Ploidy manipulation and induction of alternate cleavage patterns through inhibition of centrosome duplication in the early zebrafish embryo

Tissue-specific compensatory mechanisms maintain tissue architecture and body size independent of cell size in polyploid zebrafish

Genome duplications and ploidy transitions have occurred in nearly every major taxon of eukaryotes, but they are far more common in plants than in animals. Due to the conservation of the nuclear:cytoplasmic volume ratio increased DNA content results in larger cells. In plants, polyploid organisms are larger than diploids as cell number remains relatively constant. Conversely, vertebrate body size does not correlate with cell size and ploidy as vertebrates compensate for increased cell size to maintain tissue architecture and body size. This has historically been explained by a simple reduction in cell number that matches the increase in cell size maintaining body size as ploidy increases, but here we show that the compensatory mechanisms that maintain body size in triploid zebrafish are tissue-specific: A) erythrocytes respond in the classical pattern with a reduced number of larger erythrocytes in circulation, B) muscle, a tissue comprised of polynucleated muscle fibers, compensates by reducing the number of larger nuclei such that myofiber and myotome size in unaffected by ploidy, and C) vascular tissue compensates by thickening blood vessel walls, possibly at the expense of luminal diameter. Understanding the physiological implications of ploidy on tissue function requires a detailed description of the specific mechanisms of morphological compensation occurring in each tissue to understand how ploidy changes affect development and physiology.

Heat Shock Procedure Affects Cell Division-Associated Genes in Gynogenetic Manipulation

Heat shock procedure is crucial for gynogenetic manipulation leading to diploidization of the maternal genomes; however, the underlying molecular mechanism especially the transcriptomic changes during this procedure has still not been unveiled yet. Here, the artificial gynogenesis of zebrafish (Danio rerio) using inactivated sperm from rare minnow (Gobiocypris rarus) was conducted. We found that artificial gynogenetic manipulation, including pseudo-fertilization and heat shock, decreased hatching rates, whereas heat shock treatment alone had medium hatching rates. The first cleavage changed the expression of genes associated with RNA transcription and protein synthesis. A co-expression network regulated by hub genes GIT1, Sepsecs, and FLNB was significantly correlated with heat shock procedure. The cyclin family and cyclin-dependent kinase-related genes were lowly expressed in embryos from gynogenetic zebrafish, and genes involved in controlling the cell cycle and genomic stability were significantly altered by the gynogenetic treatment. Our results show the effects of artificial gynogenesis on embryos and describe changes in gene expression that suggest drastic changes take place in cell division by heat shock procedure. These findings will contribute to an understanding of the molecular basis for germplasm improving, including the purifying effect and allogynogenetic biological effect by gynogenesis.

Cell division geometries as central organizers of early embryo development

Early cellular patterning is a critical step of embryonic development that determines the proper progression of morphogenesis in all metazoans. It relies on a series of rapid reductive divisions occurring simultaneously with the specification of the fate of different subsets of cells. Multiple species developmental strategies emerged in the form of a unique cleavage pattern with stereotyped division geometries. Cleavage geometries have long been associated to the emergence of canonical developmental features such as cell cycle asynchrony, zygotic genome activation and fate specification. Yet, the direct causal role of division positioning on blastomere cell behavior remain partially understood. Oriented and/or asymmetric divisions define blastomere cell sizes, contacts and positions, with potential immediate impact on cellular decisions, lineage specification and morphogenesis. Division positions also instruct daughter cells polarity, mechanics and geometries, thereby influencing subsequent division events, in an emergent interplay that may pattern early embryos independently of firm deterministic genetic programs. We here review the recent literature which helped to delineate mechanisms and functions of division positioning in early embryos.

Artificial whole genome duplication in paleopolyploid sturgeons yields highest documented chromosome number in vertebrates

Critically endangered sturgeons, having undergone three whole genome duplication events, represent an exceptional example of ploidy plasticity in vertebrates. Three extant ploidy groups, combined with autopolyploidization, interspecific hybridization and the fertility of hybrids are important issues in sturgeon conservation and aquaculture. Here we demonstrate that the sturgeon genome can undergo numerous alterations of ploidy without severe physiological consequences, producing progeny with a range of ploidy levels and extremely high chromosome numbers. Artificial suppression of the first mitotic division alone, or in combination with suppression of the second meiotic division of functionally tetraploid zygotes (4n, C-value = 4.15) of Siberian sturgeon Acipenser baerii and Russian sturgeon A. gueldenstaedtii resulted in progeny of various ploidy levels—diploid/hexaploid (2n/6n) mosaics, hexaploid, octoploid juveniles (8n), and dodecaploid (12n) larvae. Counts between 477 to 520 chromosomes in octoploid juveniles of both sturgeons confirmed the modal chromosome numbers of parental species had been doubled. This exceeds the highest previously documented chromosome count among vertebrates 2n ~ 446 in the cyprinid fish Ptychobarbus dipogon.

