Table 1.

Sex determination mechanisms, examples of sexual dimorphisms in the nervous system, and core cell death pathways in nematodes (Caenorhabditis elegans), fruit flies (Drosophila melanogaster) and mice (Mus musculus).

	C. elegans	Drosophila	Mus musculus
Sexes	Male (XO), Hermaphrodite (XX)	Male (XY), Female (XX)	Male (XY), Female (XX)
Sex determining and differentiating mechanisms	X chromosome to autosome ratio determines production of a secreted protein (Her-1) that in turn regulates the activity of Tra-2 and Tra-1.	X chromosome to autosome ratio determines Sxl and Tra production leading to the alternate splicing of fruitless and doublesex.	Testis-determining gene on the Y chromosome causes formation of testes leading to the secretion of hormones that act throughout the body. Also direct sex chromosome effects.
Neural sex differences	HSN (egg-laying), CEM, RN (chemo- and mechanosensory detection of hermaphrodite), CP motoneurons (male copulatoy behavior).	fru-mAL (pheromone processing), P1 (courtship initiation), MIND (innervate male-specific muscle)	SNB (innvervate male-specific muscle), BNSTp (pheromone processing, sex behavior, stress), AVPV (control of ovulation).
Canonical cell death genes	egl 1/ced 9/ced 4/ced 3	hid/grim/reaper	Pro-survival and pro-death members of the Bcl-2 family

Abbreviations: AVPV, anteroventral periventricular nucleus; BNSTp, principal nucleus of the bed nucleus of the stria terminalis; CEM, cephalic companion neurons; fru-mAL, fruitless expressing neurons medially located above antennal lobe; HSN, hermaphrodite-specific neurons, MIND, muscle of Lawrence-inducing motoneuron; P1, fruitless expressing neurons in posterior region-1, RN, ray sensory neurons; SNB, spinal nucleus of the bulbocavernosus; Tra, transformer; Sxl, sex lethal.