Social regulation of reproduction: control or signal?

Traditionally, dominant breeders have been considered to be able to control the reproduction of other individuals in multimember groups that have high variance in reproductive success/reproductive skew (e.g., forced sterility/coercion of conspecifics in eusocial animals; sex-change suppression in sequential hermaphrodites). These actions are typically presented as active impositions by reproductively dominant individuals. However, how can individuals regulate the reproductive physiology of others? Alternatively, all contestants make reproductive decisions, and less successful individuals self-downregulate reproduction in the presence of dominant breeders. Shifting perspective from a top-down manipulation to a broader view, which includes all contenders, and using a multitaxon approach, we propose a unifying framework for the resolution of reproductive skew conflicts based on signalling rather than control, along a continuum of levels of strategic regulation of reproduction.

Demographic inference provides insights into the extirpation and ecological dominance of eusocial snapping shrimps

Although eusocial animals often achieve ecological dominance in the ecosystems where they occur, many populations are unstable, resulting in local extinction. Both patterns may be linked to the characteristic demography of eusocial species-high reproductive skew and reproductive division of labor support stable effective population sizes that make eusocial groups more competitive in some species, but also lower effective population sizes that increase susceptibility to population collapse in others. Here, we examine the relationship between demography and social organization in Synalpheus snapping shrimps, a group in which eusociality has evolved recently and repeatedly. We show using coalescent demographic modelling that eusocial species have had lower but more stable effective population sizes across 100,000 generations. Our results are consistent with the idea that stable population sizes may enable competitive dominance in eusocial shrimps, but they also suggest that recent population declines are likely caused by eusocial shrimps' heightened sensitivity to environmental changes, perhaps as a result of their low effective population sizes and localized dispersal. Thus, although the unique life histories and demography of eusocial shrimps have likely contributed to their persistence and ecological dominance over evolutionary timescales, these social traits may also make them vulnerable to contemporary environmental change.

Strategic growth in social vertebrates

Individual differences in growth and size of vertebrates often represent adaptive, plastic responses to contrasts in ecological conditions. Recent studies show that vertebrates can also modify their growth and size in an adaptive fashion in response to fine-grain changes in social conditions (which we refer to as strategic growth). Here, we review experimental evidence for strategic growth in social vertebrates. We describe a set of conditions under which strategic growth commonly occurs, and highlight potential examples of convergent evolution of strategic growth across the tree of life. This synthesis has implications for the way we think about organismal growth and size, because it underscores that the size of individuals can often be fine-tuned to their social environment.

Eusociality in snapping shrimps is associated with larger genomes and an accumulation of transposable elements

Despite progress uncovering the genomic underpinnings of sociality, much less is known about how social living affects the genome. In different insect lineages, for example, eusocial species show both positive and negative associations between genome size and structure, highlighting the dynamic nature of the genome. Here, we explore the relationship between sociality and genome architecture in Synalpheus snapping shrimps that exhibit multiple origins of eusociality and extreme interspecific variation in genome size. Our goal is to determine whether eusociality leads to an accumulation of repetitive elements and an increase in genome size, presumably due to reduced effective population sizes resulting from a reproductive division of labor, or whether an initial accumulation of repetitive elements leads to larger genomes and independently promotes the evolution of eusociality through adaptive evolution. Using phylogenetically informed analyses, we find that eusocial species have larger genomes with more transposable elements (TEs) and microsatellite repeats than noneusocial species. Interestingly, different TE subclasses contribute to the accumulation in different species. Phylogenetic path analysis testing alternative causal relationships between sociality and genome architecture is most consistent with the hypothesis that TEs modulate the relationship between sociality and genome architecture. Although eusociality appears to influence TE accumulation, ancestral state reconstruction suggests moderate TE abundances in ancestral species could have fueled the initial transitions to eusociality. Ultimately, we highlight a complex and dynamic relationship between genome and social evolution, demonstrating that sociality can influence the evolution of the genome, likely through changes in demography related to patterns of reproductive skew.

