Interpreting the dependence of mutation rates on age and time

Interpreting the dependence of mutation rates on age and timeZiyue Gao, Minyoung J. Wyman, Guy Sella, Molly Przeworski
(Submitted on 24 Jul 2015)

Mutations can arise from the chance misincorporation of nucleotides during DNA replication or from DNA lesions that are not repaired correctly. We introduce a model that relates the source of mutations to their accumulation with cell divisions, providing a framework for understanding how mutation rates depend on sex, age and absolute time. We show that the accrual of mutations should track cell divisions not only when mutations are replicative in origin but also when they are non-replicative and repaired efficiently. One implication is that the higher incidence of cancer in rapidly renewing tissues, an observation ascribed to replication errors, could instead reflect exogenous or endogenous mutagens. We further find that only mutations that arise from inefficiently repaired lesions will accrue according to absolute time; thus, in the absence of selection on mutation rates, the phylogenetic “molecular clock” should not be expected to run steadily across species.

The Nicrophorus vespilloides genome and methylome, a beetle with complex social behavior

The Nicrophorus vespilloides genome and methylome, a beetle with complex social behavior
Christopher B Cunningham, Lexiang Ji, R. Axel W Wiberg, Jennifer M Shelton, Elizabeth C McKinney, Darren J Parker, Richard B Meagher, Kyle M Benowitz, Eileen M Roy-Zokan, Michael G Ritchie, Susan J Brown, Robert J Schmitz, Allen J Moore

Testing for conserved and novel mechanisms underlying phenotypic evolution requires a diversity of genomes available for comparison spanning multiple independent lineages. For example, complex social behavior in insects has been investigated primarily with eusocial lineages, nearly all of which are Hymenoptera. If conserved genomic influences on sociality do exist, we need data from a wider range of taxa that also vary in their levels of sociality. Here we present information on the genome of the subsocial beetle Nicrophorus vespilloides, a species long used to investigate evolutionary questions of complex social behavior. We used this genome to address two questions. First, does life history predict overlap in gene models more strongly than phylogenetic groupings? Second, like other insects with highly developed social behavior but unlike other beetles, does N. vespilloides have DNA methylation? We found the overlap in gene models was similar between N. vespilloides and all other insect groups regardless of life history. Unlike previous studies of beetles, we found strong evidence of DNA methylation, which allows this species to be used to address questions about the potential role of methylation in social behavior. The addition of this genome adds a coleopteran resource to answer questions about the evolution and mechanistic basis of sociality.

Stable recombination hotspots in birds

Stable recombination hotspots in birds
Sonal Singhal, Ellen Leffler, Keerthi Sannareddy, Isaac Turner, Oliver Venn, Daniel Hooper, Alva Strand, Qiye Li, Brian Raney, Christopher Balakrishnan, Simon Griffith, Gil McVean, Molly Przeworski

Although the DNA-binding protein PRDM9 plays a critical role in the specification of meiotic recombination hotspots in mice and apes, it appears to be absent from many vertebrate species, including birds. To learn about the determinants of fine-scale recombination rates and their evolution in natural populations lacking PRDM9, we inferred fine-scale recombination maps from population resequencing data for two bird species, the zebra finch Taeniopygia guttata, and the long-tailed finch, Poephila acuticauda, whose divergence is on par with that between human and chimpanzee. We find that both bird species have hotspots, and these are enriched near CpG islands and transcription start sites. In sharp contrast to what is seen in mice and apes, the hotspots are largely shared between the two species, with indirect evidence of conservation extending across bird species tens of millions of years diverged. These observations link the evolution of hotspots to their genetic architecture, suggesting that in the absence of PRDM9 binding specificity, accessibility of the genome to the cellular recombination machinery, particularly around functional genomic elements, both enables increased recombination and constrains its evolution.

Conflict and cooperation in eukaryogenesis: implications for the timing of endosymbiosis and the evolution of sex

Conflict and cooperation in eukaryogenesis: implications for the timing of endosymbiosis and the evolution of sex
Arunas L Radzvilavicius, Neil W Blackstone

