The diet of organisms generally provides a sufficient supply of energy and building materials for healthy growth and development, but should also contain essential nutrients. Species differ in their exogenous requirements, but it is not clear why some species are able to synthesize essential nutrients, while others are not. The unsaturated fatty acid, linoleic acid (LA; 18:2n-6) plays an important role in functions such as cell physiology, immunity, and reproduction, and is an essential nutrient in diverse organisms. LA is readily synthesized in bacteria, protozoa and plants, but it was long thought that all animals lacked the ability to synthesize LA de novo and thus required a dietary source of this fatty acid. Over the years, however, an increasing number of studies have shown active LA synthesis in animals, including insects, nematodes and pulmonates. Despite continued interest in LA metabolism, it has remained unclear why some organisms can synthesize LA while others cannot. Here, we review the mechanisms by which LA is synthesized and which biological functions LA supports in different organisms to answer the question why LA synthesis was lost and repeatedly gained during the evolution of distinct invertebrate groups. We propose several hypotheses and compile data from the available literature to identify which factors promote LA synthesis within a phylogenetic framework. We have not found a clear link between our proposed hypotheses and LA synthesis; therefore we suggest that LA synthesis may be facilitated through bifunctionality of desaturase enzymes or evolved through a combination of different selective pressures.

中文翻译：

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动物亚油酸合成的进化视角。


生物体的饮食通常为健康生长发育提供充足的能量和建筑材料，但也应含有必需的营养素。物种的外源需求不同，但尚不清楚为什么有些物种能够合成必需营养素，而另一些物种却不能。不饱和脂肪酸亚油酸（LA；18:2n-6）在细胞生理、免疫和生殖等功能中发挥着重要作用，是多种生物体的必需营养素。 LA 很容易在细菌、原生动物和植物中合成，但长期以来人们一直认为所有动物都缺乏从头合成 LA 的能力，因此需要这种脂肪酸的膳食来源。然而，多年来，越来越多的研究表明，包括昆虫、线虫和肺动物在内的动物体内 LA 合成活跃。尽管人们对 LA 代谢的兴趣持续存在，但仍不清楚为什么有些生物体可以合成 LA，而另一些生物体却不能。在这里，我们回顾了LA的合成机制以及LA在不同生物体中支持的生物功能，以回答为什么在不同无脊椎动物群体的进化过程中LA合成会丢失并反复获得的问题。我们提出了几个假设，并从现有文献中收集数据，以确定哪些因素在系统发育框架内促进 LA 合成。我们还没有发现我们提出的假设和 LA 合成之间存在明确的联系；因此，我们认为 LA 的合成可能是通过去饱和酶的双功能来促进的，或者是通过不同选择压力的组合来进化的。