Mallatt J, Sullivan J
Department of Zoology, Washington State University, Pullman 99164-4236, USA.
Mol Biol Evol. 1998 Dec;15(12):1706-18. doi: 10.1093/oxfordjournals.molbev.a025897.
Resolving the interrelationships of three major extant lineages of vertebrates (hagfishes, lampreys, and gnathostomes) is a particularly important issue in evolution, because the basal resolution critically influences our understanding of primitive vertebrate characters. A consensus has emerged over the last 20 years that lampreys are the sister group to the gnathostomes and the hagfishes represent an ancient, basal lineage. This hypothesis has essentially displaced the classical hypothesis of monophyly of the cyclostomes (lampreys plus hagfishes). To test these hypotheses, we compared nearly complete ribosomal DNA sequences from each of these major lineages, as well as those from a cephalochordate and a urochordate, which represent a paraphyletic outgroup for assessing the basal vertebrate relationships. For this comparison, 92%-99% complete 28S rDNA sequences were obtained from the lancelet Branchiostoma floridae, the hagfish Eptatretus stouti, the lamprey Petromyzon marinus, and cartilaginous fishes Hydrolagus colliei and Squalus acanthias and were then analyzed with previously reported 28S and 18S rDNA sequences from other chordates. We conducted conventional (nonparametric) bootstrap analyses, under maximum-likelihood, parsimony, and minimum-evolution (using LogDet distances) criteria, of both 28S and 18S rDNA sequences considered separately and combined. All these analyses provide moderate to very strong support for the monophyly of the cyclostomes. Furthermore, the currently accepted hypothesis of a lamprey-gnathostome clade is moderately rejected by the Kishino-Hasegawa test (P = 0.099) and resoundingly rejected by parametric bootstrap tests (P < 0.01) in favor of monophyly of living cyclostomes. Another significant finding is that the hagfish E. stouti has the longest 28S rDNA gene known in any organism (> 5,200 nt).
解析现存的三大主要脊椎动物谱系（盲鳗、七鳃鳗和有颌类）之间的相互关系是进化领域一个尤为重要的问题，因为基础解析对我们理解原始脊椎动物特征至关重要。在过去20年里，已形成一种共识，即七鳃鳗是有颌类的姐妹群，而盲鳗代表一个古老的基础谱系。这一假说已基本取代了圆口纲（七鳃鳗加盲鳗）单系性的经典假说。为验证这些假说，我们比较了这些主要谱系中每个谱系近乎完整的核糖体DNA序列，以及来自头索动物和尾索动物的序列，头索动物和尾索动物代表一个并系外类群，用于评估基础脊椎动物的关系。为进行此比较，从佛罗里达文昌鱼、斯氏黏盲鳗、海七鳃鳗、银鲛和棘鲨中获得了92% - 99%完整的28S rDNA序列，然后与先前报道的其他脊索动物的28S和18S rDNA序列进行分析。我们分别对28S和18S rDNA序列以及二者组合进行了常规（非参数）自展分析，分析依据最大似然法、简约法和最小进化法（使用对数行列式距离）标准。所有这些分析为圆口纲的单系性提供了中等至非常有力的支持。此外，目前被接受的七鳃鳗 - 有颌类分支假说在木村 - 长谷川检验中被中等程度拒绝（P = 0.099），在参数自展检验中被彻底拒绝（P < 0.01），支持现存圆口纲的单系性。另一个重要发现是，斯氏黏盲鳗的28S rDNA基因是已知所有生物中最长的（> 5200 nt）。
