The challenges of delivering genetically modified crops with nutritional enhancement traits.

The potential for using genetic modification (GM) to enhance the nutritional composition of crops (for either direct human consumption or as animal feed) has been recognized since the dawn of the GM era, with such 'output' traits being considered as distinct, if not potentially superior, to 'input' traits such as herbicide tolerance and insect resistance. However, while input traits have successfully been used and now form the basis of GM agriculture, output trait GM crops are still lagging behind after 20 years. This is despite the demonstrable benefits that some nutritionally enhanced crops would bring and the proven value of GM technologies. This Review considers the present state of nutritional enhancement through GM, highlighting two high-profile examples of nutritional enhancement-Golden Rice and omega-3 fish oil crops-systematically evaluating the progress, problems and pitfalls associated with the development of these traits. This includes not just the underlying metabolic engineering, but also the requirements to demonstrate efficacy and field performance of the crops and consideration of regulatory, intellectual property and consumer acceptance issues.

Left/right asymmetric collective migration of parapineal cells is mediated by focal FGF signaling activity in leading cells.

The ability of cells to collectively interpret surrounding environmental signals underpins their capacity to coordinate their migration in various contexts, including embryonic development and cancer metastasis. One tractable model for studying collective migration is the parapineal, a left-sided group of neurons that arises from bilaterally positioned precursors that undergo a collective migration to the left side of the brain. In zebrafish, the migration of these cells requires Fgf8 and, in this study, we resolve how FGF signaling correlates with-and impacts the migratory dynamics of-the parapineal cell collective. The temporal and spatial dynamics of an FGF reporter transgene reveal that FGF signaling is activated in only few parapineal cells usually located at the leading edge of the parapineal during its migration. Overexpressing a constitutively active Fgf receptor compromises parapineal migration in wild-type embryos, while it partially restores both parapineal migration and mosaic expression of the FGF reporter transgene in fgf8-/- mutant embryos. Focal activation of FGF signaling in few parapineal cells is sufficient to promote the migration of the whole parapineal collective. Finally, we show that asymmetric Nodal signaling contributes to the restriction and leftwards bias of FGF pathway activation. Our data indicate that the first overt morphological asymmetry in the zebrafish brain is promoted by FGF pathway activation in cells that lead the collective migration of the parapineal to the left. This study shows that cell-state differences in FGF signaling in front versus rear cells is required to promote migration in a model of FGF-dependent collective migration.

Tcf7l2 is required for left-right asymmetric differentiation of habenular neurons.

BACKGROUND: Although left-right asymmetries are common features of nervous systems, their developmental bases are largely unknown. In the zebrafish epithalamus, dorsal habenular neurons adopt medial (dHbm) and lateral (dHbl) subnuclear character at very different frequencies on the left and right sides. The left-sided parapineal promotes the elaboration of dHbl character in the left habenula, albeit by an unknown mechanism. Likewise, the genetic pathways acting within habenular neurons to control their asymmetric differentiated character are unknown. RESULTS: In a forward genetic screen for mutations that result in loss of habenular asymmetry, we identified two mutant alleles of tcf7l2, a gene that encodes a transcriptional regulator of Wnt signaling. In tcf7l2 mutants, most neurons on both sides differentiate with dHbl identity. Consequently, the habenulae develop symmetrically, with both sides adopting a pronounced leftward character. Tcf7l2 acts cell automously in nascent equipotential neurons, and on the right side, it promotes dHbm and suppresses dHbl differentiation. On the left, the parapineal prevents this Tcf7l2-dependent process, thereby promoting dHbl differentiation. CONCLUSIONS: Tcf7l2 is essential for lateralized fate selection by habenular neurons that can differentiate along two alternative pathways, thereby leading to major neural circuit asymmetries.

Regulation of expression of zebrafish (Danio rerio) insulin-like growth factor 2 receptor: implications for evolution at the IGF2R locus.

