Meredith Robert W, Gatesy John, Murphy William J, Ryder Oliver A, Springer Mark S
Department of Biology, University of California Riverside, Riverside, California, United States of America.
PLoS Genet. 2009 Sep;5(9):e1000634. doi: 10.1371/journal.pgen.1000634. Epub 2009 Sep 4.
Vestigial structures occur at both the anatomical and molecular levels, but studies documenting the co-occurrence of morphological degeneration in the fossil record and molecular decay in the genome are rare. Here, we use morphology, the fossil record, and phylogenetics to predict the occurrence of "molecular fossils" of the enamelin (ENAM) gene in four different orders of placental mammals (Tubulidentata, Pholidota, Cetacea, Xenarthra) with toothless and/or enamelless taxa. Our results support the "molecular fossil" hypothesis and demonstrate the occurrence of frameshift mutations and/or stop codons in all toothless and enamelless taxa. We then use a novel method based on selection intensity estimates for codons (omega) to calculate the timing of iterated enamel loss in the fossil record of aardvarks and pangolins, and further show that the molecular evolutionary history of ENAM predicts the occurrence of enamel in basal representatives of Xenarthra (sloths, anteaters, armadillos) even though frameshift mutations are ubiquitous in ENAM sequences of living xenarthrans. The molecular decay of ENAM parallels the morphological degeneration of enamel in the fossil record of placental mammals and provides manifest evidence for the predictive power of Darwin's theory.
痕迹结构在解剖学和分子水平上均有出现，但记录化石记录中形态退化与基因组中分子衰变同时发生的研究却很罕见。在此，我们利用形态学、化石记录和系统发育学来预测牙釉蛋白（ENAM）基因的“分子化石”在四种不同目胎盘哺乳动物（管齿目、鳞甲目、鲸目、异关节目）中无牙和/或无釉质类群中的出现情况。我们的结果支持“分子化石”假说，并证明在所有无牙和无釉质类群中都存在移码突变和/或终止密码子。然后，我们使用一种基于密码子选择强度估计值（ω）的新方法来计算土豚和穿山甲化石记录中反复出现的釉质缺失时间，并进一步表明，尽管移码突变在现存异关节类动物的ENAM序列中普遍存在，但ENAM的分子进化史预测了异关节目（树懒、食蚁兽、犰狳）基部代表动物中釉质的出现。ENAM的分子衰变与胎盘哺乳动物化石记录中釉质的形态退化并行，并为达尔文理论的预测能力提供了明显证据。
