Mating strategies shape tropical plants’ invasive ability
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Ageratum conyzoides, which is self-incompatible in South America where it is native, and self-compatible in India where it is invasive
view moreCredit: Ravi Jambhekar
A recent study from the Centre for Ecological Sciences (CES), Indian Institute of Science (IISc) has found strong evidence that a plant’s ability to reproduce on its own – through self-fertilisation – is one of the key traits that helps it become invasive.
Most flowering plants need pollen from another individual of the same species to produce seeds. However, plants of some species can reproduce using their own pollen, or even without pollen, allowing even a single plant to establish a new population. This idea, known as Baker’s law, suggests that plants capable of this type of “uniparental reproduction” or self-fertilisation should be better invaders.
To test this idea, Saskya van Nouhuys, Associate Professor at CES, and Narashiman Nagendra Rao, former MSc student at CES, examined 28 species from the daisy family (Asteraceae), comparing 11 invasive species, eight non-invasive alien species, and nine native species. They collected plants from disturbed habitats such as roadsides and open spaces across Karnataka and Tamil Nadu over a year. Seeds or plant cuttings from at least five individuals of each species were grown on campus, while a few hard-to-grow native species were studied in the wild, with a total of about 900 plants examined.
The team tested whether the plants could reproduce without cross-pollination by comparing self-pollinated and naturally pollinated flowers, and then assessed seed viability and germination. Using fluorescent microscopy, they determined whether seeds formed through self-fertilisation or apomixis (without fertilisation), and classified each species by its reproductive strategy.
What they found was that all of the 11 invasive Asteraceae species studied could reproduce uniparentally, while most native and non-invasive alien species remained self-incompatible – they generally required pollen from another plant. “Uniparental reproduction is a conceptually simple trait. It has been exciting to see such clear evidence of its advantage for invasive species,” says van Nouhuys.
Two especially aggressive invaders, Ageratum conyzoides and Bidens pilosa, showed an even more striking pattern. In their native range of Mexico, these species were largely self-incompatible, but in India – where they are alien – they had evolved to become uniparental, suggesting that individuals capable of self-fertilisation were favoured during the invasion process.
“Before our experiments, the idea of reproductive strategies shifting during invasion seemed like a very far-fetched idea to me and I thought that previous evidences of such shifts were very rare occurrences,” says Narashiman. “The results from our experiments and that of our collaborators absolutely baffled me.”
As invasive species continue to spread worldwide, reproductive strategy should become a routine part of weed-risk assessment programmes used to predict which introduced plants are most likely to become future invaders, the researchers suggest.
“Invasive species are called invasive for a reason,” says van Nouhuys. “They establish and then flourish in a new location. When this happens, existing species decline or disappear entirely, which changes the whole landscape.”
Journal
Biological Invasions
Article Title
More invasive than non-invasive Asteraceae plants can reproduce uniparentally
Flower bagged to exclude pollen
Pollen germination on the stigma viewed through a florescent microscope filter after Aniline blue staining
Experiments in progress
Credit
Narashiman Nagendra Rao
