Gallery and acoustic traits related to female body size mediate male mate choice in a bark beetle

Expression analysis of genes related to cold tolerance in Dendroctonus valens

Pine beetles are well known in North America for their widespread devastation of pine forests. However, Dendroctonus valens LeConte is an important invasive forest pest in China also. Adults and larvae of this bark beetle mainly winter at the trunks and roots of Pinus tabuliformis and Pinus sylvestris; larvae, in particular, result in pine weakness or even death. Since the species was introduced from the United States to Shanxi in 1998, its distribution has spread northward. In 2017, it invaded a large area at the junction of Liaoning, Inner Mongolia and Hebei provinces, showing strong cold tolerance. To identify genes relevant to cold tolerance and the process of overwintering, we sequenced the transcriptomes of wintering and non-wintering adult and larval D. valens using the Illumina HiSeq platform. Differential expression analysis methods for other non-model organisms were used to compare transcript abundances in adults and larvae at two time periods, followed by the identification of functions and metabolic pathways related to genes associated with cold tolerance. We detected 4,387 and 6,091 differentially expressed genes (DEGs) between sampling dates in larvae and adults, respectively, and 1,140 common DEGs, including genes encoding protein phosphatase, very long-chain fatty acids protein, cytochrome P450, and putative leucine-rich repeat-containing proteins. In a Gene Ontology (GO) enrichment analysis, 1,140 genes were assigned to 44 terms, with significant enrichment for cellulase activity, hydrolase activity, and carbohydrate metabolism. Kyoto Encyclopedia of Genes and Genomes (KEGG) classification and enrichment analyses showed that the lysosomal and purine metabolism pathways involved the most DEGs, the highly enriched terms included autophagy-animal, pentose and glucuronate interconversions and lysosomal processes. We identified 140 candidate genes associated with cold tolerance, including genes with established roles in this trait (e.g., genes encoding trehalose transporter, fructose-1,6-bisphosphatase, and trehalase). Our comparative transcriptome analysis of adult and larval D. valens in different conditions provides basic data for the discovery of key genes and molecular mechanisms underlying cold tolerance.

Fighting and aggressive sound determines larger male to win male-male competition in a bark beetle

Intrasexual selection occurs in male-male competition over access to females and usually results in the larger male winning. While much research has documented that size matters, little is known about how the larger male wins. Dendroctonus valens is an aggregating monogamous bark beetle in which males have large variation in body size and display intense competition over females. Behavioral observation showed two males fight each other within the gallery by pushing/shoving and stridulated more when two males encountered each other. Experiments using two different-sized males synchronously competing showed that larger males won 95% of contests. Reciprocal displacement experiments using muted and intact males of different or equal size were used to simulate male-male competition. Larger males displaced the smaller resident male in 90% of contests, while smaller males prevailed over larger residents in 6.7% of contests. With both males silenced, larger males displaced smaller males in 80% of contests, while smaller males prevailed in 8% of contests. Further experiments using equal-sized males showed aggressive sound-emitting males displaced muted males in 67% of contests, yet intact males displaced other intact males in only 37.5% of contests. Sound analysis showed sound pressure level is an honest signal of body size and males chose soft sounds over loud aggressive sounds in assays. Therefore, D. valens males have evolved dual behaviors, fighting and aggressive sounds associated with body size, to assess rivals to compete for a partner, gaining insights in male-male competition for this species and for other animals.

Physical contact, volatiles, and acoustic signals contribute to monogamy in an invasive aggregating bark beetle

The behavioral strategies and mechanisms by which some insects maintain monogamous mating systems are not well understood. We investigated the mating system of the bark beetle Dendroctonus valens, and identified several contributing mechanisms. Field and laboratory observations suggest the adults commonly form permanent bonds during host colonization. Moreover, it showed mated females that remained paired with males produced more offspring than mated females that were alone in galleries. In bioassays, a second female commonly entered a gallery constructed by a prior female. Videos show she commonly reached the location of the first female, but they did not engage in actual fighting. Rather, the second female typically departs to form her own gallery. Acoustic signaling likewise does not appear to influence female-female encounters, based on controlled muting experiments. Instead, the intruder appears to perceive the resident's presence by physical contact. Both acoustic signals and volatiles released by females during gallery constructing were shown to attract males. After a male joined a female in a gallery, the male-produced aggressive sounds, which were shown by playback to deter other males from entering the gallery. Unlike female-female interactions, resident males use their head and rear to push intruders out of galleries. Additionally, volatiles released by males during feeding repelled intruding males, discouraging them from entering the gallery. Males also construct plugs that block the entrance, which may prevent subsequent males and predators from entering the gallery. Thus, D. valens has evolved multifaceted mechanisms contributing to single pairings that confer benefits to both sexes.

