⌬5 and ⌬6 fatty acid desaturases are critical enzymes in the pathways for the biosynthesis of the polyunsaturated fatty acids arachidonic, eicosapentaenoic, and docosahexaenoic acids. They are encoded by distinct genes in mammals and Caenorhabditis elegans. This paper describes a cDNA isolated from zebrafish (Danio rerio) with high similarity to mammalian ⌬6 desaturase genes. The 1,590-bp sequence specifies a protein that, in common with other fatty acid desaturases, contains an N-terminal cytochrome b5 domain and three histidine boxes, believed to be involved in catalysis. When the zebrafish cDNA was expressed in Saccharomyces cerevisiae it conferred the ability to convert linoleic acid (18:2n-6) and ␣-linolenic acid (18:3n-3) to their corresponding ⌬6 desaturated products, 18:3n-6 and 18:4n-3. However, in addition it conferred on the yeast the ability to convert di-homo-␥-linoleic acid (20:3n-6) and eicosatetraenoic acid (20:4n-3) to arachidonic acid (20:4n-6) and eicosapentaenoic acid (20:5n-3), respectively, indicating that the zebrafish gene encodes an enzyme having both ⌬5 and ⌬6 desaturase activity. The zebrafish ⌬5͞⌬6 desaturase may represent a component of a prototypic vertebrate polyunsaturated fatty acids biosynthesis pathway.

A cattle genetic linkage map was constructed which marks about 90% of the expected length of the cattle genome. Over 200 DNA polymorphisms were genotyped in cattle families which comprise 295 individuals in full sibling pedigrees. One hundred and seventy-one loci were found linked to one other locus. Twenty nine of the 30 chromosome pairs are represented by at least one of the 36 linkage groups. Less than a 50 cM difference was found in the male and female genetic maps. The conserved loci on this map show as many differences in gene order compared to humans as is found between humans and mice. The conservation is consistent with the patterns of karyotypic evolution found in the rodents, primates and artiodactyls. This map will be important for localizing quantitative trait loci and provides a basis for further mapping.

Fig. 7The first diglyceride produced by PtdCho should have contained an oxygen atom, thus the vertical dashed line from the CH 2 -O bond has been moved to the O-P bond. The same is true for PtdOH, so the vertical dashed line from the P-O bond has been moved to the O-CH 2 bond. For Lyso-PtdCho (16:0), the vertical dashed line from the O-CH bond has been moved to the O=C-O bond. In addition, the carbon number of EPA has been corrected to 20, instead of 22, near the CH 2 -C=O bond.

A cattle genetic linkage map was constructed which covers more than 95 percent of the bovine genome at medium density. Seven hundred and forty six DNA polymorphisms were genotyped in cattle families which comprise 347 individuals in full sibling pedigrees. Seven hundred and three of the loci are linked to at least one other locus. All linkage groups are assigned to chromosomes, and all are orientated with regards to the centromere. There is little overall difference in the lengths of the bull and cow linkage maps although there are individual differences between maps of chromosomes. One hundred and sixty polymorphisms are in or near genes, and the resultant genome-wide comparative analyses indicate that while there is greater conservation of synteny between cattle and humans compared with mice, the conservation of gene order between cattle and humans is much less than would be expected from the conservation of synteny. This map provides a basis for high-resolution mapping of the bovine genome with physical resources such as Yeast and Bacterial Artificial Chromosomes as well as providing the underpinning for the interpolation of information from the Human Genome Project.

Fish are the only major dietary source for humans of omega-3 highly unsaturated fatty acids (HUFA) and, with declining fisheries, farmed fish such as Atlantic salmon (Salmo salar) constitute an increasing proportion of the fish in the human diet. However, the current high use of fish oils, derived from wild capture marine fisheries, in aquaculture feeds is not sustainable in the longer term, and will constrain continuing growth of aquaculture activities. A greater understanding of how fish metabolise and biosynthesise HUFA may lead to effective use of more sustainable aquaculture diets. The study described here contributes to an effort to determine the molecular genetics of the HUFA biosynthetic pathway in salmon, with the overall aim being to determine mechanisms for optimising the use of vegetable oils in Atlantic salmon culture. In this paper we Upon expression in yeast, SalElo showed broad substrate specificity for polyunsaturated fatty acids with a range of chain lengths, with the rank order being C 18 > C 20 > C 22 . Thus, all fatty acid elongase activities required for the biosynthesis of docosahexaenoic acid (22:6n-3) from 18:3n-3 are displayed by this one polypeptide product.