The complex eukaryotic cell is a result of an ancient endosymbiosis and one of the major evolutionary transitions. The timing of key eukaryotic innovations relative to the acquisition of mitochondria remains subject to considerable debate, yet the evolutionary process itself might constrain the order of these events. Endosymbiosis entailed levels-of-selection conflicts, and mechanisms of conflict mediation had to evolve for eukaryogenesis to proceed. The initial mechanisms of conflict mediation were based on the pathways inherited from prokaryotic symbionts and led to metabolic homeostasis in the eukaryotic cell, while later mechanisms (e.g., mitochondrial gene transfer) contributed to the expansion of the eukaryotic genome. Perhaps the greatest opportunity for conflict arose with the emergence of sex involving whole-cell fusion. While early evolution of cell fusion may have affected symbiont acquisition, sex together with the competitive symbiont behaviour would have destabilised the emerging higher-level unit. Cytoplasmic mixing, on the other hand, would have been beneficial for selfish endosymbionts, capable of using their own metabolism to manipulate the life history of the host. Given the results of our mathematical modelling, we argue that sex represents a rather late proto- eukaryotic innovation, allowing for the growth of the chimeric nucleus and contributing to the successful completion of the evolutionary transition.

Morphological data is lacking for living mammals

Morphological data is lacking for living mammals
Thomas Guillerme, Natalie Cooper

Combining living and fossil in the same analysis data is crucial for studying changes in global biodiversity through time. One method allowing to combine this data is the Total Evidence method that uses both molecular data for living species and morphological data for both living and fossil species. With this method, a good overlap of morphological data between living and fossil taxa is crucial for accurately inferring the phylogenies’ topology. Since the advent of DNA, molecular data has become easily and widely available. However, despite two centuries of morphological studies, scientists using and generating such data mainly focus on palaeontological data. Therefore, there is a gap in our knowledge of neontological morphological data even in well studied groups such as mammals. In this study, we quantify the morphological data available for living mammal taxa. We then analyse the structure of the available data by testing if it is clustered or evenly spread across the phylogeny. We found that 78% of mammalian orders have less than 25% data available at the species level. However, we found that the available is often randomly distributed among these orders apart from six of them where the data is clustered

Non-paradoxical evolutionary stability of the recombination initiation landscape in Saccharomycetes

Non-paradoxical evolutionary stability of the recombination initiation landscape in SaccharomycetesIsabel Lam, Scott Keeney

The nonrandom distribution of meiotic recombination shapes heredity and genetic diversification. A widely held view is that individual hotspots — favored sites of recombination initiation — are always ephemeral because they evolve rapidly toward extinction. An alternative view, often ignored or dismissed as implausible, predicts conservation of the positions of hotspots if they are chromosomal features under selective constraint, such as gene promoters. Here we empirically test opposite predictions of these theories by comparing genome-wide maps of meiotic recombination initiation from widely divergent species in the Saccharomyces clade. We find that the frequent overlap of hotspots with promoters is true of the species tested and, consequently, hotspot positions are well conserved. Remarkably, however, the relative strength of individual hotspots is also highly conserved, as are larger-scale features of the distribution of recombination initiation. This stability, not predicted by prior models, suggests that the particular shape of the yeast recombination landscape is adaptive, and helps in understanding evolutionary dynamics of recombination in other species.

Genetic loci with parent of origin effects cause hybrid seed lethality between Mimulus species

Genetic loci with parent of origin effects cause hybrid seed lethality between Mimulus species
Austin G Garner, Amanda M Kenney, Lila Fishman, Andrea L Sweigart

The classic finding in both flowering plants and mammals that hybrid lethality often depends on parent of origin effects suggests that divergence in the underlying loci might be an important source of hybrid incompatibilities between species. In flowering plants, there is now good evidence from diverse taxa that seed lethality arising from interploidy crosses is often caused by endosperm defects associated with deregulated imprinted genes. A similar seed lethality phenotype occurs in many crosses between closely related diploid species, but the genetic basis of this form of early-acting F1 postzygotic reproductive isolation is largely unknown. Here, we show that F1 hybrid seed lethality is an exceptionally strong isolating barrier between two closely related Mimulus species, M. guttatus and M. tilingii, with reciprocal crosses producing less than 1% viable seeds. Using a powerful crossing design and high-resolution genetic mapping, we identify both maternally- and paternally-derived loci that contribute to hybrid seed incompatibility. Strikingly, these two sets of loci are largely non-overlapping, providing strong evidence that genes with parent of origin effects are the primary driver of F1 hybrid seed lethality between M. guttatus and M. tilingii. We find a highly polygenic basis for both parental components of hybrid seed lethality suggesting that multiple incompatibility loci have accumulated to cause strong postzygotic isolation between these closely related species. Our genetic mapping experiment also reveals hybrid transmission ratio distortion and chromosomal differentiation, two additional correlates of functional and genomic divergence between species.