Sex and breeding stage differences in neurogenomic profiles reflect hormone signaling in a socially polyandrous shorebird

This study reveals that in sex-role reversed northern jacanas, sex differences in brain gene expression are driven by complex genetic and hormonal interactions rather than a simple reversal of typical sex-biased patterns, with male-biased genes predominantly located on the Z-chromosome and specific hormone signaling pathways differentially regulating female competition and male parental care.

The Gist

Imagine a world where the usual rules of the dating game are flipped upside down. In most bird species, the males are the flashy ones who fight to impress the females, while the females sit back, choose the best partner, and then do all the hard work of raising the babies.

But in the Northern Jacana, a colorful shorebird from Panama, the script is reversed. The females are the big, tough, colorful fighters who compete aggressively to mate with multiple males. The males are the ones who stay home, build the nests, and raise the chicks alone.

This paper asks a fascinating question: If the roles are swapped, does the "software" inside their brains get swapped too?

The Big Question: Is it the Hardware or the Software?

Think of a bird's brain like a computer.

• The Hardware (Sex): This is the physical sex of the bird (Male or Female), determined by their chromosomes (like the Z and W chromosomes in birds).
• The Software (Role): This is the behavior (Fighting for mates vs. Raising babies).

The scientists wanted to know: If a female is acting like a "typical male" (fighting for mates), does her brain start looking like a male's brain? Or does her brain stay "female" because of her hardware, even if she's running "male" software?

How They Did It

The researchers went to a wetland in Panama and caught these birds. They sorted them into three groups:

1. Fighting Females: The moms who are out there competing for mates.
2. Courting Males: The dads who are currently trying to find a new mate.
3. Parenting Males: The dads who are currently sitting on eggs or feeding babies.

They took tiny samples of two specific parts of the bird's brain that control social behavior (think of these as the "command centers" for love and fighting). Then, they used high-tech sequencing to read the "instruction manuals" (genes) inside those brain cells to see which ones were turned on or off.

The Surprising Findings

Here is what they discovered, translated into everyday terms:

1. The "Male" Brain is Still Dominant
Even though the females are doing the "male" job of fighting, their brains didn't suddenly look like male brains. In fact, most of the differences they found were genes that were turned on in males.

• Analogy: Imagine a factory. Even if the female workers are doing the heavy lifting usually done by men, the factory's blueprint (the Z chromosome) still has more instructions for "male" tasks. About 60-70% of the genes that differed between sexes were located on the Z chromosome, which males have two copies of, and females only have one. Because birds don't perfectly balance these copies, males naturally have higher levels of these genes.

2. The "Fighting" and "Parenting" Brains Look Different
The scientists thought that a Fighting Female and a Courting Male would have very similar brains because they are both in "competition mode."

• The Twist: They were wrong! The brains of Fighting Females and Courting Males were actually quite different.
• The Real Difference: The biggest difference in the "fighting" part of the brain was actually between the Fighting Female and the Parenting Male.
• Analogy: It's like expecting a soccer player and a basketball player to have the same muscle structure because they both play sports. But instead, the soccer player's muscles look more like a swimmer's than the basketball player's. The "role" (fighting vs. parenting) didn't make the brains look alike; the biological sex still mattered most.

3. The Hormone "Volume Knobs"
The study found some specific "volume knobs" (receptors) that were turned up or down in interesting ways:

• The Androgen Knob (Testosterone): Females had a higher sensitivity to testosterone (more receptors) than the dads who were raising babies. This might explain how females can be so aggressive even if their actual testosterone levels aren't super high. They just have better "antennas" to hear the signal.
• The Prolactin Knob (Parenting): Males had higher levels of the "parenting hormone" receptors, regardless of whether they were currently courting or parenting. This suggests their brains are always "primed" for fatherhood.

4. The "Switch" is Subtle
When the researchers compared Courting Males to Parenting Males, they found very few differences in their genes.

