Inflorescence evolution in Santalales: integrating morphological characters and molecular phylogenetics.

PREMISE OF THE STUDY: The sandalwood order (Santalales) includes members that present a diverse array of inflorescence types, some of which are unique among angiosperms. This diversity presents not only interpretational challenges but also opportunities to test fundamental concepts in plant morphology. Here we used modern phylogenetic approaches to address the evolution of inflorescences in the sandalwood order. METHODS: Phylogenetic analyses of two nuclear and three chloroplast genes were conducted on representatives of 146 of the 163 genera in the order. A matrix was constructed that scored nine characters dealing with inflorescences. One character, "trios", that encompasses any grouping of three flowers (i.e., both dichasia and triads) was optimized on samples of the posterior distribution of trees from the Bayesian analysis using BayesTraits. Three nodes were examined: the most recent common ancestors of (A) all ingroup members, (B) Loranthaceae, and (C) Opiliaceae, Santalaceae s.l., and Viscaceae. KEY RESULTS: The phylogenetic analysis resulted in many fully resolved nodes across Santalales with strong support for 18 clades previously named as families. The trios character was not supported for nodes A and C, whereas it was supported for node B where this partial inflorescence type is best described as a triad. CONCLUSIONS: Essentially every major inflorescence type can be found in Santalales; however, the dichasium, a type of partial inflorescence, is rarely seen and is not plesiomorphic for the order. In the family Erythropalaceae, inflorescences are mostly in small, axillary fascicles or cymes. Successive families show both cymose and racemose types and compound systems (e.g., thyrses). Inflorescences in Amphorogynaceae and Viscaceae are not dichasial and in general are difficult to compare to "standard" inflorescences.

High-resolution transcript profiling reveals shoot abscission process of spruce dwarf mistletoe Arceuthobium sichuanense in response to ethephon.

Arceuthobium (dwarf mistletoes) are hemiparasites that may cause great damage to infected trees belonging to Pinaceae and Cupressaceae. Currently, dwarf mistletoe control involves the use of the ethylene-producing product ethephon (ETH), which acts by inducing dwarf mistletoe shoot abscission. However, the process by which ETH functions is mostly unknown. Therefore, the transcriptome of the ETH-exposed plants was compared to non-exposed controls to identify genes associated with the response to ethephon. In this study, the reference transcriptome was contained 120,316 annotated unigenes, with a total of 21,764 ETH-responsive differentially expressed unigenes were identified. These ETH-associated genes clustered into 20 distinctly expressed pattern groups, providing a view of molecular events with good spatial and temporal resolution. As expected, the greatest number of unigenes with changed expression were observed at the onset of abscission, suggesting induction by ethylene. ETH also affected genes associated with shoot abscission processes including hormone biosynthesis and signaling, cell wall hydrolysis and modification, lipid transference, and more. The comprehensive transcriptome data set provides a wealth of genomic resources for dwarf mistletoe communities and contributes to a better understanding of the molecular regulatory mechanism of ethylene-caused shoots abscission.

Population structure of a vector-borne plant parasite.

Parasites are among the most diverse groups of life on Earth, yet complex natural histories often preclude studies of their speciation processes. The biology of parasitic plants facilitates in situ collection of data on both genetic structure and the mechanisms responsible for that structure. Here, we studied the role of mating, dispersal and establishment in host race formation of a parasitic plant. We investigated the population genetics of a vector-borne desert mistletoe (Phoradendron californicum) across two legume host tree species (Senegalia greggii and Prosopis velutina) in the Sonoran desert using microsatellites. Consistent with host race formation, we found strong host-associated genetic structure in sympatry, little genetic variation due to geographic site and weak isolation by distance. We hypothesize that genetic differentiation results from differences in the timing of mistletoe flowering by host species, as we found initial flowering date of individual mistletoes correlated with genetic ancestry. Hybrids with intermediate ancestry were detected genetically. Individuals likely resulting from recent, successful establishment events following dispersal between the host species were detected at frequencies similar to hybrids between host races. Therefore, barriers to gene flow between the host races may have been stronger at mating than at dispersal. We also found higher inbreeding and within-host individual relatedness values for mistletoes on the more rare and isolated host species (S. greggii). Our study spanned spatial scales to address how interactions with both vectors and hosts influence parasitic plant structure with implications for parasite virulence evolution and speciation.

Medicinal parasitic plants on diverse hosts with their usages and barcodes.

