gigas we identified its centromere histone variant, Cg-CenH3, that shows stage-specific distribution in gonadal cells. In order to investigate the DNA composition of genomic regions associated with the two specific chromatin types, we employed chromatin immunoprecipitation followed by high-throughput next-generation sequencing of the Cg-CenH3- and H3K9me3-associated sequences. CenH3-associated sequences were assigned to six groups of repetitive elements, while H3K9me3-associated-ones were assigned only to three. Those associated with CenH3 indicate the lack of uniformity in the chromosomal distribution of sequences building the centromeres, being also in the same time dispersed throughout the genome. The heterochromatin of C. gigas exhibited general paucity and limited chromosomal localization as predicted, with H3K9me3-associated sequences being predominantly constituted of DNA transposons.

Pan-cancer analysis of whole genomes identifies driver rearrangements promoted by LINE-1 retrotransposition.

About half of all cancers have somatic integrations of retrotransposons. Here, to characterize their role in oncogenesis, we analyzed the patterns and mechanisms of somatic retrotransposition in 2,954 cancer genomes from 38 histological cancer subtypes within the framework of the Pan-Cancer Analysis of Whole Genomes (PCAWG) project. We identified 19,166 somatically acquired retrotransposition events, which affected 35% of samples and spanned a range of event types. Long interspersed nuclear element (LINE-1; L1 hereafter) insertions emerged as the first most frequent type of somatic structural variation in esophageal adenocarcinoma, and the second most frequent in head-and-neck and colorectal cancers. Aberrant L1 integrations can delete megabase-scale regions of a chromosome, which sometimes leads to the removal of tumor-suppressor genes, and can induce complex translocations and large-scale duplications. Somatic retrotranspositions can also initiate breakage-fusion-bridge cycles, leading to high-level amplification of oncogenes. These observations illuminate a relevant role of L1 retrotransposition in remodeling the cancer genome, with potential implications for the development of human tumors.

Terminal-Repeat Retrotransposons in Miniature (TRIMs) in bivalves.

Terminal repeat retrotransposons in miniature (TRIMs) are small non-autonomous LTR retrotransposons consisting of two terminal direct repeats surrounding a short internal domain. The detection and characterization of these elements has been mainly limited to plants. Here we present the first finding of a TRIM element in bivalves, and among the first known in the kingdom Animalia. Class Bivalvia has high ecological and commercial importance in marine ecosystems and aquaculture, and, in recent years, an increasing number of genomic studies has addressed to these organisms. We have identified biv-TRIM in several bivalve species: Donax trunculus, Ruditapes decussatus, R. philippinarum, Venerupis corrugata, Polititapes rhomboides, Venus verrucosa, Dosinia exoleta, Glycymeris glycymeris, Cerastoderma edule, Magallana gigas, Mytilus galloprovincialis. biv-TRIM has several characteristics typical for this group of elements, exhibiting different variations. In addition to canonically structured elements, solo-TDRs and tandem repeats were detected. The presence of this element in the genome of each species is <1%. The phylogenetic analysis showed a complex clustering pattern of biv-TRIM elements, and indicates the involvement of horizontal transfer in the spreading of this element.

Cytogenetics in Arctica islandica (Bivalvia, Arctidae): the Longest Lived Non-Colonial Metazoan.

Due to its extraordinary longevity and wide distribution, the ocean quahog Arctica islandica has become an important species model in both aging and environmental change research. Notwithstanding that, most genetic studies on ocean quahogs have been focused on fishery related, phylogeographic and phylogenetic aspects but nothing is known about their chromosomes. In this work, the chromosomes of the ocean quahog Arctica islandica were analysed by means of 4′,6-diamidino-2-phenylindole (DAPI)/propidium iodide (PI) staining and fluorescent in situ hybridization (FISH) with rDNA, histone gene and telomeric probes. Whilst both 5S rDNA and 45S rDNA were clustered at single subcentromeric locations on the long arms of chromosome pairs 2 and 12, respectively, histone gene clusters located on the short arms of chromosome pairs 7, 10 and 17. As happens with most bivalves, the location of the vertebrate type telomeric sequence clusters was restricted to chromosome ends. The knowledge of the karyotype can facilitate the anchoring of genomic sequences to specific chromosome pairs in this species.

