Long term environmental stability drives reduced stress tolerance in salt lake invertebrates

The capacity of species to tolerate physical stressors is critical in a world of increasing environmental instability, however, past selective environments should dramatically impact on future stress tolerance, particularly in isolated populations. Through stabilising selection, long-term environmental stasis may reduce physiological tolerance, creating an evolutionary legacy where populations are less fit if environments change. Few empirical studies have investigated this evolutionary legacy of past selection, and of particular interest whether stabilising selection in a benign environment reduces stress tolerance in natural systems. Here we use multiple populations of salt-lake invertebrates (Coxiella striata, Austrochiltonia subtenuis) with either stable or fluctuating environmental histories to investigate the relationship between stabilising selection and environmental stress resistance. Tolerance to both salinity and temperature stress were examined in invertebrate populations from lakes with long-term (decadal) stable environments and compared with populations from lakes with extreme salinity variations. Individuals from stable environments demonstrated significantly lower survival under both increasing salinity and temperature stresses when compared with environmentally unstable populations. Our results support the hypothesis that the evolutionary legacy from stabilising selection in constant environments leads to reduced stress tolerance. This finding demonstrates that under an increasingly variable climate, the evolutionary legacies of populations will be critical for future survival and adaptation.

Stabilising selection causes grossly altered but stable karyotypes in metastatic colorectal cancer

Aneuploidy, defined as the loss and gain of whole and part chromosomes, is a near-ubiquitous feature of cancer genomes, is prognostic, and likely an important determinant of cancer cell biology. In colorectal cancer (CRC), aneuploidy is found in virtually all tumours, including precursor adenomas. However, the temporal evolutionary dynamics that select for aneuploidy remain broadly uncharacterised. Here we perform genomic analysis of 755 samples from a total of 167 patients with colorectal-derived neoplastic lesions that cross-sectionally represent the distinct stages of tumour evolution, and longitudinally track individual tumours through metastasis and treatment. Precancer lesions (adenomas) exhibited low levels of aneuploidy but high intra-tumour heterogeneity, whereas cancers had high aneuploidy but low heterogeneity, indicating that progression is through a genetic bottleneck that suppresses diversity. Individual CRC glands from the same tumour have similar karyotypes, despite prior evidence of ongoing instability at the cell level. Pseudo-stable aneuploid genomes were observed in metastatic lesions sampled from liver and other organs, after chemo- or targeted therapies, and late recurrences detected many years after the diagnosis of a primary tumour. Modelling indicates that these data are consistent with the action of stabilising selection that ‘traps’ cancer cell genomes on a fitness peak defined by the specific pattern of aneuploidy. These data show that the initial progression of CRC requires the traversal of a rugged fitness landscape and subsequent genomic evolution, including metastatic dissemination and therapeutic resistance, is constrained by stabilising selection.

Fluctuating asymmetry in an ornamental trait in the cave weta, Pleioplectron simplex Hutton (Orthoptera: Rhaphidophoridae): no role for parasites

Using digitised pictures of the pronotum, we investigated the influence of nematomorph and mermithid parasites on the expression (intensity and fluctuating asymmetry) of cuticular ornamentation in the cave weta, Pleioplectron simplex. Because P. simplex is nocturnal, this ornamentation is probably not subject to strong directional or stabilising selection. Based on measurements of brightness, we found no significant difference in the amount of pigmented area between unparasitised and parasitised males or females. Parasitised individuals were not significantly more asymmetrical than unparasitised ones. This result was not due to measurement error. Among parasitised wetas, brightness and the fluctuating asymmetry were not related to the length of the parasite, even when the size of the host was taken into account. These results are discussed in relation to current ideas concerning the influence of parasites on fluctuating asymmetry of traits in the host.