Selection for multiple mating in females due to mates that reduce female fitness

If females are unable to discriminate among males before mating,remating by females that store sperm may have evolved as a hedgeagainst having only "costly" mates (less preferred males thatreduce her fitness). However, the benefit of remating is notguaranteed because she can also mate by chance with anothercostly male. We devised a model to explain the evolution offemale remating by representing female fitness as a functionof the proportion of costly mates. We examined the effect ofa linear, a concave-up, and a concave-down fitness functionand found that only the latter favors the evolution of femaleremating. With a concave-down function, females mating with50% costly mates have nearly the same fitness as do femaleswith none. A biological interpretation for a concave-down functionis that sperm from good males are better at competing with spermfrom costly males or are more preferred by females. A concave-upfunction implies the reverse, whereas a linear function willoccur when sperm are equally competitive. We review specificsituations in nature that might produce a concave-down functionand find evidence that sterility and intragenomic conflict aretwo phenomena capable of driving the evolution of female rematingby our model.     

Should mothers provision their offspring equally? A manipulative field test

Within‐brood variation in offspring size is universal, but its causes are unclear. Theoretical explanations for within‐brood variation commonly invoke bet‐hedging, although alternatives consider the role of sibling competition. Despite abundant theory, empirical manipulations of within‐brood variation in offspring size are rare. Using a field experiment, we investigate the consequences of unequal maternal provisioning for both maternal and offspring fitness in a marine invertebrate. We create experimental broods of siblings with identical mean, but different variance, in offspring size, and different sibling densities. Overall, more‐variable broods had higher mean performance than less‐variable broods, suggesting benefits of unequal provisioning that arise independently of bet‐hedging. Complementarity effects drove these benefits, apparently because offspring‐size variation promotes resource partitioning. We suggest that when siblings compete for the same resources, and offspring size affects niche usage, the production of more‐variable broods can provide greater fitness returns given the same maternal investment; a process unanticipated by the current theory.     

Multitasking and the evolution of optimal clutch size in fluctuating environments

Adaptive studies of avian clutch size variation across environmental gradients have resulted in what has become known as the fecundity gradient paradox, the observation that clutch size typically decreases with increasing breeding season length along latitudinal gradients, but increases with increasing breeding season length along elevational gradients. These puzzling findings challenge the common belief that organisms should reduce their clutch size in favor of additional nesting attempts as the length of the breeding season increases, an approach typically described as a bet‐hedging strategy. Here, we propose an alternative hypothesis—the multitasking hypothesis—and show that laying smaller clutches represents a multitasking strategy of switching between breeding and recovery from breeding. Both our individual‐based and analytical models demonstrate that a small clutch size strategy is favored during shorter breeding seasons because less time and energy are wasted under the severe time constraints associated with breeding multiply within a season. Our model also shows that a within‐generation bet‐hedging strategy is not favored by natural selection, even under a high risk of predation and in long breeding seasons. Thus, saving time—wasting less time as a result of an inability to complete a breeding cycle at the end of breeding season—is likely to be the primary benefit favoring the evolution of small avian clutch sizes during short breeding seasons. We also synthesize the seasonality hypothesis (pronounced seasonality leads to larger clutch size) and clutch size‐dependent predation hypothesis (larger clutch size causes higher predation risks) within our multitasking hypothesis to develop an integrative model to help resolve the paradox of contrasting patterns of clutch size along elevational and latitudinal gradients. Ultimately, our models provide a new perspective for understanding life‐history evolution under fluctuating environments.     

The chemical basis of mate recognition in two parasitoid wasp species of the genus Nasonia

Triatomines (Hemiptera: Reduviidae) are vectors of Trypanosoma cruzi Chagas, the etiological agent of Chagas's disease. They display pre‐adult development delay – that is, a development time much longer than on average – which usually has been considered as a maladaptive trait. However, this hypothesis has not been tested. We carried out an experiment under controlled laboratory conditions to (1) test whether a development delay exists in the fifth nymphal stage of Rhodnius prolixus Stål (Hemiptera: Reduviidae, Rhodniini), and (2) measure any fitness cost related to such delay by estimating the relationship between individual development time and other life‐history traits. We analyzed the development time with various continuous statistical distributions (normal, log‐normal, Weibull, gamma, Pareto, Burr, and log‐logistic). Using goodness‐of‐fit tests, the best fit was obtained with asymmetrical distributions, with the Burr distribution showing the best fit to the data. We concluded that a development delay exists in stage five of R. prolixus without fitness cost. The combination of our results and previous work suggests that such a delay could be viewed as an adaptive response to environmental stochasticity and/or density‐dependence rather than as a maladaptive trait. We propose further investigations to provide a conclusive test of adaptive delay in triatomines.     

Bet hedging or not? A guide to proper classification of microbial survival strategies

Bacteria have developed an impressive ability to survive and propagate in highly diverse and changing environments by evolving phenotypic heterogeneity. Phenotypic heterogeneity ensures that a subpopulation is well prepared for environmental changes. The expression bet hedging is commonly (but often incorrectly) used by molecular biologists to describe any observed phenotypic heterogeneity. In evolutionary biology, however, bet hedging denotes a risk-spreading strategy displayed by isogenic populations that evolved in unpredictably changing environments. Opposed to other survival strategies, bet hedging evolves because the selection environment changes and favours different phenotypes at different times. Consequently, in bet hedging populations all phenotypes perform differently well at any time, depending on the selection pressures present. Moreover, bet hedging is the only strategy in which temporal variance of offspring numbers per individual is minimized. Our paper aims to provide a guide for the correct use of the term bet hedging in molecular biology.     

