Fertility and ploidy of gametes of allodiploid and allotriploid loaches produced by diploid Misgurnus anguillicaudatus females and Paramisgurnus dabryanus males

Mitotic defects lead to unreduced sperm formation in cdk1-/- mutants

Unreduced gametes, that are important for species evolution and agricultural development, are generally believed to be formed by meiotic defects. However, we found that male diploid loach (Misgurnus anguillicaudatus) could produce not only haploid sperms, but also unreduced sperms, after cyclin-dependent kinase 1 gene (cdk1, one of the most important kinases in regulating cell mitosis) deletion. Observations on synaptonemal complexes of spermatocyte in prophase of meiosis and spermatogonia suggested that the number of chromosomes in some spermatogonia of cdk1^-/- loach doubled, leading to unreduced diploid sperm production. Then, transcriptome analysis revealed aberrant expressions of some cell cycle-related genes (such as ppp1c and gadd45) in spermatogonia of cdk1^-/- loach relative to wild-type loach. An in vitro and in vivo experiment further validated that Cdk1 deletion in diploid loach resulted in mitotic defects, leading to unreduced diploid sperm formation. In addition, we found that cdk1^-/- zebrafish could also produce unreduced diploid sperms. This study provides important information on revealing the molecular mechanisms behind unreduced gamete formation through mitotic defects, and lays the foundation for a novel strategy for fish polyploidy creation by using cdk1 mutants to produce unreduced sperms, which can then be used to obtain polyploidy, proposed to benefit aquaculture.

Formation of the synaptonemal complex in a gynogenetic allodiploid hybrid fish

Introduction: The correct pairing and separation of homologous chromosomes during meiosis is crucial to ensure both genetic stability and genetic diversity within species. In allodiploid organisms, synapsis often fails, leading to sterility. However, a gynogenetic allodiploid hybrid clone line (GDH), derived by crossing red crucian carp (Carassius auratus ♀) and common carp (Cyprinus carpio ♂), stably produces diploid eggs. Because the GDH line carries 100 chromosomes with 50 chromosomes from the red crucian carp (RCC; ♀, 2n = 2x = 100) and 50 chromosomes from the common carp (CC; C. carpio L., ♂, 2n = 2x = 100), it is interesting to study the mechanisms of homologous chromosome pairing during meiosis in GDH individuals. Methods: By using fluorescence in situ hybridization (FISH) with a probe specific to the red crucian carp to label homologous chromosomes, we identified the synaptonemal complex via immunofluorescence assay of synaptonemal complex protein 3 (SCP3). Results: FISH results indicated that, during early ovarian development, the GDH oogonium had two sets of chromosomes with only one set from Carassius auratus, leading to the failure formation of normal bivalents and the subsequently blocking of meiosis. This inhibition lasted at least 5 months. After this long period of inhibition, pairs of germ cells fused, doubling the chromosomes such that the oocyte contained two sets of chromosomes from each parent. After chromosome doubling at 10 months old, homologous chromosomes and the synaptonemal complex were identified. Discussion: Causally, meiosis proceeded normally and eventually formed diploid germ cells. These results further clarify the mechanisms by which meiosis proceeds in hybrids.

An updated review of cold shock and cold stress in fish

Temperature is critical in regulating virtually all biological functions in fish. Low temperature stress (cold shock/stress) is an often-overlooked challenge that many fish face as a result of both natural events and anthropogenic activities. In this study, we present an updated review of the cold shock literature based on a comprehensive literature search, following an initial review on the subject by M.R. Donaldson and colleagues, published in a 2008 volume of this journal. We focus on how knowledge on cold shock and fish has evolved over the past decade, describing advances in the understanding of the generalized stress response in fish under cold stress, what metrics may be used to quantify cold stress and what knowledge gaps remain to be addressed in future research. We also describe the relevance of cold shock as it pertains to environmental managers, policymakers and industry professionals, including practical applications of cold shock. Although substantial progress has been made in addressing some of the knowledge gaps identified a decade ago, other topics (e.g., population-level effects and interactions between primary, secondary and tertiary stress responses) have received little or no attention despite their significance to fish biology and thermal stress. Approaches using combinations of primary, secondary and tertiary stress responses are crucial as a research priority to better understand the mechanisms underlying cold shock responses, from short-term physiological changes to individual- and population-level effects, thereby providing researchers with better means of quantifying cold shock in laboratory and field settings.

