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Cattle, pig and sheep oocytes isolated from healthy cumulus-oocyte complexes were pooled, within species, to provide samples of immature denuded oocytes with intact zona pellucida (n = 1000 per sample) for determination of fatty acid mass and composition in total lipid, constituent phospholipid and triglyceride. Acyl-containing lipid extracts, transmethylated in the presence of a reference penta-decaenoic acid (15:0), yielded fatty acid methyl esters which were analysed by gas chromatography. Mean (± SEM) fatty acid content in samples of pig oocytes (161 ± 18 µg per 1000 oocytes) was greater than that in cattle (63 ± 6 µg; P < 0.01) and sheep oocytes (89 ± 7 µg; P < 0.05). Of 24 fatty acids detected, palmitic (16:0; 25-35%, w/w), stearic (18:0; 14-16%) and oleic (18:1n-9; 22-26%) acids were most prominent in all three species. Saturated fatty acids (mean = 45-55%, w/w) were more abundant than mono-(27-34%) or polyunsaturates (11-21%). Fatty acids of the n-6 series, notably linoleic (18:2n-6; 5-8%, w/w) and arachidonic acid (20:4n-6; 1-3%), were the most abundant polyunsaturates. Phospholipid consistently accounted for a quarter of all fatty acids in the three species, but ruminant oocytes had a lower complement of polyunsaturates (14-19%, w/w) in this fraction than pig oocytes (34%, w/w) which, for example, had a three-to fourfold greater linoleic acid content. An estimated 74 ng of fatty acid was sequestered in the triglyceride fraction of individual pig oocytes compared with 23-25 ng in ruminant oocytes (P < 0.01). It is concluded that the greater fatty acid content of pig oocytes is primarily due to more abundant triglyceride reserves. Furthermore, this speciesspecific difference, and that in respect of polyunsaturated fatty acid reserves, may underlie the contrasting chilling, culture and cryopreservation sensitivities of embryos derived from pig and ruminant (cattle, sheep) oocytes.

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1. Within‐clutch variation in parental investment of yolk lipid (including essential fatty acid composition and fat‐soluble vitamin content) was investigated in whole clutches of three eggs of Lesser Black‐backed Gulls, Larus fuscus. 2. Although there were no significant differences between years in egg size or yolk mass among eggs of the same laying order, eggs collected in 1996 had significantly lower lipid content than those collected in 1997. 3. In both years third‐laid c‐eggs were smaller, had a smaller mass of yolk and had significantly lower yolk lipid mass compared with earlier laid (a‐ and b‐) eggs. 4. Despite this lower lipid mass of c‐eggs, of the different lipid fractions only cholesterol content of c‐eggs was significantly lower than that of a‐ or b‐eggs. 5. Earlier laid eggs had greater amounts of arachidonic acid in the phospholipid and cholesterol ester fractions of yolk lipid, and considerably greater quantities of vitamin E and carotenoids. 6. Conversely, levels of linoleic acid and vitamin A were equalized among eggs of different laying sequence as both these essential nutrients have primarily systemic roles. 7. The probable effect of increased antioxidant (vitamin E and carotenoid) content of earlier‐laid eggs is to enhance embryonic and neonatal growth and development, reduce susceptibility to pathogens and increase the ability of the embryo to withstand the oxidative stress associated with hatching. 8. The results suggest that there is maternal bias of essential nutrients to offspring with the highest reproductive value in order to maximize reproductive success.

The aim of this review is to collate data relevant to understanding the evolution of viviparity in general, and complex placentae in particular. The wide range of reproductive modes exhibited by lizards provides a solid model system for investigating the evolution of viviparity. Within the lizards are oviparous species, viviparous species that have a very simple placenta and little nutrient uptake from the mother during pregnancy (lecithotrophic viviparity), through a range of species that have intermediate placental complexities and placental nutrient provision, to species that lay microlecithal eggs and most nutrients are provided across the placenta during development (obligate placentotrophy). In its commonest form, lecithotrophic viviparity, some uptake of water, inorganic ions and oxygen occurs from the mother to the embryo during pregnancy. In contrast, the evolution of complex placentae is rare, but has evolved at least five times. Where there is still predominantly a reliance on egg yolk, the omphaloplacenta seems to be paramount in the provision of nutrition to the embryo via histotrophy, whereas the chorioallantoic placenta is more likely involved in gas exchange. Reliance on provision of substantial organic nutrient is correlated with the regional specialisation of the chorioallantoic placenta to form a placentome for nutrient uptake, particularly lipids, and the further development of the gas exchange capabilities of the other parts of the chorioallantois.