Comparative transcriptomics of ice-crawlers demonstrates cold specialization constrains niche evolution in a relict lineage

Abstract

Key changes in ecological niche space are often critical to understanding how lineages diversify during adaptive radiations. However, the converse, or understanding why some lineages are depauperate and relictual, is more challenging, as many factors may constrain niche evolution. In the case of the insect order Grylloblattodea, highly conserved thermal breadth is assumed to be closely tied to their relictual status, but has not been formerly tested. Here, we investigate whether evolutionary constraints in the physiological tolerance of temperature can help explain relictualism in this lineage. Using a comparative transcriptomics approach, we investigate gene expression following acute heat and cold stress across members of Grylloblattodea and their sister group, Mantophasmatodea. We additionally examine patterns of protein evolution, to identify candidate genes of positive selection. We demonstrate that cold specialization in Grylloblattodea has been accompanied by the loss of the inducible heat shock response under both acute heat and cold stress. Additionally, there is widespread evidence of selection on protein-coding genes consistent with evolutionary constraints due to cold specialization. This includes positive selection on genes involved in trehalose transport, metabolic function, mitochondrial function, oxygen reduction, oxidative stress, and protein synthesis. These patterns of molecular adaptation suggest that Grylloblattodea have undergone evolutionary trade-offs to survive in cold habitats and should be considered highly vulnerable to climate change. Finally, our transcriptomic data provide a robust backbone phylogeny for generic relationships within Grylloblattodea and Mantophasmatodea. Major phylogenetic splits in each group relate to arid conditions driving biogeographical patterns, with support for a sister-group relationship between North American Grylloblatta and Altai-Sayan Grylloblattella, and a range disjunction in Namibia splitting major clades within Mantophasmatodea.

Keywords: Grylloblattodea; adaptation; gene regulation; insect phylogenomics; niche conservatism; protein evolution.

FIGURE 1

Multispecies coalescent trees inferred from astral‐ii showing the phylogenetic relationships of Mantophasmatodea and Grylloblattodea. Results are shown for (a) dataset 1 from orthograph, with 877 genes, and (b) dataset 3, merging 422 unique genes from dataset 1 and 2 with complete taxon sampling. Numbers at nodes indicate local posterior probability, and branch lengths are represented in coalescent units

FIGURE 2

The maximum likelihood tree inferred from the concatenated amino acid dataset of 3,022 genes. This ML tree received the best log‐likelihood score 39 times across 50 independent tree searches. Node support values were estimated based on 100 bootstrap replicates. Branch lengths are unscaled

FIGURE 3

Volcano plots of the differential expression analysis in acute temperature stress experiments. Plots show the statistical significance (y‐axis, log FDR value) and relative expression of genes (x‐axis, log fold change) under acute heat or acute cold stress relative to control samples. Significantly differentially expressed genes are indicated in red

FIGURE 4

Heat map of gene expression for members of the heat shock protein (hsp) gene family. Mean results are shown for control, acute cold, and acute heat treatments for: (a) Karoophasma biedouwense, (b) Galloisiana yezoensis, (c) Grylloblattella pravdini, (d) Grylloblatta bifratrilecta, (e) Grylloblatta marmoreus, (f) Grylloblatta sp. "North Cascades" at Whitechuck Mountain, and (g) Grylloblatta sp. "Lillburn Cave." Expression values represent normalized within‐species trimmed mean of M (TMM) values, and are shown on a log₂‐scale color gradient from off‐white to red. Refer to Table S14 for observed TMM values