Gene drives and population persistence vs elimination: The impact of spatial structure and inbreeding at low density

Synthetic gene drive constructs are being developed to control disease vectors, invasive species, and other pest species. In a well-mixed random mating population a sufficiently strong gene drive is expected to eliminate a target population, but it is not clear whether the same is true when spatial processes play a role. In species with an appropriate biology it is possible that drive-induced reductions in density might lead to increased inbreeding, reducing the efficacy of drive, eventually leading to suppression rather than elimination, regardless of how strong the drive is. To investigate this question we analyse a series of explicitly solvable stochastic models considering a range of scenarios for the relative timing of mating, reproduction, and dispersal and analyse the impact of two different types of gene drive, a Driving Y chromosome and a homing construct targeting an essential gene. We find in all cases a sufficiently strong Driving Y will go to fixation and the population will be eliminated, except in the one life history scenario (reproduction and mating in patches followed by dispersal) where low density leads to increased inbreeding, in which case the population persists indefinitely, tending to either a stable equilibrium or a limit cycle. These dynamics arise because Driving Y males have reduced mating success, particularly at low densities, due to having fewer sisters to mate with. Increased inbreeding at low densities can also prevent a homing construct from eliminating a population. For both types of drive, if there is strong inbreeding depression, then the population cannot be rescued by inbreeding and it is eliminated. These results highlight the potentially critical role that low-density-induced inbreeding and inbreeding depression (and, by extension, other sources of Allee effects) can have on the eventual impact of a gene drive on a target population.

Pollinating fig wasps' simple solutions to complex sex ratio problems: a review

Local mate competition (LMC) favours female biased clutch sex ratios because it reduces competition between brothers and provides extra mating opportunities for sons. Fig wasps seem to fit LMC model assumptions and lay female-biased sex ratios as predicted. These female biased sex ratios increase fitness greatly. In line with predictions, their sex ratios become less female-biased as the number of mothers laying in the same fig increases. However, this variation results in comparatively small fitness benefits compared to just biased ratios and data suggest substantial mismatches with LMC theory. The mismatches are due to several factors. (1) Multiple foundresses typically lay too many daughters. (2) Single foundress sex ratios are explained by sequential oviposition and ladies-last models. (3) Mortality that typically exceeds 10% may decouple the link between primary sex ratios, the focus of model predictions, and secondary sex ratios of adult wasps that are counted by researchers. (4) Model assumptions are frequently violated: (a) clutch sizes are unequal, (b) oviposition may not be simultaneous (c) cryptic/multiple wasp species inhabit the same host, (d) foundress numbers are systematically undercounted, (e) inbreeding coefficient calculations are inaccurate, and (f) male wasps sometimes disperse. These data and calculations suggest that alternative explanations must be considered seriously. Substantial data show that wasps typically lay most of their male eggs first followed by mostly female eggs require a new approach. These "slope" strategies result in more accurate sex ratios that are automatically adjusted to foundress number, own and relative clutch sizes and to sequential clutches. This effect will alter sex ratios in all species once the egg capacity of a fig is crossed or when interference reduces clutch sizes. In addition to this passive response, the females of about half the studied species have a conditional response that reduces female bias under higher foundress numbers by laying more sons. Therefore, wasps seem to use a very simple strategy that increases their fitness. Natural selection could have optimized parameters of the slope strategy and possibly the existence of the slope strategy itself. Variation in the slope strategy that is the result of natural selection is adaptive. Research should therefore focus on quantifying variables of this slope strategy. Currently, it is unclear how much of the variation is adaptive as opposed to being coincidental by-products.

Cooperative interactions among females can lead to even more extraordinary sex ratios

Hamilton's local mate competition theory provided an explanation for extraordinary female-biased sex ratios in a range of organisms. When mating takes place locally, in structured populations, a female-biased sex ratio is favored to reduce competition between related males, and to provide more mates for males. However, there are a number of wasp species in which the sex ratios appear to more female biased than predicted by Hamilton's theory. It has been hypothesized that the additional female bias in these wasp species results from cooperative interactions between females. We investigated theoretically the extent to which cooperation between related females can interact with local mate competition to favor even more female-biased sex ratios. We found that (i) cooperation between females can lead to sex ratios that are more female biased than predicted by local competition theory alone, and (ii) sex ratios can be more female biased when the cooperation occurs from offspring to mothers before dispersal, rather than cooperation between siblings after dispersal. Our models formally confirm the verbal predictions made in previous experimental studies, which could be applied to a range of organisms. Specifically, cooperation can help explain sex ratio biases in Sclerodermus and Melittobia wasps, although quantitative comparisons between predictions and data suggest that some additional factors may be operating.

