Macroscale analysis of mistletoe host ranges in the Andean-Patagonian forest

Spatial aggregation patterns in four mistletoe species: ecological and environmental determinants

Plant spatial distribution is an important topic in ecology as it determines species coexistence and biodiversity dynamics. Usually, plants show clustered distributions in nature. Mistletoes are a good example of aggregated distributions, as they form dense aggregations due to several factors (availability of competent hosts, seed dispersal vectors, microclimate conditions). We analysed four native mistletoe species with divergent life histories and host ranges: Desmaria mutabilis and Tristerix corymbosus from the temperate rainforests of southern Chile; and Tristerix aphyllus and Tristerix verticillatus from the northern semi-desert zone. While T. corymbosus and T. verticillatus have a wide host range, T. aphyllus and D. mutabilis are specialists that can parasitize only a few plant species. We hypothesized that specialized species would be more aggregated due to ecological and environmental restrictions. We used heterogeneous Poisson models to quantify spatial aggregation. Three of the four mistletoe species were spatially clustered at both environments, with aggregation being stronger in the temperate rainforest of southern Chile and particularly in the host-specialist species. Our results suggest that environmental constraints are more important than ecological constraints (host range) in shaping mistletoe spatial structure. Mistletoe aggregated spatial distribution depends primarily on the environment that they inhabit, which conditions host spatial availability, and arrangement.

Parasite-host network analysis provides insights into the evolution of two mistletoe lineages (Loranthaceae and Santalaceae)

Mistletoes are ecologically important parasitic plants, with > 1600 species from five lineages worldwide. Mistletoe lineages exhibit distinct patterns of species diversification and host specificity, however, the mechanisms underlying these differences are poorly understood. In this study, we analysed a comprehensive parasite-host network, including 280 host species from 60 families and 22 mistletoe species from two lineages (Santalaceae and Loranthaceae) in Xishuangbanna, located in a biodiversity hotspot of tropical Asia. We identified the factors that predict the infection strength of mistletoes. We also detected host specificity and the phylogenetic signal of mistletoes and their hosts. We found that this interaction network could be largely explained by a model based on the relative abundance of species. Host infection was positively correlated with diameter at breast height and tree coverage, but negatively correlated with wood density. Overall, closely related mistletoe species tend to interact more often with similar hosts. However, the two lineages showed a significantly different network pattern. Rates of host generality were higher in Loranthaceae than in Santalaceae, although neither lineage showed phylogenetic signal for host generality. This study demonstrates that the neutral interaction hypothesis provides suitable predictions of the mistletoe-host interaction network, and mistletoe species show significant phylogenetic signals for their hosts. Our findings also indicate that high species diversification in Loranthaceae may be explained by high rates of host generality and the evolutionary history shared by Loranthaceae species with diverse host plants in the tropics.

Geographical context outweighs habitat disturbance effects in explaining mistletoe population genetic differentiation at a regional scale

Genetic differentiation depends on ecological and evolutionary processes that operate at different spatial and temporal scales. While the geographical context is likely to determine large-scale genetic variation patterns, habitat disturbance events will probably influence small-scale genetic diversity and gene flow patterns. Therefore, the genetic diversity patterns that we observe today result from the combination of both processes, but they are rarely assessed simultaneously. We determined the population structure and genetic diversity of a hemiparasitic mistletoe (Tristerix corymbosus) from the temperate rainforests of southern Chile to determine the effects of geographical context and habitat disturbance at a regional scale and if it is affected by the abundance and occurrence of its seed disperser mutualist (the arboreal marsupial Dromiciops gliroides). We genotyped 359 individuals from 12 populations using single nucleotide polymorphisms, across three different geographical contexts and four disturbance conditions. We also used camera traps to estimate the abundance and occurrence of the seed disperser. Our results suggest that genetic differences among populations are related more to geographical context than to habitat disturbance. However, as disturbance increased, D. gliroides abundance and occurrence decreased, and mistletoe inbreeding index (F_IS ) increased. We also found highly uneven gene flow among study sites. Despite the high levels of disturbance that these temperate rainforests are facing, our results suggest that mistletoe genetic differentiation at a regional scale was more influenced by historical events. However, habitat disturbance can indirectly affect mistletoe population genetic differentiation via the seed dispersal process, which may increase levels of inbreeding.

Gender dimorphism in the virulence of a dioecious mistletoe

Parasite virulence often differs between male and female hosts. However, less is known about how virulence might differ between male and female parasites. Here, I show that female plants of the dioecious mistletoe Misodendrum quadrifolium (Misodendraceae) grow larger than male plants. Correspondingly, females reduce the photosynthetic capacity of infected host branches more than males. Results indicate that in addition to playing an important role in determining host susceptibility to parasitism, gender can also play an important role in determining the virulence of dioecious parasites.

Host range dynamics at different scales: host use by a hemiparasite across its geographic distribution

The complexity of natural communities is the result of interactions among species that coexist within them. Parasitic interactions are among the most common species interaction types, and analysis of parasite-host ranges can advance understanding of how host-parasite pairs structure community interactions across their geographic distributions. Using network analysis and host preference relative index, we analysed host use by the South American mistletoe, Tristerix corymbosus (Loranthaceae), in 22 localities among two biomes: Chilean matorral and temperate forest. The total number of host species recorded was 27, and 40% of these species were non-native. The non-native Populus sp. was shared between biomes. There was a positive relationship between host range and potential host species richness at the studied localities. On average, the mistletoe parasitized each host species relative to its abundance. However, some host species in some localities are more parasitized than expected. Network structure showed a differences in host use between the two biomes: Aristotelia chilensis was central in the temperate forest, with Populus sp. in the Chilean matorral. Host use intensity in the Chilean matorral was higher for non-native species. Tristerix corymbosus has a wide host range and could be considered a generalist parasite across its full geographic distribution, but at local scales, host preferences differed among localities and are related to host coverage. Alterations in community composition, due to natural events or human activities, can modify the availability of possible hosts. Hence, the mistletoe with the described characteristics may be able to change its infection preference while maintaining the interaction functionality.