Biology subjects

Rafajlovic, M.

Publications and source records attributed to Rafajlovic, M..

11 recordsLinked to original sources

bioRxiv2026

Do sex differences in autosomal recombination rates facilitate divergence?

Recombination rate varies within and between individuals. One form of such variations is seen between sexes in dioecious populations, with males typically exhibiting a smaller recombination rate than females. This is true both for sex chromosomes and autosomes (so-called heterochiasmy). Although a large body of theory exists on the role of sex chromosomes in adaptation and population divergence, much less is known about the role of heterochiasmy. Recently, it has been suggested that heterochiasmy can facilitate local adaptation and divergence, but if, and when this is true has not been systematically studied theoretically to date. Here we use Individual-based simulations to assess the effect of sex differences in autosomal recombination rates on the process of divergence and adaptation in populations subject to divergent selection and migration. We found evidence supporting that sex differences in autosomal recombination rate between adaptive loci can facilitate, and especially maintain, divergence, but this is true only under very limited conditions, involving strong selection, high sex-averaged effective recombination rates and relatively high rates of migration compared to the strength of selection. We further found that this effect, when present, is typically weak but is amplified in cases of highly polygenic adaptation in comparison to cases with a few adaptive loci of strong effect. We conclude that, in most cases, sex differences in autosomal recombination rate alone are unlikely to noticeably contribute to the process of adaptation and divergence. Further studies are needed to evaluate their effect in combination with other processes not considered in the present study, such as assortative mating between the alike mates, or recombination suppression in heterozygotes. TeaserIn dioecious populations, recombination rate typically differs between males and females. This is true both for sex chromosomes and autosomes. While much theoretical research has focused on understanding how recombination rate differences in sex chromosomes shape local adaptation and divergence, we lack theoretical knowledge of the potential role of sex differences in autosomal recombination rates. Recombination has a dual role in local adaptation. Strong recombination can effectively purge deleterious alleles, but it can also break apart beneficial allele complexes (and vice versa for weak recombination). Thus, one may expect that in the presence of both strong and weak recombination exhibited by females, and males, respectively, population divergence can be efficiently facilitated. But is this true? Here, we study this question theoretically using computer simulations. Our main finding is that sex differences in autosomal recombination can facilitate divergence, but this effect is typically weak and present only under very stringent conditions.

evolutionary biology↗

bioRxiv2023

Coupling of 12 chromosomal inversions maintains a strong barrier to gene flow between ecotypes

Chromosomal rearrangements lead to the coupling of reproductive barriers, but whether and how they contribute to completion of speciation remains unclear. Marine snails of the genus Littorina repeatedly form hybrid zones between taxa segregating for inversion arrangements, providing opportunities to study this question. Here, we analysed two adjacent transects across hybrid zones between large and dwarf ecotypes of Littorina fabalis covering wave exposure gradients in a Swedish island. Applying whole-genome sequences we found 12 putative inversions reaching near differential fixation between the opposite ends of each transect, and being in strong linkage disequilibrium. These inversions cover 20% of the genome and carry 93% of divergent SNPs. Bimodal hybrid zones in both transects indicate that the two ecotypes of Littorina fabalis maintain their genetic and phenotypic integrity following contact due to strong coupling between inversion clines that strengthened the reproductive barrier. The bimodality resulting from the linked inversions extends into collinear parts of the genome, suggesting a genome-wide coupling. Demographic inference suggests that the coupling built up during a period of allopatry, and has been maintained for more than 1K generations after secondary contact. Overall, this study shows that the coupling of multiple chromosomal inversions contributes to strong reproductive isolation. Importantly, two of the inversions overlap with inverted genomic regions associated with ecotype differences in a closely-related species (L. saxatilis), suggesting the same regions, with similar structural variants, repeatedly contribute to ecotype evolution in distinct species.

evolutionary biology↗

bioRxiv2022

Adaptive, maladaptive, neutral, or absent plasticity: Hidden caveats of reaction norms

Adaptive phenotypic plasticity may improve the response of individuals when faced with new environmental conditions. Typically, empirical evidence for plasticity is based on phenotypic reaction norms obtained in reciprocal transplant experiments. In such experiments, individuals from their native environment are transplanted into a different environment, and a number of trait values, potentially implicated in individuals response to the new environment, are measured. However, the interpretations of reaction norms may differ depending on the nature of the assessed traits, which may not be known beforehand. For example, for traits that contribute to local adaptation, adaptive plasticity implies non-zero slopes of reaction norms. By contrast, for traits that are correlated to fitness, high tolerance to different environments (possibly due to adaptive plasticity in traits that contribute to adaptation) may, instead, result in flat reaction norms. Here we investigate reaction norms for adaptive versus fitness-correlated traits, and how they may affect the conclusions regarding the contribution of plasticity. To this end, we first simulate range expansion along an environmental gradient where plasticity evolves to different values locally and then perform reciprocal transplant experiments in silico. We show that reaction norms alone cannot inform us whether the assessed trait exhibits locally adaptive, maladaptive, neutral or no plasticity, without any additional knowledge of the traits assessed and species biology. We use the insights from the model to analyse and interpret empirical data from reciprocal transplant experiments involving the marine isopod Idotea balthica sampled from two geographical locations with different salinities, concluding that the low-salinity population likely has reduced adaptive plasticity relative to the high-salinity population. Overall, we conclude that, when interpreting results from reciprocal transplant experiments, it is necessary to consider whether traits assessed are locally adaptive with respect to the environmental variable accounted for in the experiments, or correlated to fitness.

