Tricky flowers: how floral variation in a food-deceptive orchid is maintained over generations

Flowers come in all different shapes, sizes, and colours, though within a species most flowers tend to look quite similar. Not Orchis purpurea though! Populations of this orchid species vary considerably in their flower colour (from very light to very dark purple, with different spotty patterns) and size (the flowering stalk can range from 25 cm up to 80 cm tall). A recent study by Jacquemyn & Brys explored how this floral variation is maintained over generations.

But first, you might be wondering what advantage this floral variation provides to a food-deceptive orchid like Orchis purpurea. For most flowers, we might expect that a particular floral colour or shape would be better for attracting pollinators, and that these advantageous traits would be naturally selected for over several generations. As Jacquemyn & Brys write, “the colour, odour, shape and size of flowers are the main signals that plants use to attract pollinators searching for rewards” and “the sensory systems of insects are specifically adapted to detect and learn variation in floral traits”. However, in the case of food-deceptive orchid species that try to trick pollinators, “pronounced variation in the colour, size or shape of flowers may be a successful strategy to lure pollinators and to guarantee fruit set, as it will take some time before pollinators realize that the flowers offer no reward.” Very tricky!

So, how is this floral variation maintained? To find out, Jacquemyn & Brys investigated patterns of fruit set and selection gradients in two natural populations of Orchis purpurea over 16 years. They performed artificial hand pollination experiments, and calculated annual variation in selection gradients for colour traits, flower size, and plant size. They found that fruit set was low, with only 12% of all flowers developing into fruit, and severely pollinator limited. Self-pollination led to reduced production of viable seeds and thus severe inbreeding depression. They also found that opportunities for selection varied strongly across years, and were greatest in the years when fruit set was very low. Selection pressures on flower traits such as spur length, labellum size and flower colour were negligible, but in contrast, there was strong and consistent positive selection on floral display. Overall, Jacquemyn & Brys concluded that the floral variation observed in populations of Orchis purpurea is maintained through a consistent lack of strong selection pressure on these traits through time.