Medicinal properties of parasitic plants were investigated by means of ethnobotanical study in some areas of northeastern Thailand. Important traditional usages are: Scurrula atropurpurea nourishes blood, Dendrophthoe pentandra decreases high blood pressure, and Helixanthera parasitica treats liver disease. Their systematics were also determined. The research is based on findings obtained from 100 parasite-host pairs. Of these, eight parasitic species were recorded; they are members of two families, viz. family Loranthaceae, namely D. lanosa, D. pentandra, H. parasitica, Macrosolen brandisianus, M. cochinchinensis and S. atropurpurea, and family Viscaceae, namely Viscum articulatum and V. ovalifolium. In addition, each parasitic species is found on diverse hosts, indicating non-host-parasitic specificity. Species-specific tagging of all species studied was carried out using the rbcL and psbA-trnH chloroplast regions. These tag sequences are submitted to GenBank databases under accession numbers JN687563-JN687578. Genetic distances calculated from nucleotide variations in a couple of species of each genus, Dendrophthoe, Macrosolen, and Viscum, were 0.032, 0.067 and 0.036 in the rbcL region, and 0.269, 0.073 and 0.264 in the psbA-trnH spacer region, respectively. These variations will be used for further identification of incomplete plant parts or other forms such as capsule, powder, dried or chopped pieces.

On the brink of holoparasitism: plastome evolution in dwarf mistletoes (Arceuthobium, Viscaceae).

Chloroplast sequences spanning rps7 to 23S rDNA in Arceuthobium campylopodum and A. pendens were generated and compared to Arabidopsis and seven other parasitic plants. Pseudogenes for trnV, trnI (GAU), and trnA (UGC) were seen in both Arceuthobium species, paralleling the situation in the holoparasite Epifagus (Orobanchaceae). These tRNA genes were intact, however, in two other members of Santalales (Ximenia and Phoradendron). The 16S-23S rDNA intergenic spacer was sequenced for 13 additional species of Arceuthobium representing both Old and New World taxa. All species examined had pseudogenes for trnI and trnA, however, deletions in these tRNAs have occurred in different regions among various lineages of the genus. The aligned 16S-23S rDNA intergenic spacer was analyzed using maximum parsimony and compared with nuclear ITS rDNA using a similar suite of species. Overall species relationships were generally congruent, although two cases of potential lineage sorting or chloroplast capture were detected. Arceuthobium is a valuable genetic model to contrast with holoparasites because, despite significant alteration and truncation of its plastome, it still maintains photosynthetic function.

Comparative population structure and genetic diversity of Arceuthobium americanum (Viscaceae) and its Pinus host species: insight into host-parasite evolution in parasitic angiosperms.

In a recent study we revealed that the parasitic angiosperm Arceuthobium americanum is comprised of three distinct genetic races, each associated with a different host in regions of allopatry. In order to assess the role of host identity and geographical isolation on race formation in A. americanum, we compared the genetic population structure of this parasite with that of its three principal hosts, Pinus banksiana, Pinus contorta var. latifolia and Pinus contorta var. murrayana. Despite the fact that A. americanum was divided into three genetic races, hosts were divided into only two genetic groups: (i) Pinus banksiana and hybrids, and (ii) P. contorta var. latifolia and var. murrayana. These findings suggest that factors such as geographical isolation and adaptation to different environmental conditions are important for race formation in the absence of host-driven selection pressures. To assess factors impacting population structure at the fine-scale, genetic and geographical distance matrices of host and parasite were compared within A. americanum races. The lack of a relationship between genetic and geographical distance matrices suggests that isolation-by-distance plays a negligible role at this level. The effect of geographical isolation may have been diminished because of the influence of factors such as random seed dispersal by animal vectors or adaptation to nongeographically patterned environmental conditions. Host-parasite interactions might also have impacted the fine-scale structure of A. americanum because the parasite and host were found to have similar patterns of gene flow.

The discovery of three genetic races of the dwarf mistletoe Arceuthobium americanum (Viscaceae) provides insight into the evolution of parasitic angiosperms.

A population genetic approach was used to explore the evolutionary biology of the parasitic angiosperm Arceuthobium americanum Nutt. ex Engelm. (Viscaceae). Arceuthobium americanum infects three principal hosts and has the most extensive geographical range of any North American dwarf mistletoe. Based on the lack of apparent morphological and phenological differences between populations of A. americanum, past researchers have found no evidence for recognizing infraspecific taxa. In this study, molecular analysis using amplified fragment length polymorphism (AFLP) analysis indicated that A. americanum is divided into three distinct genetic races, each associated with a different host taxon in regions of allopatry: (i) Pinus banksiana in western Canada; (ii) Pinus contorta var. murrayana in the Sierra Nevada and Cascade Mountain ranges in the western US; and (iii) Pinus contorta var. latifolia in the western US and Canada. These observations suggest that host identity, geographical isolation and environmental factors have contributed to race formation in A. americanum. The lack of fine-scale patterning within each of the A. americanum races is attributed to random dispersal of seeds over long distances by animal vectors. Historical factors such as glaciations and founder events have also influenced structuring and genetic diversity in A. americanum populations. Given sufficient time, it is possible that these races will become reproductively isolated and undergo speciation.