Alternative Strategies: The Evolution of Switch Points

Sexual selection on population-level mating opportunities drives morph ratios in a fig wasp with extreme male dimorphism

Background

Alternative mating tactics are widespread in animals and associated with extreme morphological polymorphism in some insects. Some fig wasps have both highly modified wingless males and dispersing winged males. Wingless males mate inside figs before females disperse, while winged males mate elsewhere after dispersal. Hamilton proposed a model for this system with morphs determined by alternative alleles. This has an equilibrium where the proportion of winged males equals the proportion of females dispersing unmated; i.e. the proportion of matings that they obtain. Previously, we have shown qualitative support for this prediction across nine wing-dimorphic fig wasp species. Here I test the quantitative prediction in the fig wasp Pseudidarnes minerva. In addition, some fig wasp species that lack winged males, but have two wingless morphs, show a conditional strategy with morph determination influenced by the number of wasps developing in a patch. I also test for this alternative pattern in the wing-dimorphic P. minerva.

Results

I sampled 114 figs that contained a mean of 2.1 P. minerva wasps from 44 trees across four sites in Sydney, Australia. At the whole population level, the proportion of winged males (0.84 or 0.79 corrected for sampling bias) did not differ significantly from the proportion of unmated females (0.84), providing strong quantitative support for the prediction of Hamilton’s model. In addition, there was no evidence for other factors, such as local mate competition or fighting between wingless males, that could violate simplifying assumptions of the model. Meanwhile, the proportion of winged males was not correlated with the number of wasps per fig, providing no evidence for a conditional strategy.

Conclusion

The morph ratio in P. minerva is consistent with Hamilton’s simple Mendelian strategy model, where morph ratios are set by average mating opportunities at the population level. This contrasts with some fig wasps from another subfamily that show conditional morph determination, allowing finer scale adaptation to fig-level mating opportunities. However, these conditional cases do not involve wing polymorphism. Male polymorphism is common and variable in fig wasps and has evolved independently in multiple lineages with apparently different underlying mechanisms.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01898-3.

Heterogeneous genetic basis of age at maturity in salmonid fishes

Understanding the genetic basis of repeated evolution of the same phenotype across taxa is a fundamental aim in evolutionary biology and has applications in conservation and management. However, the extent to which interspecific life-history trait polymorphisms share evolutionary pathways remains underexplored. Here, we address this gap by studying the genetic basis of a key life-history trait, age at maturity, in four species of Pacific salmonids (genus Oncorhynchus) that exhibit intra- and interspecific variation in this trait-Chinook Salmon, Coho Salmon, Sockeye Salmon, and Steelhead Trout. We tested for associations in all four species between age at maturity and two genome regions, six6 and vgll3, that are strongly associated with the same trait in Atlantic Salmon (Salmo salar). We also conducted a genome-wide association analysis in Steelhead to assess whether additional regions were associated with this trait. We found the genetic basis of age at maturity to be heterogeneous across salmonid species. Significant associations between six6 and age at maturity were observed in two of the four species, Sockeye and Steelhead, with the association in Steelhead being particularly strong in both sexes (p = 4.46 × 10^-9 after adjusting for genomic inflation). However, no significant associations were detected between age at maturity and the vgll3 genome region in any of the species, despite its strong association with the same trait in Atlantic Salmon. We discuss possible explanations for the heterogeneous nature of the genetic architecture of this key life-history trait, as well as the implications of our findings for conservation and management.

The role of genetic diversity in the evolution and maintenance of environmentally-cued, male alternative reproductive tactics

BACKGROUND

Alternative reproductive tactics (ARTs) are taxonomically pervasive strategies adopted by individuals to maximize reproductive success within populations. Even for conditionally-dependent traits, consensus postulates most ARTs involve both genetic and environmental interactions (GEIs), but to date, quantifying genetic variation underlying the threshold disposing an individual to switch phenotypes in response to an environmental cue has been a difficult undertaking. Our study aims to investigate the origins and maintenance of ARTs within environmentally disparate populations of the microscopic bulb mite, Rhizoglyphus robini, that express 'fighter' and 'scrambler' male morphs mediated by a complex combination of environmental and genetic factors.

RESULTS

Using never-before-published individual genetic profiling, we found all individuals across populations are highly inbred with the exception of scrambler males in stressed environments. In fact within the poor environment, scrambler males and females showed no significant difference in genetic differentiation (Fst) compared to all other comparisons, and although fighters were highly divergent from the rest of the population in both poor or rich environments (e.g., Fst, STRUCTURE), fighters demonstrated approximately three times less genetic divergence from the population in poor environments. AMOVA analyses further corroborated significant genetic differentiation across subpopulations, between morphs and sexes, and among subpopulations within each environment.

CONCLUSION

Our study provides new insights into the origin of ARTs in the bulb mite, highlighting the importance of GEIs: genetic correlations, epistatic interactions, and sex-specific inbreeding depression across environmental stressors. Asymmetric reproductive output, coupled with the purging of highly inbred individuals during environmental oscillations, also facilitates genetic variation within populations, despite evidence for strong directional selection. This cryptic genetic variation also conceivably facilitates stable population persistence even in the face of spatially or temporally unstable environmental challenges. Ultimately, understanding the genetic context that maintains thresholds, even for conditionally-dependent ARTs, will enhance our understanding of within population variation and our ability to predict responses to selection.

