Pollination biology and sexual selection in plants

Plants often spend what seems like an absurd amount of resources to attract pollinators. Showy displays and sometimes overwhelming odours likely constitute traits under strong sexual selection to increase fitness by enhancing the spread and/or reception of pollen, indicating that this can indeed be very important for the plant. Flowers and their pollinators form classic examples of a mutualistic interaction, but in reality the interaction may range from mutualism to complete exploitation of one part by the other. Flowers may cheat insects into pollinating them without providing any reward, and conversely insects or other consumers may use the resources (e.g. nectar, seeds or other nutritious treats) provided by the flower without performing pollination services in return. The relationships between flowers and their pollinators may also range from specialist interactions between a single species of plant and pollinator, respectively, to interactions involving a multitude of pollinators visiting many different plants.

 

Pollination biology captures the essence of the evolutionary struggle, and also has important applied consequences, as most of our agricultural crops, from fruits to cereals, depend on  sexual reproduction by means of pollination.

 

Relationships between sexually selected flower traits

As stated above, showy floral displays and strong scents likely constitute sexually selected traits to increase pollen dispersal by the male part of the flower. How important are different sexually selected characters, such as pollen growth, Flower size, Odour amounts.... in different systems; is there genetic variation in the production of different traits, and how are these sexually selected traits correlated?

 

Mechanisms for flower recognition

Some pollinators are generalists, visiting many different types of flowers, while others specialize on a single species. What mechanisms govern the level of specialization, and what sensory cues do pollinators use to distinguish between flowers of different species and of differing quality?

 

Sexual conflict during pollen transfer

Sexual conflicts can be expressed between two plant individuals, or between the two aspects of a hermaphroditic plant. Conflicts between plant individuals can be expressed during pollen competition. In angiosperms, closed carpels and transfer of male gametes from the stigma to the ovules via a pollen tube (siphonogamy) separate pollen arrival on the stigma from conception, creating an arena for pollen competition and female choice after pollen transfer. At this prezygotic stage of the life cycle the female function of one plant is represented by its pistil, while the male function of another plant is represented by its pollen (cf. sperm competition). Because pollen performance in the pistil is highly dependent on the female reproductive function male reproductive success may be improved by manipulation of the female function.  Floral adaptation could also be constrained by sexual conflicts within individuals, as successful adaptations within a hermaphroditic plant require positive fitness effects when summed over the male and female functions. Fitness of both female (seed production) and male (siring success) reproductive functions will be influenced by how much pollen is transferred to conspecific stigmas. In many cases, selection on traits influencing pollen export is in harmony with those affecting pollen import, but nevertheless it can be hypothesised that the reproductive functions differ in what kind of pollination system is optimal. What is then the advantage of limiting the number of pollinators and how might the optimal level of specialisation differ between the two reproductive functions?

 

Funding

The Linnaeus initiative "Insect Chemical Ecology, Ethology and Evolution" ICE3

 

Personnel

Mattias Larsson

 

Collaboration

Åsa Lankinen (Lund University, Sweden)

 

Selected References

Lankinen Å, Larsson MC. Conflicting selection pressures on reproductive functions and speciation in plants. Evolutionary Ecology (in press)