Pollination and mating systems of Apodanthaceae and the distribution of reproductive traits in parasitic angiosperms

Rethinking convergence in plant parasitism through the lens of molecular and population genetic processes

The autotrophic lifestyle of photosynthetic plants has profoundly shaped their body plan, physiology, and gene repertoire. Shifts to parasitism and heterotrophy have evolved at least 12 times in more than 4000 species, and this transition has consequently left major evolutionary footprints among these parasitic lineages. Features that are otherwise rare at the molecular level and beyond have evolved repetitively, including reduced vegetative bodies, carrion-mimicking during reproduction, and the incorporation of alien genetic material. Here, I propose an integrated conceptual model, referred to as the funnel model, to define the general evolutionary trajectory of parasitic plants and provide a mechanistic explanation for their convergent evolution. This model connects our empirical understanding of gene regulatory networks in flowering plants with classical theories of molecular and population genetics. It emphasizes that the cascading effects brought about by the loss of photosynthesis may be a major force constraining the physiological capacity of parasitic plants and shaping their genomic landscapes. Here I review recent studies on the anatomy, physiology, and genetics of parasitic plants that lend support to this photosynthesis-centered funnel model. Focusing on nonphotosynthetic holoparasites, I elucidate how they may inevitably reach an evolutionary terminal status (i.e., extinction) and highlight the utility of a general, explicitly described and falsifiable model for future studies of parasitic plants.

The bright side of parasitic plants: what are they good for?

Parasitic plants are mostly viewed as pests. This is caused by several species causing serious damage to agriculture and forestry. There is however much more to parasitic plants than presumed weeds. Many parasitic plans exert even positive effects on natural ecosystems and human society, which we review in this paper. Plant parasitism generally reduces the growth and fitness of the hosts. The network created by a parasitic plant attached to multiple host plant individuals may however trigger transferring systemic signals among these. Parasitic plants have repeatedly been documented to play the role of keystone species in the ecosystems. Harmful effects on community dominants, including invasive species, may facilitate species coexistence and thus increase biodiversity. Many parasitic plants enhance nutrient cycling and provide resources to other organisms like herbivores or pollinators, which contributes to facilitation cascades in the ecosystems. There is also a long tradition of human use of parasitic plants for medicinal and cultural purposes worldwide. Few species provide edible fruits. Several parasitic plants are even cultivated by agriculture/forestry for efficient harvesting of their products. Horticultural use of some parasitic plant species has also been considered. While providing multiple benefits, parasitic plants should always be used with care. In particular, parasitic plant species should not be cultivated outside their native geographical range to avoid the risk of their uncontrolled spread and the resulting damage to ecosystems.

Scent chemistry and pollinators in the holoparasitic plant Cynomorium songaricum (Cynomoriaceae)

Holoparasitic plants are interesting heterotrophic angiosperms. However, carrion- or faeces-mimicking is rarely described for such plants. There is no information on the pollination biology of Cynomoriaceae, despite the fact that these plants are rare and vulnerable. This is the first study to reveal pollination in a member of this family, Cynomorium songaricum, a root holoparasite with a distinctive and putrid floral odour. From 2016 to 2018, we studied the floral volatiles, floral visitors and pollinators, behavioural responses of visitors to floral volatiles, breeding system, flowering phenology and floral biology of two wild populations of C. songaricum in Alxa, Inner Mongolia, China. A total of 42 volatiles were identified in inflorescences of C. songaricum. Among these volatiles are compounds known as typical carrion scents, such as p-cresol, indole, dimethyl disulphide and 1-octen-3-ol. C. songaricum is pollinated by various Diptera, such as Musca domestica, M. stabulans (Muscidae), Delia setigera, D. platura (Anthomyiidae), Lucilia sericata, L. caesar (Calliphoridae), Wohlfahrtia indigens, Sarcophaga noverca, S. crassipalpis and Sarcophila meridionalis (Sarcophagidae). The inflorescence scent of C. songaricum attracted these pollinators. The plants significantly benefit from insect pollination, although wind can be a pollen vector in the absence of pollinators. C. songaricum is a cross-pollinated, self-incompatible plant. Our findings suggest that C. songaricum releases malodorous volatiles to attract Diptera to achieve pollination. This new example lays the foundation for further comparative studies in other members of this plant group and contributes to a better understanding of fly-pollinated, carrion mimicking plants.

