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SEMIOCHEMICALS, SEXUAL DECEPTION AND SPECIATION

Introduction to Pollinator-Driven Ecological Speciation

Sexually deceptive orchids have long been considered prime candidates for pollinator-driven speciation. Indeed, with the accumulating evidence that floral volatile chemistry is of paramount importance for pollinator attraction and specificity, the plausibility of pollinator-driven speciation in sexually deceptive orchids has been reinforced, and hypotheses on the speciation process refined in both empirical studies (e.g., Schiestl and Ayasse, 2002, Peakall et ah, 2010, Xu et ah, 2011, Peakall and Whitehead, 2014) and in a series of reviews (e.g., Schliiter and Schiestl, 2008, Schiestl and Schliiter, 2009, Ayasse et ah, 2011).

Central to the current hypotheses on the evolution of sexual deception are the following assumptions: (1) pollinator specificity is controlled by floral odor chemistry, (2) pollinator- mediated divergent selection for floral odor chemistry is highly likely, (3) reproductive isolation is predominantly achieved by the pollinator (as strong ethological floral isolation), (4) genic speciation is plausible. Below we briefly evaluate the empirical evidence supporting these assumptions.

Is Pollinator Specificity Controlled by Floral Odor Chemistry?

The often highly specialized pollination of orchids sets them apart from many plant groups with more generalized pollination, where a given plant species has many different species of pollinator. For example, Schiestl and Schliiter (2009) found the mean number of pollinator species per orchid species was just 2.3 (across 31 studies spanning diverse genera). The degree of pollinator specialization is even higher among sexually deceptive systems, with the mean number of pollinators per orchid species estimated to be 1.2 in Ophrys, 1.14 in Caladenia, and 1.0 in Chiloglottis (Schiestl and Schliiter, 2009), with more recent estimates of 1.1 for Chiloglottis (Peakall et al., 2010), 1.75 for Drakaea and 1.05 for Caladenia (estimated from supplementary data of Phillips et al., 2017).

Male Neozeleboria thynnine wasp responses to choice tests revealing variation among Chiloglottis orchid pollinator species in serniochemical specificity and preference

FIGURE 15.4 Male Neozeleboria thynnine wasp responses to choice tests revealing variation among Chiloglottis orchid pollinator species in serniochemical specificity and preference. The results show the total responses of three closely related wasp pollinators to choice tests offering chiloglottone 1 (Chilo 1), chiloglot- tone 3 (Chilo 3) and an equal blend of the two compounds (Blend 1:1). (a) Male Neozeleboria monticola the pollinator of C. valida, (b) N. sp. (impatiens2) the pollinator of C. aff.jeanesii, (c) N. sp. (nitidulal) the pollinator of C. chlorantha. (Based on a subset of the data shown in Figure 5 of Peakall, R. et al., New Phytol., 188.437-450. 2010.)

Presently, the strongest experimental evidence that pollinator specificity is controlled by chemistry is found in Chiloglottis, where the outcome of extensive bioassays have confirmed that pollinator specificity is either controlled by single compounds, or specific blends of two compounds (Peakall et al., 2010; Figures 15.2 and 15.4). In other sexually deceptive orchid genera, a strong role for chemistry in controlling pollinator specificity is indicated by the specific blends, and the often unusual compounds involved (Bohman et al. 2016, this chapter).

 
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