Interdisciplinary Centre of Marine and Environmental Research (CIIMAR), CIMAR Associate Laboratory, University of Porto, Portugal.
BMC Evol Biol. 2011 May 19;11:132. doi: 10.1186/1471-2148-11-132.
Stearoyl-CoA desaturases (SCDs) are key enzymes involved in de novo monounsaturated fatty acid synthesis. They catalyze the desaturation of saturated fatty acyl-CoA substrates at the delta-9 position, generating essential components of phospholipids, triglycerides, cholesterol esters and wax esters. Despite being crucial for interpreting SCDs roles across species, the evolutionary history of the SCD gene family in vertebrates has yet to be elucidated, in particular their isoform diversity, origin and function. This work aims to contribute to this fundamental effort.
We show here, through comparative genomics and phylogenetics that the SCD gene family underwent an unexpectedly complex history of duplication and loss events. Paralogy analysis hints that SCD1 and SCD5 genes emerged as part of the whole genome duplications (2R) that occurred at the stem of the vertebrate lineage. The SCD1 gene family expanded in rodents with the parallel loss of SCD5 in the Muridae family. The SCD1 gene expansion is also observed in the Lagomorpha although without the SCD5 loss. In the amphibian Xenopus tropicalis we find a single SCD1 gene but not SCD5, though this could be due to genome incompleteness. In the analysed teleost species no SCD5 is found, while the surrounding SCD5-less locus is conserved in comparison to tetrapods. In addition, the teleost SCD1 gene repertoire expanded to two copies as a result of the teleost specific genome duplication (3R). Finally, we describe clear orthologues of SCD1 and SCD5 in the chondrichthian, Scyliorhinus canicula, a representative of the oldest extant jawed vertebrate clade. Expression analysis in S. canicula shows that whilst SCD1 is ubiquitous, SCD5 is mainly expressed in the brain, a pattern which might indicate an evolutionary conserved function.
We conclude that the SCD1 and SCD5 genes emerged as part of the 2R genome duplications. We propose that the evolutionary conserved gene expression between distinct lineages underpins the importance of SCD activity in the brain (and probably the pancreas), in a yet to be defined role. We argue that an expression independent of an external stimulus, such as diet induced activity, emerged as a novel function in vertebrate ancestry allocated to the SCD5 isoform in various tissues (e.g. brain and pancreas), and it was selectively maintained throughout vertebrate evolution.
硬脂酰辅酶 A 去饱和酶（SCDs）是从头合成单不饱和脂肪酸的关键酶。它们催化饱和脂肪酸辅酶 A 底物在 δ-9 位的去饱和，生成磷脂、甘油三酯、胆固醇酯和蜡酯的必需成分。尽管 SCD 对于解释物种间的作用至关重要，但脊椎动物 SCD 基因家族的进化历史尚未阐明，特别是它们的同工型多样性、起源和功能。这项工作旨在为此做出贡献。
通过比较基因组学和系统发育分析，我们在这里表明 SCD 基因家族经历了一个出乎意料的复杂的复制和丢失事件的历史。并分析提示 SCD1 和 SCD5 基因是脊椎动物谱系 2R 全基因组复制的一部分。SCD1 基因家族在啮齿动物中扩张，而在鼠科家族中 SCD5 平行丢失。SCD1 基因的扩张也发生在兔形目动物中，尽管没有 SCD5 的丢失。在分析的两栖动物非洲爪蟾中，我们只发现了一个 SCD1 基因，但没有 SCD5，尽管这可能是由于基因组不完整。在所分析的硬骨鱼类中，没有发现 SCD5，而周围的 SCD5 缺失基因座与四足动物相比是保守的。此外，由于硬骨鱼特异性基因组复制（3R），硬骨鱼 SCD1 基因库扩展到两个拷贝。最后，我们在软骨鱼鲨属中描述了 SCD1 和 SCD5 的明确直系同源物，鲨属是最古老的现存有颌脊椎动物分支的代表。S. canicula 的表达分析表明，虽然 SCD1 是普遍存在的，但 SCD5 主要在大脑中表达，这种模式可能表明其具有进化保守的功能。
我们得出结论，SCD1 和 SCD5 基因是 2R 基因组复制的一部分。我们提出，不同谱系之间保守的基因表达支持 SCD 活性在大脑（可能还有胰腺）中的重要性，其作用尚待确定。我们认为，一种不受外部刺激（如饮食诱导的活性）影响的表达，作为一个新的功能出现在脊椎动物祖先中，分配给各种组织（如大脑和胰腺）中的 SCD5 同工型，并在整个脊椎动物进化过程中被选择性地保留。
