Discussion
Nebria riversi has a smaller genome than most sequenced beetles to date, which is associated with a reduction in transposable element content and a truncation in the distribution of intron length. A comparison of genome structure shows that the number of predicted genes remains within the range of other beetles and the number of collinear genes is similar (but modest) across sequenced coleopteran lineages. These results, in addition to patterns of gene family expansion, are primarily associated with molecular changes involved in cold specialization (see below). In contrast, there is more limited evidence of genome novelty in Carabidae, relative to other Coleoptera. Rapidly expanding ortholog groups associated with DNA and RNA binding suggest evolutionary divergence in gene regulation, but this is not unexpected given the long evolutionary history (~300 Myr) separating Carabidae from other Coleoptera (Schmitz et al. 2016). The expansion of the troponin orthogroup might be linked to muscle fiber performance of predatory ground beetles, but also could be linked to cold adaptation (see below). Contracting gene families (Table S7 ), especially the glutathione S-transferase genes, and ion channel genes, but potentially the genes linked to monooxygenase activity and membrane transport, likely distinguish the predatory lifestyle of ground beetles from the herbivorous lifestyle of most Polyphaga (Seppeyet al. 2019a). These gene families are known to play a role in detoxifying plant secondary chemicals, and likely diversified in Polyphaga as a result of the plant-insect coevolutionary arms race (Ehrlich & Raven 1964). Consistent with the transcriptomic analysis of Seppey et al. (2019a), many of the adaptive lineage-specific gene expansions in beetles (Figure 1 ) occur in Polyphaga, rather than Adephaga, and involve plant dietary innovations.