Eusociality through conflict dissolution

Eusociality, where largely unreproductive offspring help their mothers reproduce, is a major form of social organization. An increasingly documented feature of eusociality is that mothers induce their offspring to help by means of hormones, pheromones or behavioural displays, with evidence often indicating that offspring help voluntarily. The co-occurrence of maternal influence and offspring voluntary help may be explained by what we call the converted helping hypothesis, whereby maternally manipulated helping subsequently becomes voluntary. Such hypothesis requires that parent-offspring conflict is eventually dissolved-for instance, if the benefit of helping increases sufficiently over evolutionary time. We show that help provided by maternally manipulated offspring can enable the mother to sufficiently increase her fertility to transform parent-offspring conflict into parent-offspring agreement. This conflict-dissolution mechanism requires that helpers alleviate maternal life-history trade-offs, and results in reproductive division of labour, high queen fertility and honest queen signalling suppressing worker reproduction-thus exceptionally recovering diverse features of eusociality. As such trade-off alleviation seemingly holds widely across eusocial taxa, this mechanism offers a potentially general explanation for the origin of eusociality, the prevalence of maternal influence, and the offspring's willingness to help. Overall, our results explain how a major evolutionary transition can happen from ancestral conflict.

Eusociality Shapes Convergent Patterns of Molecular Evolution across Mitochondrial Genomes of Snapping Shrimps

Eusociality is a highly conspicuous and ecologically impactful behavioral syndrome that has evolved independently across multiple animal lineages. So far, comparative genomic analyses of advanced sociality have been mostly limited to insects. Here, we study the only clade of animals known to exhibit eusociality in the marine realm-lineages of socially diverse snapping shrimps in the genus Synalpheus. To investigate the molecular impact of sociality, we assembled the mitochondrial genomes of eight Synalpheus species that represent three independent origins of eusociality and analyzed patterns of molecular evolution in protein-coding genes. Synonymous substitution rates are lower and potential signals of relaxed purifying selection are higher in eusocial relative to noneusocial taxa. Our results suggest that mitochondrial genome evolution was shaped by eusociality-linked traits-extended generation times and reduced effective population sizes that are hallmarks of advanced animal societies. This is the first direct evidence of eusociality impacting genome evolution in marine taxa. Our results also strongly support the idea that eusociality can shape genome evolution through profound changes in life history and demography.

Comparative mitochondrial genomics of sponge-dwelling snapping shrimps in the genus Synalpheus: Exploring differences between eusocial and non-eusocial species and insights into phylogenetic relationships in caridean shrimps

The genus Synalpheus is a cosmopolitan clade of marine shrimps found in most tropical regions. Species in this genus exhibit a range of social organizations, including pair-forming, communal breeding, and eusociality, the latter only known to have evolved within this genus in the marine realm. This study examines the complete mitochondrial genomes of seven species of Synalpheus and explores differences between eusocial and non-eusocial species considering that eusociality has been shown before to affect the strength of purifying selection in mitochondrial protein coding genes. The AT-rich mitochondrial genomes of Synalpheus range from 15,421 bp to 15,782 bp in length and comprise, invariably, 13 protein-coding genes (PCGs), two ribosomal RNA genes, and 22 transfer RNA genes. A 648 bp to 994 bp long intergenic space is assumed to be the D-loop. Mitochondrial gene synteny is identical among the studied shrimps. No major differences occur between eusocial and non-eusocial species in nucleotide composition and codon usage profiles of PCGs and in the secondary structure of tRNA genes. Maximum likelihood phylogenetic analysis of the complete concatenated PCG complement of 90 species supports the monophyly of the genus Synalpheus and its family Alpheidae. Moreover, the monophyletic status of the caridean families Alvinocaridae, Atyidae, Thoridae, Lysmatidae, Palaemonidae, and Pandalidae within caridean shrimps are fully or highly supported by the analysis. We therefore conclude that mitochondrial genomes contain sufficient phylogenetic information to resolve relationships at high taxonomic levels within the Caridea. Our analysis of mitochondrial genomes in the genus Synalpheus contributes to the understanding of the coevolution between genomic architecture and sociality in caridean shrimps and other marine organisms.

