The Interaction of Natural Selection and GC Skew May Drive the Fast Evolution of a Sand Rat Homeobox Gene

Shared properties of gene transfer agent and core genes revealed by comparative genomics of Alphaproteobacteria

Gene transfer agents (GTAs) are phage-like particles that transfer pieces of cellular genomic DNA to other cells. Homologues of the Rhodobacter capsulatus GTA (RcGTA) structural genes are widely distributed in the Alphaproteobacteria and particularly well conserved in the order Rhodobacterales. Possible reasons for their widespread conservation are still being discussed. It has been suggested that these alphaproteobacterial elements originate from a prophage that was present in an ancestral bacterium and subsequently evolved into a GTA that is now widely maintained in extant descendant lineages. Here, we analysed genomic properties that might relate to the conservation of these alphaproteobacterial GTAs. This revealed that the chromosomal locations of the GTA gene clusters are biased. They primarily occur on the leading strand of DNA replication, at large distances from long repetitive elements, and thus are in regions of lower plasticity, and in areas of extreme GC skew, which also accumulate core genes. These extreme GC skew regions arise from the preferential use of codons with an excess of G over C, a distinct phenomenon from the elevated GC content that has previously been found to be associated with GTA genes. The observed properties, along with their high level of conservation, show that GTA genes share multiple features with core genes in the examined lineages of the Alphaproteobacteria.

An Evaluation of Cardiac Health in the Spontaneously Hypertensive Rat Colony: Implications of Evolutionary Driven Increases in Concentric Hypertrophy

BACKGROUND

The Spontaneously Hypertensive Rat (SHR) Colony was established in 1963 and is the most commonly used rodent model for studying heart failure (HF). Ideally, animal models should recapitulate the clinical disease as closely as possible. Any drift in a genetic model may create a new model that no longer adequately represents the human pathology. Further, instability overtime may lead to conflicting data between laboratories and/or irreproducible results. While systolic blood pressure (SBP) is closely monitored during inbreeding, the sequelae of HF (e.g., cardiac hypertrophy) are not. Thus, the object of this review was to investigate whether the hypertension-induced sequelae of HF in the SHR have remained stable after decades of inbreeding.

METHODS

A systematic review was performed to evaluate indices of cardiovascular health in the SHR over the past 60 years. For post hoc statistical analyses, studies were separated into 2 cohorts: Initial (mid to late 1900s) and Current (early 2000s to present) Colony SHRs. Wistar-Kyoto rats (WKY) were used as controls.

RESULTS

SBP was consistent between Initial and Current Colony SHRs. However, Current Colony SHRs presented with increased concentric hypertrophy (i.e., elevated heart weight and posterior wall thickness) while cardiac output remained consistent. Since these changes were not observed in the WKY controls, cardiac-derived changes in Current Colony SHRs were unlikely due to differences in environmental conditions.

CONCLUSIONS

Together, these data firmly establish a cardiac-based phenotypic shift in the SHR model and provide important insights into the beneficial function of concentric hypertrophy in hypertension-induced HF.

Compromised Function of the Pancreatic Transcription Factor PDX1 in a Lineage of Desert Rodents

Gerbils are a subfamily of rodents living in arid regions of Asia and Africa. Recent studies have shown that several gerbil species have unusual amino acid changes in the PDX1 protein, a homeodomain transcription factor essential for pancreatic development and β-cell function. These changes were linked to strong GC-bias in the genome that may be caused by GC-biased gene conversion, and it has been hypothesized that this caused accumulation of deleterious changes. Here we use two approaches to examine if the unusual changes are adaptive or deleterious. First, we compare PDX1 protein sequences between 38 rodents to test for association with habitat. We show the PDX1 homeodomain is almost totally conserved in rodents, apart from gerbils, regardless of habitat. Second, we use ectopic gene overexpression and gene editing in cell culture to compare functional properties of PDX1 proteins. We show that the divergent gerbil PDX1 protein inefficiently binds an insulin gene promoter and ineffectively regulates insulin expression in response to high glucose in rat cells. The protein has, however, retained the ability to regulate some other β-cell genes. We suggest that during the evolution of gerbils, the selection-blind process of biased gene conversion pushed fixation of mutations adversely affecting function of a normally conserved homeodomain protein. We argue these changes were not entirely adaptive and may be associated with metabolic disorders in gerbil species on high carbohydrate diets. This unusual pattern of molecular evolution could have had a constraining effect on habitat and diet choice in the gerbil lineage.