Male mate choice for large gravid spots in a livebearing fish

Male mate choice occurs in a wide range of species, and males can increase their reproductive success by distinguishing between females based on their fecundity (e.g., large body size) or their expected sperm competition risk (e.g., virgins). However, patterns of male mate choice could be mitigated by variation in female physiological receptivity, as males can benefit by directing their mating efforts toward females that are at a point in their reproductive cycle when fertilization probability is highest. Here, we perform three experiments to assess whether male mate choice is influenced by cues of female physiological receptivity, fecundity, or sperm competition risk in the pygmy halfbeak (Dermogenys collettei), a small livebearing fish. Female halfbeaks possess a "gravid spot"-an orange abdominal marking that is caused by pigmentation of the females' skin and variation in embryo development and pigmentation during pregnancy. We show that gravid spot size increases toward parturition and is largest right before giving birth, independent of abdominal width or body size. Males consistently chose females with large gravid spots over females with small gravid spots. In contrast, males did not prefer larger females over smaller females or virgin females over mated females. As female halfbeaks store sperm prior to fertilizations, we suggest that males use the size of the gravid spot as a cue to direct their mating efforts to those females where the chance of fertilization is highest.

A Review of the Ecology and Management of Black Turpentine Beetle (Coleoptera: Curculionidae)

The black turpentine beetle, Dendroctonus terebrans Olivier is the largest pine-infesting bark beetle native to the southern and eastern United States. It generally reproduces in fresh stumps and bases of trees weakened or killed by other biotic or abiotic agents, although it can also infest and sometimes kills apparently healthy trees. Its numbers can build when large amounts of host material become available (typically through a disturbance), and black turpentine beetle-caused mortality at a local scale can become considerable. Here, we provide a complete review of the literature on this species, including its taxonomy, host, life history, chemical ecology, arthropod and microbial associates, and management options. We also provide original data on numbers of instars, acoustic signals, and pheromone chirality in this species. Our survey of the existing literature revealed that key biological characteristics of black turpentine beetles are known, but interactions with closely associated organisms, economic and ecological impacts, and improvements to monitoring and management practices have been only partially investigated.

Fitness costs of mating with preferred females in a scramble mating system

Little is known about the operation of male mate choice in systems with perceived high costs to male choosiness. Scramble mating systems are one type of system in which male choice is often considered too costly to be selected. However, in many scramble mating systems, there are also potentially high rewards of male choosiness, as females vary dramatically in reproductive output and males typically mate once per season and/or per lifetime. Using scramble mating wood frogs (Rana sylvatica), we tested whether males gain fitness benefits by mating with preferred females. We conducted choice trials (1 male presented simultaneously with 2 females) and permitted males to mate with their preferred or nonpreferred female. Offspring of preferred and nonpreferred females were reared in the laboratory and field, and we quantified various fitness-relevant parameters, including survivorship and growth rates. Across multiple parameters measured, matings with preferred females produced fewer and lower-quality offspring than did those with nonpreferred females. Our results are inconsistent with the idea that mate choice confers benefits on the choosing sex. We instead propose that, in scramble systems, males will be more likely to amplex females that are easier to capture, which may correlate with lower quality but increases male likelihood of successfully mating. Such male choice may not favor increased fitness when the operational sex ratio is less biased toward males in scramble mating systems but is, instead, a bet-hedging tactic benefitting males when available females are limited.

Variation in Mating Dynamics across Five Species of Leiobunine Harvestmen (Arachnida: Opliones)

The study of mating choices often focuses on correlates of traits to the overall outcome of a mating interaction. However, mating interactions can proceed through a series of stages, with opportunities for assessment at each stage. We compared whether male or female size predicted mating interaction outcome across several stages of mating in five species of North American leiobunine harvestmen (commonly known as daddy longlegs). Leiobunine harvestmen have been previously shown to exhibit incredible morphological diversity consistent with a spectrum of male⁻female antagonism. Across all of the species, we found a general progression of female size predicting the outcome (success and timing) of early stages of interactions, and male size or male size relative to female size predicting the outcome and timing of later stages of interactions. We also found that size was not a strong predictor of outcome in the two species on the lower end of the antagonism spectrum. The variation in how female and male size predicted outcomes across species and stages of mating suggests that multiple mechanisms may operate to shape mating dynamics within and across species. Given the close relatedness of the species studied, the patterns we uncovered suggest a rapid evolution of the traits and processes predicting the outcome of mating interactions.

Male mate choice via cuticular hydrocarbon pheromones drives reproductive isolation between Drosophila species

Mate discrimination is a key mechanism restricting gene flow between species. While studied extensively with respect to female mate choice, mechanisms of male mate choice between species are far less studied. Thus, we have little knowledge of the relative frequency, importance, or overall contribution of male mate discrimination to reproductive isolation. In the present study, we estimated the relative contributions of male and female choice to reproductive isolation between Drosophila simulans and D. sechellia, and show that male mate discrimination accounts for the majority of the current isolation between these species. We further demonstrate that males discriminate based on female cuticular hydrocarbon pheromones, and collect evidence supporting the hypothesis that male mate discrimination may alleviate the costs associated with heterospecific courtship and mating. Our findings highlight the potentially significant contribution of male mate choice to the formation of reproductive isolating barriers, and thus the speciation process.