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.

Transgene-mediated skeletal phenotypic variation in zebrafish.

When considering relationships between genotype and phenotype we frequently ignore the fact that the genome of a typical animal, notably including that of a fish and a human, harbours a huge amount of foreign DNA. Such DNA, in the form of transposable elements, can affect genome function in a major way, and transgene biology needs to be included in our understanding of the genome. Here we examine an unexpected phenotypic effect of the chromosomally integrated transgene fli1a-F-hsp70l:Gal4VP16 that serves as a model for transgene function generally. We examine larval fras1 mutant zebrafish (Danio rerio). Gal4VP16 is a potent transcriptional activator that is already well known for toxicity and mediating unusual transcriptional effects. In the presence of the transgene, phenotypes in the neural crest-derived craniofacial skeleton, notably fusions and shape changes associated with loss of function fras1 mutations, are made more severe, as we quantify by scoring phenotypic penetrance, the fraction of mutants expressing the trait. A very interesting feature is that the enhancements are highly specific for fras1 mutant phenotypes, occurring in the apparent absence of more widespread changes. Except for the features due to the fras1 mutation, the transgene-bearing larvae appear generally healthy and to be developing normally. The transgene behaves as a genetic partial dominant: a single copy is sufficient for the enhancements, yet, for some traits, two copies may exert a stronger effect. We made new strains bearing independent insertions of the fli1a-F-hsp70l:Gal4VP16 transgene in new locations in the genome, and observed increased severities of the same phenotypes as observed for the original insertion. This finding suggests that sequences within the transgene, for example Gal4VP16, are responsible for the enhancements, rather than the effect on neighbouring host sequences (such as an insertional mutation). The specificity and biological action underlying the traits are subjects of considerable interest for further investigation, as we discuss. Our findings show that work with transgenes needs to be undertaken with caution and attention to detail.

Role of mef2ca in developmental buffering of the zebrafish larval hyoid dermal skeleton.

Phenotypic robustness requires a process of developmental buffering that is largely not understood, but which can be disrupted by mutations. Here we show that in mef2ca(b1086) loss of function mutant embryos and early larvae, development of craniofacial hyoid bones, the opercle (Op) and branchiostegal ray (BR), becomes remarkably unstable; the large magnitude of the instability serves as a positive attribute to learn about features of this developmental buffering. The OpBR mutant phenotype variably includes bone expansion and fusion, Op duplication, and BR homeosis. Formation of a novel bone strut, or a bone bridge connecting the Op and BR together occurs frequently. We find no evidence that the phenotypic stability in the wild type is provided by redundancy between mef2ca and its co-ortholog mef2cb, or that it is related to the selector (homeotic) gene function of mef2ca. Changes in dorsal-ventral patterning of the hyoid arch also might not contribute to phenotypic instability in mutants. However, subsequent development of the bone lineage itself, including osteoblast differentiation and morphogenetic outgrowth, shows marked variation. Hence, steps along the developmental trajectory appear differentially sensitive to the loss of buffering, providing focus for the future study.

Zebrafish Mef2ca and Mef2cb are essential for both first and second heart field cardiomyocyte differentiation.

Mef2 transcription factors have been strongly linked with early heart development. D-mef2 is required for heart formation in Drosophila, but whether Mef2 is essential for vertebrate cardiomyocyte (CM) differentiation is unclear. In mice, although Mef2c is expressed in all CMs, targeted deletion of Mef2c causes lethal loss of second heart field (SHF) derivatives and failure of cardiac looping, but first heart field CMs can differentiate. Here we examine Mef2 function in early heart development in zebrafish. Two Mef2c genes exist in zebrafish, mef2ca and mef2cb. Both are expressed similarly in the bilateral heart fields but mef2cb is strongly expressed in the heart poles at the primitive heart tube stage. By using fish mutants for mef2ca and mef2cb and antisense morpholinos to knock down either or both Mef2cs, we show that Mef2ca and Mef2cb have essential but redundant roles in myocardial differentiation. Loss of both Mef2ca and Mef2cb function does not interfere with early cardiogenic markers such as nkx2.5, gata4 and hand2 but results in a dramatic loss of expression of sarcomeric genes and myocardial markers such as bmp4, nppa, smyd1b and late nkx2.5 mRNA. Rare residual CMs observed in mef2ca;mef2cb double mutants are ablated by a morpholino capable of knocking down other Mef2s. Mef2cb over-expression activates bmp4 within the cardiogenic region, but no ectopic CMs are formed. Surprisingly, anterior mesoderm and other tissues become skeletal muscle. Mef2ca single mutants have delayed heart development, but form an apparently normal heart. Mef2cb single mutants have a functional heart and are viable adults. Our results show that the key role of Mef2c in myocardial differentiation is conserved throughout the vertebrate heart.

