About a month ago my labmate and I published a paper with our advisor titled “The effects of synthetic estrogen exposure on the sexually dimorphic liver transcriptome of the sex-role-reversed Gulf pipefish”. It’s published in an open-access journal, PLoS One, so everyone with internet can access it! However, I wanted to write a more-accessible summary of the paper here on my blog to help non-specialists understand what the paper was about.
To describe the paper, I’ll de-construct the title, starting with the end. We did this study in a species of marine fish, called the Gulf pipefish (scientific name Syngnathus scovelli). Pipefish have male pregnancy, so the males provide all of the parental care, and in this species the males can only brood a clutch of eggs from a single female, but females can impregnate multiple males (for more information on pipefish, check out my citizen science project, Pipefish World!). Because males then become the limiting factor in reproduction, females end up competing for males and males become the choosy sex, so we talk about the species as being sex-role-reversed. In this study, we extracted RNA from the liver of pregnant males, non-pregnant males, and females. RNA is a molecule similar to DNA that acts as a messenger between the DNA sequence and the rest of the cell, and tells the cell what proteins to make based on the code in the DNA. When a gene has RNA in the cell we describe that gene as being expressed. So we extracted the RNA and sequenced it to generate a transcriptome, which is all of the RNA that is expressed at the time of RNA extraction in a particular tissue, in this case the liver. We then looked at how RNA was in the liver for each gene in both sexes and discovered that males and females have different levels of expression for different genes, meaning that the liver is a sexually dimorphic tissue. This is actually not as surprising as it may seem at first, since many other species (including mammals like humans) have sexually dimorphic livers when it comes to DNA expression. One interesting feature of the Gulf pipefish liver transcriptome is that, unlike behavioral roles in this species, the gene expression roles seem to follow conventional patterns of sex roles. In other words, although the species is behaviorally sex-role-reversed, their sexually dimorphic organ, the liver, does not show a reversal compared to other species (genes expressed in females are still expressed in female pipefish). That’s pretty cool, but it leaves a lot of questions still unanswered–namely, what genetic mechanisms actually underlie sex-role-reversal?
The final component of the paper was investigating how exposure to synthetic estrogen, a hormone analagous to the estrogen produced by our bodies, affects the patterns of gene expression in the pipefish liver. We know from previous studies (e.g. this one by a former labmate or this one by my labmate who is the first author of this paper) that synthetic estrogen can alter and disrupt the mating system of the Gulf pipefish, and in high enough doses it feminizes male pipefish so that they no longer have a brood pouch and start developing the female ornamentation patterns. So we were interested to see whether the sexually dimorphic liver responds to this feminizing hormone in expected ways. By comparing the expression levels in pregnant males, non-pregnant males, and females that were both exposed and not exposed, we were able to show that exposure to estrogen makes the male transcriptome more female-like, in that female-biased genes (like some that are involved in the production of eggs) are up-regulated in exposed males.
So why does this matter? Well, understanding how sex-role-reversal occurs at the mechanistic/genetic level is a major goal of evolutionary biology, because it is a major shift in the targets of sexual selection. And sexual selection and sexual dimorphism are often related, since sexual selection can drive the proliferation of exaggerated traits in one sex. So understanding gene expression in a sexually dimorphic liver in a sex-role-reversed species is a good first step: we can say that the liver is likely not directly involved in the sex-role-reversed behaviors of the Gulf pipefish. However, it may play a role in sexual selection, especially because it produces egg-related proteins, which could be experiencing rapid evolution because of female-female competition or to keep up with rapidly evolving sperm proteins. Additionally, we now know that synthetic estrogen at low levels feminizes the male liver. This is important because synthetic estrogen is one of many endocrine disruptors that are polluting water systems worldwide and affecting reproductive and endocrine systems in many species, including Gulf pipefish. It is possible that pipefish could be used as an indicator species by screening gene expression in the livers to identify whether a particular population has been exposed to endocrine disruptors.