Bisexual Fertile Triploid Zebrafish (Danio rerio): a Rare Case

Previous studies have suggested that artificially induced triploid zebrafish are exclusively male-biased. Owing to greatly inhibited gonadal development for the artificially induced triploid fish, they are regarded to be sterile in general. In this article, partially fertile bisexual triploid zebrafish are produced by suppressing extrusion of the second polar body by heat shock. Histological observation confirms that the early gonadal development of these triploid zebrafish is normal. Backcrossing and self-crossing are used to demonstrate that both the female and male triploid zebrafish have partial reproductive ability. Their dynamic of chromosomes during meiosis is revealed from the chromosome preparations of gonads. Examination of the expressed gonadal development-related genes shows some molecular evidence of the normal gonadal development in the triploid zebrafish. Clearly, these fertile bisexual triploid zebrafish can provide a unique system to study sex determination, as well as aneuploidy associated human diseases such as infertility and pregnancy loss.

Comparative analysis of testis transcriptomes associated with male infertility in triploid cyprinid fish

Spermatogenesis involves a series of cellular transformations and thousands of regulated genes. Previously, we showed that the triploid fish (3nBY) cannot produce mature spermatozoa. In the present study, evaluation of the testis microstructure revealed that germ cells of 3nBY could develop into round spermatids, but then degenerated, resulting in male infertility. In this study we comparatively analysed the testis transcriptomes from 3nBY and its diploid parent YB and identified a series of differentially expressed genes (DEGs) that were enriched in the Wnt signalling pathway and the apoptotic and ubiquitin-mediated proteolysis processes in 3nBY. Gene ontology functional analyses revealed that some DEGs in 3nBY were directly associated with the process of gamete generation, development and sperm flagellum assembly. In addition, the expression of a number of genes related to meiosis (Inhibitor Of DNA Binding 2 (ID2), Ovo Like Transcriptional Repressor 1 (OVOL1)), mitochondria (ATP1b (ATPase Na+/K+ Transporting Subunit Beta 1), ATP2a (ATPase, Ca++ Transporting, Cardiac Muscle, Slow Twitch 2), ATP5a (ATP Synthase F1 Subunit Alpha), Mitochondrially Encoded Cytochrome C Oxidase I (COX1), NADH Dehydrogenase Subunit 4 (ND4)) and chromatin structure (Histone 1 (H1), Histone 2a (H2A), Histone 2b (H2B), Histone 3 (H3), Histone 4 (H4)) was lower in the testes of 3nBY, whereas the expression of genes encoding ubiquitin (Ubiquitin Conjugating Enzymes (UBEs), Ring Finger Proteins (RNFs)) and apoptosis (CASPs (Caspase 3, Caspase 7,Caspase 8), BCLs (B-Cell Lymphoma 3, B-Cell CLL/Lymphoma 2, B Cell CLL/Lymphoma 10)) proteins involved in spermatid degeneration was higher. These data suggest that the disrupted expression of genes associated with spermatogenesis and the increased expression of mitochondrial ubiquitin, which initiates cell apoptosis, may result in spermatid degeneration in male 3nBY. This study provides information regarding the potential molecular regulatory mechanisms underlying male infertility in polyploid fish.

Asymmetric expression patterns reveal a strong maternal effect and dosage compensation in polyploid hybrid fish

Background

Hybridization and polyploidization are regarded as the major driving forces in plant speciation, diversification, and ecological adaptation. Our knowledge regarding the mechanisms of duplicated-gene regulation following genomic merging or doubling is primarily derived from plants and is sparse for vertebrates.

Results

We successfully obtained an F1 generation (including allodiploid hybrids and triploid hybrids) from female Megalobrama amblycephala Yih (BSB, 2n = 48) × male Xenocypri davidi Bleeker (YB, 2n = 48). The duplicated-gene expression patterns of the two types of hybrids were explored using RNA-Seq data. In total, 5.44 × 10⁸ (69.32 GB) clean reads and 499,631 assembled unigenes were obtained from the testis transcriptomes. The sequence similarity analysis of 4265 orthologs revealed that the merged genomes were dominantly expressed in different ploidy hybrids. The differentially expressed genes in the two types of hybrids were asymmetric compared with those in both parents. Furthermore, the genome-wide expression level dominance (ELD) was biased toward the maternal BSB genome in both the allodiploid and triploid hybrids. In addition, the dosage-compensation mechanisms that reduced the triploid expression levels to the diploid state were determined in the triploid hybrids.

Conclusions

Our results indicate that divergent genomes undergo strong interactions and domination in allopolyploid offspring. Genomic merger has a greater effect on the gene-expression patterns than genomic doubling. The various expression mechanisms (including maternal effect and dosage compensation) in different ploidy hybrids suggest that the initial genomic merger and doubling play important roles in polyploidy adaptation and evolution.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4883-7) contains supplementary material, which is available to authorized users.