Astringent drugs for bleedings and diarrhoea: The history of Cynomorium coccineum (Maltese Mushroom)

ETHNOPHARMACOLOGICAL RELEVANCE

The botanical identity of the ancient vernacular cynomorium does not correspond to the modern scientific genus while it is not clear how many species of hipocistis (Cytinus sp.) were differentiated by the ancient physicians and whether Cynomorium coccineum was subsumed. The early history of therapeutic uses related to the herbal drugs derived from these parasitic taxa is therefore not easily accessible. Cynomorium coccineum became an important pharmaceutical commodity after the Siege of Malta but its importance decreased in the 18th century and now is considered obsolete.

MATERIAL AND METHODS

We compare the morphological, ecological and therapeutic information of Cynomorium and other parasitizing plant taxa across the past 2000 years and contextualize their uses with the pharmacological properties of their principal metabolites focusing on the raise and fall of C. coccineum as a medicine.

RESULTS

The therapeutic uses of C. coccineum, the Maltese mushroom, seem to become clearly traceable since the Canon of Medicine by Avicenna. Styptic and astringent drugs such as Cynomorium, Cytinus but also gall apples and many others have been selected for their protein-linking capacity leading to the formation of a protective layer on the mucous membranes, which can be used to reduce the secretion of water and electrolytes in case of diarrhoea, dysentery and external bleedings. Whether C. coccineum is effective as a systemically applied anti-haemorrhagic drug is questionable.

CONCLUSION

It appears that the vernacular cynomorium of the ancients corresponds to an edible Orobanche sp. while it remains doubtful whether the vernacular hipocistis was next to Cytinus sp. also applied to C. coccineum as evidence of C. coccineum parasitizing Cistus sp. is scarce. The isolation of gallic acid used as a styptic and the increasing availability of chemical styptics in the 18th century together with the availability of effective alternative anti-diarrhoeic drugs with a more reliable supply very probably led to the decline of the importance of the Maltese mushroom in pharmacy during the 18th century. The effectiveness of gallic acid as a systemic anti-haemorrhagic remains uncertain.

Sequential horizontal gene transfers from different hosts in a widespread Eurasian parasitic plant, Cynomorium coccineum

PREMISE

Parasitic plants with large geographic ranges, and different hosts in parts of their range, may acquire horizontally transferred genes (HGTs), which might sometimes leave a footprint of gradual host and range expansion. Cynomorium coccineum, the only member of the Saxifragales family Cynomoriaceae, is a root holoparasite that occurs in water-stressed habitats from western China to the Canary Islands. It parasitizes at least 10 angiosperm families from different orders, some of them only in parts of its range. This parasite therefore offers an opportunity to trace HGTs as long as parasite-host pairs can be obtained and sequenced.

METHODS

By sequencing mitochondrial, plastid, and nuclear loci from parasite-host pairs from throughout the parasite's range and with prior information from completely assembled mitochondrial and plastid genomes, we detected 10 HGTs of five mitochondrial genes.

RESULTS

The 10 HGTs appear to have occurred sequentially as C. coccineum expanded from East to West. Molecular-clock models yield Cynomorium stem ages between 66 and 156 Myr, with relaxed clocks converging on 66-67 Myr. Chinese Sapindales, probably Nitraria, were the first source of transferred genes, followed by Iranian and Mediterranean Caryophyllales. The most recently acquired gene appears to come from a Tamarix host in the Iberian Peninsula.

CONCLUSIONS

Data on HGTs that have accumulated over the past 15 years, along with this discovery of multiple HGTs within a single widespread species, underline the need for more whole-genome data from parasite-host pairs to investigate whether and how transferred copies coexist with, or replace, native functional genes.

Flora das cangas da Serra dos Carajás, Pará, Brasil: Apodanthaceae

Resumo Este estudo engloba o tratamento florístico de Apodanthaceae para as cangas da Serra dos Carajás, no estado do Pará. Inclui descrição, ilustração, fotografias, distribuição, comentários morfológicos e taxonômicos de Pilostyles blanchetii, a única espécie da família registrada na área de estudo.

