Australasian Showy Mistletoes
Amylotheca dictyophleba at la baie d’Oro, L'Île-des-Pins, New Caledonia. S Mathews photo.
Showy mistletoes are obligate parasites. Their level of host specificity therefore influences the extent of their 1) geographic range and their capacity for range shifts in response to environmental change; 2) dispersal routes; 3) local abundance; 4) likelihood of population persistence; and 5) the set of organisms with which they interact.
Most Loranthaceae use multiple species of hosts, but some are clear generalists, while others show specificity at various phylogenetic scales, from the level of major flowering plant clade to genus-level specificity. Our work so far, focused on the Australian showy mistletoes, has revealed that mistletoes were most commonly reported on hosts in the rosid clade of flowering plants, suggesting that relationships of mistletoes with rainforest lineages may have been established early in the history of Australian Loranthaceae and that some lineages co‐diversified with their hosts in arid regions (Santiago-Rosario et al. 2024).
RIGHT: Summary bipartite networks of mistletoe species interactions with their host orders for (A) Amyema, (B) Amylotheca, (C) Decaisnina, (D) Dendrophthoe, (E) Lysiana, and (F) Muellerina. Mistletoe species are shown in the top half of each circle and records of host orders are shown at the bottom half of each circle. Host orders are colored by clades with asterid orders in hues of red, rosids hues of blue, basal eudicots hues of yellow, magnoliids hues of green, and conifers hues of brown. Dilleniales and Santalales were assigned single colors, orange, and lime green, respectively . Ticks indicate increments in the number of observations and differ by mistletoe genus.
Host Specificity
Mistletoe (Decaisnina brittenii) attached to host stem via multiple haustoria. S Mathews photo.
Mistletoe (Amyema cambagei) germinanat. K Harms photo.
Mistletoe interactions: Organisms & Traits
Adult Imperial Jezebel (Delias harpalyce) emerging from chrysalis on mistletoe (Amyema pendula). S Mathews video.
The interaction between a mistletoe and its host is influenced by the large number of interactions that each partner has with other organisms. To understand the evolution of showy mistletoes requires insights into how these interactions have affected the ability of parasite and host to coevolve and for the relationship to persist. Each partner has mutualistic and antagonistic partners that may have indirect effects on mistletoe-host coevolution.
Thus, we are interested in the evolution of traits relating to pollination, dispersal, and protection. The majority of showy mistletoes are pollinated and dispersed by birds, and across Australasia, Central and South America, and Africa they interact with both specialist and generalist pollinators and disperser. Specific floral and fruit traits favor specialist pollinators and dispersers, and they exclude some but not all generalists. In some environments, mistletoes must attract pollinators while simultaneously avoiding predation by large flocks of flower-eating birds? What traits are involved? When did these traits evolve?
Providing the phylogenetic and biogeographic context needed to investigate the evolution of these traits is a major undertaking in the lab.