Modelling the evolution of the phosphoglucose isomerase (Pgi) locus in butterfly metapopulations: project proposal.
Background: Metapopulation theory, as developed by R. Levins in 1969, concerns the dynamics of metapopulations, which consist of several distinct local populations that exhibit some degree of interaction and occupy isolated areas of suitable habitat, referred to as patches. Each local population occupies a patch and has a finite lifespan – which is dependent on the probability of extinction due to fluctuations in population size as a result of demographic stochasticity – with some patches left unoccupied and open to potential colonisation (Haag et al., 2005). Patch extinction and colonisation rate is related to the size of the patch population (and therefore carrying capacity) and the distance between patches (Hanski & Gaggiotti, 2004). The metapopulation generally remains at a stable equilibrium: as populations in some patches go extinct, other unoccupied patches are colonised and form new populations (Hanski, 1994). Patches are formed by spatial heterogeneity of the habitat, which may be the result of habitat fragmentation, geographical separation or isolation caused by inhabitable terrain over a portion of the range (Levins, 1969). Many spatial, behavioural and genetic factors affect metapopulation equilibrium – this study will examine dispersal propensity as a result of genetic variance of individuals. Understanding the evolution of dispersal within populations is vital when analysing the dynamics of a metapopulation.
Dispersal may arise within a population for several reasons, the evolution of which is well-studied and experimentally proven (Clobert et al., 2001). Dispersal evolves, or disappears, as a result of the combined effect of several selective pressures and driving forces for and against dispersal – the spread of dispersal ability within a population depends heavily on both the environment and competition between individuals (Heino & Hanski, 2001). A habitat with great temporal or spatial variation in quality or suitability is likely to increase the level of individuals with genotypes facilitating greater dispersal (Friedenberg, 2003), as are those where competition between individuals is high (Clobert et al., 2001). Another selective factor which increases the propensity of dispersal is avoidance of inbreeding, which may occur simultaneously with kin competition at high population levels, and as such dispersal is favoured at high population densities (Szulkin & Sheldon, 2008).
Dispersal includes costs, risks and trade-offs; therefore dispersal is not always beneficial to a population and may not arise. Dispersal is associated with increased mortality, particularly in archipelago systems where migrants must travel long distances over the sea (Johnson et al., 2009). Dispersal may also lower fecundity, as the energetic cost of travelling and the time spent migrating (especially in sustained flight) necessitates an elevated metabolic rate and extensive resource use – resources which otherwise could have been allocated to reproduction and other costly activities (Hanski et al., 2006). For this reason dispersal is selected against at low population densities, since propensity to disperse is expected to increase the likelihood of extinction of small populations, and carries no benefit when resources are plentiful (Hanski & Gaggiotti, 2004).
A single nucleotide polymorphism (SNP) at the phosphoglucose isomerase (Pgi) locus, which has two alleles – A and C – has been proven to show a strong association with individual variation of dispersal and fitness, with balancing selection via a heterozygote advantage conferring increased dispersal ability and fitness to migrant females relative to homozygotes of both allele types (Mattila & Hanski, 2014; Niitepõld et al., 2009; Orsini et al., 2009). The archipelago system in the Åland Islands in Finland has an extensive and well-studied metapopulation of Glanville fritillary butterflies (Melitaea cinxia), with islands forming habitable patches isolated from each other by the surrounding sea (Saastamoinen et al., 2009), and it is this metapopulation that will form the basis of this study. The study aims, by individual-based metapopulation modelling, to investigate the mechanisms of evolution of the polymorphism at the Pgi locus by analysing the effects of spatial heterogeneity on the spread and maintenance of the beneficial allele on an experimental basis. Particularly, I aim to determine whether the novel allele arose by a single mutation of large effect or by several mutations of cumulative effect; something which as yet has not been investigated and would provide great insight into the evolution of dispersal. Findings would also be applicable to other systems and metapopulations.
Hypotheses: There are two key testable hypotheses in this study, integral to understanding the evolution of the phosphoglucose isomerase locus and associated alleles, as follows:
- Increasing spatial heterogeneity of the environment by variance in patch size and distance will increase the propensity to disperse, resulting in an increased frequency of heterozygote individuals at equilibrium, as well as an increased frequency of the mutant allele in newer, isolated populations.
- The dispersal increasing C allele at the Pgi locus arose by a single mutation of large effect, with balancing selection via heterozygote advantage.
Methods: This study will be carried out by analysis of the effect of variable conditions on allele frequency at the Pgi locus at metapopulation equilibrium in an individual-based metapopulation model. The model will be constructed in the freely available NetLogo 5.0.5 modelling environment software (available at https://ccl.northwestern.edu/netlogo/).
The model will consist of a series of island patches isolated by sea and a parent patch with individual butterflies with attributes parameterised from existing experimental data collected in a previous study (Zheng et al., 2009) for the Glanville fritillary butterfly metapopulation in the Finnish Åland Islands. Three ‘breeds’ will be applied, of genotype AA, AC and CC, with male and female variants and assumed sexual reproduction and random mating. Each individual will be allocated specified attributes of dispersal ability, fecundity and survival – with AC heterozygotes possessing increased dispersal capacity – and colonisation and extinction of patches will be probabilistic as a function of population size. Patch carrying capacity and distance will be varied over several runs of the model; the output values of mortality and allele frequencies of each population at equilibrium will be recorded, and the effects of this spatial heterogeneity on dispersal and allele frequency will be analysed.
To determine the mechanism by which the single nucleotide polymorphism at the Pgi locus arose, the dispersal benefit gained by heterozygote females in a portion of separate model runs will be increased incrementally by small amounts, and this will be contrasted to model runs of one large increase in dispersal ability. The output variables in this instance will again be mortality and allele frequencies at metapopulation equilibrium, and the spread and maintenance of the dispersal benefiting C allele of the two suggested mechanisms of evolution will be compared to the parameterised equilibrium frequencies, in order to evaluate the validity of each.
A Student’s t-test will be employed to establish statistical significance of normalised differences in equilibrium allele frequencies between model runs.
Predictions: In concordance with previous studies (Mattila & Hanski, 2014; Zheng et al., 2009) it is expected that the mutant C allele will have a higher frequency in newly established, isolated populations and a lower frequency in older populations below patch carrying capacity, as the less-dispersive allele will be selected for when dispersal is not as beneficial to the individual as higher fecundity (Hanski et al., 2006). Increased spatial heterogeneity is expected to result in increased local population extinction risk, and therefore in increasing frequency of migrant females possessing the mutant C allele and higher propensity for dispersal (Mattila & Hanski, 2014).
It is anticipated that the single nucleotide polymorphism at the Pgi locus must have arisen by a single mutation of large effect, as multiple mutations of small cumulative effect would likely not initially provide a large enough benefit to dispersal ability to be selected for in favour of the less dispersive A allele, regardless of population and patch size.
Time allocation: Time available will be allocated to specific parts of the project following the plan below, which allows time for submission of relevant sections and feedback from the project supervisor for each.
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Zheng, C., Ovaskainen, O. and Hanski, I. (2009) Modelling single nucleotide effects in phosphoglucose isomerase on dispersal in the Glanville fritillary butterfly: coupling of ecological and evolutionary dynamics. Phil. Trans. R. Soc. B, 364, 1519-1532.