Identification of triploid individuals and clonal lines in Carassius auratus complex using microsatellites

Differential interferon system gene expression profiles in susceptible and resistant gynogenetic clones of gibel carp challenged with herpesvirus CaHV

Interferon (IFN) system plays a vital role in the first line of defense against viruses. In this study, we first identified multiple transcripts of 15 IFN system genes, including PRRs (TLR2, TLR3, RIG-I, and LGP2), PRR-mediated IFN signal pathway (MyD88, MITA, and MAVS), IFN regulatory factors (IRF1, IRF3, IRF7, and IRF9), IFNs (IFNφ1 and IFNφ3), and ISGs (Mx and viperin), and one transcript of TLR9 in de novo transcriptome assembly data of gibel carp head-kidney. Multiple nucleotide alignments and phylogenetic analysis of common region showed that the transcripts of every of the 15 IFN system genes were classified into two homologs with distinctly divergent sequences, indicating that hexaploid gibel carp may be an allopolyploid. During Carassius auratus herpesvirus (CaHV) infection, gibel carp resistant clone H significantly suppressed CaHV replication with markedly less viral loads than those in highly susceptible clone A⁺ and moderately resistant clone F. Then, qPCR analyses were performed to reveal their differential and dynamic expression changes during CaHV infection in head kidney, spleen and liver among three gibel carp gynogenetic clones. Through qPCR and hierarchical clustering analysis, 8 genes, such as RIG-Is, LGP2s, IRF1-B, IRF3s, IRF7s, IRF9-B, Mxs, and viperins, were identified as candidate resistant-related genes. They remarkably increased their expression in immune tissues of three clones after CaHV infection. Significantly, the up-regulation folds of these genes in clone A⁺, F and H were related to their resistance ability to CaHV, progressively increasing from susceptible clone to resistant clone at 1 dpi. The positive correlation to the resistance ability suggested that resistant clone H immediately triggered stronger IFN response. IFNφ3 showed a different dynamic change and was sharply induced in moderately resistant clone F at 3 dpi. The other 5 IFN system genes (TLR2, TLR3, TLR9, MyD88, and MITA) maintained a low expression level after CaHV challenge. Interestingly, the A or B copies/homologs of almost these IFN system genes exhibited differential transcript abundance in immune tissue after CaHV challenge, suggesting A or B homologs might occur dominant or biased expression of homeologs during gibel carp evolution. These data provide candidate resistant-related genes for disease-resistance breeding of gibel carp.

Development of novel EST-SSR markers for ploidy identification based on de novo transcriptome assembly for Misgurnus anguillicaudatus

The co-existence of several ploidy types in natural populations makes the cyprinid loach Misgurnus anguillicaudatus an exciting model system to study the genetic and phenotypic consequences of ploidy variations. A first step in such effort is to identify the specific ploidy of an individual. Currently popular methods of karyotyping via cytological preparation or flow cytometry require a large amount of tissue (such as blood) samples, which can be damaging or fatal to the fishes. Here, we developed novel microsatellite markers (SSR markers) from M. anguillicaudatus and show that they can effectively discriminate ploidy using samples collected in a minimally invasive way. Specifically, we generated whole genome transcriptomes from multiple M. anguillicaudatus using the Illumina paired-end sequencing. Approximately 150 million raw reads were assembled into 76,544 non-redundant unigenes. A total of 8,194 potential SSR markers were identified. We selected 98 pairs with more than five tandem repeats for further assays. Out of 45 putative EST-SSR markers that successfully amplified and harbored polymorphism in diploids, 11 markers displayed high variability in tetraploids. We further demonstrate that a set of five EST-SSR markers selected from these are sufficient to distinguish ploidy levels, by first validating them on 69 reference specimens with known ploidy levels and then subsequently using fresh-collected 96 ploidy-unknown specimens. The results from EST-SSR markers are highly concordant with those from independent flow cytometry analysis. The novel EST-SSR markers developed here should facilitate genetic studies of polyploidy in the emerging model system M. anguillicaudatus.

Distinct herpesvirus resistances and immune responses of three gynogenetic clones of gibel carp revealed by comprehensive transcriptomes

Background

Gibel carp is an important aquaculture species in China, and a herpesvirus, called as Carassius auratus herpesvirus (CaHV), has hampered the aquaculture development. Diverse gynogenetic clones of gibel carp have been identified or created, and some of them have been used as aquaculture varieties, but their resistances to herpesvirus and the underlying mechanism remain unknown.