• Analogy: You might expect a dad's brain to undergo a massive overhaul when he goes from "dating mode" to "dad mode." But in Jacanas, it seems like the dad's brain is already set up for parenting. The switch between courting and parenting is more like turning a dimmer switch on a light, rather than flipping a light switch from off to on.

The Bottom Line

The main takeaway is that nature doesn't just hit "undo" and "redo" when sex roles are reversed.

Social polyandry (where females fight and males parent) isn't a simple case of females becoming males and males becoming females at the genetic level. Instead, it's a complex dance. The birds are using the same basic genetic "hardware" (the Z chromosome and autosomes) but tweaking the "software" (hormone sensitivity and specific gene networks) to allow females to fight and males to parent.

In short: You can change the job description, but you can't easily rewrite the operating system. The Northern Jacana shows us that biology is flexible, but it's also deeply rooted in the fundamental differences between being male and female.

Technical Summary

1. Problem Statement and Objective

Context: In "sex-role reversed" species like the northern jacana (Jacana spinosa), females compete for multiple mates while males provide uniparental care. This system decouples biological sex from traditional behavioral roles (female competition vs. male parental care), offering a unique model to investigate whether behavioral phenotypes are driven by gonadal sex or specific social roles.
Knowledge Gap: While previous studies have examined hormone levels or specific traits in these species, there is a lack of genome-wide transcriptomic data comparing sex and breeding stages (courting vs. parenting) in socially polyandrous birds. It remains unclear if the neurogenomic profiles of competitive females resemble those of courting males (who also compete) or if sex differences persist regardless of behavioral state.
Objective: The authors aimed to characterize sex- and stage-specific gene expression patterns in two key brain regions associated with social behavior (the preoptic area of the hypothalamus and the nucleus taeniae) to determine if social polyandry results in a simple reversal of sex-biased gene expression or a more complex interaction of genetic and hormonal mechanisms.

2. Methodology

Study System:

• Species: Northern jacana (Jacana spinosa), a socially polyandrous shorebird.
• Location: La Barqueta, Chiriqui, Panama.
• Sample Groups: 12 territorial females, 5 courting males, and 7 parenting (incubating) males.
• Behavioral Assays: Aggression was measured using a 5-minute resident-intruder assay (taxidermy decoys and playback). Morphological traits (wing spur length, body mass, tarsus length) were recorded.

Genomic Resources:

• De Novo Genome Assembly: The authors generated a high-quality, chromosome-level reference genome for the northern jacana using PacBio HiFi long-read sequencing and Hi-C scaffolding.
  • Assembly: 41 mapped chromosomes (including Z and W), ~95% of scaffolds placed.
  • Annotation: Lifted over from the chicken (Gallus gallus) genome, identifying 14,588 genes.
  • Sex Chromosome Identification: Z- and W-linked scaffolds were identified using depth-of-coverage analysis from 5 males and 5 females, validated against chicken/zebra finch genomes and expression data.

Experimental Workflow:

• Tissue Collection: Microdissection of two specific brain regions: the Preoptic Area (POA) of the hypothalamus and the Nucleus Taeniae (TnA) (avian homologue of the medial amygdala).
• RNA-seq: Bulk RNA sequencing was performed on ~28 million paired-end reads per sample.
• Analysis Pipeline:
  • Differential Expression (DE): DESeq2 was used to compare three dyads: (1) Females vs. Courting Males, (2) Females vs. Parenting Males, and (3) Courting vs. Parenting Males.
  • Co-expression Networks: Weighted Gene Co-expression Network Analysis (WGCNA) was conducted to identify gene modules correlated with sex, breeding stage, aggression metrics, and morphological size.
  • Functional Annotation: Gene Ontology (GO) enrichment analysis was performed using PantherGO.

3. Key Contributions

1. First Chromosome-Level Genome: Provided the first high-quality, chromosome-level reference genome for the Jacanidae family, enabling precise mapping of Z- and W-linked genes.
2. Neurogenomic Profiling: Conducted the first genome-wide study of gene expression in a socially polyandrous bird, focusing on specific social behavior network nodes.
3. Disentangling Sex vs. Role: Explicitly tested the hypothesis that competitive females and courting males share similar transcriptomic profiles, providing data to distinguish between gonadal sex effects and social role effects.
4. Hormone-Receptor Decoupling: Highlighted that behavioral differences may be driven by receptor sensitivity (e.g., Androgen Receptor) rather than circulating hormone levels alone.

