Neural, not gonadal, origin of brain sex differences in a gynandromorphic finch – Brief Review

This is a study about birds, not humans. We cannot generalize from bird brains to human brains. The results can only suggest possibilities to study.

It is, however, a fascinating study. The authors found that the bird’s brain cells were influenced by their genetic sex, not just their hormones.

They looked at a rare case of a bird whose brain was genetically male in one half and genetically female in the other half. The two halves had different neural song circuits, with one half being more masculine. Since both halves of the brain were exposed to the same hormones, the difference may have been due to the genes of the brain cells.

The study suggests that the sex chromosomes in the brain can themselves affect the brain.

The authors conclude:

“These results provide the strongest evidence to date that sex differences in the song circuit of zebra finches originate partly because of differences in the actions of sex chromosome genes acting locally within the brain.”

Furthermore, there may be other animals whose sex chromosome genes play a role in creating sex differences.

“This system thus represents one of a growing number of model systems in which the actions of sex chromosome genes are implicated in sexual differentiation of nongonadal tissues in birds and mammals. Other sexually dimorphic phenotypes influenced by the genetic sex of cells include the size of mammalian embryos (28), external genitalia of tammar wallabies (29), aggression in mice (30, 31), phenotype of midbrain and hypothalamic cells in rats and mice (32, 33), and vasopressinergic innervation of the lateral septum of mice (34). Further work is needed to resolve how the hormonal and cell-autonomous mechanisms interact to produce sex differences in brain phenotype and disease.”

The study has one weakness, however: the  sample size is small, as small as you can get, in fact. This result might turn out to be an anomaly.

Original Study:

Neural, not gonadal, origin of brain sex differences in a gynandromorphic finch by Robert J. Agate, William Grisham, Juli Wade, Suzanne MannJohn WingfieldCarolyn SchanenAarno Palotie, and Arthur P. Arnold in Proceedings of the National Academy of Sciences of the United States of America, vol. 100 no. 8,  April 15th, 2003, 4873–4878.