Gametic selection favours polyandry and selfing.

Competition among pollen or sperm (gametic selection) can cause evolution. Mating systems shape the intensity of gametic selection by determining the competitors involved, which can in turn cause the mating system itself to evolve. We model the bidirectional relationship between gametic selection and mating systems, focusing on variation in female mating frequency (monandry-polyandry) and self-fertilisation (selfing-outcrossing). First, we find that monandry and selfing both reduce the efficiency of gametic selection in removing deleterious alleles. This means that selfing can increase mutation load, in contrast to cases without gametic selection where selfing purges deleterious mutations and decreases mutation load. Second, we explore how mating systems evolve via their effect on gametic selection. By manipulating gametic selection, polyandry can evolve to increase the fitness of the offspring produced. However, this indirect advantage of post-copulatory sexual selection is weak and is likely to be overwhelmed by any direct fitness effects of mating systems. Nevertheless, gametic selection can be potentially decisive for selfing evolution because it significantly reduces inbreeding depression, which favours selfing. Thus, the presence of gametic selection could be a key factor driving selfing evolution.

Paternal starvation affects metabolic gene expression during zebrafish offspring development and lifelong fitness.

Dietary restriction in the form of fasting is a putative key to a healthier and longer life, but these benefits may come at a trade-off with reproductive fitness and may affect the following generation(s). The potential inter- and transgenerational effects of long-term fasting and starvation are particularly poorly understood in vertebrates when they originate from the paternal line. We utilised the externally fertilising zebrafish amenable to a split-egg clutch design to explore the male-specific effects of fasting/starvation on fertility and fitness of offspring independently of maternal contribution. Eighteen days of fasting resulted in reduced fertility in exposed males. While average offspring survival was not affected, we detected increased larval growth rate in F1 offspring from starved males and more malformed embryos at 24 h post-fertilisation in F2 offspring produced by F1 offspring from starved males. Comparing the transcriptomes of F1 embryos sired by starved and fed fathers revealed robust and reproducible increased expression of muscle composition genes but lower expression of lipid metabolism and lysosome genes in embryos from starved fathers. A large proportion of these genes showed enrichment in the yolk syncytial layer suggesting gene regulatory responses associated with metabolism of nutrients through paternal effects on extra-embryonic tissues which are loaded with maternal factors. We compared the embryo transcriptomes to published adult transcriptome datasets and found comparable repressive effects of starvation on metabolism-associated genes. These similarities suggest a physiologically relevant, directed and potentially adaptive response transmitted by the father, independently from the offspring's nutritional state, which was defined by the mother.

Germline mutation rate is elevated in young and old parents in Caenorhabditis remanei.

The effect of parental age on germline mutation rate across generations is not fully understood. While some studies report a positive linear relationship of mutation rate with increasing age, others suggest that mutation rate varies with age but not in a linear fashion. We investigated the effect of parental age on germline mutations by generating replicated mutation accumulation lines in Caenorhabditis remanei at three parental ages ("Young T1" [Day 1], "Peak T2" [Day 2], and "Old T5" [Day 5] parents). We conducted whole-genome resequencing and variant calling to compare differences in mutation rates after three generations of mutation accumulation. We found that Peak T2 lines had an overall reduced mutation rate compared to Young T1 and Old T5 lines, but this pattern of the effect varied depending on the variant impact. Specifically, we found no high-impact variants in Peak T2 lines, and modifiers and up- and downstream gene variants were less frequent in these lines. These results suggest that animals at the peak of reproduction have better DNA maintenance and repair compared to young and old animals. We propose that C. remanei start to reproduce before they optimize their DNA maintenance and repair, trading the benefits of earlier onset of reproduction against offspring mutation load. The increase in offspring mutation load with age likely represents germline senescence.

Pre-fertilization gamete thermal environment influences reproductive success, unmasking opposing sex-specific responses in Atlantic salmon (Salmo salar).

