Figure 1.

Sexual differentiation of neuron number in worms, flies and mice. The columns represent four different possibilities: male-specific cell groups (cells completely absent in females), female-specific (absent in males), as well as male-or female-biased (greater cell numbers in one sex). In worms, sex differences in neuronal cell number are absolute. Male-specific neurons (e.g., CEMs) die in XX hermaphrodites (first column; healthy cells represented by colored circles and dead cells by stippling) while female-specific neurons (e.g. HSNs) die in XO males (second column). Flies provide examples of male-specific neurons (first column; e.g. P1 cluster) as well as quantitative sex differences favoring males (third column; e.g. mAL cluster). Compared to the vast literature on male behavior, much less is known about the neural substrate of female behavior; we are not aware of examples of female specific or female-biased cell groups in flies (question marks in columns two and four). In mice, sex differences are not absolute and cell numbers are vastly greater. Mice have no known examples of male-or female-specific neurons (columns one and two), unless one considers the subset of spinal motoneurons in the SNB that innervate only the bulbocavernosus muscle. There are examples of male-biased (e.g., BNSTp) and female-biased (e.g. AVPV) sex differences (columns three and four); in both cases, cell groups are comprised of many different phenotypes (different colored cells in columns three and four). Sex differences may be seen in overall cell number, in specific cell types, or both. As discussed in the text, the magnitude of the differences and the molecular mechanisms underlying sex differences in cell number can vary for different cell phenotypes within single sexually dimorphic nuclei.