Induction of Gynogenetic and Androgenetic Haploid and Doubled Haploid Development in the Brown Trout (Salmo trutta Linnaeus 1758)

Transcript level of telomerase reverse-transcriptase (TERT) gene in the rainbow trout (Oncorhynchus mykiss) eggs with different developmental competence for gynogenesis

Expression of the telomerase reverse-transcriptase (TERT) gene and activity of telomerase have been reported in the somatic tissues and gonads in fish irrespective of their age and size. Nevertheless, little is known about TERT expression in the fish eggs. In the current study, the presence of the TERT transcripts was confirmed in the rainbow trout ovulated eggs before and after activation with nonirradiated and UV-irradiated (gynogenesis) sperm. Eggs originating from eight females had high and comparable quality expressed by similar hatching rates. However, survival of the gynogenetic larvae that hatched from eggs activated with UV-irradiated sperm and further exposed to the high hydrostatic pressure (HHP) shock for duplication of the maternal chromosomes varied between females from 2.1 ± 0.4 to 40.5 ± 2.2%. Increased level of TERT transcripts was observed in eggs originating from two females, and gametes from only one of them showed improved competence for gynogenesis (27.3 ± 1.9%). In turn, eggs from the female that exhibited the highest survival after gynogenetic activation were characterized by the lowest expression of the TERT gene. Telomerase in rainbow trout eggs may compensate erosion of the telomeres during early embryonic development; however, its upregulation does not assure better development after gynogenetic activation.

Quality of fish eggs and production of androgenetic and gynogenetic doubled haploids (DHs)

Induced development of haploid embryos (H) with only paternal (androgenesis) or maternal (gynogenesis) chromosomes requires irradiation of eggs before fertilization or activation of eggs with irradiated spermatozoa, respectively. To provide doubled haploids (DHs), androgenetic and gynogenetic haploid zygotes need to be subjected to the thermal or high hydrostatic pressure (HHP) shock to suppress the first mitotic cleavage and to double paternal or maternal haploid set of chromosomes. Androgenesis and mitotic gynogenesis (mito-gynogenesis) result in the generation of fully homozygous individuals in a single generation. DHs have been utilized in selective breeding programs, in studies concerning the phenotypic consequences of recessive alleles and to evaluate the impact of sex chromosomes on the early ontogeny. Moreover, the use of DHs for the NGS approach radically improves de novo the assembly of the genomes. However, reduced survival of the doubled haploids limits the wide application of androgenotes and gynogenotes. The high mortality of DHs may be only partly explained by the expression of recessive traits. Observed inter-clutch variation in the survival of DHs developing in eggs originating from different females make it necessary to take a closer look at the quality of the eggs used during induced androgenesis and gynogenesis. Moreover, the developmental competence of eggs that are subjected to irradiation before fertilization in order to deactivate maternal chromosomes when undergoing induced androgenesis and exposed to the physical shock after fertilization that leads to the duplication of the zygotes in both mito-gynogenesis and androgenesis may be also altered as irradiation and sublethal values of temperatures and hydrostatic pressure are considered as harmful for the cell organelles and biomolecules. Here, recently provided results concerning the morphological, biochemical, genomic, and transcriptomic characteristics of fish eggs showing high and low competence for androgenesis and mito-gynogenesis are reviewed.

Genome-wide analysis facilitates estimation of the amount of male contribution in meiotic gynogenetic three-spined stickleback (Gasterosteus aculeatus)

Gynogenetic embryos - those inheriting only maternal DNA - can be experimentally created by fertilizing eggs with radiation-treated sperm containing inactivated paternal chromosomes. Diploidy in the zygotes can be maintained through prevention of the second meiosis or restored by preventing the first mitosis after the maternal chromosome complement has been replicated. These gynogenetic organisms are useful in many fields including aquaculture, evolutionary biology and genomics. Although gynogenetic organisms have been created in numerous species, the completeness of uni-parental inheritance has often been assumed rather than thoroughly quantified across the genome. Instead, when tests of uni-parental inheritance occur, they typically rely on well-studied genetically determined phenotypes that represent a very small sub-set of the genome. Only assessing small genomic regions for paternal inheritance leaves the question of whether some paternal contributions to offspring might still have occurred. In this study, the authors quantify the efficacy of creating gynogenetic diploid three-spined stickleback fish (Gasterosteus aculeatus). To this end, the authors mirrored previous assessments of paternal contribution using well-studied genetically determined phenotypes including sex and genetically dominant morphological traits but expanded on previous studies using dense restriction site-associated DNA sequencing (RAD-seq) markers in parents and offspring to assess paternal inheritance genome-wide. In the gynogenetic diploids, the authors found no male genotypes underlying their phenotypes of interest - sex and dominant phenotypic traits. Using genome-wide assessments of paternal contribution, nevertheless, the authors found evidence of a small, yet potentially important, amount of paternally "leaked" genetic material. The application of this genome-wide approach identifies the need for more widespread assessment of paternal contributions to gynogenetic animals and promises benefits for many aspects of aquaculture, evolutionary biology and genomics.