evolutionary biology↗

bioRxiv2022

Clones on the run - the genomics of a recently expanded facultative asexual species

Why, in facultative asexual species, marginal populations are often richer in clones than are core populations, remains unclear. Cloning freezes genotypes but hampers recombination and local adaptation. During expansion, clones are favoured over non-selfing sexuals by uniparental reproduction. To better understand the dynamics of clones and sexual lineage, we used genome-wide sequencing to analyse a recently expanded seaweed. We found large clones and sexual populations mixed close to range margins. Clones had evolved repeatedly from sexual populations but were unexpectedly low in genetic variation. Modelling suggested clones form from sexual populations after repeated bottlenecks at the expansion front. A clonal wave of depauperate genotypes thereafter spread ahead of the sexual population. As we observed, these early formed clones may survive side-by-side sexual individuals, which suggests they lost their sexual capacity. Our study illustrates how range expansion can result in complex and dynamic patterns of genetic variation in facultative asexual species. TeaserWe use genome data and modelling to find out why large clones are only found at range margins in a recently expanded seaweed

evolutionary biology↗

bioRxiv2022

Species' ranges and the steepening-gradient hypothesis

One hypothesis invoked to explain limits to species ranges is a mismatch in environmental conditions between the central and marginal areas of species ranges. Low population size at the margins causes genetic drift to outplay selection locally, and limits the accumulation of genetic variance, so that adaptation is hindered locally. Earlier theoretical work shows that, for a population expanding over a spatially heterogeneous environment without any geographical barriers, adaptation will fail abruptly and sharp range margins will establish only when the underlying environmental conditions change more and more severely across space, whereas an environment changing constantly will result either in infinite expansion or rapid global extinction. Here, we extend this "steepening-gradient hypothesis" to encompass situations when multiple (up to three) environmental factors impose selection on separate adaptive traits. We show that multiple selection gradients steepen each other and that it is sufficient that just one of the gradients steepen in space for sharp range margins to form. This is true even if this gradient is shallow throughout the realised range. Thus, despite its detrimental role in forming range margins, it could be overlooked in field studies. Finally, by decomposing an environmental gradient to selection on two (or three) adaptive traits, we show that a population can withstand harsher environmental conditions than when selection acts on one adaptive trait alone. This finding argues for the evolution of novel traits in harsh environments.

evolutionary biology↗

bioRxiv2021

The role of phenotypic plasticity in the establishment of range margins

It has been argued that adaptive phenotypic plasticity may facilitate range expansions over spatially and temporally variable environments. However, plasticity may induce fitness costs. This may hinder the evolution of plasticity. Earlier modelling studies examined the role of plasticity during range expansions of populations with fixed genetic variance. However, genetic variance evolves in natural populations. This may critically alter model outcomes. We ask: How does the capacity for plasticity in populations with evolving genetic variance alter range margins that populations without the capacity for plasticity are expected to attain? We answered this question using computer simulations and analytical approximations. We found a critical plasticity cost above which the capacity for plasticity has no impact on the expected range of the population. Below the critical cost, by contrast, plasticity facilitates range expansion, extending the range in comparison to that expected for populations without plasticity. We further found that populations may evolve plasticity to buffer temporal environmental fluctuations, but only when the plasticity cost is below the critical cost. Thus, the cost of plasticity is a key factor involved in range expansions of populations with the potential to express plastic response in the adaptive trait.

evolutionary biology↗

bioRxiv2020

Assessing the role of inversions in maintaining genomic differentiation after secondary contact: local adaptation, genetic incompatibilities, and drift

Due to their effects on reducing recombination, chromosomal inversions may play an important role in speciation by establishing and/or maintaining linked blocks of genes causing reproductive isolation (RI) between populations. These views fit empirical data indicating that inversions typically harbour loci involved in RI. However, previous computer simulations of infinite populations with 2-4 loci involved in RI implied that, even with gene flux as low as 10-8 between alternative arrangements, inversions may not have large, qualitative advantages over collinear regions in maintaining population differentiation after secondary contact. Here, we report that finite population sizes can help counteract the homogenizing consequences of gene flux, especially when several fitness-related loci reside within the inversion. In these cases, the persistence time of differentiation after secondary contact can be similar to when gene flux is absent, and notably longer than the persistence time without inversions. Thus, despite gene flux, population differentiation may be maintained for up to 100,000 generations, during which time new incompatibilities and/or local adaptations might accumulate and facilitate progress towards speciation. How often these conditions are met in nature remains to be determined.

