Spirostomum is a widely distributed heterotrichean genus composed of well-known species with described ecology and phylogenetic affinities. The morphological classification of Spirostomum species is mostly based on the body size/shape, number of cortical granule rows and macronuclear characteristics. These features along with molecular phylogenies based on ribosomal genes divide the genus into two phylogroups, one including species with a compact macronucleus, and another including species with a moniliform macronucleus. Here, we present our observations on atypical Spirostomum specimens with unusually two distinct macronuclei and shortened adoral zone of membranelles. These atypical forms appeared in the cultures of S. minus and S. yagiui, sampled at different sites in South America (Chile and Brazil) and associated with unrelated substrate types. Morphological observations of living and stained cells, 18S rRNA gene analyses, and a thorough investigation of the literature suggest that the atypical phenotype may be a result of uncommon pathways during the conjugative process. Thus, we demonstrate that studies of ciliate natural populations and their morphological variations, especially from undersampled biogeographical regions, can reveal the boundaries of widely used morphological characters for Spirostomum taxonomy and species identification.

Spirostomum是一种广泛分布的异毛类属，由具有描述的生态学和系统发育亲缘关系的知名物种组成。Spirostomum物种的形态分类主要基于体型/形状、皮质颗粒行的数量和大核特征。这些特征以及基于核糖体基因的分子系统发育将该属分为两个系统群，一个包括具有紧密大核的物种，另一个包括具有念珠状大核的物种。在这里，我们展示了我们对非典型Spirostomum标本的观察结果，这些标本具有不同寻常的两个不同的大核和缩短的膜贴壁区。这些非典型形式出现在S. minus和S. yagiui，在南美洲（智利和巴西）的不同地点取样，与不相关的底物类型相关。活细胞和染色细胞的形态学观察、18S rRNA 基因分析以及对文献的彻底调查表明，非典型表型可能是接合过程中不常见途径的结果。因此，我们证明纤毛虫自然种群及其形态变异的研究，特别是来自欠采样生物地理区域的研究，可以揭示广泛用于螺口虫分类和物种鉴定的形态特征的边界。