Strong static magnetic field delayed the early development of zebrafish

One of the major topics in magnetobiology is the biological effects of strong static magnetic field (SMF) on living organisms. However, there has been a paucity of the comprehensive study of the long-term effects of strong SMF on an animal's development. Here, we explored this question with zebrafish, an excellent model organism for developmental study. In our research, zebrafish eggs, just after fertilization, were exposed to a 9.0 T SMF for 24 h, the critical period of post-fertilization development from cleavage to segmentation. The effects of strong SMF exposure on the following developmental progress of zebrafish were studied until 6 days post-fertilization (dpf). Results showed that 9.0 T SMF exposure did not influence the survival or the general developmental scenario of zebrafish embryos. However, it slowed down the developmental pace of the whole animal, and the late developers would catch up with their control peers after the SMF was removed. We proposed a mechanical model and deduced that the development delaying effect was caused by the interference of SMF in microtubule and spindle positioning during mitosis, especially in early cleavages. Our research data provide insights into how strong SMF influences the developing organisms through basic physical interactions with intracellular macromolecules.

Experimental Manipulation of Ploidy in Zebrafish Embryos and Its Application in Genetic Screens

Metazoan animals are typically diploid, possessing two sets of a chromosome in the somatic cells of an organism. In naturally diploid species, alteration from the endogenous diploid state is usually embryonic lethal. However, the ability to experimentally manipulate ploidy of animal embryos has fundamental as well as applied biology advantages. In this chapter we describe experimental procedures to convert normally diploid zebrafish embryos into haploid or tetraploid states. We also describe methodologies to verify the ploidy of embryos and the utility of ploidy manipulation in expediting the isolation of mutations using both forward and reverse genetic strategies in zebrafish.

Effects of homozygosity on sex determination in zebrafish Danio rerio

Gynogenetic zebrafish Danio rerio were obtained by activating D. rerio oocytes with UV irradiated common carp Cyprinus carpio sperm and then applying one of four different shocks [two (early) meiotic and two (late) mitotic shocks]. Gynogens produced by three of the shocks survived to maturity. All adult gynogens (n = 52) except one were found to be male. There was no difference in growth rate between the biparental controls and gynogens produced through the most effective shock, thereby eliminating growth rate as a possible cause of the skewed sex ratio. Gynogen males had reduced fertility compared to biparental controls, with about half of gynogens being unable to reproduce through natural spawning (all controls reproduced successfully). Gynogen males that did reproduce gave lower fertilization rates compared with controls. This demonstrates the negative effects of increased homozygosity on male reproductive function. Families sired by meiotic gynogen males were more likely to be female biased (33% of families) compared with families sired by biparental control males (11%). In addition to confirming the polygenic nature of sex determination in D. rerio, these observations suggest that recessive or over-dominant male-determining alleles are present in domesticated D. rerio populations.

Transient window of resilience during early development minimizes teratogenic effects of heat in zebrafish embryos

Background: Transient heat shock during early development is an established experimental paradigm for doubling the genome of the zebrafish zygote, which has practical applications in expedited identification of recessive mutations in genetic screens. Despite the simplicity of the strategy and the genetic tractability of zebrafish, heat shock has not been used for genome doubling since the proof‐of‐principle experiments done in the 1980s. This is because of poor survival of embryos that ensue from transient heat shocks and gross developmental abnormalities in the few survivors, which is incompatible with phenotype driven screens. Results: We show that heat shocks during early zebrafish development uncouple the second cycle of DNA and centrosome duplication. Interestingly, the developmental time of the heat shock that triggers the dissociation between DNA and centrosome duplication cycles significantly affect the potential of embryos to survive and attain normal morphology. The potential to develop normally after a heat shock alters in a developmental time span of 2 min in zebrafish embryos, a phenomenon that has not been reported in any species. Conclusions: The existence of heat resilient developmental windows and reduced heat teratogenicity during these windows could be an effective step forward in practical application of transient heat for experimental manipulation of ploidy in zebrafish. More broadly, heat resilience before zygotic genome activation suggests that metazoan embryos may possess innate protective features against heat beyond the canonical heat shock response. Developmental Dynamics 247:992–1004, 2018. © 2018 Wiley Periodicals, Inc.

Zebrafish embryos at the end of pronuclear fusion and before initiation of zygotic mitosis are resistant to teratogenic effects of heat.

The teratogenic heat resilient window exists transiently during the maternally controlled phase of development.

Heat shock during the teratogenic heat resilient window enables generation of morphologically normal zebrafish tetraploids.

Diploidization of haploids by transient heat shocks during the teratogenic heat resilient windows aids in effective generation of gynogenic diploids.