The insulin-like growth factor 2 receptor (IGF2R) is an unusual multifunctional receptor that interacts with a diverse variety of ligands. While the receptor has been well-characterized in mammals, little is known of its biology in other vertebrates. In this report, we characterize the expression of the zebrafish (Danio rerio) ortholog of the IGF2R gene. We show that two distinct, cell-type-specific promoters drive transcription of zebrafish igf2r and that these encode receptor isoforms that differ in their amino termini. Both promoters are active in adult fish and during embryonic development, but the proximal promoter generates more abundant transcripts. The 5'-UTR of the more abundantly expressed transcript contains several AUGs upstream of the main start codon, and these negatively regulate translation of a downstream reporter gene. Comparative sequence analysis shows that upstream AUGs (uAUGs) are a feature of IGF2R genes in several other vertebrates, including Xiphophorus, Xenopus, chicken, platypus, and opossum, but not in eutherian mammals. The IGF2R is imprinted in marsupial and placental mammals, and this transcriptional control of receptor abundance was proposed to have evolved following acquisition of an insulin-like growth factor 2 (IGF2) binding site by the ancestral receptor. Our observations suggest that receptor abundance was regulated at translation in ancestral vertebrates, before acquisition of an IGF2 binding site. We propose that evolution of imprinting at the mammalian IGF2R may have facilitated the loss of negative regulation of translation conferred by uAUGs.

A gene-based radiation hybrid map of the gilthead sea bream Sparus aurata refines and exploits conserved synteny with Tetraodon nigroviridis.

BACKGROUND: Comparative teleost studies are of great interest since they are important in aquaculture and in evolutionary issues. Comparing genomes of fully sequenced model fish species with those of farmed fish species through comparative mapping offers shortcuts for quantitative trait loci (QTL) detections and for studying genome evolution through the identification of regions of conserved synteny in teleosts. Here a comparative mapping study is presented by radiation hybrid (RH) mapping genes of the gilthead sea bream Sparus aurata, a non-model teleost fish of commercial and evolutionary interest, as it represents the worldwide distributed species-rich family of Sparidae. RESULTS: An additional 74 microsatellite markers and 428 gene-based markers appropriate for comparative mapping studies were mapped on the existing RH map of Sparus aurata. The anchoring of the RH map to the genetic linkage map resulted in 24 groups matching the karyotype of Sparus aurata. Homologous sequences to Tetraodon were identified for 301 of the gene-based markers positioned on the RH map of Sparus aurata. Comparison between Sparus aurata RH groups and Tetraodon chromosomes (karyotype of Tetraodon consists of 21 chromosomes) in this study reveals an unambiguous one-to-one relationship suggesting that three Tetraodon chromosomes correspond to six Sparus aurata radiation hybrid groups. The exploitation of this conserved synteny relationship is furthermore demonstrated by in silico mapping of gilthead sea bream expressed sequence tags (EST) that give a significant similarity hit to Tetraodon. CONCLUSION: The addition of primarily gene-based markers increased substantially the density of the existing RH map and facilitated comparative analysis. The anchoring of this gene-based radiation hybrid map to the genome maps of model species broadened the pool of candidate genes that mainly control growth, disease resistance, sex determination and reversal, reproduction as well as environmental tolerance in this species, all traits of great importance for QTL mapping and marker assisted selection. Furthermore this comparative mapping approach will facilitate to give insights into chromosome evolution and into the genetic make up of the gilthead sea bream.

A genetic linkage map of the hermaphrodite teleost fish Sparus aurata L.

The gilthead sea bream (Sparus aurata L.) is a marine fish of great importance for fisheries and aquaculture. It has also a peculiar sex-determination system, being a protandrous hermaphrodite. Here we report the construction of a first-generation genetic linkage map for S. aurata, based on 204 microsatellite markers. Twenty-six linkage groups (LG) were found. The total map length was 1241.9 cM. The ratio between sex-specific map lengths was 1:1.2 (male:female). Comparison with a preliminary radiation hybrid (RH) map reveals a good concordance, as all markers located in a single LG are located in a single RH group, except for Ad-25 and CId-31. Comparison with the Tetraodon nigroviridis genome revealed a considerable number of evolutionary conserved regions (ECRs) between the two species. The mean size of ECRs was 182 bp (sequence identity 60-90%). Forty-one ECRs have a known chromosomal location in the pufferfish genome. Despite the limited number of anchoring points, significant syntenic relationships were found. The linkage map presented here provides a robust comparative framework for QTL analysis in S. aurata and is a step toward the identification of genetic loci involved both in the determination of economically important traits and in the individual timing of sex reversal.