Fabry S, Köhler A, Coleman A W
Universität Regensburg, Genetik, D-93040 Regensburg, Germany.
J Mol Evol. 1999 Jan;48(1):94-101. doi: 10.1007/pl00006449.
The morphologically uniform species Gonium pectorale is a colonial green flagellate of worldwide distribution. The affinities of 25 isolates from 18 sites on five continents were assessed by both DNA sequence comparisons and sexual compatibility. Complete sequences were obtained (i) for the internal transcribed spacer ITS-1 and ITS-2 regions of ribosomal DNA and (ii) for each of three single-copy spliceosomal introns, two in a small G protein and one in the actin gene. ITS sequences appeared to homogenize sufficiently rapidly to behave as a single copy gene. Intron sequence differences between isolates in this species reached nucleotide substitution saturation, while ITS sequences did not. Parsimony and evolutionary distance analysis of the two types of DNA data gave essentially the same tree conformation. By all these criteria, the group of G. pectorale isolates fell into two main clades, A and B. Clade A, with isolates from four continents, was comprised of four subclades of quite closely related isolates, plus one strain of ambiguous affinity. Clade B was comprised of two subclades represented by South African and South American isolates, respectively; thus, only subclades of clade B showed geographical localization. With respect to mating, all isolates except one homothallic strain and one apparently sterile strain fell into either one or the other of two mating types. Pairings in all possible combinations revealed that isolates from the same site formed abundant zygotes, which germinated to produce new, sexually active organisms. Zygotes were also formed in many pairings of other combinations, including crosses of clade A with clade B organisms, but none of the latter produced viable germlings. The ability to mate and produce viable progeny that were themselves capable of sexual reproduction was restricted to members of subclades established on the basis of DNA sequence similarities. Thus, the grades of difference in both nuclear intron sequences and rDNA ITS sequences paralleled those observed in the sexual analysis.
形态上一致的胸状盘藻（Gonium pectorale）是一种分布于全球的群体绿藻鞭毛虫。通过DNA序列比较和有性兼容性评估了来自五大洲18个地点的25个分离株的亲缘关系。获得了核糖体DNA的内部转录间隔区ITS - 1和ITS - 2区域的完整序列，以及三个单拷贝剪接体内含子的序列，其中两个位于小G蛋白中，一个位于肌动蛋白基因中。ITS序列似乎足够快地同质化，表现为单拷贝基因。该物种分离株之间的内含子序列差异达到核苷酸替代饱和，而ITS序列没有。对这两种类型的DNA数据进行简约分析和进化距离分析得到了基本相同的树形结构。根据所有这些标准，胸状盘藻分离株群体分为两个主要分支，A和B。分支A包含来自四大洲的分离株，由四个亲缘关系相当密切的亚分支以及一个亲缘关系不明确的菌株组成。分支B由分别以南美和南非分离株为代表的两个亚分支组成；因此，只有分支B的亚分支显示出地理定位。关于交配，除了一个同宗配合菌株和一个明显不育的菌株外，所有分离株都属于两种交配类型之一。所有可能组合的配对显示，来自同一地点的分离株形成了大量合子，这些合子萌发产生新的、有性活性的生物体。合子也在许多其他组合的配对中形成，包括A分支与B分支生物体的杂交，但后者都没有产生可存活的幼苗。交配并产生本身能够进行有性繁殖的可存活后代的能力仅限于基于DNA序列相似性建立的亚分支成员。因此，核内含子序列和rDNA ITS序列的差异等级与有性分析中观察到的差异等级平行。
