Neutral processes forming large clones during colonization of new areas

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.

The effect of genetic architecture and selfing on the capacity of a population to expand its range

“…Because of the large population size of individuals with near-optimal fitness, it would be highly unlikely for locally maladapted alleles originating from the source population to establish in this colony. A similar effect (albeit with clonal reproduction rather than selfing, and in a model with a homogeneous environment) was observed in a modelling study by Rafajlović et al (2017) where wide spread long-lived clonal colonies were formed during range expansion of a partly clonal population. A well-known disadvantage of selfing is the increased frequency of individuals homozygous for recessive deleterious alleles (Charlesworth and Charlesworth 1987;Pujol et al 2009).…”

“…Here we ask how the ability to self-fertilise affects the capability of a population to expand its range compared to when selfing is not possible. Selfing is common among plants and some hermaphrodite animals (Jarne and Charlesworth 1993), and it is known that uniparental reproduction can increase the speed of range expansion, at least under spatially homogeneous conditions (Baker 1955;Tomlinson 1966;Pannell and Barrett 1998;Rafajlović et al 2017). However, the ability for a single selfing individual to establish a new colony might come at a price of locally reduced genetic variation.…”

The evolutionary history of Nebraska deer mice: local adaptation in the face of strong gene flow

“…Thus, the fewer loci necessary for the diverging locally adaptive phenotype, the lower the resulting migration load. More recently, Yeaman & Whitlock (2011) argued that with gene flow, the genetic architecture underlying an adaptive phenotype is expected to have fewer and larger-effect alleles compared to neutral expectations under models without migration (i.e., an exponential distribution of effect sizes (and see Rafajlovic et al 2017)). As recombination between these locally beneficial alleles may result in maladapted intermediate phenotypes, this model further predicts a genomic clustering of the underlying mutations (Maynard Smith 1977;Lenormand & Otto 2000).…”

Tolerance to climate change of the clonally reproducing endemic Baltic seaweed, Fucus radicans: is phenotypic plasticity enough?

“…Clonal reproduction has typically been associated with dispersal to new regions and with the persistence of low‐density populations in range‐margin habitats, and this is also the case with Fucus radicans (Rafajlović et al. and references therein). Asexual reproduction is probably an asset in early colonization, but it eventually leads to a decrease in genetic diversity, as a clonal population harbors less standing genetic variation than one that reproduces sexually.…”