The Australian Mistletoes

The Australian Loranthaceae comprise 64 species in 11 genera. A feature of Australian loranthaceous mistletoes is the close vegetative similarity, especially of the leaves, between many species of mistletoe and their usual hosts (Barlow and Wiens, 1977). The parasitic relationship often involves several dominant Australian genera of host trees such as Eucalyptus, Acacia, and Casuarina, and a number of mistletoe genera, including Amyema, Lysiana, Muellerina, Diplatia, and Dendrophthoe (Barlow and Wiens, 1977). Like other plant and animal parasites, mistletoes live in an intimate association with their hosts and derive nutrition from the host, and, of course, share a life-long association with a single host individual (Norton and Carpenter, 1998).

Needle-leaved Mistletoe. Photo: David Watson

           The Australian mistletoes show considerable variation in host specificity that makes for an interesting topic of study (Norton and Carpenter, 1998; Barlow and Wiens, 1977). Several species have very low host specificity and occur on many hosts of considerable taxonomic diversity (Norton and Carpenter, 1998). In tropical forest habitats, high host specificity is not likely advantageous because species dominance in the forest is low. There is likely selective pressure for the physiological capacity to infect and grow on a wide range of host species (Barlow and Wiens, 1977). On the other end of the spectrum, however, there are species of Australian mistletoe that are specific to very few, or even one species of host. In open habitats, where the dominance of one or a few tree species is usual, natural selection favours close physiological adaptations in a mistletoe for growth on the predominant host species (Norton and Carpenter, 1998). In other words, the evolution of host specificity is largely correlated with the development of ecological dominance in plant communities (Barlow and Wiens, 1977).

Eucalypt on left, Mistletoe on right (Barlow, 2012)

Mistletoe on left, Eucalypt on right (Barlow, 2012)

                              

           It has been debated whether or not this phenomenon is true mimicry or just visual similarity, however, research has supported a firm genetic basis on which this mimicry could be founded. Analysis of the genetic variation within and between mistletoe races, compared to that occurring between mistletoe species, might indicate speciation that is host-specific (Norton and Carpenter, 1998). Phylogenetic comparison of mistletoes and their hosts would reveal the relative importance of co-speciation and host-switching events in mistletoe speciation (Norton and Carpenter, 1998). Further research is needed to be sure of this mimetic-genetic relation and its implications for parasite-host evolution and ecology.

References

Barlow B 2012, “Do mistletoes show cryptic mimicry of their hosts?”  Mistletoes in Australia, An Australian Government Initiative.

Barlow BA and Wiens D 1977, “Host-Parasite Resemblance in Australian Mistletoes: The Case for Cryptic Mimicry,” Evolution, vol. 31, no. 1, pp. 69-84.

Norton DA and Carpenter MA 1998, “Mistletoes as parasites: host specificity and speciation,” TREE, vol. 13, no. 3, pp. 101-105.