Genomics of sex allocation in the parasitoid wasp Nasonia vitripennis

BACKGROUND

Whilst adaptive facultative sex allocation has been widely studied at the phenotypic level across a broad range of organisms, we still know remarkably little about its genetic architecture. Here, we explore the genome-wide basis of sex ratio variation in the parasitoid wasp Nasonia vitripennis, perhaps the best studied organism in terms of sex allocation, and well known for its response to local mate competition.

RESULTS

We performed a genome-wide association study (GWAS) for single foundress sex ratios using iso-female lines derived from the recently developed outbred N. vitripennis laboratory strain HVRx. The iso-female lines capture a sample of the genetic variation in HVRx and we present them as the first iteration of the Nasonia vitripennis Genome Reference Panel (NVGRP 1.0). This panel provides an assessment of the standing genetic variation for sex ratio in the study population. Using the NVGRP, we discovered a cluster of 18 linked SNPs, encompassing 9 annotated loci associated with sex ratio variation. Furthermore, we found evidence that sex ratio has a shared genetic basis with clutch size on three different chromosomes.

CONCLUSIONS

Our approach provides a thorough description of the quantitative genetic basis of sex ratio variation in Nasonia at the genome level and reveals a number of inter-related candidate loci underlying sex allocation regulation.

The costs and benefits of multiple mating in a mostly monandrous wasp

The taxonomically widespread nature of polyandry remains a puzzle. Much of the empirical work regarding the costs and benefits of multiple mating to females has, for obvious reasons, relied on species that are already highly polyandrous. However, this makes it difficult to separate the processes that maintain the current level of polyandry from the processes that facilitate its expression and initiated its evolution. Here we consider the costs and benefits of polyandry in Nasonia vitripennis, a species of parasitoid wasp that is "mostly monandrous" in the wild, but which evolves polyandry under laboratory culture conditions. In a series of six experiments, we show that females gain a direct fecundity and longevity benefit from mating multiply with virgin males. Conversely, mating multiply with previously mated males actually results in a fecundity cost. Sexual harassment may also represent a significant cost of reproduction. Harassment was, however, only costly during oviposition, resulting in reduced fecundity, longevity, and disrupted sex allocation. Our results show that ecological changes, in our case associated with differences in the local mating structure in the laboratory can alter the costs and benefits of mating and harassment and potentially lead to shifts in mating patterns.

No sex-biased dispersal in a primate with an uncommon social system-cooperative polyandry

An influential hypothesis proposed by Greenwood (1980) suggests that different mating systems result in female and male-biased dispersal, respectively, in birds and mammals. However, other aspects of social structure and behavior can also shape sex-biased dispersal. Although sex-specific patterns of kin cooperation are expected to affect the benefits of philopatry and dispersal patterns, empirical evidence is scarce. Unlike many mammals, Saguinus geoffroyi (Geoffroy's tamarin) has a breeding system in which typically multiple males mate with a single breeding female. Males typically form cooperative reproductive partnerships between relatives, whereas females generally compete for reproductive opportunities. This system of cooperative polyandry is predicted to result in female-biased dispersal, providing an opportunity to test the current hypotheses of sex-biased dispersal. Here we test for evidence of sex-biased dispersal in S. geoffroyi using demographic and genetic data from three populations. We find no sex bias in natal dispersal, contrary to the prediction based on the mating patterns. This pattern was consistent after controlling for the effects of historical population structure. Limited breeding opportunities within social groups likely drive both males and females to disperse, suggesting that dispersal is intimately related to the social context. The integration of genetic and field data revealed that tamarins are another exception to the presumed pattern of male-biased dispersal in mammals. A shift in focus from mating systems to social behavior, which plays a role in most all processes expected to influence sex-bias in dispersal, will be a fruitful target for research both within species and across taxa.

Evidence of Taxa-, Clone-, and Kin-discrimination in Protists: Ecological and Evolutionary Implications

Unicellular eukaryotes, or protists, are among the most ancient organisms on Earth. Protists belong to multiple taxonomic groups; they are widely distributed geographically and in all environments. Their ability to discriminate among con- and heterospecifics has been documented during the past decade. Here we discuss exemplar cases of taxa-, clone-, and possible kin-discrimination in five major lineages: Mycetozoa (Dictyostelium, Polysphondylium), Dikarya (Saccharomyces), Ciliophora (Tetrahymena), Apicomplexa (Plasmodium) and Archamoebae (Entamoeba). We summarize the proposed genetic mechanisms involved in discrimination-mediated aggregation (self versus different), including the csA, FLO and trg (formerly lag) genes, and the Proliferation Activation Factors (PAFs), which facilitate clustering in some protistan taxa. We caution about the experimental challenges intrinsic to studying recognition in protists, and highlight the opportunities for exploring the ecology and evolution of complex forms of cell-cell communication, including social behavior, in a polyphyletic, still superficially understood group of organisms. Because unicellular eukaryotes are the evolutionary precursors of multicellular life, we infer that their mechanisms of taxa-, clone-, and possible kin-discrimination gave origin to the complex diversification and sophistication of traits associated with species and kin recognition in plants, fungi, invertebrates and vertebrates.

