Sex Determination

Core Idea

Sex determination is the developmental process that directs an initially bipotential gonad to develop as either a testis or an ovary, which then drives secondary sexual differentiation of the entire body. In mammals, the SRY gene on the Y chromosome activates Sox9 in the bipotential gonad, initiating a testis-determining cascade (Sertoli cell differentiation, testosterone production, Mullerian duct regression). In the absence of SRY, the default pathway (Wnt4/RSPO1/beta-catenin) drives ovarian development. Other animals use different mechanisms: ZW chromosomal system in birds, X:autosome ratio in Drosophila, temperature-dependent determination in many reptiles. Sex determination reveals how a single genetic or environmental signal can redirect an entire developmental trajectory.

Explainer

The development of sex is one of the most dramatic binary decisions in biology — from a single undifferentiated primordium, the gonad becomes either a testis or an ovary, and this choice cascades through the entire body to produce male or female anatomy, physiology, and behavior. The mechanisms that drive this decision vary remarkably across the animal kingdom, but the downstream effectors — the transcription factors and signaling pathways that build testes versus ovaries — are deeply conserved.

In mammals, the bipotential gonad develops from intermediate mesoderm and initially contains both the potential testis-forming cells (supporting cell precursors) and the potential ovary-forming cells. The switch is SRY (Sex-determining Region of the Y chromosome), a transcription factor that activates Sox9 in the gonadal somatic cells. Sox9 drives their differentiation into Sertoli cells — the key orchestrators of testis development. Sertoli cells organize the gonad into testis cords, produce Anti-Mullerian Hormone (AMH, which degenerates the female reproductive tract precursor), and signal to Leydig cells to produce testosterone (which drives male external genital development and brain masculinization). Without SRY (XX genotype), the gonad follows the ovarian pathway: Wnt4 and RSPO1 activate beta-catenin signaling, driving Foxl2 expression and granulosa cell differentiation. The ovary produces estrogen, which drives female tract development and secondary sexual characteristics.

In birds, the system is chromosomally reversed: females are ZW and males are ZZ. The dosage of the Z-linked gene DMRT1 determines sex — two copies (ZZ) drive testis development, one copy (ZW) permits ovary development. In Drosophila, sex is determined by the ratio of X chromosomes to autosomes (not by the Y chromosome), acting through the Sex-lethal (Sxl) RNA splicing cascade. In many reptiles and some fish, temperature during a critical developmental window determines sex — typically by influencing the expression of aromatase (which converts testosterone to estrogen) or other components of the conserved sex-determination molecular toolkit.

A remarkable recent discovery is that sex determination is not a one-time event. In adult mouse gonads, testis identity requires continuous expression of DMRT1, which represses the ovarian program (Foxl2). Deleting DMRT1 in adult testes causes Sertoli cells to transdifferentiate into granulosa-like (ovarian) cells. Conversely, deleting Foxl2 in adult ovaries causes granulosa cells to transdifferentiate into Sertoli-like (testicular) cells. The two sexual programs are in continuous mutual antagonism, and the adult gonad actively maintains its identity rather than passively retaining it. This ongoing maintenance requirement means that adult gonadal sex is more plastic than previously assumed — a finding with implications for understanding disorders of sex development, the effects of endocrine disruptors, and the remarkable natural sex changes observed in some fish species.