Collagen Sequence Analysis of the Extinct Giant Ground Sloths Lestodon and Megatherium

For over 200 years, fossils of bizarre extinct creatures have been described from the Americas that have ranged from giant ground sloths to the ‘native’ South American ungulates, groups of mammals that evolved in relative isolation on South America. Ground sloths belong to the South American xenarthrans, a group with modern although morphologically and ecologically very different representatives (anteaters, armadillos and sloths), which has been proposed to be one of the four main eutherian clades. Recently, proteomics analyses of bone collagen have recently been used to yield a molecular phylogeny for a range of mammals including the unusual ‘Malagasy aardvark’ shown to be most closely related to the afrotherian tenrecs, and the south American ungulates supporting their morphological association with condylarths. However, proteomics results generate partial sequence information that could impact upon the phylogenetic placement that has not been appropriately tested. For comparison, this paper examines the phylogenetic potential of proteomics-based sequencing through the analysis of collagen extracted from two extinct giant ground sloths, Lestodon and Megatherium. The ground sloths were placed as sister taxa to extant sloths, but with a closer relationship between Lestodon and the extant sloths than the basal Megatherium. These results highlight that proteomics methods could yield plausible phylogenies that share similarities with other methods, but have the potential to be more useful in fossils beyond the limits of ancient DNA survival.两百余年来，美洲地区已有多类奇异已灭绝生物的化石被研究报道，类群涵盖巨型大地懒，以及在南美相对独立演化的原生南美有蹄类等哺乳动物类群。大地懒隶属于南美异关节总目（Xenarthra），该类群的现生代表为食蚁兽、犰狳与树懒——尽管这些现生类群在形态与生态上均差异显著，但异关节总目曾被认为是四大真兽类主要演化支之一。近年来，研究人员通过骨胶原蛋白质组学（proteomics）分析，为多类哺乳动物构建了分子系统发育树：其中包括被证实与非洲兽总目马岛猬类亲缘关系最近的奇异“马达加斯加土豚”，以及支持形态学研究将南美有蹄类归入踝节目（condylarths）类群的相关类群。然而，蛋白质组学分析仅能获取部分序列信息，其对物种系统发育位置的影响尚未得到充分验证。为开展对比研究，本研究通过分析两具已灭绝巨型大地懒（Lestodon与Megatherium）体内提取的骨胶原，探讨了基于蛋白质组学测序的系统发育分析潜力。研究结果显示，大地懒类与现生树懒构成姊妹群，其中Lestodon与现生树懒的亲缘关系比基干类群Megatherium更近。本研究结果表明，蛋白质组学方法可构建出与其他分析方法结果相近的可靠系统发育树，且在超出古DNA保存极限的化石样本中具备更广阔的应用前景。