Cardiac troponin T is essential in sarcomere assembly and cardiac contractility.

Mutations of the gene (TNNT2) encoding the thin-filament contractile protein cardiac troponin T are responsible for 15% of all cases of familial hypertrophic cardiomyopathy, the leading cause of sudden death in young athletes. Mutant proteins are thought to act through a dominant-negative mode that impairs function of heart muscle. TNNT2 mutations can also lead to dilated cardiomyopathy, a leading cause of heart failure. Despite the importance of cardiac troponin T in human disease, its loss-of-function phenotype has not been described. We show that the zebrafish silent heart (sih) mutation affects the gene tnnt2. We characterize two mutated alleles of sih that severely reduce tnnt2 expression: one affects mRNA splicing, and the other affects gene transcription. Tnnt2, together with alpha-tropomyosin (Tpma) and cardiac troponins C and I (Tnni3), forms a calcium-sensitive regulatory complex within sarcomeres. Unexpectedly, in addition to loss of Tnnt2 expression in sih mutant hearts, we observed a significant reduction in Tpma and Tnni3, and consequently, severe sarcomere defects. This interdependence of thin-filament protein expression led us to postulate that some mutations in tnnt2 may trigger misregulation of thin-filament protein expression, resulting in sarcomere loss and myocyte disarray, the life-threatening hallmarks of TNNT2 mutations in mice and humans.

Making gynogenetic diploid zebrafish by early pressure.

Heterozygosity in diploid eukaryotes often makes genetic studies cumbersome. Methods that produce viable homozygous diploid offspring directly from heterozygous females allow F1 mutagenized females to be screened directly for deleterious mutations in an accelerated forward genetic screen. Streisinger et al. described methods for making gynogenetic (homozygous) diploid zebrafish by activating zebrafish eggs with ultraviolet light-inactivated sperm and preventing either the second meiotic or the first zygotic cell division using physical treatments (heat or pressure) that deploymerize microtubules. The "early pressure" (EP) method blocks the meiosis II, which occurs shortly after fertilization. The EP method produces a high percentage of viable embryos that can develop to fertile adults of either sex. The method generates embryos that are homozygous at all loci except those that were separated from their centromere by recombination during meiosis I. Homozygous mutations are detected in EP clutches at between 50% for centromeric loci and less than 1% for telomeric loci. This method is reproduced verbatim from the Zebrafish Book.

Evolution and development of facial bone morphology in threespine sticklebacks.

How do developmental mechanisms evolve to control changing skeletal morphology, the shapes and sizes of individual bones? We address this question with studies of the opercle (OP), a large facial bone that has undergone marked morphological evolution in the ray-finned fish. Attributes for developmental analysis motivated us to examine how OP shape and size evolve and develop in threespine sticklebacks, a model system for understanding vertebrate evolution. We find that when Alaskan anadromous fish take up permanent residence in lakes, they evolve smaller and reshaped OPs. The change is a reduction in the amount of bone laid down along one body axis, and it arises at or shortly after the onset of OP development. A quantitative trait locus is present on linkage group 19 that contributes in a major way to this phenotype.

Combined haploid and insertional mutation screen in the zebrafish.

To identify genes required for development of the brain and somites, we performed a pilot screen of gynogenetic haploid zebrafish embryos produced from mothers mutagenized by viral insertion. We describe an efficient method to identify new mutations and the affected gene. In addition, we report the results of a small-scale screen that identified five genes required for brain development, including novel alleles of nagie oko, pou5f1, ribosomal protein L36, and n-cadherin, as well as a novel allele of the laminin g1 gene that is required for normal skeletal muscle fiber organization and somite patterning.