4. Key Results

A. Sex-Biased Gene Expression:

• Magnitude: Hundreds of differentially expressed genes (DEGs) were found between sexes (3–4% of the transcriptome).
• Direction: The majority of DEGs were male-biased (83–85%).
• Chromosomal Distribution:
  • Male-biased genes: Predominantly located on the Z chromosome (60–73%), likely due to incomplete dosage compensation in birds (males are ZZ, females are ZW).
  • Female-biased genes: Predominantly located on autosomes (91–93%).
• Contrary to Hypothesis: The greatest difference in autosomal gene expression was between females and courting males (in the POA), not between females and parenting males. This suggests that despite shared competitive behaviors, the underlying transcriptomes of females and courting males are distinct.

B. Breeding Stage Differences (Males):

• Low Plasticity: Very few genes differed between courting and parenting males (19 genes in POA, 0 in TnA).
• Candidate Genes:
  • Courting Males: Higher expression of Prolactin-Releasing Hormone (PRLH) and Prodynorphin (PDYN).
  • Parenting Males: Higher expression of genes related to thyroid and Insulin-like Growth Factor (IGF) pathways (IGFBP3, PAPPA2, IYD).

C. Hormone Signaling and Receptors:

• Androgen Receptor (AR): Higher expression in females than parenting males (POA), suggesting females may have heightened sensitivity to androgens despite potentially lower circulating testosterone.
• Prolactin Receptor (PRLR): A Z-linked gene with male-biased expression in both brain regions. Expression was marginally higher in parenting males, supporting a role in paternal care.
• Estrogen Receptor 1 (ESR1): Higher in females (POA).

D. Co-expression Networks (WGCNA):

• Sex-Associated Modules: Two modules (Brown in POA, Blue in TnA) were strongly associated with sex and enriched for Z-linked genes. These correlated negatively with body size (traits larger in females).
• Aggression-Associated Modules: Several modules correlated with aggression (e.g., distance to decoy, vocalizations) but were not strictly sex-specific.
  • POA Cyan Module: Correlated with aggression; enriched for CNS myelination.
  • TnA Pink Module: Correlated with aggression; enriched for cilium beat frequency and sperm axoneme assembly (potentially reflecting neurobiological microtubule processes).

E. Tissue Specificity:

• Most genes were expressed across all tissues (blood, gonad, brain), but a significant portion (32%) showed high tissue specificity (Tau > 0.8). Z-linked and autosomal genes showed similar ratios of uneven expression across tissues.

5. Significance and Conclusion

• Complexity of Sex-Role Reversal: The study concludes that social polyandry is not a simple reversal of sex-biased gene expression. Instead, it involves complex interactions where gonadal sex (driven largely by Z-chromosome dosage) and social roles (mediated by specific hormone receptor sensitivities) interact.
• Mechanism of Competition: The finding that females have higher Androgen Receptor (AR) expression than parenting males, despite similar or lower testosterone levels, suggests that receptor sensitivity is a critical mechanism driving female competition in sex-role reversed species.
• Stability of Parental State: The minimal transcriptomic shift between courting and parenting males suggests that male jacanas may maintain a "ready-to-parent" physiological state throughout the breeding season, or that the transition involves rapid neuromodulation rather than broad transcriptional reprogramming.
• Future Directions: The authors suggest that future work should compare polyandrous species with monogamous relatives to identify the specific genetic transitions required for sex-role reversal and utilize single-cell sequencing to resolve cell-type-specific expression differences.

In summary, this paper provides a foundational neurogenomic resource for the northern jacana, demonstrating that while sex chromosomes drive broad male-biased expression, the behavioral nuances of sex-role reversal are mediated by specific, often female-biased, autosomal pathways and hormone receptor dynamics.