Highly reactive chemicals meet haploidization

Mutation of the sperm-specific phospholipase A and treatment of pollen with reactive oxygen species (ROS) reagents lead to the induction of maize haploids. ZmPOD65, a gene associated with sperm-specific ROS metabolism, also exhibits a haploidization effect.

Distant hybridization and gynogenesis between Nile tilapia Oreochromis niloticus and Jaguar cichlid Parachromis managuensis

To investigate the distant hybridization and gynogenesis between Nile tilapia Oreochromis niloticus and Jaguar cichlid Parachromis managuensis, reciprocal crossing was first performed between the two species. No offspring, however, were viable when there were these hybridizations. Gynogenesis was induced in O. niloticus and P. managuensis using ultraviolet (UV)-irradiated spermatozoa from P. managuensis and O. niloticus, respectively. The morphology during embryonic development indicated gynogenetic offspring of both O. niloticus and the P. managuensis were normal and deformed, and the results from flow cytometric analysis indicated normal fry were diploid and deformed fry were haploid. Gynogenetic O. niloticus and P. managuensis had the same DNA content and chromosome number as their species of origin, indicating that gynogenetic individuals were produced in both species. The presence of only females for both gynogenetic P. managuensis and O. niloticus was indicative of an XX genotype in the female P. managuensis and O. niloticus. Results from studies on genetic diversity indicated the average heterozygosity of the gynogenetic diploid population of O. niloticus were less than that of the cultured population, but the genetic homozygosity of the gynogenetic diploid population of O. niloticus was greater than that of the cultured population after one generation of gynogenesis, which achieved the goal of rapidly establishing genetic homozygosity.

Optimization of ultraviolet irradiation of Clarias gariepinus (Burchell, 1822) eggs for androgen production

The optimum distance and duration of ultraviolet (UV) irradiation for the complete inactivation of African catfish Clarias gariepinus egg nucleus was investigated in this study. The UV light was suspended above the unfertilized eggs at four distances (5, 10, 20 and 30 cm) and for five durations (1, 2, 3, 4 and 5 min). Then, the irradiated eggs were activated with sperm from diploid C. gariepinus and cold shocked at 5°C for 5 min just moments before cell cleavage. Ploidy analysis was performed using karyotype chromosome counting. The results obtained suggested that the further the distance, the better the hatchability rate, however prolonged duration seemed to significantly reduced hatchability. All treatments with surviving progenies at the end of the study showed evidence of successfully diploid gynogen (2n = 56) induction at different percentages. However, the optimal protocol that gave a moderately high hatchability/survival rate and completely induced gynogens was exposure of the eggs to UV irradiation at 20 cm for 1 min. It was concluded that the distance and duration of UV irradiation affects gynogenetic induction in African catfish C. gariepinus.

Production and verification of the first Atlantic salmon (Salmo salar L.) clonal lines

Background

In several fish species homozygous and heterozygous clonal lines have been produced using gynogenetic and androgenetic techniques. These lines are standardized and can be reproduced over generations. In rainbow trout such lines have existed for decades and has become important research tools in genome studies as well as in studies of commercially important traits. The Atlantic salmon is one of the best studied fish species globally, but all experiments are done on fish of wild or domesticated origin and access to standardized immortal fish lines would be of great benefit. Here, we describe the protocols developed to produce mitotic gynogenes, and from these the first clonal lines in Atlantic salmon.

Results

Atlantic salmon eggs fertilized with UV irradiated sperm combined with a pressure shock applied at 4700–4800 minC at 8 °C gave all homozygous (doubled haploid) gynogenetic progeny with high survival. From the six first maturing females, five all homozygous clonal lines were produced by meiotic gynogenesis and were verified as clonal and identical to their mother with microsatellite markers.

Conclusions

We have now produced the first documented cloned Atlantic salmon lines. This work demonstrates the potential for production of further Atlantic salmon clonal lines, potentially with distinct characteristics. Such lines will provide an important resource for further elucidation of phenotypic and genetic traits in this globally important species.

Further evidence for paternal DNA transmission in gynogenetic grass carp

Gynogenesis is an important breeding method in aquaculture and has been widely applied to many fish species. If gynogenetic progenies are to inherit paternal partial genomic DNA, this will increase genetic variation and will provide a useful outcome for breeding. In this study, we investigated the genetic variation in homeobox (Hox) gene clusters (HoxA4a, HoxA9a, HoxA11b, HoxB1b, HoxC4a, HoxC6b, and HoxD10a) among koi carp (Cyprinus carpio haematopterus, KOC; the stimulation sperm source), grass carp (Ctenopharyngodon idellus), and gynogenetic grass carp (GGC). We found paternal DNA (a special DNA fragment and HoxC6b) derived from KOC and a recombinant gene belonging to HoxC6b in GGC. We are the first to report the recombinant HoxC6b in GGC. Our study provides further evidence for paternal DNA transmission to gynogenetic progenies, which is a finding with great significance for the genetic breeding of fish.

