It's survival of the fittest – even in the world of seeds. New research conducted by an international team of scientists including the University of New Mexico's Tim Lowrey, professor of Biology, reveals a complex plant-animal interaction of long-standing that has depended exclusively for its evolutionary longevity on mechanical factors.
The material properties of the thick-shelled seed, from the plant family Annonaceae (Soursop, Pawpaw, and Custard Apples), have evolved into a fine balance that allows the seed to germinate while impeding predators, including tiny weevils and large great apes, that want to devour it for its tasty oils and nutrients.
The article "Evolutionary optimization of material properties of a tropical seed," appeared online last week in the Journal of the Royal Society Interface. No such study has previously demonstrated a central role for mechanics in the evolutionary stability of a plant-animal interaction, a subject dominated largely by chemical investigation.
"In tropical forests, there are many plants with large seeds that are rich in oils and other nutrients, which are used to nourish the germinating seedling," said Lowrey, who is also a UNM Regents' Lecturer. "Since these seeds are packed with energy-rich nutrients, there are many seed predators in tropical forests. The seed that we studied has three main predators from very different animal groups including orangutans, squirrels and beetles. Orangutans exert tremendous pressure with their teeth to crack open the seed. Squirrels gnaw and beetles use their mandibles to bore through the shell. Over the years, the material properties of the plant have evolved in response to how these very different predators gain access to the seed nutrients."
The Annonaceae is one of the largest and most important plant families in tropical rain forests located in Indonesia, Malaysia, Singapore and Thailand. Ecological research on orangutans in peat swamp forest in Indonesia showed exactly how orangutans, great apes with posterior teeth similar to those of some ancestors of modern humans, manage to break open the seed.
Several Southeast Asian representatives of the family, including Mezzettia parviflora - whose seeds the researchers studied, have evolved a novel seed covering that they have used to develop a thick and complex shell to surround the plant embryo. A combination of macro-, micro- and nano- mechanical testing was used to establish the mechanical properties of the shell.
"The physical properties of the shell resulting from millions of years of evolution protect the seed from predators," said Lowrey. "A special layer in the seed coat has likely evolved due to selection pressure by primate predators, while still allowing for germination. We looked at the process of how it splits open and germinates, despite being hard enough to resist the blows from a hammer without cracking. There are zones of tissues that allow water to enter and to allow the seed to germinate."
Germination of the avocado-sized seed can take two to three weeks. For both germination and predation, mathematical formulae were developed to show how the design of the seed allows it to open, but not be opened, so that predators can't penetrate the seed.
In addition to Lowrey, the international team includes scientists from Kuwait University including Peter Lucas, John Gaskins from the University of Virginia, Matthew Begley from Cal-Santa Barbara, researchers from the United Kingdom (Cambridge & Oxford) and OuTrop, the Orangutan Tropical Peatland Project, a large tropical research foundation.
"We think that we've gone a lot further with the analysis of seed fracture than previous investigations of seed predators have managed," said Lucas, lead author on the paper. "We had to draw together evidence from a whole variety of disciplines in order to make the case that mechanics is critical, including not just techniques in materials science, but also animal and botanical ecology and palaeontology. The seed has a complex architecture and it took us a long time to unravel its mechanical behaviour."
The group went further to try to examine whether there was a coincidence between the timing of the evolution of the seed and its current predators.
"The timing of the evolutionary aspects of this interaction need a lot more work. There is much more to do on this – it opens up a whole area of investigation," added Lucas.
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