Abstract
BackgroundThe 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.ResultsHere 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.ConclusionSome 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.
Highlights
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
Many of the support values are high for individual genes and almost every node is very well supported in the combined analysis
Maximum-likelihood analyses were performed on the individual gene datasets; these analyses gave nearly congruent topologies that agreed at well-supported in-group nodes (Figure 3)
Summary
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. (Convolvulaceae), commonly known as dodders, are epiphytic vines that invade the stems of their host with haustorial feeding structures at the points of contact. They lack expanded leaves, some species are noticeably chlorophyllous, especially as seedlings and in maturing fruits. Between 150 and 200 species of Cuscuta have been described, and they are distributed widely on every continent except Antarctica [1] These parasites have no roots at maturity and their leaves are reduced to minute scales. The last full monograph of the genus completed by Yuncker [1] recognized nine species in Monogyna, distributed primarily in Eurasia and Africa with one species, Cuscuta exaltata Engelmann, having a disjunctive distribution in the southern United States in the scrub habitat of Florida and Texas. Subgenus Grammica, with 121 species recognized by Yuncker, is almost completely limited to the New World, with a handful of exceptions in Asia, Africa and the Pacific islands, including Tasmania and Australia
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