The effect of female mating status on male offspring traits

In haplodiploid insects, males develop from unfertilized eggs; consequently, unmated females can reproduce. In a patchy, highly structured population, where brothers compete for mates and the reproductive return through sons is lower, females should minimize the number of male offspring. Consequently, unmated females are likely to have a reduced fitness compared to mated females. Here, we tested the oviposition behaviour of the haplodiploid beetle Coccotrypes dactyliperda. In this species, the unmated female can mate with her son to produce daughters. We predicted that unmated females could increase their fitness by (1) producing only few and small sons sufficient for mother-son mating and (2) dispersing to a patch occupied by conspecific females in order to increase their or their sons' chance of mating. We demonstrate that (1) unmated females are common (23 % of all females), (2) they oviposit more frequently than mated females in occupied patches, (3) unmated females oviposit more eggs than mated females-this is in spite of the trade-offs, evident in this study, between the number of sons and the number of the mother's future offspring after mating, (4) unmated females have a higher proportion of dispersing sons, and (5) sons of unmated females are smaller than sons of mated females. We conclude that the incidence of unmated females in the structured populations of C. dactyliperda is explained by plasticity in their oviposition behaviour. We discuss conditions where a high incidence of unmated females can persist as a successful strategy in structured populations.

Sex allocation adjustment to mating group size in a simultaneous hermaphrodite

Sex allocation theory is considered as a touchstone of evolutionary biology, providing some of the best supported examples for Darwinian adaptation. In particular, Hamilton's local mate competition theory has been shown to generate precise predictions for extraordinary sex ratios observed in many separate-sexed organisms. In analogy to local mate competition, Charnov's mating group size model predicts how sex allocation in simultaneous hermaphrodites is affected by the mating group size (i.e., the number of mating partners plus one). Until now, studies have not directly explored the relationship between mating group size and sex allocation, which we here achieve in the simultaneously hermaphroditic flatworm Macrostomum lignano. Using transgenic focal worms with ubiquitous expression of green-fluorescent protein (GFP), we assessed the number of wild-type mating partners carrying GFP+ sperm from these focal worms when raised in different social group sizes. This allowed us to test directly how mating group size was related to the sex allocation of focal worms. We find that the proportion of male investment initially increases with increasing mating group size, but then saturates as predicted by theory. To our knowledge, this is the first direct test of the mating group size model in a simultaneously hermaphroditic animal.

Brood sex ratio variance, developmental mortality and virginity in a gregarious parasitoid wasp

Females of the parasitoid wasp Goniozus nephantidis paralyse host caterpillars and lay a clutch of up to 18 eggs onto the host integument. The known biology of G. nephantidis suggests that matings occur exclusively between siblings from the same brood. This leads to the prediction that brood sex ratios should be highly female-biased and have low variance. Sex ratios are indeed female-biased, with the mean proportion of males equal to 0.093. However, while sex ratio variance is significantly less than binomial, many broods contain no males at emergence. During development 28% of G. nephantidis offspring die. Male mortality offers a potential explanation for all-female (= virgin) broods. For the clutch sizes and mortality observed, theory predicts that <10% of females will emerge from all-female broods but the empirical value is much higher. The prediction that the prevalence of virginity decreases with increasing clutch size is, however, supported. We consider alternative explanations for the observed proportion of all-female broods, but this appears to be neither an artefact of the laboratory environment nor due to incorrect assumptions about G. nephantidis life history. Although its reproductive biology has been much investigated and its sex ratio matches some theoretical predictions, we conclude that a fuller understanding of G. nephantidis sex ratio requires a deeper knowledge of its field biology.

Variable sex ratio strategy of Telenomus heliothidis (Hymenoptera: Scelionidae): adaptation to host and conspecific density

The sex allocation behavior of the solitary egg parasitoid Telenomus heliothidis Ashmead was investigated by examining the response of females reared in isolation and under crowded conditions. Females reared in isolation adjusted their sex ratio with foundress and host number per patch in accordance with the predictions of local mate competition (LMC) theory. However, females did not shift their sex ratio in response to conspecifics foraging on the same host patch or to contact with previously parasitized hosts. Instead, shifts were associated with encounter rate and a sequence of oviposition. Females maintained under crowded conditions responded to host patches much differently. One-day-old females which had lived under crowded conditions for 24 h produced sex ratios similar to those of continuously isolated females. However, females reared under crowded conditions for 7 days consistently produced unbiased sex ratios, and exhibited a different sequence of oviposition. This shift appeared to be due directly to crowding rather than age, oviposition experience or sperm depletion since the effect could be reversed by subsequent isolation.