The environment gametes perform in just before fertilization is increasingly recognized to affect offspring fitness, yet the contributions of male and female gametes and their adaptive significance remain largely unexplored. Here, we investigated gametic thermal plasticity and its effects on hatching success and embryo performance in Atlantic salmon (Salmo salar). Eggs and sperm were incubated overnight at 2°C or 8°C, temperatures within the optimal thermal range of this species. Crosses between warm- and cold-incubated gametes were compared using a full-factorial design, with half of each clutch reared in cold temperatures and the other in warm temperatures. This allowed disentangling single-sex interaction effects when pre-fertilization temperature of gametes mismatched embryonic conditions. Pre-fertilization temperature influenced hatch timing and synchrony, and matching sperm and embryo temperatures resulted in earlier hatching. Warm incubation benefited eggs but harmed sperm, reducing the hatching success and, overall, gametic thermal plasticity did not enhance offspring fitness, indicating vulnerability to thermal changes. We highlight the sensitivity of male gametes to higher temperatures, and that gamete acclimation may not effectively buffer against deleterious effects of thermal fluctuations. From an applied angle, we propose the differential storage of male and female gametes as a tool to enhance sustainability within the hatcheries.

Fasting increases investment in soma upon refeeding at the cost of gamete quality in zebrafish.

Fasting increases lifespan in invertebrates, improves biomarkers of health in vertebrates and is increasingly proposed as a promising route to improve human health. Nevertheless, little is known about how fasted animals use resources upon refeeding, and how such decisions affect putative trade-offs between somatic growth and repair, reproduction and gamete quality. Such fasting-induced trade-offs are based on strong theoretical foundations and have been recently discovered in invertebrates, but the data on vertebrates are lacking. Here, we report that fasted female zebrafish, Danio rerio, increase investment in soma upon refeeding, but it comes at a cost of egg quality. Specifically, an increase in fin regrowth was accompanied by a reduction in 24 h post-fertilization offspring survival. Refed males showed a reduction in sperm velocity and impaired 24 h post-fertilization offspring survival. These findings underscore the necessity of considering the impact on reproduction when assessing evolutionary and biomedical implications of lifespan-extending treatments in females and males and call for careful evaluation of the effects of intermittent fasting on fertilization.

Frequency-dependent viscosity of salmon ovarian fluid has biophysical implications for sperm-egg interactions.

Gamete-level sexual selection of externally fertilising species is usually achieved by modifying sperm behaviour with mechanisms that alter the chemical environment in which gametes perform. In fish, this can be accomplished through the ovarian fluid, a substance released with the eggs at spawning. While the biochemical effects of ovarian fluid in relation to sperm energetics have been investigated, the influence of the physical environment in which sperm compete remains poorly explored. Our objective was therefore to gain insights on the physical structure of this fluid and potential impacts on reproduction. Using soft-matter physics approaches of steady-state and oscillatory viscosity measurements, we subjected wild Atlantic salmon ovarian fluids to variable shear stresses and frequencies resembling those exerted by sperm swimming through the fluid near eggs. We show that this fluid, which in its relaxed state is a gel-like substance, displays a non-Newtonian viscoelastic and shear-thinning profile, where the viscosity decreases with increasing shear rates. We concurrently find that this fluid obeys the Cox-Merz rule below 7.6 Hz and infringes it above this level, thus indicating a shear-thickening phase where viscosity increases provided it is probed gently enough. This suggests the presence of a unique frequency-dependent structural network with relevant implications for sperm energetics and fertilisation dynamics. This article has an associated ECR Spotlight interview with Marco Graziano.

The potential role of the mobile and non-coding genomes in adaptive response.

The tightly regulated feedback loops linking small RNAs (sRNAs) and transposable elements (TEs) offer the opportunity for an adaptive response to changing environments at the molecular level. Environmentally induced changes in TE and sRNA profiles may affect expression of coding genes and trigger an organismic and transgenerational response. Understanding this link may provide a mechanistic explanation for how species can adapt to changing climates and may offer novel molecular targets for biomedical and agricultural applications.

Multigenerational downregulation of insulin/IGF-1 signaling in adulthood improves lineage survival, reproduction, and fitness in Caenorhabditis elegans supporting the developmental theory of ageing.

