Where did giant buttercup come from?
Giant buttercup taking over valuable pasture.
Finding the exact area of origin for some of our weeds is important detective work because it enables us to pinpoint where to seek biocontrol agents that are most closely linked to the plant and therefore more likely to establish and be effective. When it comes to giant buttercup (Ranunculus acris), a weed that dominates many dairy pastures in high rainfall areas of New Zealand, we found that it was not a simple task to pinpoint its origin. The R. acris complex is a group of closely related plants that extend across European countries as far east as Asia and Japan. The plant has also naturalised in a number of other countries including Canada, South Africa and USA. Although giant buttercup was introduced into New Zealand around the time of early European settlement, it was not known for sure where ‘our’ plants originated from.
As it turns out, recent studies have found that the plant is quite variable in its form and genetic structure, making it challenging to work on. Understanding genetic relationships between populations of plants would usually involve sequencing the nuclear genome (DNA within the cell nuclei) but because of the presence of multiple sets of chromosomes (polyploidy) in this plant, that method is not an option. Instead, examining the variation within the R. acris complex has involved extracting the genetic information contained within chloroplasts, the part of the cell responsible for photosynthesis amongst other functions.
Initial investigations led by molecular biologist Gary Houliston found that giant buttercup chloroplasts have remarkably diverse genetic information, which is inherited from parent chloroplast material. Gary refers to the variability within this independent genetic information as chloroplast haplotype diversity. “Typically chloroplast haplotypes do not vary much within plant species or even the same genera so it was quite unusual to find such a high diversity in the chloroplasts of giant buttercup growing at one site here in New Zealand,” said Gary. “This points towards multiple introductions of giant buttercup into New Zealand,” he explained.
Gary’s more recent research compares chloroplast diversity in the New Zealand populations of R. acris to the chloroplast diversity in the plant’s native range, including areas that he suspected were the likely source of the New Zealand plants. In total, Gary looked at 57 samples sourced from around the world. Much to his surprise, Gary found that the samples sourced from other countries had equally high chloroplast diversity. “We would expect that over time, the diversity in the genetic material would diminish through drift – a process where variation is lost over time. This is often the case for invasive species that are doing well in a new environment, especially if they underwent a bottleneck on arrival,” explained Gary. “Giant buttercup established in New Zealand close to 150 years ago and to find such a high degree of variation is therefore quite surprising,” said Gary.
Samples from Central Europe proved to be a good match to most of those found in New Zealand, but Gary has also found common haplotypes between the Canadian samples and those from New Zealand. “It is highly likely that most of the introduced R. acris in New Zealand has come from the United Kingdom, but that there have been multiple introductions,” said Gary. However, he is still puzzling over why the high haplotype diversity in both Europe and New Zealand has been maintained over time, and it could be a while before we get to the bottom of this.
Given the extensive problem that this weed causes in dairy pastures, and its evolved resistance to phenoxy and possibly other classes of herbicide, a number of alternative control methods are being sought. Massey University and AgResearch have been working on methods for dairy farms, where the plants are avoided by cattle due to their bitter taste. A mycoherbicide based on the fungus Sclerotinia sclerotiorum has been developed but commercialisation of this product is awaiting development of a cost-effective formulation. One of the most promising options is pre-graze mowing. Although biocontrol is not out of the question, this is likely to be a difficult and costly option. “The large number of native Ranunculus here means we need to be very cautious when selecting potential agents,” said Simon Fowler. “Plus all the potential agents look likely to be difficult to work with.” At present it seems that giant buttercup is not widespread enough to warrant the cost of developing a full biocontrol programme.
This project was funded by AgResearch’s Undermining Weeds programme