Higher rates of sex evolve under K-selection

Evolution Under Dietary Restriction Increases Reproduction at the Cost of Decreased Somatic Growth

Dietary restriction (DR) is cited as the most reliable means of extending life span in a wide range of taxa, yet the evolutionary basis of the DR effect on life span remains unclear. The resource reallocation hypothesis proposes that the longevity-extending response of DR is adaptive and stems from the reallocation of resources from reproduction to somatic maintenance under food-limited conditions. However, if DR continues for multiple generations, such a response becomes maladaptive, and genotypes with higher fecundity should be selectively favored over genotypes with longer longevity. To test this prediction, we exposed replicate populations of the rotifer Brachionus dorcas, a model organism for aging and experimental evolution studies, to DR and ad-libitum (AL) diets for 100 days. During the selection experiment, AL-selected populations showed higher growth rates and mictic ratios than DR-selected populations. After approximately 27 asexual generations of selection, populations with a DR selection history had a higher net reproductive rate but lower body volume and ingestion rate in the absence of survival costs than populations with an AL selection history when they were assayed on an AL diet. Our results are inconsistent with the prediction that evolution on sustained DR increases reproduction and reduces life span, and show for the first time that sustained DR selects for clones with higher energy investment in reproduction but lower investment in somatic growth.

An experimental test of the growth rate hypothesis as a predictive framework for microevolutionary adaptation

The growth rate hypothesis (GRH) posits that the relative body phosphorus content of an organism is positively related to somatic growth rate, as protein synthesis, which is necessary for growth, requires P-rich rRNA. This hypothesis has strong support at the interspecific level. Here, we explore the use of the GRH to predict microevolutionary responses in consumer body stoichiometry. For this, we subjected populations of the rotifer Brachionus calyciflorus to selection for fast population growth rate (PGR) in P-rich (HPF) and P-poor (LPF) food environments. With common garden transplant experiments, we demonstrate that in HP populations evolution toward increased PGR was concomitant with an increase in relative phosphorus content. In contrast, LP populations evolved higher PGR without an increase in relative phosphorus content. We conclude that the GRH has the potential to predict microevolutionary change, but that its application is contingent on the environmental context. Our results highlight the potential of cryptic evolution in determining the performance response of populations to elemental limitation of their food resources.

Rapid adaptation of Brachionus dorcas (Rotifera) to tetracycline antibiotic stress

Although natural populations can rapidly adapt to selection pressures, the fitness consequences of selection are controversial. In this study, a selection experiment was conducted with replicate populations of Brachionus dorcas that were exposed to two sublethal concentrations (26.8 and 78.3 mg/L) of oxytetracycline (OTC), followed by two common garden experiments (population growth and life table experiments). During the 102-day (approximately 36 asexual generations) selection experiment, a markedly increased growth rate but a significantly decreased mictic ratio over time in the populations exposed to OTC when compared to the control populations suggested that the former adapted to the selection pressures and that a trade-off exists between asexual and sexual reproduction. The high and stable population growth rates after 90 days of OTC selection illustrate an example of evolutionary rescue. After 102 days of selection, OTC-selected populations showed higher population growth rates than the control populations when exposed to OTC, indicating significantly increased tolerance. OTC-selected populations showed a lower average growth rate, longer average generation time and life expectancy at hatching, and higher average net reproduction rate and proportion of mictic offspring than the control populations in the absence of OTC, which indicate that OTC selection results in two fitness costs and three fitness gains and that the effect of OTC selection on fitness differs with the measured fitness variables. Both the evolutionary potential of populations under the stress of higher concentrations of OTC and the fitness costs and gains of selection in the absence of OTC indicate that past exposures to pollutants cannot be neglected when evaluating the effects of current stressors on natural populations.

Higher rates of sex evolve during adaptation to more complex environments

A leading hypothesis for the evolutionary maintenance of sexual reproduction proposes that sex is advantageous because it facilitates adaptation. Changes in the environment stimulate adaptation but not all changes are equivalent; a change may occur along one or multiple environmental dimensions. In two evolution experiments with the facultatively sexual rotifer Brachionus calyciflorus, we test how environmental complexity affects the evolution of sex by adapting replicate populations to various environments that differ from the original along one, two, or three environmental dimensions. Three different estimates of fitness (growth, lifetime reproduction, and population density) confirmed that populations adapted to their new environment. Growth measures revealed an intriguing cost of complex adaptations: populations that adapted to more complex environments lost greater amounts of fitness in the original environment. Furthermore, both experiments showed that B. calyciflorus became more sexual when adapting to a greater number of environmental dimensions. Common garden experiments confirmed that observed changes in sex were heritable. As environments in nature are inherently complex these findings help explain why sex is maintained in natural populations.

What does the geography of parthenogenesis teach us about sex?

Theory predicts that sexual reproduction is difficult to maintain if asexuality is an option, yet sex is very common. To understand why, it is important to pay attention to repeatably occurring conditions that favour transitions to, or persistence of, asexuality. Geographic parthenogenesis is a term that has been applied to describe a large variety of patterns where sexual and related asexual forms differ in their geographic distribution. Often asexuality is stated to occur in a habitat that is, in some sense, marginal, but the interpretation differs across studies: parthenogens might not only predominate near the margin of the sexuals' distribution, but might also extend far beyond the sexual range; they may be disproportionately found in newly colonizable areas (e.g. areas previously glaciated), or in habitats where abiotic selection pressures are relatively stronger than biotic ones (e.g. cold, dry). Here, we review the various patterns proposed in the literature, the hypotheses put forward to explain them, and the assumptions they rely on. Surprisingly, few mathematical models consider geographic parthenogenesis as their focal question, but all models for the evolution of sex could be evaluated in this framework if the (often ecological) causal factors vary predictably with geography. We also recommend broadening the taxa studied beyond the traditional favourites.This article is part of the themed issue 'Weird sex: the underappreciated diversity of sexual reproduction'.