evolutionary biology↗

bioRxiv2020

Assortative mating, sexual selection and their consequences for gene flow in Littorina

When divergent populations are connected by gene flow, the establishment of complete reproductive isolation usually requires the joint action of multiple barrier effects. One example where multiple barrier effects are coupled consists of a single trait that is under divergent natural selection and also mediates assortative mating. Such multiple-effect traits can strongly reduce gene flow. However, there are few cases where patterns of assortative mating have been described quantitatively and their impact on gene flow has been determined. Two ecotypes of the coastal marine snail, Littorina saxatilis, occur in North Atlantic rocky-shore habitats dominated by either crab predation or wave action. There is evidence for divergent natural selection acting on size, and size-assortative mating has previously been documented. Here, we analyze the mating pattern in L. saxatilis with respect to size in intensively-sampled transects across boundaries between the habitats. We show that the mating pattern is mostly conserved between ecotypes and that it generates both assortment and directional sexual selection for small male size. Using simulations, we show that the mating pattern can contribute to reproductive isolation between ecotypes but the barrier to gene flow is likely strengthened more by sexual selection than by assortment.

evolutionary biology↗

bioRxiv2020

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

Previous theoretical work on range expansions over heterogeneous environments showed that there is a critical environmental gradient where range expansion stops. For populations with freely recombining loci underlying the trait under selection (hereafter adaptive loci), the critical gradient in one-dimensional habitats depends on the fitness cost of dispersal, and the strength of selection relative to genetic drift. Here, we extend the previous work in two directions and ask: What is the role of the genetic architecture of adaptive loci during range expansions? And what effect does the ability of selfing have on range expansions? To answer these questions, we use computer simulations. We demonstrate that, while reduced recombination rates between adaptive loci slow down range expansions due to poor purging of locally deleterious alleles at the expansion front, they may also allow a species to occupy a greater range. In fact, for some parameter values, we find that a population with freely recombining adaptive loci experiences global extinction, whereas reduced recombination rates allow for a successful expansion over a wide geographic range. In addition, we find that allowance of selfing may improve the ability of populations to expand their ranges. We discuss the mechanisms underlying these results.

evolutionary biology↗

bioRxiv2019

The impact of global selection on local adaptation and reproductive isolation.

Despite the homogenising effect of strong gene flow between two populations, adaptation under symmetric divergent selection pressures results in partial reproductive isolation: adaptive substitutions act as local barriers to gene flow, and if divergent selection continues unimpeded, this will result in complete reproductive isolation of the two populations, i.e. speciation. However, a key issue in framing the process of speciation as a tension between local adaptation and the homogenising force of gene flow is that the mutation process is blind to changes in the environment and therefore tends to limit adaptation. Here we investigate how globally beneficial mutations (GBMs) affect divergent local adaptation and reproductive isolation. When phenotypic divergence is finite, we show that the presence of GBMs limits local adaptation, generating a persistent genetic load at the loci which contribute to the trait under divergent selection and reducing genome-wide divergence. Furthermore, we show that while GBMs cannot prohibit the process of continuous differentiation, they induce a substantial delay in the genome-wide shutdown of gene flow.

evolutionary biology↗

bioRxiv2019

Spatial variation in introgression along a toad hybrid zone in France

The barrier effect is a restriction of gene flow between diverged populations by barrier genes. Restriction of gene flow and asymmetric introgression over multiple transects indicates species wide (genetic) adaptations, whereas transect-specific barrier loci may indicate local adaptation to gene flow. Asymmetric introgression can be caused by selection, hybrid zone movement, asymmetric reproductive isolation, or a combination of these. We study two widely separated transects (northwest and southeast France) for the 900 km long hybrid zone between Bufo bufo and B. spinosus toads, using ~1200 markers from restriction-site associated DNA (RAD) sequencing data. Genomic and geographic clines were used to identify outlier markers which show restricted or elevated introgression. Twenty-six barrier markers are shared between transects (the union of 56 and 123 barrier markers identified in each transect), which is more than would be expected by chance. However, the number of barrier markers is twice as high in the southeast transect. In the northwest transect a high amount of (asymmetric) introgression from B. spinosus into B. bufo is consistent with hybrid zone movement or asymmetric reproductive isolation. In the southeast transect, introgression is symmetric and consistent with a stable hybrid zone. Differences between transects may be related to genetic sub-structure within B. bufo. A longer period of secondary contact in southeast France appears to result in a relatively stronger barrier effect than in the northwest. The Bufo hybrid zone provides an excellent opportunity to separate a general barrier to gene flow from local reductions in gene flow.

evolutionary biology↗