Induced androgenetic development in rainbow trout and transcriptome analysis of irradiated eggs

Ionizing radiation is administered to damage nuclear genome in fish eggs during induced androgenesis. In this study, we examined whether 350 Gy of X-ray applied to damage chromosomes in the rainbow trout eggs affects maternal RNA. Shortly after irradiation, we did not find any symptoms of RNA degradation in the treated eggs. Significant (p < 0.01) differences between non-irradiated and irradiated eggs concerned only a few transcripts including increased expression of immediate early response 2 (IER2) and early growth response 1 (EGR1) genes observed in the irradiated eggs. Both genes belong to the group of “immediate early genes” that respond quickly to the diverse extracellular stimuli. Elevated expression of these genes was accompanied by decreased level of ssa-miR-10b-5p and ssa-miR-21b-5p (p < 0.05), for which IER2 and EGR1 are target genes. The level of RNA in the fertilized irradiated eggs was highly significantly lower than in the non-irradiated eggs (p < 0.001) and in the unfertilized irradiated eggs (p < 0.0001). However, transcriptome profiles of fertilized non-irradiated eggs and fertilized irradiated eggs did not differ significantly. Thus, we assume that reduced abundance of mRNA in the fertilized irradiated eggs was associated with post-translational degradation and clearance of the maternal transcripts rather than from the irradiation of eggs.

Transcriptome Analysis of Rainbow Trout (Oncorhynchus mykiss) Eggs Subjected to the High Hydrostatic Pressure Treatment

High hydrostatic pressure (HHP) causes depolymerization of the spindle microtubules. HHP applied to fish eggs prevents extrusion of the second polar body and inhibits the first cell cleavage, and it is used to produce triploids and diploid gynogenetic and androgenetic individuals. HHP has been also found to affect biomolecules including nucleic acids, and it may be presumed that HHP administered to the rainbow trout (Oncorhynchus mykiss) eggs disturbs cytoplasmic maternal RNA indispensable for the early embryogenesis. To verify this assumption, quality and quantity of RNA extracted from the rainbow trout eggs subjected to the high hydrostatic pressure shock were analyzed. Provided results exhibited that maternal transcriptome was resistant to a three-minute exposure to 65.5 MPa of HHP treatment. Some trend showing increase of the RNA integrity was observed in the HHP-treated eggs; however, the difference was not statistically significant. Alterations in the expression profiles in the rainbow trout eggs subjected to HHP were also negligible. Greater differences in the maternal gene expression were observed between eggs from different clutches than between HHP-treated and untreated eggs from the same clutch. It may be assumed that exposure to HHP shock was too short to modify significantly maternal transcripts in the rainbow trout eggs.

Genome incompatibility between rainbow trout (Oncorhynchus mykiss) and sea trout (Salmo trutta) and induction of the interspecies gynogenesis

Rainbow trout (Oncorhynchus mykiss Walbaum) and sea trout (Salmo trutta Linnaeus, 1758) show large karyotypic differences and their hybrid offspring is not viable due to unstable karyotype and chromosome fragmentation. However, gametes from these two species were used to induce gynogenetic development. Rainbow trout eggs activated by UV-irradiated sea trout sperm were subjected to high hydrostatic pressure (HHP) shock to prevent release of the 2nd polar body (early shock) or to inhibit the first cleavage (late shock) in order to produce diploid meiotic gynogenotes and gynogenetic doubled haploids (DHs), respectively. Cytogenetic analysis proved fish that development was induced by the sea trout spermatozoa were rainbow trout. In turn, molecular examination confirmed homozygosity of the gynogenetic DHs. Presumed appearance of the recessive alleles resulted in lower survival of the gynogenetic DH larvae (~25%) when compared to survival of the heterozygous (meiotic) gynogenotes (c. 50%). Our results proved that genomic incompatibilities between studied trout species result in the hybrid unviability. However, artificial gynogenesis including activation of rainbow trout eggs with UV-irradiated sea trout spermatozoa was successfully induced. As both species are unable to cross, application of the UV-irradiated sea trout spermatozoa to activate rainbow trout development assures only maternal inheritance with no contamination by the residues of the paternal chromosomes.

