Global warming now and then: the speed and load factors affecting the success or failure of a range expansion

A geographic range expansion is the colonization of new areas previously unoccupied. It is a common response to climate change, and it has a large impact on the levels of biodiversity and the biogeographic patterns. Several interacting environmental and biological factors contribute to the success (or failure) of an expansion, and we focus here on the colonization rate and the genetic load. In particular, using as a model an Italian endangered endemic species (the Apennine yellow-bellied toad, Bombina pachypus) that expanded northwards along the peninsula after the last glaciation, and an individual-based spatially explicit simulation model, we will study the genomic consequences of a successful postglacial expansion, infer the major characteristics of this process, and predict the impact of geographic range expansions over different temporal scales.

Our main goals are 1) estimate the expansion load in our model species, i.e. the differential loss of genetic variation and the accumulation of deleterious mutations along the expansion route; 2) develop a realistic simulation model capable of generating genomic patterns compatible with those observed in the real system; 3) use our realistic simulation model to understand how the rate of climate change (comparing past and contemporary global warming rates) and the velocity of the species to colonize new suitable habitats (mimicking the dispersal ability of a generic species) affect the expansion load and thus the probability of success of the colonization.

Extensive sampling of approximately 300 toads covering the entire distribution range and genomic typing at approximately 24 megabases of genic and intergenic regions will allow us to estimate the neutral and adaptive genomic variation patterns and clines. The genetic load will be evaluated using bioinformatic approaches that predict the deleteriousness of a mutation and considering also the level of expression of different genes and the chromatin accessibility of intergenic regions. The relationships between the load of an individual, separately computed in its realized and masked components affecting the fitness in present or future generations, respectively, and its geographic position along the expansion route, will be analyzed. The results will be relevant to predict the current extinction risks of this species in different geographic locations.

The simulation model will be developed using a forward approach that allows the inclusion of several life-history, genomic, demographic, and geographic features. This model, tested and adjusted using the empirical data obtained in the Apennine yellow-bellied toad, will be useful i) to estimate the main factors producing a successful or unsuccessful expansion (such as the level of genetic load in the source population, the rate of climate change and the dispersal process), and ii) as a predictive tool in climate change biology and the management of biological invasions.