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Rapid enhancer and slow promoter evolution have been demonstrated through comparative genomics. However, it is not clear how this information is encoded genetically and if this can be used to place evolution in a predictive context. Part of the challenge is that our understanding of the potential for regulatory evolution is biased primarily toward natural variation or limited experimental perturbations. Here, to explore the evolutionary capacity of promoter variation, we surveyed an unbiased mutation library for three promoters in Drosophila melanogaster . We found that mutations in promoters had limited to no effect on spatial patterns of gene expression. Compared to developmental enhancers, promoters are more robust to mutations and have more access to mutations that can increase gene expression, suggesting that their low activity might be a result of selection. Consistent with these observations, increasing the promoter activity at the endogenous locus of shavenbaby led to increased transcription yet limited phenotypic changes. Taken together, developmental promoters may encode robust transcriptional outputs allowing evolvability through the integration of diverse developmental enhancers. This article is part of the theme issue ‘Interdisciplinary approaches to predicting evolutionary biology’.

Gene regulation is a ubiquitous mechanism by which organisms respond to their environment. While organisms are often found to be adapted to the environments they experience, the role of gene regulation in environmental adaptation is not often known. In this study, we examine divergence incis-regulatory effects between twoSaccharomycesspecies,S. cerevisiaeandS. uvarum, that have substantially diverged in their thermal growth profile. We measured allele specific expression (ASE) in the species’ hybrid at three temperatures, the highest of which is lethal toS. uvarumbut not the hybrid orS. cerevisiae. We find thatS. uvarumalleles can be expressed at the same level asS. cerevisiaealleles at high temperature and mostcis-acting differences in gene expression are not dependent on temperature. While a small set of 136 genes show temperature-dependent ASE, we find no indication that signatures of directionalcis-regulatory evolution are associated with temperature. Within promoter regions we find binding sites enriched upstream of temperature responsive genes, but only weak correlations between binding site and expression divergence. Our results indicate that temperature divergence betweenS. cerevisiaeandS. uvarumhas not caused widespread divergence incis-regulatory activity, but point to a small subset of genes where the species’ alleles show differences in magnitude or opposite responses to temperature. The difficulty of explaining divergence incis-regulatory sequences with models of transcription factor binding sites and nucleosome positioning highlights the importance of identifying mutations that underliecis-regulatory divergence between species.

Large genomes with elevated mutation rates are prone to accumulating deleterious mutations more rapidly than natural selection can purge (Muller's ratchet). As a consequence, it may lead to the extinction of small populations. Relative to most unicellular organisms, cancer cells, with large and nonrecombining genome and high mutation rate, could be particularly susceptible to such "mutational meltdown." However, the most common type of mutation in organismal evolution, namely, deleterious mutation, has received relatively little attention in the cancer biology literature. Here, by monitoring single-cell clones from HeLa cell lines, we characterize deleterious mutations that retard the rate of cell proliferation. The main mutation events are copy number variations (CNVs), which, estimated from fitness data, happen at a rate of 0.29 event per cell division on average. The mean fitness reduction, estimated reaching 18% per mutation, is very high. HeLa cell populations therefore have very substantial genetic load and, at this level, natural population would likely face mutational meltdown. We suspect that HeLa cell populations may avoid extinction only after the population size becomes large enough. Because CNVs are common in most cell lines and tumor tissues, the observations hint at cancer cells' vulnerability, which could be exploited by therapeutic strategies.

A growing body of research suggests that the mitochondrial genome (mtDNA) is important for temperature adaptation. In the yeast genus Saccharomyces, species have diverged in temperature tolerance, driving their use in high- or low-temperature fermentations. Here, we experimentally test the role of mtDNA in temperature tolerance in synthetic and industrial hybrids (Saccharomyces cerevisiae × Saccharomyces eubayanus or Saccharomyces pastorianus), which cold-brew lager beer. We find that the relative temperature tolerances of hybrids correspond to the parent donating mtDNA, allowing us to modulate lager strain temperature preferences. The strong influence of mitotype on the temperature tolerance of otherwise identical hybrid strains provides support for the mitochondrial climactic adaptation hypothesis in yeasts and demonstrates how mitotype has influenced the world’s most commonly fermented beverage., This work was supported by the USDA National Institute of Food and Agriculture (Hatch project no. 1003258), the NSF (grant no. DEB-1253634), and in part by the DOE Great Lakes Bioenergy Research Center (DOE BER Office of Science; nos. DE-SC0018409 and DE-FC02-07ER64494). E.P.B. was supported by a Louis and Elsa Thomsen Wisconsin Distinguished Graduate Fellowship. C.T.H. is a Pew Scholar in the Biomedical Sciences and a Vilas Faculty Early Career Investigator, supported by the Pew Charitable Trusts and the Vilas Trust Estate. D.P. is a Marie Sklodowska-Curie fellow of the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 747775). J.C.F. was supported by the NIH (no. GM080669)

