By studying a rare hermaphroditic bird, researchers have shown that the sexual genotype of brain cells contributes directly to sex differences in the brains of birds, beyond the well-established action of sex hormones.

Credit: Photo ©National Academy of Sciences

Although it is accepted that genes on the sex chromosomes determine whether an animal will develop male or female reproductive organs, the origin of sex differences in brain function remains unclear. There have been suggestions that somatic sexual differentiation results exclusively from the effects of circulating sex hormones, rather than from the direct effects of sex chromosome genes. However, recent studies, in which researchers manipulated sex hormone levels in developing birds, have suggested that genes may be active in sex determination as well. The discovery of a bilateral gynandromorphic zebra finch (Taeniopygia guttata), which exhibited male characteristics on the right side of its body and female characteristics on the left, provided researchers with the perfect opportunity to test this hypothesis.

This odd bird, the result of an unknown developmental error, was hatched in a colony at Rockefeller University (New York, NY) and given to a group led by Arthur P. Arnold at the University of California, Los Angeles for analysis. Although the bird's behavior was male in that it copulated with females and sang courtship songs, Arnold's group found that the bird's sexual genotype and phenotype were laterally divided throughout its body (Proc. Natl. Acad. Sci. USA, published online, doi: 10.1073/pnas.0636925100).

In addition to having characteristically male brightly colored plumage on the right and dull typically female feathers on the left, the finch's song circuit—the brain area responsible for the males' singing of courtship songs—was 82% larger on the “male” side than on the “female” side. The bird's vocal organ was intermediate in size relative to male and female controls. Genomic PCR and mRNA analysis revealed that cells on the bird's right side were genetically male, while those on the left side were genetically female; furthermore, female sex chromosome-linked genes were only expressed on the left side.

Because both halves of the finch's brain were exposed to the same hormonal environment, “We now suspect, based on this and other work, that the sex chromosomes present within brain cells also help cause sex differences in brain development,” Arnold tells Lab Animal. He goes on to caution, “One can't immediately generalize these results to other species .... However, there is evidence from studies on mammals that supports the same idea.”