Mistletoes - cryptic mimicry

An evolutionary process?

Convergence of form in response to common environmental factors

Convergence of host and mistletoe leaf structure and function through adaptation to the same environmental conditions may have occurred in some situations. For example, rain forest mistletoes are often difficult to distinguish vegetatively from their hosts, and mimicry could therefore be suspected. But rain forest mistletoes normally have very low host specificity, growing on a wide range of hosts from different plant families.

Nothothixos leiophyllus with rainforest-like leaves

This would be expected, given the species diversity in rain forests and the general absence of dominance by one or a few species. High host specificity would be a disadvantage because of the low probability of effective seed dispersal to another suitable host tree.

The case for cryptic mimicry can be argued largely on the grounds that the morphological form of the mistletoe has a firm genetic basis, and is not determined or influenced by any physiological interaction such as transfer of growth substances from the host (see below under nutrient metabolism). The form of the mistletoe is therefore the result of natural selection.

Secondly, the resemblance between host and parasite in our open forests cannot be simply due to convergence in leaf structure and function through adaptation to the same environmental conditions (but see box). Similar leaf shape does not always reflect similar leaf function. Mistletoe leaves have a much higher transpiration rate than their hosts, and have a different cycle of stomatal opening and closing. Furthermore, strikingly different leaf forms are found among the mistletoes of one geographical area, just as they are among the hosts.

Written by Bryan Barlow, updated 1 December, 2011 by webmaster, ANBG (anbg-info@anbg.gov.au)