Australasian Showy Mistletoes
Amylotheca dictyophleba at la baie d’Oro, L'Île-des-Pins, New Caledonia
An interesting feature of some showy mistletoes is the striking degree of phenotypic similarity between the mistletoe and its host. Is this a case of mimicry? Not necessarily. Other explanations, such as coincidence, are plausible, but the processes by which similarity is achieved remain unknown. We are using imaging and computer vision to quantify phenotypic similarity and will model the evolution of this trait on our species phylogenies.
Amyema pendular (center) on a eucalypt host. S Mathews photo.
The mingled leaves of the mistletoe (top, Amyema quandang) and its host (bottom, Acacia georgiae). S Mathews photo.
Why do some mistletoes look so much like their hosts?
Showy mistletoes (Loranthaceae) are obligate hemiparasites of woody plants, unable to survive without connecting to the vascular system of their host. The degree of host specificity in mistletoe-host interactions is thus a fundamental determinant of mistletoe diversity, persistence, geographic distribution, and abundance, and these plant-plant interactions bear on eco-evolutionary dynamics and conservation. So far studies of host specificity of Loranthaceae in Australia have focused mostly on host range (taxon counts) to explore whether a mistletoe occurs on a small or large number of hosts. Additional insights into specificity can, however, be obtained by quantifying mistletoe prevalence on taxa in its host range and by exploring specificity in a phylogenetic context.
In this study we used multiple measures of host specificity to infer continental-scale patterns of host use by Australian Loranthaceae using occurrence records from the Atlas of Living Australia. After filtering a download of the occurrence records, we 1) tallied the number of host taxa used by each mistletoe genus in our study to estimate host taxon richness, 2) asked whether mistletoe genera appeared to be more prevalent on one or a few taxa in their host range, and 3) asked whether a particular mistletoe genus occurred on host taxa that were more closely related than expected by estimating the phylogenetic diversity of hosts of mistletoe taxa. Because we expected sampling coverage of mistletoe hosts in the ALA sample to be incomplete, we used rarefaction curves to evaluate whether more sampling of host genera, families, and orders would be needed to be confident of host taxon richness estimated from the ALA records.
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.