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The chimera marmoset story reported here and elsewhere is fascinating; I was stunned by the possibility, not heavily noted in a lot of press, that male cells might have made it into the germline of a female - that is XY cells from a male might have developed into eggs in his female twin sister resulting in a live birth. Germline transmission of one's brothers cells is interesting enough, but the idea of XY eggs is particularly interesting -- not just for the fact that a female had her brother's baby without all that messy incest.

Another interesting chimera story, this one in humans, was noted in the media earlier this week. In a unique event researchers ascertained that a pair of twins, one phenotypically male and one hermaphroditic with ambiguous genitalia, were both actually XX/XY chimeras of each other. The twins developed, DNA analysis suggests, from a single egg fertilized by two sperm. See the paper here.

And unlike in most observed human chimeras the mixed cells appear in several populations In addition to some of the philosophical quandaries such events raise about the definition of self from a genetic standpoint (Carl Zimmer did a nice job with this in the Times Tuesday), both studies speak to crucial developmental strictures even if they are simply rare or unique cases. The marmosets suggest that it might not be so difficult for a male germ cell to develop into a functional egg exposing a bit more about natural stem cell plasticity, and the human story may shed light on the 'default' developmental program of sex, which has generally been assumed to be female (although that tide has been turning).