Floral Isolation, Specialized Pollination, and Pollinator Behavior in Orchids

Abstract

Floral isolation is a form of prepollination reproductive isolation mediated by floral morphology (morphological isolation) and pollinator behavior (ethological isolation). Here we review mechanisms and evolutionary consequences of floral isolation in various pollination systems. Furthermore, we compare key features of floral isolation, i.e., pollinator sharing and specialization in pollination, in different orchid pollination systems. In orchid pollination, pollinator sharing is generally low, indicating strong floral isolation. The pollinators' motivation to visit flowers (specifically) can be due to both foraging or reproductive behavior. In both types of behavior, innate preferences for floral signals can be quickly overruled by learning. In pollination systems in which reproductive behavior of pollinators triggers flower visits, lower pollinator sharing was evident compared with systems with foraging behavior, probably because pollinators displaying reproductive behavior show higher fidelity in their visitation patterns. Orchids pollinated through reproductive behavior also use fewer pollinators than orchids pollinated through foraging behavior. No association between specialization and pollinator sharing was found. Thus, generalized pollination does not impede floral isolation, as orchids with many pollinators may nonetheless have low pollinator sharing. Specialization in pollination was, however, linked to orchid species richness in our analysis. Flower size, spur, and column morphology are most important for morphological isolation, and floral scent is most important for ethological isolation. These traits may be based on few genes, implying that floral isolation can be brought about by few genes of large effect.

Noctuid moths as potential hybridization agents for Platanthera orchids

“…Thus, in general, foraging pollinators are identified via indirect methods, for example the identification of an orchid's pollinarium on moths' hairy bodies (Darwin 1877, Maad & Nilsson 2004, Nilsson 1983). Such research is also essential to better understand the potential for hybridization in (orchid) plant systems (Cozzolino & Widmer 2005, Schiestl & Schlüter 2009). Insects visiting orchid flowers can be divided into three main groups: (i) 'flower-visitors, which land on a flower, without any pollinaria attached to their body; (ii) 'potential pollinators', if pollinia adhere somewhere on the insect's body (e.g.…”

“…Additionally, two morphological traits are especially discriminant between both species: the length of the caudicles and the distance between the viscidia (Nilsson 1983). The latter is particularly significant because it leads to the placement of pollinaria on different parts of the moths' heads (Claessens & Kleynen 2006, Esposito, Vereecken, Rinaldi, Laurent & Tyteca unpublished, Maad & Nilsson 2004, Nilsson 1983, Schiestl & Schlüter 2009). Generally, moths will probe deep into the spur to reach the nectar until the head comes into contact with the sticky discs at the base of the pollinaria.…”

Stearoyl-acyl carrier protein desaturases are associated with floral isolation in sexually deceptive orchids

“…In practice, the challenge in studying these processes is identifying the traits under divergent selection and their genetic basis (1). In plants with strong pollinator-mediated reproductive isolation (floral isolation), however, key floral traits are direct targets of selection (1,4). By identifying the molecular mechanisms underlying these traits, genes directly involved in reproductive isolation (so-called "barrier" or "isolation" genes) or even speciation can be identified (3)(4)(5).…”

“…Strong floral isolation and high pollinator specificity make sexually deceptive orchids an excellent system for identifying barrier genes (4,6). Rewardless orchids of the genus Ophrys attract male insects by sexual mimicry, inducing mating attempts of pollinators with flowers, whereby pollen is transferred.…”

“…Alkenes (unsaturated hydrocarbons) are of special importance, and a different proportion of alkenes was found to be the major odor difference among two closely related Ophrys species attracting different pollinators (9). In Ophrys, speciation by pollinator shift has been hypothesized, and there is evidence both for pollinator-driven genetic differentiation and selection on floral hydrocarbon profiles (4,6,9,10). In particular, specific pollinators mediate strong floral isolation among the coflowering closely related species O. sphegodes and O. exaltata by effectively preventing gene flow, whereas other reproductive barriers are largely absent (11).…”