Beetle pollination (cantharophily) is characteristic of cycads and "basal" angiosperms: the early diverging lineages of Amborella and the Nymphaeales (waterlilies) as well as the magnoliids. These lineages are characterized by flowers with numerous primitive features (indefinite free floral parts, spiral arrangement of parts) which is suggestive that beetle pollination may be a primitive trait.
Features of beetle pollinated plants:
- Flowers dull-coloured or white
- Nectar guides, nectar and nectaries often absent
- Radially symmetrical
- Strong fruity or yeasty, fermented odors
- Pollen or food bodies as reward to pollinators
- Often use heat as a reward - there are thermogenic species in Nymphaeaceae, Illiciaceae, Magnoliaceae
Function of thermogenesis
Thermogenesis needs further study. There are two hypotheses as to function.
1. volatilizes floral scents
2. heat as reward, beetles are sluggish so do not generate much heat themselves
Conversely, typical eudicot pollinators such as bees and butterflies are not usually attracted by heat rewards. Bees buzz (vibrate wings very fast) and generate heat, so heat may not be such a good reward for a bee … evolutionary shift to nectar as reward?
Pollination of Amborella
- Pollinating beetles are forest litter dwelling but climb into the foliage to feed on pollen
- Pollen is the reward - no floral volatiles or nectar
- Pollinators probably visit female flowers because of the mimetic role of the staminodes (Amborella is dioecious) – otherwise why should beetles visit the female plant? Staminodes that look like functional stamens may miscue beetles into visiting because of the similarity to a male flower. Wind pollination may also occur?
Pollination of Nymphaeaceae
- Mainly beetles
- Brasenia (water shield) - mixture of wind and insects, light dry pollen that blows about in the wind (derived character)
- In the tropics Nymphaea species are typically pollinated by scarab beetles or rhinoceros beetles
Pollination of water lilies (Nymphaea and related genera)
- in some species the tepals close over the beetles at night “trapping” them inside while they are dusted with pollen. After pollen is shed and the flower opens the beetle is released. The beetles may visit another waterlily and can succeed in depositing pollen.
- Giant waterlily (Victoria) rewards beetles by actively heating them at night (thermogenesis)
Victoria amazonica (giant waterlily) of the Amazon – inner staminodes act like bars in a cage so beetles (scarab beetles) cannot get out. Some species of Nymphaea also have cage-like inner staminodes.
Pollination of Illicium and related species
- Illicium pollinated by leaf-litter and water breeding flies (Diptera) including midges (not beetles)
- Illicium floridanum has a floral odour described as being like “freshly caught fish”
- Schisandra henryi is pollinated by female gall midges (Diptera) which oviposit (lay eggs) in flowers and eat pollen
MAGNOLIIDS: Pollination of Magnolia
- In Magnolia, a fruity fragrance attracts beetles, which enter the flower carrying pollen
- Beetles crawl around on pistils, pollinating them.
- Later, the stamens fall off, landing on the petals and exposing the pollen, and the beetles crawl around eating it and getting it all over themselves.
- The flower then wilts and the beetles fly on to another opening flower.
Example: Magnolia tamaulipana – from Mexico
Shown to be thermogenic and specifically pollinated by beetles (Cyclocephala caelestis) which eat the tepals. Thermogenesis may be common in Magnolia species, but no one has systematically tested which produce heat and which ones don’t.
POLLINATION PARALLELS: BASAL ANGIOSPERMS AND CYCADS
Cycads: Cycads are gymnosperms = naked seed (i.e. no ovaries here!)
Many cycads are beetle (Coleoptera) or thrip (Thysanoptera) pollinated (often highly specifically)
- Lepidozamia peroffskyana (Zamiaceae) pollinated exclusively by Tranes weevils
- Macrozamia macdonellii pollinated exclusively by cycad-specialist thrip (Cycadothrips)
- Pollen as reward
- Heat as reward (?) and to dissipate volatiles - some cycad cones are thermogenic
What about other gymnosperms?
Other gymnosperms: the Gnetales are fly pollinated. The rest (including conifers): wind pollination.