Oliver Y. Martin

The evolution of reproductive isolation through sexual conflict.

Classical population-genetics theory suggests that reproductive isolation will evolve fastest in small isolated populations. In contrast, recent theory suggests that divergence should occur fastest in larger allopatric populations. The rationale behind this is that sexual conflict, potentially the strongest driver of speciation, is greater in larger, higher-density populations. This idea is highly controversial and has little experimental support. Here we show, using replicate fly populations with varying levels of sexual conflict, that larger, more dense populations with more sexual conflict diverged to a greater degree than small populations with relaxed conflict. This result strongly suggests that speciation can occur rapidly in large populations through increased sexual conflict.

Indium Oxide as a Superior Catalyst for Methanol Synthesis by CO2 Hydrogenation

Methanol synthesis by CO2 hydrogenation is attractive in view of avoiding the environmental implications associated with the production of the traditional syngas feedstock and mitigating global warming. However, there still is a lack of efficient catalysts for such alternative processes. Herein, we unveil the high activity, 100 % selectivity, and remarkable stability for 1000 h on stream of In2 O3 supported on ZrO2 under industrially relevant conditions. This strongly contrasts to the benchmark Cu-ZnO-Al2 O3 catalyst, which is unselective and experiences rapid deactivation. In-depth characterization of the In2 O3 -based materials points towards a mechanism rooted in the creation and annihilation of oxygen vacancies as active sites, whose amount can be modulated in situ by co-feeding CO and boosted through electronic interactions with the zirconia carrier. These results constitute a promising basis for the design of a prospective technology for sustainable methanol production.

Inbreeding Promotes Female Promiscuity

After a population bottleneck, polyandry allows females to select sperm with the best prospects for fitness. The widespread phenomenon of polyandry (mating by females with multiple males) is an evolutionary puzzle, because females can sustain costs from promiscuity, whereas full fertility can be provided by a single male. Using the red flour beetle, Tribolium castaneum, we identify major fitness benefits of polyandry to females under inbreeding, when the risks of fertilization by incompatible male haplotypes are especially high. Fifteen generations after inbred populations had passed through genetic bottlenecks, we recorded increased levels of female promiscuity compared with noninbred controls, most likely due to selection from prospective fitness gains through polyandry. These data illustrate how this common mating pattern can evolve if population genetic bottlenecks increase the risks of fitness depression due to fertilization by sperm carrying genetically incompatible haplotypes.

Inbreeding depresses sperm competitiveness, but not fertilization or mating success in male Tribolium castaneum

As populations decline to levels where reproduction among close genetic relatives becomes more probable, subsequent increases in homozygous recessive deleterious expression and/or loss of heterozygote advantage can lead to inbreeding depression. Here, we measure how inbreeding across replicate lines of the flour beetle Tribolium castaneum impacts on male reproductive fitness in the absence or presence of male–male competition. Effects on male evolution from mating pattern were removed by enforcing monogamous mating throughout. After inbreeding across eight generations, we found that male fertility in the absence of competition was unaffected. However, we found significant inbreeding depression of sperm competitiveness: non-inbred males won 57 per cent of fertilizations in competition, while inbred equivalents only sired 42 per cent. We also found that the P2 ‘offence’ role in sperm competition was significantly more depressed under inbreeding than sperm ‘defence’ (P1). Mating behaviour did not explain these differences, and there was no difference in the viability of offspring sired by inbred or non-inbred males. Sperm length variation was significantly greater in the ejaculates of inbred males. Our results show that male ability to achieve normal fertilization success was not depressed under strong inbreeding, but that inbreeding depression in these traits occurred when conditions of sperm competition were generated.

EXPERIMENTAL EVOLUTION EXPOSES FEMALE AND MALE RESPONSES TO SEXUAL SELECTION AND CONFLICT IN TRIBOLIUM CASTANEUM

Between‐individual variance in potential reproductive rate theoretically creates a load in reproducing populations by driving sexual selection of male traits for winning competitions, and female traits for resisting the costs of multiple mating. Here, using replicated experimental evolution under divergent operational sex ratios (OSR, 9:1 or 1:6 ♀:♂) we empirically identified the parallel reproductive fitness consequences for females and males in the promiscuous flour beetle Tribolium castaneum. Our results revealed clear evidence that sexual conflict resides within the T. castaneum mating system. After 20 generations of selection, females from female‐biased OSRs became vulnerable to multiple mating, and showed a steep decrease in reproductive fitness with an increasing number of control males. In contrast, females from male‐biased OSRs showed no change in reproductive fitness, irrespective of male numbers. The divergence in reproductive output was not explained by variation in female mortality. Parallel assays revealed that males also responded to experimental evolution: individuals from male‐biased OSRs obtained 27% greater reproductive success across 7‐day competition for females with a control male rival, compared to males from the female‐biased lines. Subsequent assays suggest that these differences were not due to postcopulatory sperm competitiveness, but to precopulatory/copulatory competitive male mating behavior.

Sexual selection protects against extinction

Reproduction through sex carries substantial costs, mainly because only half of sexual adults produce offspring. It has been theorized that these costs could be countered if sex allows sexual selection to clear the universal fitness constraint of mutation load. Under sexual selection, competition between (usually) males and mate choice by (usually) females create important intraspecific filters for reproductive success, so that only a subset of males gains paternity. If reproductive success under sexual selection is dependent on individual condition, which is contingent to mutation load, then sexually selected filtering through ‘genic capture’ could offset the costs of sex because it provides genetic benefits to populations. Here we test this theory experimentally by comparing whether populations with histories of strong versus weak sexual selection purge mutation load and resist extinction differently. After evolving replicate populations of the flour beetle Tribolium castaneum for 6 to 7 years under conditions that differed solely in the strengths of sexual selection, we revealed mutation load using inbreeding. Lineages from populations that had previously experienced strong sexual selection were resilient to extinction and maintained fitness under inbreeding, with some families continuing to survive after 20 generations of sib × sib mating. By contrast, lineages derived from populations that experienced weak or non-existent sexual selection showed rapid fitness declines under inbreeding, and all were extinct after generation 10. Multiple mutations across the genome with individually small effects can be difficult to clear, yet sum to a significant fitness load; our findings reveal that sexual selection reduces this load, improving population viability in the face of genetic stress.

