Kodner Robin B, Summons Roger E, Pearson Ann, King Nicole, Knoll Andrew H
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA.
Proc Natl Acad Sci U S A. 2008 Jul 22;105(29):9897-902. doi: 10.1073/pnas.0803975105. Epub 2008 Jul 15.
Molecular clocks suggest that animals originated well before they first appear as macroscopic fossils, but geologic tests of these hypotheses have been elusive. A rare steroid hydrocarbon, 24-isopropylcholestane, has been hypothesized to be a biomarker for sponges or their immediate ancestors because of its relatively high abundance in pre-Ediacaran to Early Cambrian sedimentary rocks and oils. Biolipid precursors of this sterane have been reported to be prominent in several demosponges. Whether 24-isopropylcholestane can be interpreted as a sponge (and, hence, animal) biomarker, and so provide clues about early metazoan history, depends on an understanding of the distribution of sterol biosynthesis among animals and their protistan relatives. Accordingly, we characterized the sterol profile of the choanoflagellate Monosiga brevicollis, a representative of the unicellular sister group of animals. M. brevicollis does not produce a candidate sterol precursor for 24-isopropylcholestane under our experimental growth conditions. It does, however, produce a number of other sterols, and comparative genomics confirms its biosynthetic potential to produce the full suite of compounds recovered. Consistent with the phylogenetic position of choanoflagellates, the sterol profile and biosynthetic pathway of M. brevicollis display characteristics of both fungal and poriferan sterol biosynthesis. This is an example in which genomic and biochemical information have been used together to investigate the taxonomic specificity of a fossil biomarker.
分子钟表明，动物的起源时间远早于它们首次以宏观化石形式出现的时间，但对这些假说的地质验证一直难以实现。一种罕见的类固醇碳氢化合物，24-异丙基胆甾烷，由于其在前埃迪卡拉纪至早寒武纪沉积岩和石油中相对较高的丰度，被推测是海绵或其直接祖先的生物标志物。据报道，这种甾烷的生物脂质前体在几种海绵纲动物中很突出。24-异丙基胆甾烷是否可以被解释为海绵（进而为动物）的生物标志物，从而为早期后生动物的历史提供线索，取决于对甾醇生物合成在动物及其原生生物亲属中的分布情况的了解。因此，我们对领鞭毛虫短柄单歧藻的甾醇谱进行了表征研究，领鞭毛虫是动物单细胞姐妹群的代表。在我们的实验生长条件下，短柄单歧藻不会产生24-异丙基胆甾烷的候选甾醇前体。然而，它确实会产生一些其他甾醇，比较基因组学证实了其生物合成全套已回收化合物的潜力。与领鞭毛虫的系统发育位置一致，短柄单歧藻的甾醇谱和生物合成途径显示出真菌和海绵甾醇生物合成的特征。这是一个将基因组和生化信息结合起来研究化石生物标志物分类特异性的例子。
