Expansion load reduces fitness at the range margin of an invasive plant

Genetic drift and natural selection contribute to how invasive plant populations evolve as they expand their range. However, the impact of these forces on fitness at the edges of their range is not often studied. Founder events during range expansion can intensify genetic drift, leading to the buildup (or more pronounced expression) of harmful mutations at the margins, a phenomenon known as expansion load.

On the other hand, natural selection should promote local adaptation during expansion if populations have sufficient genetic diversity.

To explore this, researchers conducted a transplant experiment using common tansy (Tanacetum vulgare) and examined genome-wide SNP data along with demographic surveys from Minnesota, USA. The common garden was located near the southern range limit, incorporating 16 populations from the northeastern invasion core to the southern and western margins, as well as environmental variations in temperature and precipitation.

The study found a strong decline in population fitness as distance increased from the invasion core, suggesting the accumulation of expansion load. This was supported by data from 173 populations, which revealed increased homozygosity (F) and reduced population size moving from the invasion core to range margins. No evidence of local adaptation was found; a strong positive correlation existed between population mean fitness and environmental distance, indicating maladaptation.

While elevated temperature reduced fitness, it did not exacerbate the expansion load. Overall, the findings support the notion that repeated population bottlenecks and substantial genetic drift result in expansion load and diminished fitness at range margins.