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.