Long Hongan, Zufall Rebecca A
Department of Biology and Biochemistry, University of Houston, Houston, Texas, 77204-5001; Department of Biology, Indiana University, Bloomington, Indiana, 47405.
J Eukaryot Microbiol. 2015 Mar-Apr;62(2):249-54. doi: 10.1111/jeu.12174. Epub 2014 Oct 13.
Ciliate nuclear architecture, in particular the sequestration of a transcriptionally silent germline genome, allows for the accumulation of mutations that are "hidden" from selection during many rounds of asexual reproduction. After sexual conjugation, these mutations are expressed, potentially resulting in highly variable phenotypes. Morphological traits are widely used in ciliate taxonomy, however, the extent to which the values of these traits are robust to change in the face of mutation remains largely unknown. In this study, we examine the effects of mutations accumulated in the germline genome to test the mutational robustness of four traits commonly used in ciliate morphological taxonomy (number of somatic kineties, number of postoral kineties, macronuclear size, and cell size). We found that the number of postoral kineties is robust to mutation, confirming that it should be preferentially used in taxonomy. By contrast, we found that, as in other unicellular and multicellular species, cell and macronucleus sizes change in response to mutation. Thus, we argue that cell and macronucleus sizes, which are widely used in taxonomy, should be treated cautiously for species identification. Finally, we found evidence of correlations between cell and macronucleus sizes and fitness, suggesting possible mutational pleiotropy. This study demonstrates the importance of, and methods for, determining mutational robustness to guide morphological taxonomy in ciliates.
纤毛虫的核结构，特别是转录沉默的种系基因组的隔离，使得在多轮无性繁殖过程中积累的突变能够“隐藏”起来不被选择。有性接合后，这些突变会表达出来，可能导致高度可变的表型。形态特征在纤毛虫分类学中被广泛使用，然而，面对突变时这些特征值的稳健程度在很大程度上仍然未知。在本研究中，我们研究了种系基因组中积累的突变的影响，以测试纤毛虫形态分类学中常用的四个特征（体动基列数、口后动基列数、大核大小和细胞大小）的突变稳健性。我们发现口后动基列数对突变具有稳健性，证实了它在分类学中应优先使用。相比之下，我们发现，与其他单细胞和多细胞物种一样，细胞和大核大小会因突变而发生变化。因此，我们认为，在分类学中广泛使用的细胞和大核大小在物种鉴定时应谨慎对待。最后，我们发现了细胞和大核大小与适应性之间存在相关性的证据，这表明可能存在突变多效性。本研究证明了确定突变稳健性以指导纤毛虫形态分类学的重要性和方法。