Runaway GC Evolution in Gerbil Genomes

Recombination increases the local GC-content in genomic regions through GC-biased gene conversion (gBGC). The recent discovery of a large genomic region with extreme GC-content in the fat sand rat Psammomys obesus provides a model to study the effects of gBGC on chromosome evolution. Here, we compare the GC-content and GC-to-AT substitution patterns across protein-coding genes of four gerbil species and two murine rodents (mouse and rat). We find that the known high-GC region is present in all the gerbils, and is characterized by high substitution rates for all mutational categories (AT-to-GC, GC-to-AT, and GC-conservative) both at synonymous and nonsynonymous sites. A higher AT-to-GC than GC-to-AT rate is consistent with the high GC-content. Additionally, we find more than 300 genes outside the known region with outlying values of AT-to-GC synonymous substitution rates in gerbils. Of these, over 30% are organized into at least 17 large clusters observable at the megabase-scale. The unusual GC-skewed substitution pattern suggests the evolution of genomic regions with very high recombination rates in the gerbil lineage, which can lead to a runaway increase in GC-content. Our results imply that rapid evolution of GC-content is possible in mammals, with gerbil species providing a powerful model to study the mechanisms of gBGC.

Phylogeny and evolution of Lasiopodomys in subfamily Arvivolinae based on mitochondrial genomics

The species of Lasiopodomys Lataste 1887 with their related genera remains undetermined owing to inconsistent morphological characteristics and molecular phylogeny. To investigate the phylogenetic relationship and speciation among species of the genus Lasiopodomys, we sequenced and annotated the whole mitochondrial genomes of three individual species, namely Lasiopodomys brandtii Radde 1861, L. mandarinus Milne-Edwards 1871, and Neodon (Lasiopodomys) fuscus Büchner 1889. The nucleotide sequences of the circular mitogenomes were identical for each individual species of L. brandtii, L. mandarinus, and N. fuscus. Each species contained 13 protein-coding genes (PCGs), 22 transfer RNAs, and 2 ribosomal RNAs, with mitochondrial genome lengths of 16,557 bp, 16,562 bp, and 16,324 bp, respectively. The mitogenomes and PCGs showed positive AT skew and negative GC skew. Mitogenomic phylogenetic analyses suggested that L. brandtii, L. mandarinus, and L. gregalis Pallas 1779 belong to the genus Lasiopodomys, whereas N. fuscus belongs to the genus Neodon grouped with N. irene. Lasiopodomys showed the closest relationship with Microtus fortis Büchner 1889 and M. kikuchii Kuroda 1920, which are considered as the paraphyletic species of genera Microtus. T_MRCA and niche model analysis revealed that Lasiopodomys may have first appeared during the early Pleistocene epoch. Further, L. gregalis separated from others over 1.53 million years ago (Ma) and then diverged into L. brandtii and L. mandarinus 0.76 Ma. The relative contribution of climatic fluctuations to speciation and selection in this group requires further research.

The nucleotide usages significantly impact synonymous codon usage in Mycoplasma hyorhinis

Five annotated genomes of Mycoplasma hyorhinis were analyzed for clarifying evolutionary dynamics driving the overall codon usage pattern. Information entropy used for estimating nucleotide usage pattern at the gene level indicates that multiple evolutionary dynamics participate in forcing nucleotide usage bias at every codon position. Moreover, nucleotide usage bias directly contributes to synonymous codon usage biases with two different extremes. The overrepresented codons tended to have A/T in the third codon position, and the underrepresented codons strongly used G/C in the third position. Furthermore, correspondence analysis and neutrality plot reflect an obvious interplay between mutation pressure and natural selection mediating codon usage in M. hyorhinis genome. Due to significant bias in usages between A/T and G/C at the gene level, different selective forces have been proposed to contribute to codon usage preference in M. hyorhinis genome, including nucleotide composition constraint derived from mutation pressure, translational selection involved in natural selection, and strand-specific mutational bias represented by different nucleotide skew index. The systemic analyses of codon usage for M. hyorhinis can enable us to better understand the mechanisms of evolution in this species.

Divergent genes in gerbils: prevalence, relation to GC-biased substitution, and phenotypic relevance

Background

Two gerbil species, sand rat (Psammomys obesus) and Mongolian jird (Meriones unguiculatus), can become obese and show signs of metabolic dysregulation when maintained on standard laboratory diets. The genetic basis of this phenotype is unknown. Recently, genome sequencing has uncovered very unusual regions of high guanine and cytosine (GC) content scattered across the sand rat genome, most likely generated by extreme and localized biased gene conversion. A key pancreatic transcription factor PDX1 is encoded by a gene in the most extreme GC-rich region, is remarkably divergent and exhibits altered biochemical properties. Here, we ask if gerbils have proteins in addition to PDX1 that are aberrantly divergent in amino acid sequence, whether they have also become divergent due to GC-biased nucleotide changes, and whether these proteins could plausibly be connected to metabolic dysfunction exhibited by gerbils.

