Disentangling Verbal Arguments: Intralocus Sexual Conflict in Haplodiploids

Genetic drift drives faster-Z evolution in the salmon louse Lepeophtheirus salmonis

How sex chromosomes evolve compared to autosomes remains an unresolved question in population genetics. Most studies focus on only a handful of taxa, resulting in uncertainty over whether observed patterns reflect general processes or idiosyncrasies in particular clades. For example, in female heterogametic (ZW) systems, bird Z chromosomes tend to evolve quickly but not adaptively, while in Lepidopterans they evolve adaptively, but not always quickly. To understand how these observations fit into broader evolutionary patterns, we explore Z chromosome evolution outside of these two well-studied clades. We utilize a publicly available genome, gene expression, population, and outgroup data in the salmon louse Lepeophtheirus salmonis, an important agricultural pest copepod. We find that the Z chromosome is faster evolving than autosomes, but that this effect is driven by increased drift rather than adaptive evolution. Due to high rates of female reproductive failure, the Z chromosome exhibits a slightly lower effective population size than the autosomes which is nonetheless to decrease efficiency of hemizygous selection acting on the Z. These results highlight the usefulness of organismal life history in calibrating population genetic expectations and demonstrate the value of the ever-expanding wealth of publicly available data to help resolve outstanding evolutionary questions.

Lack of paternal silencing and ecotype-specific expression in head and body lice hybrids

Paternal genome elimination (PGE) is a non-Mendelian inheritance system, described in numerous arthropod species, in which males develop from fertilized eggs, but their paternally inherited chromosomes are eliminated before or during spermatogenesis. Therefore, PGE males only transmit their maternally inherited set of chromosomes to their offspring. In addition to the elimination of paternal chromosomes, diverse PGE species have also repeatedly evolved the transcriptional silencing of the paternal genome, making males effectively haploid. However, it is unclear if this paternal chromosome silencing is mechanistically linked to the chromosome elimination or has evolved at a later stage, and if so, what drives the haploidization of males under PGE. In order to understand these questions, here we study the human louse, Pediculus humanus, which represents an ideal model system, as it appears to be the only instance of PGE where males eliminate, but not silence their paternal chromosomes, although the latter remains to be shown conclusively. In this study, we analyzed parent-of-origin allele-specific expression patterns in male offspring of crosses between head and body lice ecotypes. We show that hybrid adult males of P. humanus display biparental gene expression, which constitutes the first case of a species with PGE in which genetic activity of paternal chromosomes in the soma is not affected by embryonic silencing or (partial or complete) elimination. We did however also identify a small number of maternally biased genes (potentially imprinted genes), which may be involved in the elimination of paternal chromosomes during spermatogenesis. Finally, we have identified genes that show ecotype-specific expression bias. Given the low genetic diversity between ecotypes, this is suggestive for a role of epigenetic processes in ecotype differences.

Sexual antagonism in sequential hermaphrodites

Females and males may have distinct phenotypic optima, but share essentially the same complement of genes, potentially leading to trade-offs between attaining high fitness through female versus male reproductive success. Such sexual antagonism may be particularly acute in hermaphrodites, where both reproductive strategies are housed within a single individual. While previous models have focused on simultaneous hermaphroditism, we lack theory for how sexual antagonism may play out under sequential hermaphroditism, which has the additional complexities of age-structure. Here, we develop a formal theory of sexual antagonism in sequential hermaphrodites. First, we construct a general theoretical overview of the problem, then consider different types of sexually antagonistic and life-history trade-offs, under different modes of genetic inheritance (autosomal or cytoplasmic), and different forms of sequential hermaphroditism (protogynous, protoandrous or bidirectional). Finally, we provide a concrete illustration of these general patterns by developing a two-stage two-sex model, which yields conditions for both invasion of sexually antagonistic alleles and maintenance of sexually antagonistic polymorphisms.

Why is measuring and predicting fitness under genomic conflict so hard?

Genomic conflict between the sexes is caused by differences in the optimal male and female reproductive strategies, and is a major contributor to genetic, phenotypic, and life history variation. While early experimental work appeared to strongly support the sexual conflict paradigm, recent work has produced more ambiguous results. Recent advances in experimental evolution studies combined with theoretical arguments can shed light on why measuring fitness under a conflict is so challenging, including the incidental alteration of mating dynamics, demographic effects, and inherent complexity in what quantity selection maximizes. We stress that non-intuitive results do not necessarily mean the absence of conflict, and follow-up experiments to determine why a priori predictions failed can ultimately teach us more than if they had been confirmed.

Paternal genome elimination: patterns and mechanisms of drive and silencing

In thousands of arthropod species, males inherit, but subsequently eliminate the entire haploid genome of their father. However, why this peculiar reproductive strategy evolved repeatedly across diverse species and what mechanisms are involved in paternal genome elimination (PGE) remains largely unknown. In this review, we summarize what we know about the patterns of paternal chromosome elimination during various stages of development in the diverse taxa that have been studied. We also discuss some other unusual features often associated with PGE, such as the transcriptional silencing of paternally derived chromosomes in males and sex determination through the early embryonic elimination of X chromosomes. Little is known about the molecular mechanisms underlying the parent-of-origin-dependent chromosome elimination and silencing under PGE, but we discuss the insight of several studies that are pioneering this work and highlight directions for future research.