The complete mitochondrial genome of the eusocial sponge-dwelling snapping shrimp Synalpheus microneptunus

In the marine realm, eusociality is only known to have evolved within a clade of sponge-dwelling snapping shrimps in the genus Synalpheus. Deciphering the genomic underpinnings of eusociality in these marine shrimps has been limited by the sparse genomic resources in this genus. Here, we report, for a eusocial shrimp Synalpheus microneptunus, a complete mitochondrial genome (22X coverage) assembled from short Illumina 150 bp paired-end reads. The 15,603 bp long mitochondrial genome of S. microneptunus is AT-rich and includes 13 protein-coding genes (PCGs), 2 ribosomal RNA genes, 22 transfer RNA genes and an 834 bp intergenic region assumed to be the D-loop. The gene order is identical to that reported for most caridean shrimps and corresponds to the presumed Pancrustacean ground pattern. All PCGs showed signs of purifying selection, with K_A/K_S <<1 across the whole PCGs and most sliding windows within PCGs. Maximum-likelihood and Bayesian inference phylogenetic analyses of 13 PCGs and 68 terminals supports the monophyly of the Caridea and the family Alpheidae. The complete mitochondrial genome of the eusocial shrimp Synalpheus microneptunus will contribute to a better understanding of the selective pressures and rates of molecular evolution in marine eusocial animals.

Social transitions in sponge-dwelling snapping shrimp

Sociality is exceedingly rare in the marine environment, with true eusociality found only within a single genus of sponge-dwelling snapping shrimp. This genus is socially diverse and exhibits multiple independent evolutionary origins of both eusociality and communal breeding from pair-forming ancestors. Ecology was critical to the evolution of shrimp sociality, as the transition from host specialization to generalism preceded the evolution of eusociality, and the transition from small to large host sponges favored the evolution of communal breeding. Moreover, a change in life history from planktonic to non-dispersing, crawling larvae only occurred in eusocial species. Here, we present a hypothesis describing the evolutionary transitions toward sociality in shrimp that serves to illustrate how ecology and life history interact to shape social evolution more broadly.

Allometry of individual reproduction and defense in eusocial colonies: A comparative approach to trade-offs in social sponge-dwelling Synalpheus shrimps

Eusociality, one of the most complex forms of social organization, is thought to have evolved in several animal clades in response to competition for resources and reproductive opportunities. Several species of snapping shrimp in the genus Synalpheus, the only marine organisms known to exhibit eusociality, form colonies characterized by high reproductive skew, and aggressive territoriality coupled with cooperative defense. In eusocial Synalpheus colonies, individual reproduction is limited to female 'queens', whose fecundity dictates colony growth. Given that individual reproduction and defense are both energetically costly, individual and colony fitness likely depend on the optimal allocation of resources by these reproducing individuals towards these potentially competing demands. Synalpheus species, however, display varying degrees of eusociality, suggesting that reproducing females have adopted different strategies for allocation among reproduction and defense. Here, we use structural equation modeling to characterize the relationships between the allometry of queen reproductive capacity and defensive weaponry, and colony size in six eusocial Synalpheus species, estimating trade-offs between reproduction and defense. We document strong trade-offs between mass of the fighting claw (defense) and egg number (reproduction) in queens from weakly eusocial species, while the trade-off is reduced or absent in those from strongly eusocial species. These results suggest that in less cooperative species, intra-colony conflict selects for queen retention of weapons that have significant costs to fecundity, while reproducing females from highly eusocial species, i.e., those with a single queen, have been able to reduce the cost of weapons as a result of protection by other colony members.

Reproductive skew drives patterns of sexual dimorphism in sponge-dwelling snapping shrimps

Sexual dimorphism is typically a result of strong sexual selection on male traits used in male-male competition and subsequent female choice. However, in social species where reproduction is monopolized by one or a few individuals in a group, selection on secondary sexual characteristics may be strong in both sexes. Indeed, sexual dimorphism is reduced in many cooperatively breeding vertebrates and eusocial insects with totipotent workers, presumably because of increased selection on female traits. Here, we examined the relationship between sexual dimorphism and sociality in eight species of Synalpheus snapping shrimps that vary in social structure and degree of reproductive skew. In species where reproduction was shared more equitably, most members of both sexes were physiologically capable of breeding. However, in species where reproduction was monopolized by a single individual, a large proportion of females--but not males--were reproductively inactive, suggesting stronger reproductive suppression and conflict among females. Moreover, as skew increased across species, proportional size of the major chela--the primary antagonistic weapon in snapping shrimps--increased among females and sexual dimorphism in major chela size declined. Thus, as reproductive skew increases among Synalpheus, female-female competition over reproduction appears to increase, resulting in decreased sexual dimorphism in weapon size.