Adulthood-only downregulation of insulin/IGF-1 signaling (IIS), an evolutionarily conserved pathway regulating resource allocation between somatic maintenance and reproduction, increases life span without fecundity cost in the nematode, Caenorhabditis elegans. However, long-term multigenerational effects of reduced IIS remain unexplored and are proposed to carry costs for offspring quality. To test this hypothesis, we ran a mutation accumulation (MA) experiment and downregulated IIS in half of the 400 MA lines by silencing daf-2 gene expression using RNA interference (RNAi) across 40 generations. Contrary to the prediction, adulthood-only daf-2 RNAi reduced extinction of MA lines both under UV-induced and spontaneous MA. Fitness of the surviving UV-induced MA lines was higher under daf-2 RNAi. Reduced IIS increased intergenerational F1 offspring fitness under UV stress but had no quantifiable transgenerational effects. Functional hrde-1 was required for the benefits of multigenerational daf-2 RNAi. Overall, we found net benefit to fitness from multigenerational reduction of IIS and the benefits became more apparent under stress. Because reduced daf-2 expression during development carries fitness costs, we suggest that our findings are best explained by the developmental theory of ageing, which maintains that the decline in the force of selection with age results in poorly regulated gene expression in adulthood.

Morphological and ultrastructural alterations of zebrafish (Danio rerio) spermatozoa after motility activation.

Spermatozoa motility in freshwater and marine fish is mainly controlled by the difference in osmotic pressure. Specifically, zebrafish (Danio rerio) spermatozoa undergo hypoosmotic shock due to the decrease in extracellular potassium, which leads to membrane hyperpolarization and activation of flagellar motility. Previous studies have concluded that motility activation has a negative effect on the spermatozoa structure. However, no evidence exists about ultrastructural changes in zebrafish spermatozoa after motility activation. In this study, spermatozoa samples were obtained from ten adult zebrafish individuals before and 60 s after motility activation and analyzed using Scanning and Transmission Electron Microscopy. Results showed dramatic morphological and ultrastructural alterations of the zebrafish spermatozoa after activation. In particular, the spermatozoa head underwent severe morphological distortion, including swelling of the nucleus, the bursting of the plasma membrane, and the alteration of the genetic material. Midpieces were also affected after activation since rupture of the cell membrane and lysis of mitochondria occurred. Furthermore, after the hypoosmotic shock, most spermatozoa showed a coiled flagellum and a disaggregated plasma membrane. Overall, our findings show that the activation of motility leads to substantial zebrafish spermatozoa morphological and ultrastructural changes, which could modify their physiology and decrease the fertilizing potential.

Accounting for the genetic load in assisted reproductive technology.

The genetic load in the human genome has important ramifications for assisted reproductive technology (ART), human reproduction and fertility more generally. Here, we discuss these topics in the light of evolutionary genetic theory, the technological revolution in ART and the advances in the fields of genomics and bioinformatics.

Evolutionary history of sexual selection affects microRNA profiles in Drosophila sperm.

The presence of small RNAs in sperm is a relatively recent discovery and little is currently known about their importance and functions. Environmental changes including social conditions and dietary manipulations are known to affect the composition and expression of some small RNAs in sperm and may elicit a physiological stress response resulting in an associated change in gamete miRNA profiles. Here, we tested how microRNA profiles in sperm are affected by variation in both sexual selection and dietary regimes in Drosophila melanogaster selection lines. The selection lines were exposed to standard versus low yeast diet treatments and three different population sex ratios (male-biased, female-biased, or equal sex) in a full-factorial design. After 38 generations of selection, all males were maintained on their selected diet and in a common garden male-only environment prior to sperm sampling. We performed transcriptome analyses on miRNAs in purified sperm samples. We found 11 differentially expressed miRNAs with the majority showing differences between male- and female-biased lines. Dietary treatment only had a significant effect on miRNA expression levels in interaction with sex ratio. Our findings suggest that long-term adaptation may affect miRNA profiles in sperm and that these may show varied interactions with short-term environmental changes.

Evolution of plasticity in production and transgenerational inheritance of small RNAs under dynamic environmental conditions.

