Zebrafish gonad mutant models reveal neuroendocrine mechanisms of brain sexual dimorphism and male mating behaviors of different brain regions
Authors: Dai Xiangyan , Pradhan Ajay , Liu Jiao , Liu Ruolan , Zhai Gang , Zhou Linyan , Dai Jiyan , Shao Feng , Yuan Zhiyong , Wang Zhijian , Yin Zhan 
Published: August 2023

Abstract
Background
Sexually dimorphic mating behaviors differ between sexes and involve gonadal hormones and possibly sexually dimorphic gene expression in the brain. However, the associations among the brain, gonad, and sexual behavior in teleosts are still unclear. Here, we utilized germ cells-free tdrd12 knockout (KO) zebrafish, and steroid synthesis enzyme cyp17a1-deficient zebrafish to investigate the differences and interplays in the brain–gonad–behavior axis, and the molecular control of brain dimorphism and male mating behaviors.

Methods
Tdrd12+/−; cyp17a1+/− double heterozygous parents were crossed to obtain tdrd12−/−; cyp17a1+/+ (tdrd12 KO), tdrd12+/+; cyp17a1−/− (cyp17a1 KO), and tdrd12−/−; cyp17a1−/− (double KO) homozygous progenies. Comparative analysis of mating behaviors were evaluated using Viewpoint zebrafish tracking software and sexual traits were thoroughly characterized based on anatomical and histological experiments in these KOs and wild types. The steroid hormone levels (testosterone, 11-ketotestosterone and 17β-estradiol) in the brains, gonads, and serum were measured using ELISA kits. To achieve a higher resolution view of the differences in region-specific expression patterns of the brain, the brains of these KOs, and control male and female fish were dissected into three regions: the forebrain, midbrain, and hindbrain for transcriptomic analysis.

Results
Qualitative analysis of mating behaviors demonstrated that tdrd12−/− fish behaved in the same manner as wild-type males to trigger oviposition behavior, while cyp17a1−/− and double knockout (KO) fish did not exhibit these behaviors. Based on the observation of sex characteristics, mating behaviors and hormone levels in these mutants, we found that the maintenance of secondary sex characteristics and male mating behavior did not depend on the presence of germ cells; rather, they depended mainly on the 11-ketotestosterone and testosterone levels secreted into the brain–gonad regulatory axis. RNA-seq analysis of different brain regions revealed that the brain transcript profile of tdrd12−/− fish was similar to that of wild-type males, especially in the forebrain and midbrain. However, the brain transcript profiles of cyp17a1−/− and double KO fish were distinct from those of wild-type males and were partially biased towards the expression pattern of the female brain. Our results revealed important candidate genes and signaling pathways, such as synaptic signaling/neurotransmission, MAPK signaling, and steroid hormone pathways, that shape brain dimorphism and modulate male mating behavior in zebrafish.

Conclusions
Our results provide comprehensive analyses and new insights regarding the endogenous interactions in the brain–gonad–behavior axis. Moreover, this study revealed the crucial candidate genes and neural signaling pathways of different brain regions that are involved in modulating brain dimorphism and male mating behavior in zebrafish, which would significantly light up the understanding the neuroendocrine and molecular mechanisms modulating brain dimorphism and male mating behavior in zebrafish and other teleost fish.

Graphical Abstract