1.1: Time-series data can distinguish between species destined for invasion and those that adapt in situ
One of the oldest postulations in the field of invasion biology is that some species are “predisposed” to invasiveness (Baker, 1965). In other words, some traits that facilitate spread in a new environment are not adaptations that arise following colonization (see Part 4), but instead exist in native populations prior to transport (Figure 2). For example, the kudzu bug (Megacopta cribraria ) was introduced from Asia to North America where it initially grew on kudzu, but within 9 months of detection had begun exploiting soy crops. Studies in the native range had shown that the genotype of the symbiont CandidatusIshikawaella capsulata (‘Ishikawaella’ hereafter) mediatesM. cribraria ’s ability to grow on soy (Hosokawa, Kikuchi, Nikoh, Shimada, & Fukatsu, 2006). Therefore, it was initially unclear whether the switch to soy was enabled by the evolution of Ishikawaella in the invasive range. Brown et al. (2014) inferred the evolution of the symbiont Ishikawaella by sequencing its genome at various locations, including the founding population in the year it was first detected, and at different time points since. Their analysis revealed that the founding population closely resembled native Japanese Ishikawaella samples known to enable growth on soy, with little evidence of allele frequency changes during invasion. This suggests that M. cribraria and its symbiont Ishikawaella had arrived in the US already able to spread on soy plantations. Though the 750kbp Ishikawaella genome is sequenced at scale with relative ease, this study exemplifies the significant and under-exploited benefits of WGR time-series data as a means of tracking genome-wide shifts in allele frequency during biological invasion. This approach could in principle be applied to any invasive species to distinguish between pre- and post-introduction adaptation.