Researchers at the Indian Institute of Science (IISc), Bengaluru, have demonstrated that a plant's ability to reproduce on its own through self-fertilisation is a key trait enabling it to become invasive. The study, published in the journal Biological Invasions, examined 28 species from the daisy family (Asteraceae), comparing 11 invasive species, eight non-invasive alien species, and nine native species.
Baker's law and uniparental reproduction
The idea, known as Baker's law, suggests that plants capable of uniparental reproduction or self-fertilisation should be better invaders. While most flowering plants require pollen from another individual of the same species to produce seeds, some species can reproduce using their own pollen or even without pollen, allowing a single plant to establish a new population.
Saskya van Nouhuys, associate professor at the Centre for Ecological Sciences (CES), IISc, and Narashiman Nagendra Rao, a former MSc student at CES, led the research. 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, totalling about 900 plants examined.
Experimental findings
The researchers tested whether the plants could reproduce without cross-pollination by comparing self-pollinated and naturally pollinated flowers, then assessed seed viability and germination. They found that all 11 invasive Asteraceae species studied could reproduce uniparentally, while most native and non-invasive alien species remained self-incompatible, usually requiring 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," van Nouhuys said.
Evolutionary shift during invasion
Two especially aggressive invaders, Ageratum conyzoides and Bidens pilosa, were found to be largely self-incompatible in their native range of Mexico. However, in India, where these species 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," Rao said. "The results from our experiments and that of our collaborators absolutely baffled me."
Implications for weed-risk assessment
As invasive species continue to spread worldwide, the researchers suggest that reproductive strategy should become a routine part of weed-risk assessment programmes, which are used to predict which introduced plants are most likely to become future invaders.
"Invasive species are called invasive for a reason. They establish and then flourish in a new location. When this happens, existing species decline or disappear entirely, which changes the whole landscape," van Nouhuys said.