Contact‐dependent growth inhibition induces high levels of antibiotic‐tolerant persister cells in clonal bacterial populations

Bacterial populations can use bet‐hedging strategies to cope with rapidly changing environments. One example is non‐growing cells in clonal bacterial populations that are able to persist antibiotic treatment. Previous studies suggest that persisters arise in bacterial populations either stochastically through variation in levels of global signalling molecules between individual cells, or in response to various stresses. Here, we show that toxins used in contact‐dependent growth inhibition (CDI) create persisters upon direct contact with cells lacking sufficient levels of CdiI immunity protein, which would otherwise bind to and neutralize toxin activity. CDI‐mediated persisters form through a feedforward cycle where the toxic activity of the CdiA toxin increases cellular (p)ppGpp levels, which results in Lon‐mediated degradation of the immunity protein and more free toxin. Thus, CDI systems mediate a population density‐dependent bet‐hedging strategy, where the fraction of non‐growing cells is increased only when there are many cells of the same genotype. This may be one of the mechanisms of how CDI systems increase the fitness of their hosts.     

Variation in developmental rates is not linked to environmental unpredictability in annual killifishes

Comparative evidence suggests that adaptive plasticity may evolve as a response to predictable environmental variation. However, less attention has been placed on unpredictable environmental variation, which is considered to affect evolutionary trajectories by increasing phenotypic variation (or bet hedging). Here, we examine the occurrence of bet hedging in egg developmental rates in seven species of annual killifish that originate from a gradient of variation in precipitation rates, under three treatment incubation temperatures (21, 23, and 25°C). In the wild, these species survive regular and seasonal habitat desiccation, as dormant eggs buried in the soil. At the onset of the rainy season, embryos must be sufficiently developed in order to hatch and complete their life cycle. We found substantial differences among species in both the mean and variation of egg development rates, as well as species‐specific plastic responses to incubation temperature. Yet, there was no clear relationship between variation in egg development time and variation in precipitation rate (environmental predictability). The exact cause of these differences therefore remains enigmatic, possibly depending on differences in other natural environmental conditions in addition to precipitation predictability. Hence, if species‐specific variances are adaptive, the relationship between development and variation in precipitation is complex and does not diverge in accordance with simple linear relationships.     

The evolution of different maternal investment strategies in two closely related desert vertebrates

We compared egg size phenotypes and tested several predictions from the optimal egg size (OES) and bet‐hedging theories in two North American desert‐dwelling sister tortoise taxa, Gopherus agassizii and G. morafkai, that inhabit different climate spaces: relatively unpredictable and more predictable climate spaces, respectively. Observed patterns in both species differed from the predictions of OES in several ways. Mean egg size increased with maternal body size in both species. Mean egg size was inversely related to clutch order in G. agassizii, a strategy more consistent with the within‐generation hypothesis arising out of bet‐hedging theory or a constraint in egg investment due to resource availability, and contrary to theories of density dependence, which posit that increasing hatchling competition from later season clutches should drive selection for larger eggs. We provide empirical evidence that one species, G. agassizii, employs a bet‐hedging strategy that is a combination of two different bet‐hedging hypotheses. Additionally, we found some evidence for G. morafkai employing a conservative bet‐hedging strategy. (e.g., lack of intra‐ and interclutch variation in egg size relative to body size). Our novel adaptive hypothesis suggests the possibility that natural selection favors smaller offspring in late‐season clutches because they experience a more benign environment or less energetically challenging environmental conditions (i.e., winter) than early clutch progeny, that emerge under harsher and more energetically challenging environmental conditions (i.e., summer). We also discuss alternative hypotheses of sexually antagonistic selection, which arise from the trade‐offs of son versus daughter production that might have different optima depending on clutch order and variation in temperature‐dependent sex determination (TSD) among clutches. Resolution of these hypotheses will require long‐term data on fitness of sons versus daughters as a function of incubation environment, data as yet unavailable for any species with TSD.     

Bet hedging in a warming ocean: predictability of maternal environment shapes offspring size variation in marine sticklebacks

Bet hedging at reproduction is expected to evolve when mothers are exposed to unpredictable cues for future environmental conditions, whereas transgenerational plasticity (TGP) should be favoured when cues reliably predict the environment offspring will experience. Since climate predictions forecast an increase in both temperature and climate variability, both TGP and bet hedging are likely to become important strategies to mediate climate change effects. Here, the potential to produce variably sized offspring in both warming and unpredictable environments was tested by investigating whether stickleback (Gasterosteus aculeatus) mothers adjusted mean offspring size and within‐clutch variation in offspring size in response to experimental manipulation of maternal thermal environment and predictability (alternating between ambient and elevated water temperatures). Reproductive output traits of F1 females were influenced by both temperature and environmental predictability. Mothers that developed at ambient temperature (17 °C) produced larger, but fewer eggs than mothers that developed at elevated temperature (21 °C), implying selection for different‐sized offspring in different environments. Mothers in unpredictable environments had smaller mean egg sizes and tended to have greater within‐female egg size variability, especially at 21 °C, suggesting that mothers may have dynamically modified the variance in offspring size to spread the risk of incorrectly predicting future environmental conditions. Both TGP and diversification influenced F2 offspring body size. F2 offspring reared at 21 °C had larger mean body sizes if their mother developed at 21 °C, but this TGP benefit was not present for offspring of 17 °C mothers reared at 17 °C, indicating that maternal TGP will be highly relevant for ocean warming scenarios in this system. Offspring of variable environment mothers were smaller but more variable in size than offspring from constant environment mothers, particularly at 21 °C. In summary, stickleback mothers may have used both TGP and diversified bet‐hedging strategies to cope with the dual stress of ocean warming and environmental uncertainty.