McNeal Joel R, Arumugunathan Kathiravetpilla, Kuehl Jennifer V, Boore Jeffrey L, Depamphilis Claude W
Department of Plant Biology, University of Georgia, Athens, GA 30602, USA.
BMC Biol. 2007 Dec 13;5:55. doi: 10.1186/1741-7007-5-55.
The genus Cuscuta L. (Convolvulaceae), commonly known as dodders, are epiphytic vines that invade the stems of their host with haustorial feeding structures at the points of contact. Although they lack expanded leaves, some species are noticeably chlorophyllous, especially as seedlings and in maturing fruits. Some species are reported as crop pests of worldwide distribution, whereas others are extremely rare and have local distributions and apparent niche specificity. A strong phylogenetic framework for this large genus is essential to understand the interesting ecological, morphological and molecular phenomena that occur within these parasites in an evolutionary context.
Here we present a well-supported phylogeny of Cuscuta using sequences of the nuclear ribosomal internal transcribed spacer and plastid rps2, rbcL and matK from representatives across most of the taxonomic diversity of the genus. We use the phylogeny to interpret morphological and plastid genome evolution within the genus. At least three currently recognized taxonomic sections are not monophyletic and subgenus Cuscuta is unequivocally paraphyletic. Plastid genes are extremely variable with regards to evolutionary constraint, with rbcL exhibiting even higher levels of purifying selection in Cuscuta than photosynthetic relatives. Nuclear genome size is highly variable within Cuscuta, particularly within subgenus Grammica, and in some cases may indicate the existence of cryptic species in this large clade of morphologically similar species.
Some morphological characters traditionally used to define major taxonomic splits within Cuscuta are homoplastic and are of limited use in defining true evolutionary groups. Chloroplast genome evolution seems to have evolved in a punctuated fashion, with episodes of loss involving suites of genes or tRNAs followed by stabilization of gene content in major clades. Nearly all species of Cuscuta retain some photosynthetic ability, most likely for nutrient apportionment to their seeds, while complete loss of photosynthesis and possible loss of the entire chloroplast genome is limited to a single small clade of outcrossing species found primarily in western South America.
菟丝子属（旋花科）植物，通常被称为菟丝子，是附生藤本植物，通过吸器状取食结构在接触点侵入寄主植物的茎。尽管它们没有展开的叶片，但一些物种明显含有叶绿素，尤其是幼苗期和果实成熟期。一些物种被报道为分布于全球的作物害虫，而其他一些则极为罕见，具有局部分布且明显的生态位特异性。对于这个大属而言，一个强有力的系统发育框架对于在进化背景下理解这些寄生植物中发生的有趣生态、形态和分子现象至关重要。
在此，我们利用核糖体核糖核酸内转录间隔区序列以及来自该属大部分分类多样性代表的质体rps2、rbcL和matK序列，构建了一个得到充分支持的菟丝子系统发育树。我们利用该系统发育树来解释该属内的形态和质体基因组进化。目前至少有三个被认可的分类组并非单系类群，且菟丝子亚属明确为并系类群。质体基因在进化限制方面极具变异性，与光合相关物种相比，rbcL在菟丝子中表现出更高水平的纯化选择。菟丝子属内核基因组大小变化很大，尤其是在 Grammica 亚属内，在某些情况下可能表明在这个形态相似物种的大分支中存在隐存种。
一些传统上用于定义菟丝子属内主要分类划分的形态特征是同塑性的，在定义真正的进化类群方面用途有限。叶绿体基因组进化似乎是以间断的方式进行的，伴随着基因或tRNA组的丢失事件，随后主要分支中的基因含量趋于稳定。几乎所有菟丝子物种都保留了一定的光合能力，很可能是为了将养分分配给种子，而光合作用的完全丧失以及整个叶绿体基因组的可能丢失仅限于主要分布在南美洲西部的一个杂交物种小分支。