Genetic analysis of phenotypic differences between species is typically limited to interfertile species. Here, we conducted a genome-wide noncomplementation screen to identify genes that contribute to a major difference in thermal growth profile between two reproductively isolated yeast species, Saccharomyces cerevisiae and Saccharomyces uvarum. The screen identified only a single nuclear-encoded gene with a moderate effect on heat tolerance, but, in contrast, revealed a large effect of mitochondrial DNA (mitotype) on both heat and cold tolerance. Recombinant mitotypes indicate that multiple genes contribute to thermal divergence, and we show that protein divergence in COX1 affects both heat and cold tolerance. Our results point to the yeast mitochondrial genome as an evolutionary hotspot for thermal divergence., This work was supported by the NIH (grant GM080669) to J.C.F. Additional support to C.T.H. was provided by the USDA National Institute of Food and Agriculture (Hatch project 1003258), the National Science Foundation (DEB-1253634), and the DOE Great Lakes Bioenergy Research Center (DOE BER Office of Science DE-SC0018409 and DE-FC02-07ER64494 to T. J. Donohue). C.T.H. is a Pew Scholar in the Biomedical Sciences and a Vilas Faculty Early Career Investigator, supported by the Pew Charitable Trusts and the Vilas Trust Estate, respectively. D.P. is a Marie Sklodowska-Curie fellow of the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 747775).

ABSRACTLarge and non-recombining genomes are prone to accumulating deleterious mutations faster than natural selection can purge (Muller’s ratchet). A possible consequence would then be the extinction of small populations. Relative to most single-cell organisms, cancer cells, with large and non-recombining genomes, could be particularly susceptible to such “mutational meltdown”. Curiously, deleterious mutations in cancer cells are rarely noticed despite the strong signals in cancer genome sequences. Here, by monitoring single-cell clones from HeLa cell lines, we characterize deleterious mutations that retard cell proliferation. The main mutational events are copy number variations (CNVs), which happen at an extraordinarily high rate of 0.29 events per cell division. The average fitness reduction, estimated to be 18% per mutation, is also very high. HeLa cell populations therefore have very substantial genetic load and, at this level, natural population would likely experience mutational meltdown. We suspect that HeLa cell populations may avoid extinction only after the population size becomes large. Because CNVs are common in most cell lines and cancer tissues, the observations hint at cancer cells’ vulnerability, which could be exploited by therapeutic strategies.

Populations accumulate deleterious mutations that have yet to be removed by natural selection. This mutated portion of the population that does not contribute to its fitness is called the genetic load and is proportional to the deleterious mutation rate. Given the genomic instability of cancer cells, the genetic load could be substantial if the instability indeed leads to loss of cell fitness. We hypothesize that because aneuploidy and copy number variations (CNVs) are the most common forms of genome instability, their fitness consequences may determine the genetic load in cancer cells. To test this hypothesis, we randomly selected single cells from a HeLa cell line and measured the cell growth rate via daily cell counting and an MTT cell proliferation assay. We then measured the genetic changes and resulting growth rate variation. We observed rapid generation of heterogeneity in the growth rate within the population. The growth rate of different progeny cells was stable indicating that it is genetically determined. Next, using whole genome sequencing of recently descended clones of a single HeLa cell, we found that cells with fewer newly acquired CNVs have higher proliferation rates vice versa. This indicated that CNV accumulation decreased cell fitness systematically. We next estimated the rate of CNV mutation by measuring the growth rate of two daughter cells from single proliferation and found that there is approximately 1 deleterious mutation in every 4 cell divisions. We speculated that due to a high, deleterious CNV mutation rate, tumor cells inevitably accumulate deleterious CNVs and a large percentage of tumor cells are genetically defective. Accordingly, we observed that the average growth rate of tumor cell populations decreased in the short term since defective cells accumulated in the population and the variation of cellular growth rates within the population increased. By modeling the process of mutation accumulation and measuring cell growth rate, we estimated that the deleterious mutation rate in HeLa cells is about 0.26-0.31 per cell division, and that HeLa cells reduce roughly 5%-6% of fitness for every cell division. The observations of a high proliferation rate and high genetic load in this representative tumor cell line indicates a “high risk, high reward” evolution strategy for tumor cells and suggests that increasing the level of genomic instability may cause the meltdown of tumor cell population by forcing cells to accelerate the cell cycle. Citation Format: Yuezheng Zhang, Xu Shen, Yawei Li, Tianqi Zhu, Yong Tao, Tao Li, Di Wang, Xueying Li, Qin Ma, Xuemei Lu, Hurng-Yi Wang, Chung-I Wu. Genetic load in cancer cell populations [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 514. doi:10.1158/1538-7445.AM2017-514