Diversity and distribution of parasitic angiosperms in China

Parasitic plants are an important component of vegetation worldwide, but their diversity and distribution in China have not been systematically reported. This study aimed to (1) explore floral characteristics of China's parasitic plants, (2) map spatial distribution of diversity of these species, and (3) explore factors influencing the distribution pattern. We compiled a nationwide species list of parasitic plants in China, and for each species, we recorded its phylogeny, endemism, and life form (e.g., herb vs. shrub; hemiparasite vs. holoparasite). Species richness and area-corrected species richness were calculated for 28 provinces, covering 98.89% of China's terrestrial area. Regression analyses were performed to determine relationships between provincial area-corrected species richness of parasitic plants and provincial total species richness (including nonparasitic plants) and physical settings (altitude, midlongitude, and midlatitude). A total of 678 species of parasitic angiosperms are recorded in China, 63.13% of which are endemic. Of the total, 59.73% (405 species) are perennials, followed by shrubs/subshrubs (14.75%) and vines (1.47%). About 76.11% (516 species) are of root hemiparasites, higher than that of stem parasites (100, 14.75%), root holoparasites (9.00%), and endophytic parasites (0.15%). A significant positive relationship is found between the area-corrected species richness and the total species richness, which has been previously demonstrated to increase with decreasing longitude and latitude. Moreover, more parasitic species are found in the southwest high-altitude areas than low areas. Consistently, the area-corrected species richness increases with increasing altitude, decreasing latitude, and decreasing longitude, as indicated by regression analyses. China is rich in parasitic flora with a high proportion of endemic species. Perennials and root hemiparasites are the dominant types. The spatial distribution of parasitic plants is largely heterogeneous, with more species living in southwest China, similar to the distribution pattern of Chinese angiosperms. The positive relationship between parasitic and nonparasitic plant species richness should be addressed in the future.

Cryptic host-specific diversity among western hemisphere broomrapes (Orobanche s.l., Orobanchaceae)

BACKGROUND AND AIMS

The broomrapes, Orobanche sensu lato (Orobanchaceae), are common root parasites found across Eurasia, Africa and the Americas. All species native to the western hemisphere, recognized as Orobanche sections Gymnocaulis and Nothaphyllon, form a clade that has a centre of diversity in western North America, but also includes four disjunct species in central and southern South America. The wide ecological distribution coupled with moderate taxonomic diversity make this clade a valuable model system for studying the role, if any, of host-switching in driving the diversification of plant parasites.

METHODS

Two spacer regions of ribosomal nuclear DNA (ITS + ETS), three plastid regions and one low-copy nuclear gene were sampled from 163 exemplars of Orobanche from across the native geographic range in order to infer a detailed phylogeny. Together with comprehensive data on the parasites' native host ranges, associations between phylogenetic lineages and host specificity are tested.

KEY RESULTS

Within the two currently recognized species of O. sect. Gymnocaulis, seven strongly supported clades were found. While commonly sympatric, members of these clades each had unique host associations. Strong support for cryptic host-specific diversity was also found in sect. Nothaphyllon, while other taxonomic species were well supported. We also find strong evidence for multiple amphitropical dispersals from central North America into South America.

CONCLUSIONS

Host-switching is an important driver of diversification in western hemisphere broomrapes, where host specificity has been grossly underestimated. More broadly, host specificity and host-switching probably play fundamental roles in the speciation of parasitic plants.

Exploring new dating approaches for parasites: the worldwide Apodanthaceae (Cucurbitales) as an example

Gene trees of holoparasitic plants usually show distinctly longer branch lengths than seen in photosynthetic closest relatives. Such substitution rate jumps have made it difficult to infer the absolute divergence times of parasites. An additional problem is that parasite clades often lack a fossil record. Using nuclear and mitochondrial DNA sequences of Apodanthaceae, a worldwide family of endoparasites living inside Fabaceae and Salicaceae, we compared several dating approaches: (i) an uncorrelated lognormal (UCLN) model calibrated with outgroup fossils, (ii) ages of host lineages as a maximal age in an UCLN model, (iii) user-assigned local clocks, and (iv) outgroup-fossil-calibrated random local clocks (RLC) with varying prior probabilities on the number of permitted rate changes (RLCu and RLCp models), a variable that has never been explored. The resulting dated phylogenies include all 10 species of the family, three in Australia, one in Iran, one in Africa, and the remainder in the Americas. All clock models infer a drastic rate jump between nonparasitic outgroups and Apodanthaceae, but since they distribute the rate heterogeneity differently, they result in much-different age estimates. Bayes factors using path and stepping-stone sampling indicated that the RLCp model fit poorly, while for matR, topologically unconstrained RLCu and UCLN models did not differ significantly and for 18S, the UCLN model was preferred. Under the equally well fitting models, the Apodanthaceae appear to be a relatively old clade, with a stem age falling between 65 and 81my, the divergence of Apodanthes from Pilostyles between 36 and 57my ago, and the crown age of the Australian clade 8-18my ago. In our study system, host-age calibrations did not yield well-constrained results, but they may work better in other parasite clades. For small data sets where statistical convergence can be reached even with complex models, random local clocks should be explored as an alternative to the exclusive reliance on UCLN clocks.