Genetic conflicts and the case for licensed anthropomorphizing

The use of intentional language in biology is controversial. It has been commonly applied by researchers in behavioral ecology, who have not shied away from employing agential thinking or even anthropomorphisms, but has been rarer among researchers from more mechanistic corners of the discipline, such as population genetics. One research area where these traditions come into contact—and occasionally clash—is the study of genetic conflicts, and its history offers a good window to the debate over the use of intentional language in biology. We review this debate, paying particular attention to how this interaction has played out in work on genomic imprinting and sex chromosomes. In light of this, we advocate for a synthesis of the two approaches, a form of licensed anthropomorphizing. Here, agential thinking’s creative potential and its ability to identify the fulcrum of evolutionary pressure are combined with the rigidity of formal mathematical modeling.

Paternal genome elimination promotes altruism in viscous populations

Population viscosity has long been thought to promote the evolution of altruism. However, in the simplest scenarios, the potential for altruism is invariant with respect to dispersal-a surprising result that holds for haploidy, diploidy, and haplodiploidy (arrhenotoky). Here, we develop a kin-selection model to investigate how population viscosity affects the potential for altruism in species with male paternal genome elimination (PGE), exploring altruism enacted by both females and males, and both juveniles and adults. We find that (1) PGE promotes altruistic behaviors relative to the other inheritance systems, and to a degree that depends on the extent of paternal genome expression. (2) Under PGE, dispersal increases the potential for altruism in juveniles and decreases it in adults. (3) The genetics of PGE can lead to striking differences in sex-specific potentials for altruism, even in the absence of any sex differences in ecology.

Genomic evidence of paternal genome elimination in the globular springtail Allacma fusca

Paternal genome elimination (PGE) - a type of reproduction in which males inherit but fail to pass on their father's genome - evolved independently in six to eight arthropod clades. Thousands of species, including several important for agriculture, reproduce via this mode of reproduction. While PGE is well established in some of the clades, the evidence in globular springtails (Symphypleona) remains elusive, even though they represent the oldest and most species rich clade putatively reproducing via PGE. We sequenced genomic DNA from whole bodies of Allacma fusca males with high fractions (>27.5%) of sperm to conclusively confirm that all the sperm carry one parental haplotype only. Although it is suggestive that the single haplotype present in sperm is maternally inherited, definitive genetic proof of the parent of origin is still needed. The genomic approach we developed allows for detection of genotypic differences between germline and soma in all species with sufficiently high fraction of germline in their bodies. This opens new opportunities for scans of reproductive modes in small organisms.

Faster-haplodiploid evolution under divergence-with-gene-flow: simulations and empirical data from pine-feeding hymenopterans

Although haplodiploidy is widespread in nature, the evolutionary consequences of this mode of reproduction are not well characterized. Here, we examine how genome-wide hemizygosity and a lack of recombination in haploid males affects genomic differentiation in populations that diverge via natural selection while experiencing gene flow. First, we simulated diploid and haplodiploid "genomes" (500-kb loci) evolving under an isolation-with-migration model with mutation, drift, selection, migration, and recombination; and examined differentiation at neutral sites both tightly and loosely linked to a divergently selected site. So long as there is divergent selection and migration, sex-limited hemizygosity and recombination cause elevated differentiation (i.e., produce a "faster-haplodiploid effect") in haplodiploid populations relative to otherwise equivalent diploid populations, for both recessive and codominant mutations. Second, we used genome-wide SNP data to model divergence history and describe patterns of genomic differentiation between sympatric populations of Neodiprion lecontei and N. pinetum, a pair of pine sawfly species (order: Hymenoptera; family: Diprionidae) that are specialized on different pine hosts. These analyses support a history of continuous gene exchange throughout divergence and reveal a pattern of heterogeneous genomic differentiation that is consistent with divergent selection on many unlinked loci. Third, using simulations of haplodiploid and diploid populations evolving according to the estimated divergence history of N. lecontei and N. pinetum, we found that divergent selection would lead to higher differentiation in haplodiploids. Based on these results, we hypothesize that haplodiploids undergo divergence-with-gene-flow and sympatric speciation more readily than diploids.

Sexual antagonism in haplodiploids

Females and males may face different selection pressures, such that alleles conferring a benefit in one sex may be deleterious in the other. Such sexual antagonism has received a great deal of theoretical and empirical attention, almost all of which has focused on diploids. However, a sizeable minority of animals display an alternative haplodiploid mode of inheritance, encompassing both arrhenotoky, whereby males develop from unfertilized eggs, and paternal genome elimination (PGE), whereby males receive but do not transmit a paternal genome. Alongside unusual genetics, haplodiploids often exhibit social ecologies that modulate the relative value of females and males. Here we develop a series of evolutionary-genetic models of sexual antagonism for haplodiploids, incorporating details of their molecular biology and social ecology. We find that: 1) PGE promotes female-beneficial alleles more than arrhenotoky, and to an extent determined by the timing of elimination - and degree of silencing of - the paternal genome; 2) sib-mating relatively promotes female-beneficial alleles, as do other forms of inbreeding, including limited male-dispersal, oedipal-mating, and the pseudo-hermaphroditism of Icerya purchasi; 3) resource competition between related females inhibits the invasion of female-beneficial alleles; and 4) sexual antagonism foments conflicts between parents and offspring, endosymbionts and hosts, and maternal-origin and paternal-origin genes. This article is protected by copyright. All rights reserved.