Karyotype Characterization of Nine Periwinkle Species (Gastropoda, Littorinidae).

Periwinkles of the family Littorinidae (Children, 1834) are common members of seashore littoral communities worldwide. Although the family is composed of more than 200 species belonging to 18 genera, chromosome numbers have been described in only eleven of them. A molecular cytogenetic analysis of nine periwinkle species, the rough periwinkles Littorina arcana, L. saxatilis, and L. compressa, the flat periwinkles L. obtusata and L. fabalis, the common periwinkle L. littorea, the mangrove periwinkle Littoraria angulifera, the beaded periwinkle Cenchritis muricatus, and the small periwinkle Melarhaphe neritoides was performed. All species showed diploid chromosome numbers of 2n = 34, and karyotypes were mostly composed of metacentric and submetacentric chromosome pairs. None of the periwinkle species showed chromosomal differences between male and female specimens. The chromosomal mapping of major and minor rDNA and H3 histone gene clusters by fluorescent in situ hybridization demonstrated that the patterns of distribution of these DNA sequences were conserved among closely related species and differed among less related ones. All signals occupied separated loci on different chromosome pairs without any evidence of co-localization in any of the species.

Resolving the Taxonomic Status of Chamelea gallina and C. striatula (Veneridae, Bivalvia): A Combined Molecular Cytogenetic and Phylogenetic Approach.

The striped venus clams Chamelea gallina and C. striatula are commercially important bivalves inhabiting European and North African coastal waters. The taxonomic status of these taxa has been the subject of debate for decades. In order to elucidate this issue, we generated 5S and 28S ribosomal RNA and H3 histone gene probes and mapped them by fluorescent in situ hybridization to the chromosomes of morphologically identified striped venus clams, collected from four geographically distant Atlantic and Mediterranean populations. The nucleotide variation at the three DNA markers, that is, the nuclear internal transcribed spacer 2 (ITS2), the mitochondrial cytochrome c oxidase subunit I (COI), and the large ribosomal subunit rRNA (16S) fragments, was also studied and the resultant phylogenetic trees were evaluated. Striking differences in both the chromosome distribution of these genes and the clustering of the samples on the phylogenetic trees observed provide clear evidence that C. gallina and C. striatula are separated species.

Are Pericentric Inversions Reorganizing Wedge Shell Genomes?

Wedge shells belonging to the Donacidae family are the dominant bivalves in exposed beaches in almost all areas of the world. Typically, two or more sympatric species of wedge shells differentially occupy intertidal, sublittoral, and offshore coastal waters in any given locality. A molecular cytogenetic analysis of two sympatric and closely related wedge shell species, Donax trunculus and Donax vittatus, was performed. Results showed that the karyotypes of these two species were both strikingly different and closely alike; whilst metacentric and submetacentric chromosome pairs were the main components of the karyotype of D. trunculus, 10-11 of the 19 chromosome pairs were telocentric in D. vittatus, most likely as a result of different pericentric inversions. GC-rich heterochromatic bands were present in both species. Furthermore, they showed coincidental 45S ribosomal RNA (rRNA), 5S rRNA and H3 histone gene clusters at conserved chromosomal locations, although D. trunculus had an additional 45S rDNA cluster. Intraspecific pericentric inversions were also detected in both D. trunculus and D. vittatus. The close genetic similarity of these two species together with the high degree of conservation of the 45S rRNA, 5S rRNA and H3 histone gene clusters, and GC-rich heterochromatic bands indicate that pericentric inversions contribute to the karyotype divergence in wedge shells.