Tree of Sex: a database of sexual systems

The vast majority of eukaryotic organisms reproduce sexually, yet the nature of the sexual system and the mechanism of sex determination often vary remarkably, even among closely related species. Some species of animals and plants change sex across their lifespan, some contain hermaphrodites as well as males and females, some determine sex with highly differentiated chromosomes, while others determine sex according to their environment. Testing evolutionary hypotheses regarding the causes and consequences of this diversity requires interspecific data placed in a phylogenetic context. Such comparative studies have been hampered by the lack of accessible data listing sexual systems and sex determination mechanisms across the eukaryotic tree of life. Here, we describe a database developed to facilitate access to sexual system and sex chromosome information, with data on sexual systems from 11,038 plant, 705 fish, 173 amphibian, 593 non-avian reptilian, 195 avian, 479 mammalian, and 11,556 invertebrate species.

The systematics of the worldwide endoparasite family Apodanthaceae (Cucurbitales), with a key, a map, and color photos of most species

Using morphological, nuclear, and mitochondrial data, we here revise the taxonomy of Apodanthaceae and allocate the 36 names published in the family to ten biological species in two genera, Apodanthes and Pilostyles. All species are endo-parasites that live permanently inside trees or shrubs of the families Salicaceae or Fabaceae and that only emerge to flower. Because of this life history, Apodanthaceae are among the least known families of flowering plants. Nevertheless, the World's herbaria as of 2013 hold at least 785 collections that, in combination with DNA phylogenies, permit well-founded species circumscriptions and geographic range maps. We also provide a key to all species, discuss the newly accepted or synonymized names, and make available color photos of six of the ten species.

Following Darwin's trail: interactions affecting the evolution of plant mating systems

• Since the time of Charles Darwin, the variation in floral characteristics and its effects on plant mating system evolution have fascinated scientists. Recent advances in the field of genetics, molecular biology, and ecology have been very effective in addressing questions regarding mechanisms and interactions underlying the evolution of plant mating systems using various model and nonmodel species. The depth of plant mating system research reflects the complexity and diversity seen in nature, ranging from self-compatible hermaphroditic flowers to separate sexed individuals. Further, the mechanisms involved in the evolution of plant mating systems are much more diverse and differ even among closely related species. Here, as a special section, we present a suite of original papers that range from theoretical modeling to multiyear field research that address different factors affecting plant mating systems, and their effects on shaping interactions between plants, insects, and their environment.

A multidimensional approach to understanding floral function and form

PREMISE OF THE STUDY

Variation in plant mating systems is a fundamental component of the diversity of floral form and function. Mating systems have a strong influence on the distribution and expression of genetic variation, which in turn can influence the course of mating system evolution. It has long been appreciated, however, that ecological interactions may provide much of the impetus behind these evolutionary changes. •

METHODS

This commentary reviews the Special Section in the American Journal of Botany (vol. 100, issue 6) that features novel research on the role of interactions between plants and their pollinators, seed dispersers, herbivores, and interspecific competitors in the evolution of selfing rates and gender. These studies vary in approach from empirical experiments, to phylogenetic comparisons, to theoretical models, to literature reviews as they each attempt to shed new light on longstanding questions about the selective forces and evolutionary pathways that have led to the diversified means by which plants promote or discourage self-fertilization. •

KEY RESULTS

Evidence is provided that indicates that ecological interactions (e.g., herbivory and heterospecific pollen deposition by pollinators) can strongly influence the relative advantages of selfing and outcrossing. Ecological interactions can also influence allocation to sexual functions, which will influence individual and population outcrossing rates and possibly the evolutionary path of gender expression. •

CONCLUSIONS

Mating systems evolution is clearly a multidimensional problem. Research that places the study of mating and sexual systems within a more realistic ecological context will no doubt reveal more complexity as we move toward a better understanding of plant diversity.