Results

To reveal their susceptibility differences, we firstly performed herpesvirus challenge experiments in three gynogenetic clones of gibel carp, including the leading variety clone A⁺, candidate variety clone F and wild clone H. Three clones showed distinct resistances to CaHV. Moreover, 8772, 8679 and 10,982 differentially expressed unigenes (DEUs) were identified from comparative transcriptomes between diseased individuals and control individuals of clone A⁺, F and H, respectively. Comprehensive analysis of the shared DEUs in all three clones displayed common defense pathways to the herpesvirus infection, activating IFN system and suppressing complements. KEGG pathway analysis of specifically changed DEUs in respective clones revealed distinct immune responses to the herpesvirus infection. The DEU numbers identified from clone H in KEGG immune-related pathways, such as “chemokine signaling pathway”, “Toll-like receptor signaling pathway” and others, were remarkably much more than those from clone A⁺ and F. Several IFN-related genes, including Mx1, viperin, PKR and others, showed higher increases in the resistant clone H than that in the others. IFNphi3, IFI44-like and Gig2 displayed the highest expression in clone F and IRF1 uniquely increased in susceptible clone A⁺. In contrast to strong immune defense in resistant clone H, susceptible clone A⁺ showed remarkable up-regulation of genes related to apoptosis or death, indicating that clone A⁺ failed to resist virus offensive and evidently induced apoptosis or death.

Conclusions

Our study is the first attempt to screen distinct resistances and immune responses of three gynogenetic gibel carp clones to herpesvirus infection by comprehensive transcriptomes. These differential DEUs, immune-related pathways and IFN system genes identified from susceptible and resistant clones will be beneficial to marker-assisted selection (MAS) breeding or molecular module-based resistance breeding in gibel carp.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3945-6) contains supplementary material, which is available to authorized users.

Formation of diploid and triploid hybrid groupers (hybridization of Epinephelus coioides ♀ × Epinephelus lanceolatus ♂) and their 5S gene analysis

BACKGROUND

Interspecies hybridization is widely used to achieve heterosis or hybrid vigor, which has been observed and harnessed by breeders for centuries. Natural allopolyploid hybrids generally exhibit more superior heterosis than both the diploid progenies and their parental species. However, polyploid formation processes have been long ignored, the genetic basis of heterosis in polyploids remains elusive.

RESULTS

In the present study, triploid hybrids had been demonstrated to contain two sets of chromosomes from mother species and one set from father species. Cellular polyploidization process in the embryos had been traced. The triploid hybrids might be formed by failure formation of the second polarized genome during the second meiosis stage. Four spindle centers were observed in anaphase stage of the first cell division. Three spindle centers were observed in side of cell plate after the first cell division. The 5S rDNA genes of four types of groupers were cloned and analyzed. The diploid and triploid hybrids had been proved to contain the tandem chimera structures which were recombined by maternal and paternal monomer units. The results indicated that genome re-fusion had occurred in the hybrid progenies. To further elucidate the genetic patterns of diploid and triploid hybrids, fluorescence chromosome location had been carried out, maternal 5S gene (M-386) were used as the probe. The triploid hybrids contained fewer fluorescence loci numbers than the maternal species. The results indicated that participation of paternal 5S gene in the triploid hybrid genome had degraded the match rates of M-386 probe.

CONCLUSIONS

Our study is the first to investigate the cellular formation processes of natural allopolyploids in hybrid fish, the cellular polyploidization process may be caused by failure formation of the second polarized genome during the meiosis, and our results will provide the molecular basis of hybrid vigor in interspecies hybridization.

The marbled crayfish as a paradigm for saltational speciation by autopolyploidy and parthenogenesis in animals

The parthenogenetic all-female marbled crayfish is a novel research model and potent invader of freshwater ecosystems. It is a triploid descendant of the sexually reproducing slough crayfish, Procambarus fallax, but its taxonomic status has remained unsettled. By cross-breeding experiments and parentage analysis we show here that marbled crayfish and P. fallax are reproductively separated. Both crayfish copulate readily, suggesting that the reproductive barrier is set at the cytogenetic rather than the behavioural level. Analysis of complete mitochondrial genomes of marbled crayfish from laboratory lineages and wild populations demonstrates genetic identity and indicates a single origin. Flow cytometric comparison of DNA contents of haemocytes and analysis of nuclear microsatellite loci confirm triploidy and suggest autopolyploidisation as its cause. Global DNA methylation is significantly reduced in marbled crayfish implying the involvement of molecular epigenetic mechanisms in its origination. Morphologically, both crayfish are very similar but growth and fecundity are considerably larger in marbled crayfish, making it a different animal with superior fitness. These data and the high probability of a divergent future evolution of the marbled crayfish and P. fallax clusters suggest that marbled crayfish should be considered as an independent asexual species. Our findings also establish the P. fallax-marbled crayfish pair as a novel paradigm for rare chromosomal speciation by autopolyploidy and parthenogenesis in animals and for saltational evolution in general.