In a changing environment, small RNAs (sRNAs) play an important role in the post-transcriptional regulation of gene expression and can vary in abundance depending on the conditions experienced by an individual (phenotypic plasticity) and its parents (non-genetic inheritance). Many sRNAs are unusual in that they can be produced in two ways, either using genomic DNA as the template (primary sRNAs) or existing sRNAs as the template (secondary sRNAs). Thus, organisms can evolve rapid plastic responses to their current environment by adjusting the amplification rate of sRNA templates. sRNA levels can also be transmitted transgenerationally by the direct transfer of either sRNAs or the proteins involved in amplification. Theory is needed to describe the selective forces acting on sRNA levels, accounting for the dual nature of sRNAs as regulatory elements and templates for amplification and for the potential to transmit sRNAs and their amplification agents to offspring. Here, we develop a model to study the dynamics of sRNA production and inheritance in a fluctuating environment. We tested the selective advantage of mutants capable of sRNA-mediated phenotypic plasticity within resident populations with fixed levels of sRNA transcription. Even when the resident was allowed to evolve an optimal constant rate of sRNA production, plastic amplification rates capable of responding to environmental conditions were favored. Mechanisms allowing sRNA transcripts or amplification agents to be inherited were favored primarily when parents and offspring face similar environments and when selection acts before the optimal level of sRNA can be reached within the organism. Our study provides a clear set of testable predictions for the evolution of sRNA-related mechanisms of phenotypic plasticity and transgenerational inheritance.

Transgenerational fitness effects of lifespan extension by dietary restriction in Caenorhabditis elegans.

Dietary restriction (DR) increases lifespan in a broad variety of organisms and improves health in humans. However, long-term transgenerational consequences of dietary interventions are poorly understood. Here, we investigated the effect of DR by temporary fasting (TF) on mortality risk, age-specific reproduction and fitness across three generations of descendants in Caenorhabditis elegans. We show that while TF robustly reduces mortality risk and improves late-life reproduction of the individuals subject to TF (P0), it has a wide range of both positive and negative effects on their descendants (F1-F3). Remarkably, great-grandparental exposure to TF in early life reduces fitness and increases mortality risk of F3 descendants to such an extent that TF no longer promotes a lifespan extension. These findings reveal that transgenerational trade-offs accompany the instant benefits of DR, underscoring the need to consider fitness of future generations in pursuit of healthy ageing.

Evolutionary consequences of environmental effects on gamete performance.

Variation in pre- and post-release gamete environments can influence evolutionary processes by altering fertilization outcomes and offspring traits. It is now widely accepted that offspring inherit epigenetic information from both their mothers and fathers. Genetic and epigenetic alterations to eggs and sperm-acquired post-release may also persist post-fertilization with consequences for offspring developmental success and later-life fitness. In externally fertilizing species, gametes are directly exposed to anthropogenically induced environmental impacts including pollution, ocean acidification and climate change. When fertilization occurs within the female reproductive tract, although gametes are at least partially protected from external environmental variation, the selective environment is likely to vary among females. In both scenarios, gamete traits and selection on gametes can be influenced by environmental conditions such as temperature and pollution as well as intrinsic factors such as male and female reproductive fluids, which may be altered by changes in male and female health and physiology. Here, we highlight some of the pathways through which changes in gamete environments can affect fertilization dynamics, gamete interactions and ultimately offspring fitness. We hope that by drawing attention to this important yet often overlooked source of variation, we will inspire future research into the evolutionary implications of anthropogenic interference of gamete environments including the use of assisted reproductive technologies. This article is part of the theme issue 'How does epigenetics influence the course of evolution?'

Meiosis and beyond - understanding the mechanistic and evolutionary processes shaping the germline genome.

The separation of germ cell populations from the soma is part of the evolutionary transition to multicellularity. Only genetic information present in the germ cells will be inherited by future generations, and any molecular processes affecting the germline genome are therefore likely to be passed on. Despite its prevalence across taxonomic kingdoms, we are only starting to understand details of the underlying micro-evolutionary processes occurring at the germline genome level. These include segregation, recombination, mutation and selection and can occur at any stage during germline differentiation and mitotic germline proliferation to meiosis and post-meiotic gamete maturation. Selection acting on germ cells at any stage from the diploid germ cell to the haploid gametes may cause significant deviations from Mendelian inheritance and may be more widespread than previously assumed. The mechanisms that affect and potentially alter the genomic sequence and allele frequencies in the germline are pivotal to our understanding of heritability. With the rise of new sequencing technologies, we are now able to address some of these unanswered questions. In this review, we comment on the most recent developments in this field and identify current gaps in our knowledge.

Experimental evolution of adaptive divergence under varying degrees of gene flow.