High Rate of Deformed Larvae among Gynogenetic Brown Trout (Salmo trutta m. fario) Doubled Haploids

Mitotic gynogenesis results in the production of fully homozygous individuals in a single generation. Since inbred fish were found to exhibit an increased frequency of body deformations that may affect their survival, the main focus of this research was to evaluate the ratio of individuals with spinal deformities among gynogenetic doubled haploids (DHs) brown trout as compared to nonmanipulated heterozygous individuals. Gynogenetic development was induced by the activation of brown trout eggs by UV-irradiated homologous and heterologous (rainbow trout) spermatozoa. The subsequent exposure of the activated eggs to the high hydrostatic pressure disturbed the first cleavage in gynogenetic zygotes and enabled duplication of the maternal haploid set of chromosomes. The survival rate was significantly higher among gynogenetic brown trout hatched from eggs activated with the homologous UV-irradiated spermatozoa when compared to DHs hatched from eggs activated by the heterologous spermatozoa. More than 35% of the gynogenetic larvae exhibited body deformities, mostly lordosis and scoliosis. The percentage of malformed brown trout from the control group did not exceed 15%. The increased number of deformed larvae among DHs brown trout suggested rather a genetic background of the disease related to the fish spine deformities; however, both genetic and environmental factors were discussed as a cause of such conditions in fish.

Disturbances in the ploidy level in the gynogenetic sterlet Acipenser ruthenus

Artificial mitotic gynogenesis, a chromosome set manipulation, is applied to provide the homozygous progeny with only maternal inheritance. Here, gynogenetic development was induced in the sterlet Acipenser ruthenus L. (Acipenseridae) by activation of the eggs originating from albino females with the UV-irradiated spermatozoa from wild-coloured males, followed by the heat shock applied to suppress the first mitotic division in the haploid zygotes. All experimentally obtained gynogenetic offspring possessed recessive albino coloration. Moreover, the genetic verification, based on three microsatellite DNA markers, confirmed the only maternal inheritance in the albino progeny. Cytogenetic screening enabled identification of the aneuploids, haploids, diploids, triploids, tetraploids and mosaic individuals among the gynogenetic larvae that hatched from the eggs subjected to the heat shock. Furthermore, 40% of the larvae from the haploid variants of the research that were not exposed to the temperature shock showed the diploid chromosome number. A variation of the ploidy level observed in the gynogenetic sterlets may be the consequence of the spontaneous polyploidisation that occurred in the haploid zygotes. Moreover, observation during embryogenesis showed varied stages of eggs development and the asynchronous cell cleavages that may have resulted in the chromosomal disturbances observed in the gynogenetic sterlets here.

Androgenetic development of X- and Y-chromosome bearing haploid rainbow trout embryos

Haploid fish embryos are important in studies regarding role of the recessive traits during early ontogeny. In fish species with the male heterogamety, androgenetic haploid embryos might be also useful tool in studies concerning role of the sex chromosomes during an embryonic development. Morphologically differentiated X and Y chromosomes have been found in a limited number of fish species including rainbow trout (Oncorhynchus mykiss Walbaum 1792). To evaluate role of the sex chromosomes during rainbow trout embryonic development, survival of the androgenetic haploids in the presence of X or Y sex chromosomes has been examined. Androgenetic haploid rainbow trout were produced by fertilization of X-irradiated eggs with spermatozoa derived from the normal males (XY) and neomales, that is, sex-reversed females (XX) to produce X- and Y-bearing haploids, and all X-bearing haploids, respectively. Survival rates of the androgenetic progenies of normal males and neomales examined during embryogenesis and at hatching did not differ significantly. However, all haploids died within next few days after hatching. Cytogenetic analysis of the androgenetic embryos confirmed their haploid status. Moreover, apart from the intact paternal chromosomes, residues of the irradiated maternal chromosomes observed as chromosome fragments were identified in some of the haploids. Provided results suggested that rainbow trout X and Y chromosomes despite morphological and genetic differences are at the early stage of differentiation and still share genetic information responsible for the proper embryonic development.

Activation of the Albino Sterlet Acipenser ruthenus Eggs by UV-Irradiated Bester Hybrid Spermatozoa to Provide Gynogenetic Progeny

Meiotic gynogenesis was induced in the albino form of sterlet Acipenser ruthenus by activation of eggs with UV-irradiated bester (Huso huso x Acipenser ruthenus) spermatozoa followed by inhibition of the second meiotic division performed by a heat shock. Obtained putative gynogenetic progeny were all albinos. The genetic verification based on three microsatellite DNA markers confirmed the only maternal inheritance of the progeny from the gynogenetic experimental groups. Cytogenetic analysis proved the gynogenetic sterlets were diploids. Application of the albino phenotype together with the molecular and the cytogenetic diagnostic approaches enabled to evaluate the efficiency of the spermatozoa irradiation and application of the heat shock to restore diploid state in the gynogenetic zygotes.