Hiding in plain sight: experimental evidence for birds as selective agents for host mimicry in mistletoes

Abstract

Many Australian mistletoe species are cryptic, closely resembling their host foliage and overall appearance. Seed-dispersing birds have been proposed as a selective agent for host resemblance, with cryptic mistletoes only located by thoroughly searching through canopies regardless of infection status, boosting mistletoe populations by increasing the frequency of seeds dispersed to uninfected hosts; however, this idea is as yet untested. We measured bird visitation to fruiting mistletoes (n = 20) over two consecutive days, with manual defoliation of the mistletoe occurring before observation began on the second day to determine the effect of the visual appearance of the mistletoe on potential seed-dispersing birds, expecting defoliation to reduce the number of visits. Visits to the mistletoes were compared between days of observation and dietary guild (mistletoe specialist/nonspecialist). Intact mistletoes were visited more than the defoliated mistletoes, and the dietary guilds differed in their visitation patterns. This work demonstrates that the visual acuity of seed-dispersers can distinguish subtle differences in mistletoe phenotypes within infected hosts, consistent with the hypothesis that those mistletoes that more closely resemble their hosts are more difficult to perceive from afar and therefore more likely to have their seeds dispersed to uninfected hosts.

With a little help from my friends: Hyperparasitism allows a generalist mistletoe to expand habitat use

“…Therefore, using other mistletoes as host plants may be interpreted as a variant of the “host quality hypothesis” proposed by Watson (2009). Mistletoes commonly switch hosts, and spatial host turnover is known in many species (Cook et al, 2020; Milner et al, 2020). However, using other mistletoes as hosts may be interpreted as a last resource when competent hosts are unavailable, but the ecological consequences of hyperparasitism remain unknown.…”

Exposing the error hidden in plain sight: A critique of Calder's (1983) group selectionist seed‐dispersal hypothesis for mistletoe “mimicry” of host plants

“…Cook et al. (2020, p. 526) provided additional clarification (italics added for emphasis):

… those mistletoes that most closely resemble their favored hosts would be difficult for their dispersers (predominantly birds) to discern within the canopy. Thus, rather than forming a mistletoe‐specific search image and flying from mistletoe to mistletoe or infected tree to infected tree, fruit‐eating birds would instead need to search host canopies carefully, prolonging the time spent in the canopy , and maximizing the probability of seeds from previous meals being deposited.

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Endophytic bacterial communities are associated with leaf mimicry in the vine Boquila trifoliolata

“…Whereas plausible selective agents responsible for mimicry cases may be inferred after a thorough ecological knowledge of the study system 1 – 3 , the elucidation of the physiological or molecular mechanisms behind these phenomena requires greater research efforts. Perhaps the closest case to leaf mimicry by Boquila is that of Australian mistletoes, a system where the roles of herbivores 4 – 6 and seed-dispersing birds 13 , 14 as selective agents have met supporting evidence. However, the proposed explanatory mechanism for mistletoe mimicry, which considers the sharing of morphogenetic hormones such as cytokinins 15 , 16 , is still under debate 14 , 17 .…”

“…Perhaps the closest case to leaf mimicry by Boquila is that of Australian mistletoes, a system where the roles of herbivores 4 – 6 and seed-dispersing birds 13 , 14 as selective agents have met supporting evidence. However, the proposed explanatory mechanism for mistletoe mimicry, which considers the sharing of morphogenetic hormones such as cytokinins 15 , 16 , is still under debate 14 , 17 . The facts that mimetic Australian mistletoes are hemiparasites physiologically connected to their host trees 6 , 17 , and that they often show associations with a single or a few host species 4 , 17 , make the identification of the mechanisms underlying leaf mimicry a seemingly reachable goal.…”