Hamilton's inclusive fitness maintains heritable altruism polymorphism through rb = c

How can altruism evolve or be maintained in a selfish world? Hamilton's rule shows that the former process will occur when rb > c-the benefits to the recipients of an altruistic act b, weighted by the relatedness between the social partners r, exceed the costs to the altruists c-drives altruistic genotypes spreading against nonaltruistic ones. From this rule, we infer that altruistic genotypes will persist in a population by forming a stable heritable polymorphism with nonaltruistic genotypes if rb = c makes inclusive fitness of the two morphs equal. We test this prediction using the data of 12 years of study on a cooperatively breeding bird, the Tibetan ground tit Pseudopodoces humilis, where helping is performed by males only and kin-directed. Individual variation in ever acting as a helper was heritable (h² = 0.47), and the resultant altruism polymorphism remained stable as indicated by low-level annual fluctuation of the percentage of helpers among all adult males (24-28%). Helpers' indirect fitness gains from increased lifetime reproductive success of related breeders statistically fully compensated for their lifetime direct fitness losses, suggesting that rb = c holds. While our work provides a fundamental support for Hamilton's idea, it highlights the equivalent inclusive fitness returns to altruists and nonaltruists mediated by rb = c as a theoretically and realistically important mechanism to maintain social polymorphism.

Egg size and emergence timing affect morphology and behavior in juvenile Chinook Salmon, Oncorhynchus tshawytscha

Variation in early life history traits often leads to differentially expressed morphological and behavioral phenotypes. We investigated whether variation in egg size and emergence timing influence subsequent morphology associated with migration timing in juvenile spring Chinook Salmon, Oncorhynchus tshawytscha. Based on evidence for a positive relationship between growth rate and migration timing, we predicted that fish from small eggs and fish that emerged earlier would have similar morphology to fall migrants, while fish from large eggs and individuals that emerged later would be more similar to older spring yearling migrants. We sorted eyed embryos within females into two size categories: small and large. We collected early and late‐emerging juveniles from each egg size category. We used landmark‐based geometric morphometrics and found that egg size appears to drive morphological differences. Egg size shows evidence for an absolute rather than relative effect on body morphology. Fish from small eggs were morphologically more similar to fall migrants, while fish from large eggs were morphologically more similar to older spring yearling migrants. Previous research has shown that the body morphology of fish that prefer the surface or bottom location in a tank soon after emergence also correlates with the morphological variations between wild fall and spring migrants, respectively. We found that late‐emerging fish spent more time near the surface. Our study shows that subtle differences in early life history characteristics may correlate with a diversity of future phenotypes.

Genetic architecture of threshold reaction norms for male alternative reproductive tactics in Atlantic salmon (Salmo salar L.)

Alternative mating tactics have important ecological and evolutionary implications and are determined by complex interactions between environmental and genetic factors. Here, we study the genetic effect and architecture of the variability in reproductive tactics among Atlantic salmon males which can either mature sexually early in life in freshwater or more commonly only after completing a migration at sea. We applied the latent environmental threshold model (LETM), which provides a conceptual framework linking individual status to a threshold controlling the decision to develop alternative traits, in an innovative experimental design using a semi-natural river which allowed for ecologically relevant phenotypic expression. Early male parr maturation rates varied greatly across families (10 to 93%) which translated into 90% [64–100%] of the phenotypic variation explained by genetic variation. Three significant QTLs were found for the maturation status, however only one collocated with a highly significant QTL explaining 20.6% of the variability of the maturation threshold located on chromosome 25 and encompassing a locus previously shown to be linked to sea age at maturity in anadromous Atlantic salmon. These results provide new empirical illustration of the relevance of the LETM for a better understanding of alternative mating tactics evolution in natural populations.

Shifting Thresholds: Rapid Evolution of Migratory Life Histories in Steelhead/Rainbow Trout, Oncorhynchus mykiss

Expression of phenotypic plasticity depends on reaction norms adapted to historic selective regimes; anthropogenic changes in these selection regimes necessitate contemporary evolution or declines in productivity and possibly extinction. Adaptation of conditional strategies following a change in the selection regime requires evolution of either the environmentally influenced cue (e.g., size-at-age) or the state (e.g., size threshold) at which an individual switches between alternative tactics. Using a population of steelhead (Oncorhynchus mykiss) introduced above a barrier waterfall in 1910, we evaluate how the conditional strategy to migrate evolves in response to selection against migration. We created 9 families and 917 offspring from 14 parents collected from the above- and below-barrier populations. After 1 year of common garden-rearing above-barrier offspring were 11% smaller and 32% lighter than below-barrier offspring. Using a novel analytical approach, we estimate that the mean size at which above-barrier fish switch between the resident and migrant tactic is 43% larger than below-barrier fish. As a result, above-barrier fish were 26% less likely to express the migratory tactic. Our results demonstrate how rapid and opposing changes in size-at-age and threshold size contribute to the contemporary evolution of a conditional strategy and indicate that migratory barriers may elicit rapid evolution toward the resident life history on timescales relevant for conservation and management of conditionally migratory species.

