Humidity Cue Overcomes Pollinator Avoidance Behavior and May Contribute to Host-Plant Shifts.

Abstract

Ecological specialization is often described as an evolutionarily tenuous, or dead-end, strategy, where the loss of one partner may be catastrophic for the other. Some of the most highly specialized interactions are brood-pollination mutualisms, wherein plants trade food and shelter for pollination services, often at the cost of some offspring (i.e., fertile seeds). With few exceptions, brood-pollination mutualisms are generally obligate, thus the reproduction of both plant and insect pollinator are interdependent and cannot occur without the other. In many cases these interactions are also species-specific and pairwise. Due to the severity of reproductive constraint, an evolutionary "dead-end" seems all but inevitable. However, host-shifts are remarkably common, even in brood-pollination mutualisms, and may enhance evolutionary resilience. Yet we still lack a clear understanding of mechanisms of insect localization and choice of a new host-plant in these highly specialized mutualisms. Recently, Rhopalotria furfuracea, the specialized brood-site pollinator of the cycad Zamia furfuracea, has been observed on other Zamia species in an artificial environment (i.e., a conservation garden) where it is not found in the wild. To better understand what cues are facilitating this shift, we consider both "private channels", or unique secondary metabolites thought to facilitate partner fidelity in ecologically specialized interactions, and the more general cue humidity, representing two modes of signaling for which the ecological importance has been previously described in the R. furfuracea-Z. furfuracea mutualism. We hypothesize that humidity will increase pollinator attraction to non-host plant scent. To test this we characterize via GC-MS the previously unreported scent of the non-host plant, Zamia paucijuga, that R. furfuracea has recently colonized and find that it qualitatively differs from that of Z. furfuracea. Behavior trials, consisting of two-way y-tube olfactometer choice assays find that weevils are repelled by the non-host plant volatile blend, but that the addition of humidity overcomes avoidance behavior, suggesting that less specialized traits, such as primary metabolites, may create opportunities for novel associations to develop over evolutionary time.