Abstract
The mating system plays a key role during the process of plant invasion. Contemporary evolution of uniparental reproduction (selfing or asexuality) can relieve the challenges of mate limitation in colonizing populations by providing reproductive assurance. Here we examined aspects of the genetics of colonization in Ambrosia artemisiifolia, a North American native that is invasive in China. This species has been found to possess a strong self-incompatibility system and have high outcrossing rates in North America and we examined whether there has been an evolutionary shift towards the dependence on selfing in the introduced range. Specifically, we estimated outcrossing rates in one native and five invasive populations and compared levels of genetic diversity between North America and China. Based on six microsatellite loci we found that, like the native North American population, all five Chinese populations possessed a completely outcrossing mating system. The estimates of paternity correlations were low, ranging from 0.028–0.122, which suggests that populations possessed ∼8–36 pollen donor parents contributing to each maternal plant in the invasive populations. High levels of genetic diversity for both native and invasive populations were found with the unbiased estimate of gene diversity ranging from 0.262–0.289 for both geographic ranges based on AFLP markers. Our results demonstrate that there has been no evolutionary shift from outcrossing to selfing during A. artemisiifolia's invasion of China. Furthermore, high levels of genetic variation in North America and China indicate that there has been no erosion of genetic variance due to a bottleneck during the introduction process. We suggest that the successful invasion of A. artemisiifolia into Asia was facilitated by repeated introductions from multiple source populations in the native range creating a diverse gene pool within Chinese populations.
Highlights
The main challenge for populations of self-incompatible outcrossing plants is the mate acquisition phase of sexual reproduction
Only one study in the literature clearly discovered the shift from self-incompatibility to self-compatibility in the invasive range [31].Our study provides the first detailed evidence of the mating system of A. artemisiifolia in its invasive Asian range
The main result is that the outcrossing rates are very high and do not deviate from one in both native North American and invasive Chinese populations
Summary
The main challenge for populations of self-incompatible outcrossing plants is the mate acquisition phase of sexual reproduction. Smaller and more isolated populations of self-incompatible wind-pollinated plants often experience increased pollen limitation [7] Such negative effects of low abundance on individual performance and population growth known as Allee Effects can dramatically decrease rate of spread, even preventing invasion altogether [8,9,10]. Under such conditions, natural selection may favor the adaptive evolution of selfing [5,11,12,13,14], which will allow a rapid build-up of the population from a small number of colonists. Out of five case studies in which the breeding system has been estimated in both native and invasive ranges [27,28,29,30,31], only one exhibited a shift from self-incompatibility to self-compatibility following introduction to the novel range [31]
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