Is Drosera meristocaulis a pygmy sundew? Evidence of a long-distance dispersal between Western Australia and northern South America

BACKGROUND AND AIMS

South America and Oceania possess numerous floristic similarities, often confirmed by morphological and molecular data. The carnivorous Drosera meristocaulis (Droseraceae), endemic to the Neblina highlands of northern South America, was known to share morphological characters with the pygmy sundews of Drosera sect. Bryastrum, which are endemic to Australia and New Zealand. The inclusion of D. meristocaulis in a molecular phylogenetic analysis may clarify its systematic position and offer an opportunity to investigate character evolution in Droseraceae and phylogeographic patterns between South America and Oceania.

METHODS

Drosera meristocaulis was included in a molecular phylogenetic analysis of Droseraceae, using nuclear internal transcribed spacer (ITS) and plastid rbcL and rps16 sequence data. Pollen of D. meristocaulis was studied using light microscopy and scanning electron microscopy techniques, and the karyotype was inferred from root tip meristem.

KEY RESULTS

The phylogenetic inferences (maximum parsimony, maximum likelihood and Bayesian approaches) substantiate with high statistical support the inclusion of sect. Meristocaulis and its single species, D. meristocaulis, within the Australian Drosera clade, sister to a group comprising species of sect. Bryastrum. A chromosome number of 2n = approx. 32-36 supports the phylogenetic position within the Australian clade. The undivided styles, conspicuous large setuous stipules, a cryptocotylar (hypogaeous) germination pattern and pollen tetrads with aperture of intermediate type 7-8 are key morphological traits shared between D. meristocaulis and pygmy sundews of sect. Bryastrum from Australia and New Zealand.

CONCLUSIONS

The multidisciplinary approach adopted in this study (using morphological, palynological, cytotaxonomic and molecular phylogenetic data) enabled us to elucidate the relationships of the thus far unplaced taxon D. meristocaulis. Long-distance dispersal between southwestern Oceania and northern South America is the most likely scenario to explain the phylogeographic pattern revealed.

Molecular phylogeny and character evolution of carnivorous plant families in Caryophyllales--revisited

Recent phylogenetic analyses based on single gene and combined data sets have substantially increased our knowledge of the phylogeny of Caryophyllales s.l., indicating that additional carnivorous families are related to this alliance. In earlier contributions towards a reassessment of inter- and infrafamilial relationships slowly evolving genes had been preferred for phylogenetic inference. The resulting tree topologies based on rbcL and 18S rDNA, however, were characterized by limited resolution, low internal support and topological incongruence. Therefore genomic regions evolving more rapidly have been used in subsequent studies. Comparative sequencing of the matK gene and the flanking trnK intron region as well as combined analyses based on plastid matK, atpB, rbcL, and nuclear 18S rDNA have effectively improved resolution and internal support. Tree topologies revealed Caryophyllales s.l. as monophyletic group and indicated a clear division into two sister clades, the "core" and the "non-core" Caryophyllales (with Rhabdodendraceae and Simmondsiaceae with unclear affinities). Contrary to the "core" group (with Asteropeiaceae and Physenaceae as successive sister groups), which corresponds largely to the previous circumscription of the order, the monophyly of "non-core" Caryophyllales comprising Polygonaceae, Plumbaginaceae, Frankeniaceae, and Tamaricaceae along with the carnivorous families Droseraceae, Nepenthaceae, Drosophyllaceae, Dioncophyllaceae, and Ancistrocladaceae are a recent discovery. Based on reliable tree topologies it is hypothesized that pitfall traps of Nepenthes and snap traps typical for Aldrovanda and Dionaea were derived from a common ancestor with adhesive flypaper traps. With exception of Triphyophyllum carnivory was secondarily lost in the remaining Dioncophyllaceae (Dioncophyllum, Habropetalum) and all taxa of Ancistrocladaceae.

Introduction of a nuclear marker for phylogenetic analysis of Nepenthaceae

Nepenthaceae, the pitcher plants of the Old World tropics show a remarkable diversity in SE Asia, especially on the islands of Borneo and Sumatra. This region is considered as a secondary center of diversity. Sequence analysis of the cpDNA TRNK intron supports this hypothesis showing the species of the Malay Archipelago as neighbour group to the isolated species from Sri Lanka, the Seychelles, and Madagascar. Based on phylogenetic reconstructions an origin of recent Nepenthaceae in the Indian subcontinent is assumed. A recent investigation focused on a non-plastid, translocated copy of the TRNK intron has revealed an incongruence to tree topology based on the cpDNA TRNK intron. Although the translocated copy emerged as insufficient for phylogenetic reconstruction of Nepenthaceae some taxa showed, contrary to the cpDNA dataset, relatively high distances to the rest of the taxa. These results indicated that the phylogeny of the TRNK intron could not reflect true phylogenetic relationships. We investigated the peptide transferase 1 (PTR1), to develop a phylogenetic marker that is based on a nuclear low copy gene in Nepenthes. All sequences obtained were probably functional, indicated by the ratio of point mutations of the single codon positions in exon and intron regions. Comparative analysis showed that this locus is of similar variability as the cpDNA TRNK intron and, contrary to the translocated copy of TRNK, potential useful for phylogenetic reconstruction. While in parts congruent to the plastid TRNK intron phylogeny, a higher divergence of some sequences in PRT1 and in the previously reported, non cpDNA dataset indicates that remnants of an older species stock persisted east of Wallace's line and on the Sunda Shelf. This suggests that plastid haplotypes existing today in the main distribution center of the Nepenthaceae could be descendants of more recently dispersed lineages that had been transmitted to an old species stock.

Selection of chloroplasts by laser microbeam microdissection for single-chloroplast PCR

Laser microbeam microdissection and laser pressure catapulting offer the possibility of separating cell compartments, thus allowing for contamination-free analysis. Using these methods, we were able to select single chloroplasts of Nicotiana tabacum. Starting from homogenized leaf material, chloroplasts were purified by differential centrifugation and applied directly onto a poly-ethylene-naphthalate membrane that was mounted on a microscope slide. Single chloroplasts were dissected under microscopic control and catapulted into a PCR tube. Subsequent PCR of a spacer region between the trnT and trnF genes verified the successful amplification of DNA from a single chloroplast. The advantage of this method compared to the use of capillaries or optical tweezers is that one is able to prepare high numbers of samples in a short time.