Spermine in semen of male sea lamprey acts as a sex pheromone

Abstract

Semen is fundamental for sexual reproduction. The non-sperm part of ejaculated semen, or seminal plasma, facilitates the delivery of sperm to the eggs. The seminal plasma of some species with internal fertilization contains anti-aphrodisiac molecules that deter promiscuity in post-copulatory females, conferring fitness benefits to the ejaculating male. By contrast, in some taxa with external fertilization such as fish, exposure to semen promotes spawning behaviors. However, no specific compounds in semen have been identified as aphrodisiac pheromones. We sought to identify a pheromone from the milt (fish semen) of sea lamprey ( Petromyzon marinus ), a jawless fish that spawns in lek-like aggregations in which each spermiating male defends a nest, and ovulatory females move from nest to nest to mate. We postulated that milt compounds signal to ovulatory females the presence of spawning spermiating males. We determined that spermine, an odorous polyamine initially identified from human semen, is indeed a milt pheromone. At concentrations as low as 10 −14 molar, spermine stimulated the lamprey olfactory system and attracted ovulatory females but did not attract males or pre-ovulatory females. We found spermine activated a trace amine-associated receptor (TAAR)-like receptor in the lamprey olfactory epithelium. A novel antagonist to that receptor nullified the attraction of ovulatory females to spermine. Our results elucidate a mechanism whereby a seminal plasma pheromone attracts ready-to-mate females and implicates a possible conservation of the olfactory detection of semen from jawless vertebrates to humans. Milt pheromones may also have management implications for sea lamprey populations.

A pheromone antagonist liberates female sea lamprey from a sensory trap to enable reliable communication

“…All behavioral assays used ovulated females or spermiated males. Procedures for 2-choice flume and in-stream behavioral assays were slightly modified from previous studies (33,34,45) and are described in SI Appendix. Natural odorants for behavior tests were collected from a captive population of ∼25,000 larvae as previously described (24) and from a group (14 or 20) of males held in 40 L water for 4 h. Synthesized 3kPZS and PZS were prepared by Bridge Organics.…”

“…Natural odorants for behavior tests were collected from a captive population of ∼25,000 larvae as previously described (24) and from a group (14 or 20) of males held in 40 L water for 4 h. Synthesized 3kPZS and PZS were prepared by Bridge Organics. The methods for EOG recordings are described in previous reports (34,54) and in SI Appendix. Release of 3kPZS and PZS by larvae and spermiated males was quantified using methods described by Li et al (55) and in SI Appendix.…”

“…Females in their natural spawning habitat encounter 3kPZS from larvae and males simultaneously and from each source alone; therefore, we reasoned that females searching for mates should not only prefer male pheromone over larval odor but also stop orienting toward larval odor altogether. Indeed, females in a 2-choice flume (34) showed no responses to larval odor (P = 0.6) even though it was applied at an attractive concentration of 3kPZS ( Fig. 1).…”

“…As predicted, PZS also abated behavioral preferences of ovulated females for 3kPZS in a 2-choice flume. Females were (34). A positive index value indicates attraction and a negative index value indicates repulsion.…”

Extracellular Multi-Unit Recording from the Olfactory Nerve of Teleosts

“…For example, the sea lamprey (Petromyzon marinus) has high olfactory sensitivity to specific bile acids released by larvae down to a threshold of 10 -13 M 30 ; this sensitivity allows adults to locate and identify suitable spawning grounds, and therefore act as a migratory pheromone over long distance. Similarly, ripe female sea lamprey have high olfactory sensitivity to spermine (threshold 10 -14 M), a polyamine released in the milt by males, which then attracts them to the nests of spermiating males 31 . Other fish also have olfactory sensitivity to polyamines 32 , 33 , 34 , 35 , but not with sufficiently low thresholds of detection to support a similar pheromonal role; instead, avoidance of decaying fish is suggested.…”