New and revisited insights into the promotion of methanol synthesis catalysts by CO2

CO hydrogenation, CO2 hydrogenation, and water–gas shift (WGS) reactions have been simultaneously investigated over industry-like catalysts based on Cu–ZnO–Al2O3, under methanol synthesis conditions (513 K, 5.0 MPa). For this, a novel methodology has been applied: the concentration of carbon dioxide in the syngas feed was consecutively increased (R = CO2:(CO + CO2) = 0–100) resulting in a volcano-type plot of the rate of methanol formation and forming a hysteresis loop when decreasing the CO2 concentration again. H2O co-feeding experiments revealed that the enhancement of activity can be correlated with the WGS activity linking both hydrogenation paths of CO and CO2. On the other hand, excessive amounts of surface hydroxyls seem to inhibit methanol production, explaining the drop in activity at high CO2 concentrations. An investigation of the catalytic performance was accompanied by an extensive characterisation of the fresh and used catalytic materials by X-ray diffraction, temperature-programmed reduction by H2, N2O pulse chemisorption, X-ray photoelectron spectroscopy, and Auger electron spectroscopy. It was shown that the copper surface area affects the CO2 hydrogenation; however, this parameter is unambiguously not the key descriptor for CO2-promoted methanol synthesis, which is a consequence of the synergistic interaction of zinc oxide and copper. This structural feature is further promoted by Al2O3 through stabilisation of the surface. The position of the activity maximum is determined by the surface ratio Cu : Zn. The hysteresis behaviour is a result of the continuous decrease of Cu dispersion and the fixation of copper species in its monovalent oxidation state, both detrimental for CO2 hydrogenation. CO hydrogenation is strongly affected by the Cu : Zn bulk ratio and thus the reducibility of the catalyst. These facts could be substantiated by the use of impregnated model catalysts.

SPERM COMPETITIVE ABILITY AND INDICES OF LIFETIME REPRODUCTIVE SUCCESS

Understanding the selection pressures shaping components of male reproductive success is essential for assessing the role of sexual selection on phenotypic evolution. A males competitive reproductive success is often measured in sequential mating tests by recording P1 (first mating male) and P2 (second mating male) paternity scores. How each of these scores relates to a males overall fitness, for example, lifetime reproductive success is, however, not known. This information is needed to determine whether males benefit from maximizing both P1 and P2 or by trading off P1 against P2 ability. We measured P1, P2, and an index of lifetime reproductive success (LRSi, a males competitive reproductive success measured over 12 days) for individual male Drosophila melanogaster. We found no evidence for phenotypic correlations between P1 and P2. In addition, whereas both P1 and P2 were associated with relative LRSi, only P2 predicted absolute LRSi. The results suggest that P2 was most closely linked to LRSi in the wild‐type population studied, a finding which may be common to species with strong second male sperm precedence. The study illustrates how P1 and P2 can have differing relationships with a males overall reproductive success, and highlights the importance of understanding commonly used measures of sperm competition in the currency of fitness.

Same-sex sexual behavior in insects and arachnids: prevalence, causes, and consequences

Same-sex sexual (SSS) behavior represents an evolutionary puzzle: whilst associated costs seem obvious, positive contributions to fitness remain unclear. Various adaptive explanations have been proposed and thorough reviews exist for vertebrates, but a thorough synthesis of causes for SSS behavior in invertebrates is lacking. Here we provide evidence for such behavior in ~110 species of insects and arachnids. Males are more frequently involved in SSS behavior in the laboratory than in the field, and isolation, high density, and exposure to female pheromones increase its prevalence. SSS behavior is often shorter than the equivalent heterosexual behavior. Most cases can be explained via mistaken identification by the active (courting/mounting) male. Adaptive explanations, such as sperm transfer of the mounting male via the mounted one or gaining experience by young males, are of limited general significance. The passive (being courted/mounted) male is sometimes responsible for this “mistake” by releasing sex pheromones or carrying female pheromones that were attached to his cuticle during prior mating activity. Passive males often resist courting/mating attempts. SSS behavior in arthropods is predominantly based on mistaken identification and is probably maintained because the cost of rejecting a valid opportunity to mate with a female is greater than that of mistakenly mating with a male. Many species exhibiting SSS behavior also mate with related species, another case of mistaken identification. Future research should focus on uncovering the situations/contexts in which mistaken identification is more or less costly for males.

Widespread infections by the bacterial endosymbiont Cardinium in Arachnids

Abstract Maternally inherited bacterial endosymbionts such as Wolbachia can potentially have a major impact on the reproduction of their arthropod hosts. Most previous studies have focused on the effects on insects, but recent evidence demonstrates that the endosymbionts Wolbachia, Rickettsia, and Spiroplasma are also common in spiders. Such infections potentially explain observed characteristics of reproduction in this group such as skewed sex ratios or reported cases of parthenogenesis. Here we test spiders and a range of other arachnids for infection with another, more recently described maternally acquired endosymbiont, Cardinium. We present data from a survey of spiders and other arachnids collected in the field and obtained from museum collections. Infections with Cardinium are found to be very widespread, perhaps more so than in other arthropod groups. The consequences of this and directions for future research on endosymbiont-arachnid interactions are discussed.