Adaptive divergence is the key evolutionary process generating biodiversity by means of natural selection. Yet, the conditions under which it can arise in the presence of gene flow remain contentious. To address this question, we subjected 132 sexually reproducing fission yeast populations, sourced from two independent genetic backgrounds, to disruptive ecological selection and manipulated the level of migration between environments. Contrary to theoretical expectations, adaptive divergence was most pronounced when migration was either absent (allopatry) or maximal (sympatry), but was much reduced at intermediate rates (parapatry and local mating). This effect was apparent across central life-history components (survival, asexual growth and mating) but differed in magnitude between ancestral genetic backgrounds. The evolution of some fitness components was constrained by pervasive negative correlations (trade-off between asexual growth and mating), while others changed direction under the influence of migration (for example, survival and mating). In allopatry, adaptive divergence was mainly conferred by standing genetic variation and resulted in ecological specialization. In sympatry, divergence was mainly mediated by novel mutations enriched in a subset of genes and was characterized by the repeated emergence of two strategies: an ecological generalist and an asexual growth specialist. Multiple loci showed consistent evidence for antagonistic pleiotropy across migration treatments providing a conceptual link between adaptation and divergence. This evolve-and-resequence experiment shows that rapid ecological differentiation can arise even under high rates of gene flow. It further highlights that adaptive trajectories are governed by complex interactions of gene flow, ancestral variation and genetic correlations.

Within-ejaculate sperm competition.

Sperm competition was defined by Geoff Parker 50 years ago as the competition between sperm from two or more males over the fertilization of a set of eggs. Since the publication of his seminal paper, sperm competition has developed into a large field of research, and many aspects are still being discovered. One of the relatively poorly understood aspects is the importance of selection and competition among sperm within the ejaculate of a male. The sheer number of sperm present in a male's ejaculate suggests that the competition among sibling sperm produced by the same male may be intense. In this review, we summarize Parker's theoretical models generating predictions about the evolution of sperm traits under the control of the haploid gamete as opposed to the diploid male. We review the existing evidence of within-ejaculate competition from a wide range of fields and taxa. We also discuss the conceptual and practical hurdles we have been facing to study within-ejaculate sperm competition, and how novel technologies may help in addressing some of the currently open questions. This article is part of the theme issue 'Fifty years of sperm competition'.

Long life evolves in large-brained bird lineages.

The brain is an energetically costly organ that consumes a disproportionate amount of resources. Species with larger brains relative to their body size have slower life histories, with reduced output per reproductive event and delayed development times that can be offset by increasing behavioral flexibility. The "cognitive buffer" hypothesis maintains that large brain size decreases extrinsic mortality due to greater behavioral flexibility, leading to a longer lifespan. Alternatively, slow life histories, and long lifespan can be a pre-adaptation for the evolution of larger brains. Here, we use phylogenetic path analysis to contrast different evolutionary scenarios and disentangle direct and indirect relationships between brain size, body size, life history, and longevity across 339 altricial and precocial bird species. Our results support both a direct causal link between brain size and lifespan, and an indirect effect via other life history traits. These results indicate that large brain size engenders longer life, as proposed by the "cognitive buffer" hypothesis.

Trade-off between somatic and germline repair in a vertebrate supports the expensive germ line hypothesis.

The disposable soma theory is a central tenet of the biology of aging where germline immortality comes at the cost of an aging soma [T. B. L. Kirkwood, Nature 270, 301-304 (1977); T. B. L. Kirkwood, Proc. R. Soc. Lond. B Biol. Sci. 205, 531-546 (1979); T. B. L. Kirkwood, S. N. Austad, Nature 408, 233-238 (2000)]. Limited resources and a possible trade-off between the repair and maintenance of the germ cells and growth and maintenance of the soma may explain the deterioration of the soma over time. Here we show that germline removal allows accelerated somatic healing under stress. We tested "the expensive germ line" hypothesis by generating germline-free zebrafish Danio rerio and testing the effect of the presence and absence of the germ line on somatic repair under benign and stressful conditions. We exposed male fish to sublethal low-dose ionizing radiation, a genotoxic stress affecting the soma and the germ line, and tested how fast the soma recovered following partial fin ablation. We found that somatic recovery from ablation occurred substantially faster in irradiated germline-free fish than in the control germline-carrying fish where somatic recovery was stunned. The germ line did show signs of postirradiation recovery in germline-carrying fish in several traits related to offspring number and fitness. These results support the theoretical conjecture that germline maintenance is costly and directly trades off with somatic maintenance.