Manipulation of Ploidy in Caenorhabditis elegans

Mechanisms that involve whole genome polyploidy play important roles in development and evolution; also, an abnormal generation of tetraploid cells has been associated with both the progression of cancer and the development of drug resistance. Until now, it has not been feasible to easily manipulate the ploidy of a multicellular animal without generating mostly sterile progeny. Presented here is a simple and rapid protocol for generating tetraploid Caenorhabditis elegans animals from any diploid strain. This method allows the user to create a bias in chromosome segregation during meiosis, ultimately increasing ploidy in C. elegans. This strategy relies on the transient reduction of expression of the rec-8 gene to generate diploid gametes. A rec-8 mutant produces diploid gametes that can potentially produce tetraploids upon fertilization. This tractable scheme has been used to generate tetraploid strains carrying mutations and chromosome rearrangements to gain insight into chromosomal dynamics and interactions during pairing and synapsis in meiosis. This method is efficient for generating stable tetraploid strains without genetic markers, can be applied to any diploid strain, and can be used to derive triploid C. elegans. This straightforward method is useful for investigating other fundamental biological questions relevant to genome instability, gene dosage, biological scaling, extracellular signaling, adaptation to stress, development of resistance to drugs, and mechanisms of speciation.

Effective Generation of Gynogenic Haploid Zebrafish Embryos Using Low Dosage of UV Rays

The phenomenon of phenotype manifestation when the single allele in a haploid is affected is desirable for uncovering recessive mutations expeditiously in a diploid organism. However, experimentally generated haploids manifest extensive lethality and a cluster of non-specific developmental defects known as the haploid syndrome. This precludes the use of experimentally generated haploids for genetic screens due to an insufficient number of embryos for screening and the possibility of phenotypes due to the affected gene being masked by the haploid syndrome. We show here that gynogenic haploid zebrafish can be generated by irradiation of spermatozoa with a lower UV dosage than is currently used. This strategy results in reduced haploid lethality, incidence and severity of haploid syndrome. When viewed in the context of zebrafish as a genetically tractable model organism for forward and reverse genetic strategies, these results place zebrafish in a unique niche as a vertebrate in which haploid genetic screens for developmental phenotypes could be successfully attempted.

Generic Theoretical Models to Predict Division Patterns of Cleaving Embryos

Life for all animals starts with a precise 3D choreography of reductive divisions of the fertilized egg, known as cleavage patterns. These patterns exhibit conserved geometrical features and striking interspecies invariance within certain animal classes. To identify the generic rules that may govern these morphogenetic events, we developed a 3D-modeling framework that iteratively infers blastomere division positions and orientations, and consequent multicellular arrangements. From a minimal set of parameters, our model predicts detailed features of cleavage patterns in the embryos of fishes, amphibians, echinoderms, and ascidians, as well as the genetic and physical perturbations that alter these patterns. This framework demonstrates that a geometrical system based on length-dependent microtubule forces that probe blastomere shape and yolk gradients, biased by cortical polarity domains, may dictate division patterns and overall embryo morphogenesis. These studies thus unravel the default self-organization rules governing early embryogenesis and how they are altered by deterministic regulatory layers.

Ploidy Manipulation of Zebrafish Embryos with Heat Shock 2 Treatment

Manipulation of ploidy allows for useful transformations, such as diploids to tetraploids, or haploids to diploids. In the zebrafish Danio rerio, specifically the generation of homozygous gynogenetic diploids is useful in genetic analysis because it allows the direct production of homozygotes from a single heterozygous mother. This article describes a modified protocol for ploidy duplication based on a heat pulse during the first cell cycle, Heat Shock 2 (HS2). Through inhibition of centriole duplication, this method results in a precise cell division stall during the second cell cycle. The precise one-cycle division stall, coupled to unaffected DNA duplication, results in whole genome duplication. Protocols associated with this method include egg and sperm collection, UV treatment of sperm, in vitro fertilization and heat pulse to cause a one-cell cycle division delay and ploidy duplication. A modified version of this protocol could be applied to induce ploidy changes in other animal species.

Genetic screens for mutations affecting adult traits and parental-effect genes

Forward genetics remains an important approach for the unbiased identification of factors involved in biological pathways. Forward genetic analysis in the zebrafish has until now largely been restricted to the developmental period from zygotic genome activation through the end of embryogenesis. However, the use of the zebrafish as a model system for the analysis of late larval, juvenile and adult traits, including fertility and maternal and paternal effects, continues to gain momentum. Here, we describe two approaches, based on an F3-extended family and gynogenetic methods, that allow genetic screening for, and recovery of mutations affecting post-embryonic stages, including adult traits, fertility, and parental effects. For each approach, we also describe strategies to maintain, map, and molecularly clone the identified mutations.