Neutral processes forming large clones during colonization of new areas.

M Rafajlović,J Fries,L Sundqvist,K Johannesson,D Kleinhans,R T Pereyra,A Ardehed,C Gulliksson,D Johansson,B Mehlig,P R Jonsson

doi:10.1111/jeb.13124

M Rafajlović, J Fries + Show 9 more

Open Access

https://doi.org/10.1111/jeb.13124

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Abstract

In species reproducing both sexually and asexually clones are often more common in recently established populations. Earlier studies have suggested that this pattern arises due to natural selection favouring generally or locally successful genotypes in new environments. Alternatively, as we show here, this pattern may result from neutral processes during species' range expansions. We model a dioecious species expanding into a new area in which all individuals are capable of both sexual and asexual reproduction, and all individuals have equal survival rates and dispersal distances. Even under conditions that favour sexual recruitment in the long run, colonization starts with an asexual wave. After colonization is completed, a sexual wave erodes clonal dominance. If individuals reproduce more than one season, and with only local dispersal, a few large clones typically dominate for thousands of reproductive seasons. Adding occasional long-distance dispersal, more dominant clones emerge, but they persist for a shorter period of time. The general mechanism involved is simple: edge effects at the expansion front favour asexual (uniparental) recruitment where potential mates are rare. Specifically, our model shows that neutral processes (with respect to genotype fitness) during the population expansion, such as random dispersal and demographic stochasticity, produce genotype patterns that differ from the patterns arising in a selection model. The comparison with empirical data from a post-glacially established seaweed species (Fucus radicans) shows that in this case, a neutral mechanism is strongly supported.

Highlights

Most species in northern Europe, Asia and America established their present distributions following the last glacial maximum about 17,000 years ago
Within the tested values of the model parameters, we do not find any cases of global fixation, except under the modified version of the model in which males are incapable of asexual reproduction
In the model with long-range dispersal of sexually produced seeds instead of clonal fragments, we find that local clonal structures are likely to be formed (Fig. S7), but the geographic spread of a dominant clone is smaller than when clonal fragments have the potential to disperse long distances (Fig. S8)

Summary

Introduction

Most species in northern Europe, Asia and America established their present distributions following the last glacial maximum about 17,000 years ago. The incidence of asexual recruitment is commonly higher in new territories and in young habitats (Ting & Geller, 2000; Eckert, 2002; Kearney, 2003; Kliber & Eckert, 2005; Tatarenkov et al, 2005; Liu et al, 2006; Kawecki, 2008; Silvertown, 2008; Vrijenhoek & Parker, 2009) Earlier studies suggest this bias is due to selection favouring specific genotypes that are generally, or locally, successful in the new environment (Parker et al, 1977; Vrijenhoek, 1984; Peck et al, 1998; Stenberg et al, 2003; Kearney, 2005; Hörandl, 2009). Both groups of models have, difficulties in explaining why a majority of species with asexual reproduction has preserved sexual function in the absence of stabilising selection maintaining sexual traits (Green & Noakes, 1995; Johnson et al, 2010)

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Conclusion