Phylogeny and evolution of the epiphytic Rhipsalideae (Cactaceae)

Abstract

Cactaceae are one of the major floristic components of the New World’s arid as well as seasonally moist tropical regions and at the same time one of the most popular plant families in horticulture. The taxonomic units (tribes, genera) and species limits in the Cactaceae have been difficult to define due to intergrading vegetative characters, pheno-typic plasticity and the largely uniform flower morphology. Molecular phylogenetic studies so far yielded largely unresolved trees, so relationships within Cactaceae remain in-sufficiently understood. <br /> This study focuses on the Rhipsalideae, a predominantly epiphytic Cactaceae tribe from the tropical rainforests of South -and Mesoamerica. The Rhipsalideae have not been subject of a phylogenetic study so far but are well-suited for this purpose: they are a comparatively small group and most of the taxa are well known morphologically and thus allow for the clear determination of Operational Taxonomic Units (OTUs). The Rhipsalideae are well represented in botanical collections so all but one species were available for this study. This study is therefore among the most comprehensive species-level-study for the Cactaceae. <br /> Aims: resolving species-level relationships, get better insights into species limits, find morphological characters synapomorphic or at least characteristic for the newly found clades. <br /> The phylogenetic relationships were addressed using sequence data from rapidly evolving plastid spacers (psbA-trnH, rps3-rpl16, trnS-trnG, trnQ-rps16), group II introns (trnK, rpl16, trnG) and the coding regions matK. The sequence data were analysed using Maximum Parsimony, Bayesian Inference and haplotype network construction.<br /> First, the position and circumscription of the genus Pfeiffera has been addressed. It had formerly been included in Lepismium abut there was already evidence that Pfeiffera is not part the Rhipsalideae. The phylogenetic analyses revealed Pfeiffera polyphyletic, comprising two unrelated lineages, both well resolved and highly supported. One clade includes the type species, P. ianthothele; the second contains two Pfeiffera and an erstwhile Lepismium species. The results justify generic status for this newly found clade. It includes the type species of the earlier-proposed monotypic genus Lyman¬bensonia and therefore its reinstatement is proposed in an amplified circumscription. The results further provide evidence that epiphytism evolved more frequently in Cactaceae than hitherto assumed. The taxonomic and nomen¬clatural consequences are the establishment of a separate tribe Lyman¬bensonieae, formally proposed here, to contain the genera Lyman¬bensonia and Calym¬manthium. <br /> To resolve phylogenetic relationships in the Rhipsalideae, a dataset of six fast evolving plastid regions was generated with c. 4200 nucleotides sequenced per sample for 120 accessions. The regions used were evaluated for their phylogenetic performance and species discrimination power for DNA based species recognition (DNA barcoding) based on beforehand defined operational taxonomic units (OTUs). <br /> The Rhipsalideae were found as monophyletic and contain five major clades that correspond to the genera Rhipsalis, Lepismium, Schlumbergera, Hatiora subg. Hatiora and Rhipsalidopsis. The relationships between the genera could not be clarified but otherwise the trees were well resolved down to species-level and well supported. Ninety-seven % of the OTUs could be successfully identified using c. 2500 nt. The rpl16 and trnK introns yielded the best phylogenetic signal and the best OTU identification potential. The phylogenetic performance of the markers was found to be not determined by the level of sequence variability. The species discrimination power of the markers did not always correlate with their phylogenetic performance.<br /> The reliable phylogenetic hypothesis for the Rhipsalideae provided a framework for a first detailed study of character evolution in a phylogenetic context. A matrix of 36 characters was compiled and ancestral states were reconstructed using a Bayesian approach. A focus was put on the characters associated with the epiphytic life form and the floral traits. The degree of homoplasy was found to high but many characters were homogenous within the clades. Rhipsalideae and all the highly supported clades found by the molecular phylogenetic analyses could also be defined morphologically. <br /> The distribution patterns of Rhipsalis baccifera, the most widespread cactus, were addressed using tree building methods and haplotype network algorithms based on the rps3-rpl16 spacer and the rpl16 intron. Two main groups of plastid haplotypes were found: a northern South American / Caribbean / Mesoamerican haplotype and an African haplotype. These results suggest a single dispersal of Rhipsalis baccifera to Africa and reveal high genetic diversity within its populations on both continents. To obtain further resolution among the populations, microsatellite markers for Rhipsalis baccifera have been deve-loped using 454 sequencing.<br /> The comprehensive taxon sampling and the usage of effective markers resulted in almost completely resolved species level trees which were hitherto hardly achieved in the Cactaceae. In order to resolve relationships between closely related species, com¬bined data sets of markers selected for their high phylo-genetic structure are needed.<br /> Morphological convergences are frequent in the Cactaceae. The level of homoplasy in morpho¬logical characters is high, especially regarding characters associated with an adaptation such as epiphytism. Generic classification based on single or few morphological characters consequently cannot predict actual relationships. For phylogenetic stu¬dies in the Cactaceae, the morphology-based taxonomic units consequently may be misleading to guide taxon sampling. The best solution therefore would be including all morphologically deviant groups and species in the given study. Nevertheless, the analysis of morphological characters in a phylogenetic context allowed finding synapomorphies or diagnostic characters for all the clades (genera, subgenera) found by the molecular phylogenetic analyses. An integrated approach using morphology and sequence data is therefore needed to establish sound generic limits in the Cactaceae.