Evolution of discrete phenotypes from continuous norms of reaction

Discrete phenotypic variation often involves threshold expression of a trait with polygenic inheritance. How such discrete polyphenisms evolve starting from continuously varying phenotypes has received little theoretical attention. We model the evolution of sigmoid norms of reaction in response to variation in an underlying trait or in a continuous environment to identify conditions for the evolution of discontinuity. For traits with expression depending on a randomly varying underlying factor, such as developmental noise, polyphenism is unstable under constant phenotypic selection for two selective peaks, and reaction norm evolution results in a phenotypic distribution concentrated at only one peak. But with frequency-dependent selection between two adaptive peaks, a steep threshold maintaining polyphenism can evolve. For inducible plastic traits with expression conditioned on an environmental variable that also affects phenotypic selection, the steepness of the evolved reaction norm depends both on the differentiation of the environment in time or space and on its predictability between development and selection. Together with recent measurements of genetic variance of threshold steepness, these predictions suggest that quasi-discrete phenotypic variation may often evolve from continuous norms of reactions rather than being an intrinsic property of development.

Polyandry and alternative mating tactics

Many species in the animal kingdom are characterized by alternative mating tactics (AMTs) within a sex. In males, such tactics include mate guarding versus sneaking behaviours, or territorial versus female mimicry. Although AMTs can occur in either sex, they have been most commonly described in males. This sex bias may, in part, reflect the increased opportunity for sexual selection that typically exists in males, which can result in a higher probability that AMTs evolve in that sex. Consequently, females and polyandry can play a pivotal role in governing the reproductive success associated with male AMTs and in the evolutionary dynamics of the tactics. In this review, we discuss polyandry and the evolution of AMTs. First, we define AMTs and review game theoretical and quantitative genetic approaches used to model their evolution. Second, we review several examples of AMTs, highlighting the roles that genes and environment play in phenotype expression and development of the tactics, as well as empirical approaches to differentiating among the mechanisms. Third, ecological and genetic constraints to the evolution of AMTs are discussed. Fourth, we speculate on why female AMTs are less reported on in the literature than male tactics. Fifth, we examine the effects of AMTs on breeding outcomes and female fitness, and as a source, and possibly also a consequence, of sexual conflict. We conclude by suggesting a new model for the evolution of AMTs that incorporates both environmental and genetic effects, and discuss some future avenues of research.

An alternative reproductive tactic: a parasitoid wasp gathers and guards a harem by pheromone-tagging virgins

Alternative reproductive tactics (ARTs) are the outcome of decisions to obtain copulations in reproductive competition. Mating tactics male insects exhibit can be based on their competitive ability, or be dependent on conditions such as a competitive setting and the spatial and temporal distribution of receptive females. When females are clustered and numerous, two or more mating tactics can coexist. We predicted that this concept is applicable to the egg parasitoid wasp Ooencyrtus kuvanae (Hymenoptera: Encyrtidae), because wasps emerge en masse as sexually mature adults from gypsy moth, Lymantria dispar, egg masses. We reveal that male O. kuvanae exhibit two ARTs, a mate-at-once (MAO) tactic, and a harem-gathering and -guarding (HGG) tactic. MAO males invariably and immediately mate females they encounter. HGG males (i) typically mate the first receptive female they encounter, (ii) then find and assess other females, (iii) tag those without prior male contact, and finally (iv) return to, and mate with, all females they themselves have tagged. Females do not incur a direct fitness cost by mating with multiply-mated males. HGG males rely on their speed, unique tag pheromone, and on the females' rejection of HGG males except the one that pheromone-tagged them. The tagging pheromone mediates mate recognition and assessment.

Assessing adaptive phenotypic plasticity by means of conditional strategies from empirical data: the latent environmental threshold model

Conditional strategies are the most common form of discrete phenotypic plasticity. In a conditional strategy, the phenotype expressed by an organism is determined by the difference between an environmental cue and a threshold, both of which may vary among individuals. The environmental threshold model (ETM) has been proposed as a mean to understand the evolution of conditional strategies, but has been surprisingly seldom applied to empirical studies. A hindrance for the application of the ETM is that often, the proximate cue triggering the phenotypic expression and the individual threshold are not measurable, and can only be assessed using a related observable cue. We describe a new statistical model that can be applied in this common situation. The Latent ETM (LETM) allows for a measurement error in the phenotypic expression of the individual environmental cue and a purely genetically determined threshold. We show that coupling our model with quantitative genetic methods allows an evolutionary approach including an estimation of the heritability of conditional strategies. We evaluate the performance of the LETM with a simulation study and illustrate its utility by applying it to empirical data on the size-dependent smolting process for stream-dwelling Atlantic salmon juveniles.