Variation in polyandry and its fitness consequences among populations of the red flour beetle, Tribolium castaneum

Dominance and inheritance patterns of mobility and death feigning in beetle strains selected for moving activity

Reciprocal crossing of different strains is a suitable method to investigate the dominance and inheritance of a focal trait. Herein, we performed reciprocal crossing among strains of Tribolium castaneum exhibiting a genetically high (H strain) and low (L strain) moving activity and investigated the related heritable factors in the F₁ and F₂ generations. We also evaluated death-feigning behavior, which negatively responded to artificial selection for moving activity in T. castaneum. The results obtained for the F₁ generation suggest that low moving activity and short duration of death feigning were dominant. In the F₂ generation, movement and death feigning exhibited continuous segregation. The distribution of each trait value in the F₂ generation differed from that in the parental generation, and no individuals transgressing the distribution of trait values in the parental generation emerged in the F₂ generation. These results suggest that the genetic correlation between movement and death-feigning behavior is controlled in a polygenic manner. Moreover, the examination of the proportions of both behaviors (high vs. low moving activity and long vs. short death-feigning duration) in the F₁ generation revealed that the two behaviors may be controlled by the maternal genotype, suggesting that the gene(s) that control movement and death feigning are located on the sex chromosome in T. castaneum.

The female Euscepes postfasciatus refractory period is induced by the male but length is determined by the female

Females of many animals mate multiple times during their lives (i.e., polyandry). The period between matings (mating interval) is called the refractory period (RP). In the West Indian sweet potato weevil (Euscepes postfasciatus), males use the ejaculate to induce the RP in females to prevent re-mating. By measuring the RP, a large variation of 1-49 days was observed. This variation may be due to the males (ejaculate quantity and quality) and females (ejaculate sensitivity/degradation ability and body size) and their interactions, but the exact mechanisms are currently unclear. Here, we investigated a tendency towards a particular female RP duration and the associated traits of males and females to test the following three factors responsible for variation in the length of the RP: male manipulation of ejaculate volume, individual differences in male ejaculation substances, and ejaculate sensitivity/degradation ability in females. We prepared virgin males and females to create mating pairs. The following day, another mate was introduced to the females, and the first RP was measured. The same procedure was used for measuring the second RP. The males were also provided with another female (second female), mated, and then the RP of the second female was measured. In addition, the relationship between the length of the RP and female fitness was investigated. The results showed that there was a significant positive correlation between the first and second RP in the focal females, while no significant correlation was observed between the RP of the first and second females induced by the same male. It was also found that the length of the RP did not affect female fitness. This indicated that the males did not adaptively manipulate ejaculation volume depending on the quality of the females, and variance in the length of the RP may be explained by variation in the female physiological ability against ejaculate.

An empirical test of the bet-hedging polyandry hypothesis: Female red flour beetles avoid extinction via multiple mating

Bet-hedging via polyandry (spreading the extinction risk of the female's lineage over multiple males) may explain the evolution of female multiple mating, which is found in a wide range of animal and plant taxa. This hypothesis posits that females can increase their fitness via polyandrous mating when "unsuitable" males (i.e., males causing reproductive failure for various reasons) are frequent in the population and females cannot discriminate such unsuitable mates. Although recent theoretical studies have shown that polyandry can operate as a bet-hedging strategy, empirical tests are scarce. In the present study, we tested the bet-hedging polyandry hypothesis by using the red flour beetle Tribolium castaneum. We compared female reproductive success between monandry and polyandry treatments when females mated with males randomly collected from an experimental population, including 20% irradiated (infertile) males. In addition, we evaluated geometric mean fitness across multiple generations as the index of adaptability of bet-hedging traits. Polyandrous females showed a significantly higher egg hatching rate and higher geometric mean fitness than monandrous females. These results strongly support the bet-hedging polyandry hypothesis.

When does female multiple mating evolve to adjust inbreeding? Effects of inbreeding depression, direct costs, mating constraints, and polyandry as a threshold trait

Polyandry is often hypothesized to evolve to allow females to adjust the degree to which they inbreed. Multiple factors might affect such evolution, including inbreeding depression, direct costs, constraints on male availability, and the nature of polyandry as a threshold trait. Complex models are required to evaluate when evolution of polyandry to adjust inbreeding is predicted to arise. We used a genetically explicit individual‐based model to track the joint evolution of inbreeding strategy and polyandry defined as a polygenic threshold trait. Evolution of polyandry to avoid inbreeding only occurred given strong inbreeding depression, low direct costs, and severe restrictions on initial versus additional male availability. Evolution of polyandry to prefer inbreeding only occurred given zero inbreeding depression and direct costs, and given similarly severe restrictions on male availability. However, due to its threshold nature, phenotypic polyandry was frequently expressed even when strongly selected against and hence maladaptive. Further, the degree to which females adjusted inbreeding through polyandry was typically very small, and often reflected constraints on male availability rather than adaptive reproductive strategy. Evolution of polyandry solely to adjust inbreeding might consequently be highly restricted in nature, and such evolution cannot necessarily be directly inferred from observed magnitudes of inbreeding adjustment.

Strain-specific priming of resistance in the red flour beetle, Tribolium castaneum

As invertebrates lack the molecular machinery employed by the vertebrate adaptive immune system, it was thought that they consequently lack the ability to produce lasting and specific immunity. However, in recent years, it has been demonstrated that the immune defence of invertebrates is by far more complicated and specific than previously envisioned. Lasting immunity following an initial exposure that proves protection on a secondary exposure has been shown in several species of invertebrates. This phenomenon has become known as immune priming. In the cases where it is explicitly tested, this priming can also be highly specific. In this study, we used survival assays to test for specific priming of resistance in the red flour beetle, Tribolium castaneum, using bacteria of different degrees of relatedness. Our results suggest an unexpected degree of specificity that even allows for differentiation between different strains of the same bacterium. However, our findings also demonstrate that specific priming of resistance in insects may not be ubiquitous across all bacteria.