Results

We analyzed ~ 10,000 proteins with 1-to-1 orthologues in human and rodents and identified 50 proteins that accumulated unusually high levels of amino acid change in the sand rat and 41 in Mongolian jird. We show that more than half of the aberrantly divergent proteins are associated with GC biased nucleotide change and many are in previously defined high GC regions. We highlight four aberrantly divergent gerbil proteins, PDX1, INSR, MEDAG and SPP1, that may plausibly be associated with dietary metabolism.

Conclusions

We show that through the course of gerbil evolution, many aberrantly divergent proteins have accumulated in the gerbil lineage, and GC-biased nucleotide substitution rather than positive selection is the likely cause of extreme divergence in more than half of these. Some proteins carry putatively deleterious changes that could be associated with metabolic and physiological phenotypes observed in some gerbil species. We propose that these animals provide a useful model to study the ‘tug-of-war’ between natural selection and the excessive accumulation of deleterious substitutions mutations through biased gene conversion.

A broad mutational target explains a fast rate of phenotypic evolution

The rapid evolution of a trait in a clade of organisms can be explained by the sustained action of natural selection or by a high mutational variance, that is the propensity to change under spontaneous mutation. The causes for a high mutational variance are still elusive. In some cases, fast evolution depends on the high mutation rate of one or few loci with short tandem repeats. Here, we report on the fastest evolving cell fate among vulva precursor cells in Caenorhabditis nematodes, that of P3.p. We identify and validate causal mutations underlying P3.p's high mutational variance. We find that these positions do not present any characteristics of a high mutation rate, are scattered across the genome and the corresponding genes belong to distinct biological pathways. Our data indicate that a broad mutational target size is the cause of the high mutational variance and of the corresponding fast phenotypic evolutionary rate.

Heritable characteristics or traits of a group of organisms, for example the large brain size of primates or the hooves of a horse, are determined by genes, the environment, and by the interactions between them. Traits can change over time and generations when enough mutations in these genes have spread in a species to result in visible differences.

However, some traits, such as the large brain of primates, evolve faster than others, but why this is the case has been unclear. It could be that a few specific genes important for that trait in question mutate at a high rate, or, that many genes affect the trait, creating a lot of variation for natural selection to choose from.

Here, Besnard, Picao-Osorio et al. studied the roundworm Caenorhabditis elegans to better understand the causes underlying the different rates of trait evolution. These worms have a short life cycle and evolve quickly over many generations, making them an ideal candidate for studying mutation rates in different traits.

Previous studies have shown that one of C. elegans’ six cells of the reproductive system evolves faster than the others. To investigate this further, Besnard, Picao-Osorio et al. analysed the genetic mutations driving change in this cell in 250 worm generations. The results showed that five mutations in five different genes – all responsible for different processes in the cells – were behind the supercharged evolution of this particular cell. This suggests that fast evolution results from natural selection acting upon a collection of genes, rather than one gene, and that many genes and pathways shape this trait.

In conclusion, these results demonstrate that how traits are coded at the molecular level, in one gene or many, can influence the rate at which they evolve.

Viral adaption of staphylococcal phage: A genome-based analysis of the selective preference based on codon usage Bias

Given the high therapeutic value of the staphylococcal phage, the genome co-evolution of the phage and the host has gained great attention. Though the genome-wide AT richness in staphylococcal phages has been well-studied with nucleotide usage bias, here we proved that host factor, lifestyle and taxonomy are also important factors in understanding the phage nucleotide usages bias using information entropy formula. Such correlation is especially prominent when it comes to the synonymous codon usages of staphylococcal phages, despite the overall scattered codon usage pattern represented by principal component analysis. This strong relationship is explained by nucleotide skew which testified that the usage biases of nucleotide at different codon positions are acting on synonymous codons. Therefore, our study reveals a hidden relationship of genome evolution with host limitation and phagic phenotype, providing new insight into phage genome evolution at genetic level.

Advances in the role and mechanism of BAG3 in dilated cardiomyopathy

The B cell lymphoma 2-associated anthanogene (BAG3) is an anti-apoptotic co-chaperone protein. Previous reports suggest that mutations in BAG3 are associated with dilated cardiomyopathy. This review aims to summarize the current understanding of the relationship between BAG3 mutations and dilated cardiomyopathy, primarily focusing on the role and protective mechanism of BAG3 in cardiomyocytes from individuals with dilated cardiomyopathy. The results of published studies show that BAG3 is critically important for reducing cardiomyocyte apoptosis, maintaining protein homeostasis, regulating mitochondrial stability, modulating myocardial contraction, and reducing cardiac arrhythmia, which suggests an indispensable protective mechanism of BAG3 in dilated cardiomyopathy. The significant role of BAG3 in protecting cardiomyocytes provides a new direction for the diagnosis and treatment of dilated cardiomyopathy. However, further research is required to explore the molecular mechanisms that regulate BAG3 expression, to identify a novel therapy for patients with dilated cardiomyopathy.