Wendel Jonathan F, Stewart James McD, Rettig J H
Department of Botany, Iowa State University, Ames, IA, 50011, USA.
Department of Agronomy, University of Arkansas, Fayetteville, AR, 72701, USA.
Evolution. 1991 May;45(3):694-711. doi: 10.1111/j.1558-5646.1991.tb04339.x.
Interspecific hybridization and introgression are important evolutionary processes in plants, but their full significance with respect to speciation at the diploid level remains unresolved. In this study, molecular markers from the plastid and nuclear genomes were used to document an unusual evolutionary history of Gossypium bickii Prokh. (Malvaceae). This species is one of three morphologically similar Australian cottons (along with G. austrate F. Muell. and G. nelsonii Fryx.) in section Hibiscoidea. In contrast to expectations based on previous morphological data, cladistic analysis of maternally inherited cpDNA restriction site mutations unites G. bickii with G. sturtianum J. H. Willis, a morphologically distant species in a different taxonomic section (Sturtia). Few restriction site mutations distinguish the plastomes of G. bickii and G. sturtianum, but these two cpDNAs are differentiated from those of G. australe and G. nelsonii by a minimum of 33 mutations (out of 640 sites scored). These two highly distinct clades are not supported by phylogenetic analyses of allozyme markers (from 58 populations) and restriction site mutations in nuclear ribosomal DNAs. Rather, phylogenies based on 83 nuclear markers indicate that G. bickii shares a more recent common ancestor with G. australe and G. nelsonii than it does with G. sturtianum. We suggest that the striking discrepancy between independent molecular phylogenies from two different genomes indicates a biphyletic ancestry of G. bickii. Our preferred hypothesis involves an ancient hybridization, in which G. sturtianum, or a similar species, served as the maternal parent with a paternal donor from the lineage leading to G. australe and G. nelsonii. Because we detected no G. sturtianum nuclear genes in G. bickii, we suggest that the nuclear genomic contribution of the maternal parent was subsequently eliminated from the hybrid or its descendent maternal lineage. Several possible mechanisms of cytoplasm transfer are suggested, including repeated backcrossing of the hybrid, as female, into the paternal donor lineage, selection against recombinant nuclear genomes and a form of apomixis known as semigamy. This example, and several others in Gossypium as well as other genera, attest to the evolutionary possibility of interspecific cytoplasmic transfer, and perhaps the origin of diploid species via reticulate speciation. In addition, this study offers an example of natural cytoplasmic introgression without long-term survival of nuclear genes from the maternal progenitor.
种间杂交和基因渐渗是植物中重要的进化过程，但其在二倍体水平上对于物种形成的全部意义仍未得到解决。在本研究中，利用来自质体和核基因组的分子标记来记录比克氏棉（Gossypium bickii Prokh.）（锦葵科）不同寻常的进化历史。该物种是棉属刺棉亚属（Hibiscoidea）中形态相似的三种澳大利亚棉花之一（另外两种是澳棉（G. austrate F. Muell.）和尼尔森氏棉（G. nelsonii Fryx.））。与基于先前形态学数据的预期相反，对母系遗传的叶绿体DNA限制性位点突变进行的分支分析将比克氏棉与斯特蒂棉（G. sturtianum J. H. Willis）归为一类，而斯特蒂棉在形态上差异较大，属于不同的分类亚属（Sturtia）。区分比克氏棉和斯特蒂棉质体基因组的限制性位点突变很少，但这两种叶绿体DNA与澳棉和尼尔森氏棉的叶绿体DNA相比，至少有33个突变（在640个计分位点中）。这两个高度不同的分支并不得到对58个群体的等位酶标记和核糖体DNA限制性位点突变进行的系统发育分析的支持。相反，基于83个核标记的系统发育表明，比克氏棉与澳棉和尼尔森氏棉共享的最近共同祖先比它与斯特蒂棉的更近。我们认为，来自两个不同基因组的独立分子系统发育之间的显著差异表明比克氏棉具有双系祖先。我们倾向于的假说是涉及一次古老的杂交，其中斯特蒂棉或一个类似物种作为母本，父本供体来自导致澳棉和尼尔森氏棉的谱系。因为我们在比克氏棉中未检测到斯特蒂棉的核基因，所以我们认为母本的核基因组贡献随后从杂种或其后代母系中被消除了。提出了几种可能的细胞质转移机制，包括杂种作为雌性反复回交到父本供体系谱中、对重组核基因组的选择以及一种称为半融合的无融合生殖形式。这个例子以及棉属和其他属中的其他几个例子证明了种间细胞质转移以及通过网状物种形成产生二倍体物种的进化可能性。此外，本研究提供了一个自然细胞质渐渗的例子，其中母本祖先的核基因没有长期存活。
