Close Encounters

Trapping pollinators with hot and smelly flowers: different strategies for different insects!

New study finds that scent is more pollinator-specific than heating up the flower-like parts

Sir David Attenborough and botanic gardens’s live streams have given worldwide fame to the corpse plant (Amorphophallus titanum) or otherwise known as titan arum. This plant has the largest unbranched flower in the world and produces a pungent smell, resembling something that is decomposing. Additionally, the middle, flower-like structure (spadix surrounded by a modified leaf), warms up to artfully mimic the decompositioning process. If you have a composting bin, you might have noticed that heat too.

Titan arum belongs to the Araceae family that contains many other, deceptive plant species. Two of them are Typhonium angustilobum and T. wilbertii. Both of them are Australian native plants but T. angustilobum also occurs in Southern New Guinea. These two, closely-related species are known to have thermogenic spadixes and have slightly different flowers (hidden below the spadix) that might be attracting different pollinators.

Dr Thomas Sayers from the University of Melbourne and colleagues from La Trobe University, Agriculture Victoria and Royal Botanic Gardens Victoria investigated the sensory and morphological traits of these flowers and the behaviour of the pollinators. The researchers revealed that the two plant species employ different attractions, especially scents, for beetle and fly pollinators. 

The distribution of T. angustilobum (grey) and T. wilbertii (black) plants and the main study sites (A and B in red on the left), and representative flowers (right) from the two species. Source: Sayers et al., 2021

Between 2015-2019, the scientists monitored the flowering of Typhonium angustilobum and T. wilbertii at multiple Australian locations. The researchers investigated the trapped pollinators, identified them, noted down their behaviour and whether they had pollen attached to their bodies. The researchers documented the flowering sequence of the plants in detail with thermal cameras. 

Next, Sayers and colleagues collected tissue samples from different parts of the flower-like parts (thermogenic and non-thermogenic) before, during and after the thermogenic phases of flowering to identify genes involved in the process. A few previous  studies found that Heat is produced via alternative respiratory pathways within Araceae but not much is known of which genes are involved and how important these pathways are during flowering. 

The team also dissected the flowers to make detailed observations of floral traits and measured the floral scent compounds (volatiles) produced by the two species. 

Some of the pollinators observed on T. angustilobum included Philonthus species (F-G), Megasternini and Aphodius lividus (H). After pollination, T. angustilobum produced seed aggregates. Source: Sayers et al., 2021

Sayers and colleagues counted over 1,800 beetles and five flies trapped by T. angustilobum plants, whilst T. wilbertii plants trapped 80 beetles and 570 flies. The mainly beetle-pollinated T. angustilobum flowers were overall larger and produced a pungent acrid odour. The mainly fly-pollinated T. wilbertii flowers had relatively steeper entrances and produced dung and floral scents. These traits show that whilst the two plants have overlapping distributions, they seem to be pollinated by different insects.

The thermogenesis of T. angustilobum (A) and T. wilbertii (B) following the flowering sequence. The graphs show the extent of temperature difference between non-thermogenic and thermogenic flower-like parts in the appendix (solid black line) and staminate zone (solid grey line) during the peak of thermogenesis (♀) and pollination shed (♂). Source: Sayers et al., 2021

Both species flowered in a similar pattern. The plants opened their flowers gradually in the afternoon and were the warmest at dusk when they produced most of the volatiles. The pollinators were then trapped overnight. On the following day, both species shed their pollen in the afternoon. 

As the flowering and thermogenesis seems to be identical for the two species, Sayers and colleagues suggest that scent is the most important signal to either attract beetles or flies. Then, the adapted floral traits help with capturing the right pollinator. 

“[T]hat T. wilbertii thermogenic traits are more similar to beetle pollinated T. angustilobum […], raises questions about the adaptive significance of the extent of thermogenic activity and the relative importance of different sensory signals (e.g. heat and scent) for particular insect pollinators”, Sayers and colleagues write. 

Whilst it is straightforward that the heat and dung-smell attracts flies that like to lay their eggs in dung heaps, beetles do not “flock” towards the heat necessarily.

“[B]ased on our results, we suggest that heating as a direct signal for saprophagous beetles and flies explains the occurrence of thermogenesis (an energetically expensive process) in these two rewardless brood-site mimics which may also act synergistically with scent for enhanced pollinator attraction.”

The research team also successfully identified the genes related to two different thermogenesis-related pathways (AOX and pUCP). Based on gene expression analysis, the scientists suggest that the alternative oxidase (AOX) is the main mechanism of heating for both Typhonium species. 

When watching the Insta-worthy titan arum, the questions “why does it smell like this”, “why does it get hot” and “why on Earth does it flower like this” pop up often. Whilst the Typhonium plants might not be as grandiose as The Titan, this study has revealed that scent appears to be the most selective feature for beetle or fly pollinators.

In a video made by Animalogic, the narrator described the evolution of titan arum’s flowering mechanism as “an invention of a goth wizard”. There are still many questions about which trait (thermogenesis or scents) evolved first, how and why within this exciting family of plants so continue to follow hot, smelly plant-related news in the future!