Chemical communication and its role in sexual selection across Animalia

Sexual selection has been studied as a major evolutionary driver of animal diversity for roughly 50 years. Much evidence indicates that competition for mates favors elaborate signaling traits. However, this evidence comes primarily from a few taxa, leaving sexual selection as a salient evolutionary force across Animalia largely untested. Here, we reviewed the evidence for sexual selection on communication across all animal phyla, classes, and orders with emphasis on chemoreception, the only sense shared across lifeforms. An exhaustive literature review documented evidence for sexual selection on chemosensory traits in 10 of 34 animal phyla and indications of sexual selection on chemosensory traits in an additional 13 phyla. Potential targets of sexual selection include structures and processes involved in production, delivery, and detection of chemical signals. Our review suggests sexual selection plays a widespread role in the evolution of communication and highlights the need for research that better reflects animal diversity.

Plasma metabolomic profiles reveal sex- and maturation-dependent metabolic strategies in sea lamprey (Petromyzon marinus)

INTRODUCTION

Adult sea lamprey (Petromyzon marinus) cease feeding and migrate to spawning streams where males build nests, undergo final sexual maturation, and subsequently produce and release large quantities of bile acid pheromones that attract mature females. These animals are predicted to rearrange their metabolic pathways drastically to support their reproductive strategies, presenting advantageous opportunities to examine how sex and the maturation processes affect metabolism.

OBJECTIVES

The objective is to investigate the metabolic differences between sexes and maturation states in sea lamprey that support changes in physiological functions.

METHODS

We compared plasma metabolomes of spawning and prespawning sea lamprey in both sexes using both non-targeted and targeted metabolomics approaches using UPLC/MS-MS with electrospray ionization in both positive and negative modes. The data were processed using Progenesis QI, Compound Discoverer and XCMS softwares for alignment, peak picking, and deconvolution of the peaks. Principle component analyses (PCA) and partial least squares discriminant analyses (PLS-DA) were performed using SIMCA and Metaboanalyst softwares to identify discriminating features, followed by fragmentation matching with extensive database search and pathway mapping.

RESULTS

The pheromonal bile acid biosynthesis was upregulated significantly in males compared to females. Spermiating males further upregulated bile acid biosynthesis by altering amino acid metabolisms, upregulating cofactors and nucleotide metabolisms, but downregulating carbohydrate and energy metabolisms.

CONCLUSION

Plasma metabolomes are sex- and maturation-dependent and reflect the special metabolic demands at each life stage and reproductive strategy.

Behavior of female adult Pacific lamprey (Entosphenus tridentatus) exposed to natural and synthesized odors

Conservation  and management of Pacific Lamprey Entosphenus tridentatus and other imperiled lamprey species could include the use of chemosensory cues to attract or repel migrating adults. For restoration programs, passage of adult lamprey at dams might be improved by using cues to help guide lamprey through fishway entrances. In contrast, odors might repel unwanted invasive Sea Lamprey Petromyzon marinus in the Laurentian Great Lakes from spawning habitats or improve trapping efficiency. We conducted bioassays with Pacific Lamprey in a two-choice maze to evaluate the behavioral response of pre-ovulatory adult females to introduced chemical cues and changes in flow. During overnight tests, for each female we measured the number of entries into each arm of the maze and the amount of time spent in each arm after application of natural odors from pre-spawning conspecifics (males and females) in one of the arms. Using the same methods, we also tested whether adult females were attracted to natural odor from spermiating males, to a synthesized (artificially produced) component of a Sea Lamprey sex pheromone (3-keto petromyzonol sulfate, 3kPZS), or to an attraction flow (12 L/min as reference). In all tests, the lamprey showed consistent nocturnal activity, typically moving from sunset until sunrise and remaining inactive during daylight hours. For natural odors, the number of entries and the amount of time females spent in the treatment arm were not significantly different between control and treatment periods. However, females spent significantly less time in the treatment arm with the synthesized 3kPZS than when no odor was delivered. Females showed strong, positive responses to the attraction flow and with our assay, we could identify significant behavioral responses when the differences between the control and experimental means were greater than 15-20%. The response of lampreys to sex pheromones may be species-specific, with Pacific Lamprey less likely to respond to conspecific odors than Sea Lamprey.

Diel Patterns of Pheromone Release By Male Sea Lamprey

Costs to producing sexual signals can create selective pressures on males to invest signaling effort in particular contexts. When the benefits of signaling vary consistently across time, males can optimize signal investment to specific temporal contexts using biological rhythms. Sea lamprey, Petromyzon marinus, have a semelparous life history, are primarily nocturnal, and rely on pheromone communication for reproduction; however, whether male investment in pheromone transport and release matches increases in spawning activity remains unknown. By measuring 1) 3keto-petromyzonol sulfate (3kPZS, a main pheromone component) and its biosynthetic precursor petromyzonol sulfate (PZS) in holding water and tissue samples at 6 points over the course of 24 hours, and 2) 3kPZS release over the course of several days, we demonstrate that 3kPZS release exhibits a consistent diel pattern across several days with elevated pheromone release just prior to sunset and at night. Trends in hepatic concentrations and circulatory transport of PZS and 3kPZS were consistent with patterns of 3kPZS release and suggest the possibility of direct upregulation in pheromone transport and release rather than observed release patterns being solely a byproduct of increased behavioral activity. Our results suggest males evolved a signaling strategy that synchronizes elevated pheromone release with nocturnal increases in sea lamprey behavior. This may be imperative to ensure that male signaling effort is not wasted in a species having a single, reproductive event.

Evidence that male sea lamprey increase pheromone release after perceiving a competitor

Sexual signals evolve via selective pressures arising from male-male competition and female choice, including those originating from unintended receivers that detect the signal. For example, males can acquire information from other males signaling to females and alter their own signal. Relative to visual and acoustic signals, less is known about how such communication networks influence chemical signaling among animals. In sea lamprey (Petromyzon marinus), the chemical communication system is essential for reproduction, offering a useful system to study a pheromone communication network that includes signalers and both intended and unintended receivers. Male sea lamprey aggregate on spawning grounds, where individuals build nests and signal to females using sex pheromones. We examined how exposure to a major component of the male pheromone, 3keto-petromyzonol sulfate (3kPZS), influenced male pheromone signaling, and whether females had a preference for males that altered their signal. Exposure to 3kPZS, at a concentration of 5×10-10 mol l-1, simulated the presence of other male(s) and led to increased 3kPZS release rates within 10 min, followed by a return to baseline levels within 30 min. Exposure also led to increases in hepatic synthesis and circulatory transport of pheromone components. In behavioral assays, females preferred the odor of males that had been exposed to 3kPZS; therefore, males likely benefit from upregulating 3kPZS release after detecting competition for mates. Here, we define how a specific pheromone component influences chemical signaling during intrasexual competition, and show a rare example of how changes in chemical signaling strategies resulting from male competition may influence mate choice.