The transcriptomes of the crucian carp complex (Carassius auratus) provide insights into the distinction between unisexual triploids and sexual diploids

Both sexual reproduction and unisexual reproduction are adaptive strategies for species survival and evolution. Unisexual animals have originated largely by hybridization, which tends to elevate their heterozygosity. However, the extent of genetic diversity resulting from hybridization and the genomic differences that determine the type of reproduction are poorly understood. In Carassius auratus, sexual diploids and unisexual triploids coexist. These two forms are similar morphologically but differ markedly in their modes of reproduction. Investigation of their genomic differences will be useful to study genome diversity and the development of reproductive mode. We generated transcriptomes for the unisexual and sexual populations. Genes were identified using homology searches and an ab initio method. Estimation of the synonymous substitution rate in the orthologous pairs indicated that the hybridization of gibel carp occurred 2.2 million years ago. Microsatellite genotyping in each individual from the gibel carp population indicated that most gibel carp genes were not tri-allelic. Molecular function and pathway comparisons suggested few gene expansions between them, except for the progesterone-mediated oocyte maturation pathway, which is enriched in gibel carp. Differential expression analysis identified highly expressed genes in gibel carp. The transcriptomes provide information on genetic diversity and genomic differences, which should assist future studies in functional genomics.

Evolutionary history of two divergent Dmrt1 genes reveals two rounds of polyploidy origins in gibel carp

Polyploidy lineages, despite very rare in vertebrates, have been proposed to play significant role in speciation and evolutionary success, but the occurrence history and consequences are still largely unknown. In this study, we used the conserved Dmrt1 to analyze polyploidy occurrence and evolutionary process in polyploid gibel carp. We identified two divergent Dmrt1 genes and respectively localized the two genes on three homologous chromosomes. Subsequently, the corresponding full-length cDNAs and genomic sequences of Dmrt1 genes were also characterized from the closely related species including Carassius auratus auratus and Cyprinus carpio, and their two Dmrt1 genes were respectively localized on two homologous chromosomes. Significantly, the evolutionary relationship analyses among cDNA and genomic DNA sequences of these Dmrt1 genes revealed two rounds of polyploidy origins in the gibel carp: an early polyploidy might result in an common tetraploid ancestor of Carassius auratus gibelio, Carassius auratus auratus and Cyprinus carpio before 18.49 million years ago (Mya), and an late polyploidy might occur from evolutionary branch of Carassius auratus at around 0.51 Mya, which lead to the occurrence of the hexaploid gibel carp. Therefore, this study provides clear genetic evidence for understanding occurrence time and historical process of polyploidy in polyploid vertebrates.

High male incidence and evolutionary implications of triploid form in northeast Asia Carassius auratus complex

Carassius auratus complex is believed to originate from East Eurasia and consist of diploid and triploid forms. Diploid form reproduces sexually, whereas triploid form possesses mixture modes of unisexual gynogenesis and sexual reproduction, which makes it a unique case to study evolutionary issues among vertebrates. In this study, we identified 337 triploid individuals from 386 specimens of Carassius auratus complex sampled from 4 different sites of Xingkai Lake and Suifen River on the northeast Asia transboundary areas of Russia and China, and found that triploids were ubiquitous, whereas diploids existed only in SII site of Suifen River. Triploid males were detected in all surveyed sites, and an unusually high triploid male incidence (23%) was found in the Chinese reach of Suifen River. Then, nuclear and cytoplasmic markers were used to analyze their genetic diversity and phylogenetic relationship. A total of 61 distinct tf alleles and 35 mtDNA CR haplotypes were revealed. Higher genetic diversity and divergence were confirmed in triploids than in diploids, and identical genetic background between triploid males and females was demonstrated. Moreover, evolutionary implications and roles of triploid males were suggested in population proliferation and diversity creation of the triploid form.