Symptoms of population range expansion: lessons from phenotypic and genetic differentiation in hexaploid Mercurialis annua

Background: Range expansion often results in colonisation bottlenecks that should both deplete genetic diversity and increase genetic differentiation towards the margins of a species' geographic distribution. Aims: We tested whether genetic differentiation increased among populations of the annual plant Mercurialis annua after its colonisation of the Iberian Peninsula from Morocco. Previous work showed that this colonisation resulted in a decrease of phenotypic and genetic diversity from the core in North Africa towards the distribution margins of M. annua in north-eastern and north-western Spain. Methods: Seeds were sampled from 20 populations located across the hexaploid range of M. annua. Patterns of phenotypic and genetic differentiation among experimentally grown populations were analysed and compared between the Iberian Peninsula and North Africa. Results: The level of phenotypic and genetic differentiation among populations in the expanded range of the Iberian Peninsula was similar to that in the core range in North Africa. Conclusions: Our findings imply that the observed effects of range expansion on genetic differentiation may be independent of the effects on genetic diversity. They point to the importance of taking both historic and contemporary processes of migration into account when predicting the results of range expansion.