Thursday, September 27, 2007

Germ-line chimerism and paternal care in marmosets (Callithrix kuhlii)

Ross, C., J. French, and G. Ortí. 2007. Germ Line Chimerism and Paternal Care in Marmosets (Callithrix kuhlii). Proc. Natl. Acad. Sci. USA, 104 (15): 6278–6282.

The formation of viable genetic chimeras in mammals through the transfer of cells between siblings in utero is rare. Using microsatellite DNA markers, we show here that chimerism in marmoset (Callithrix kuhlii) twins is not limited to blood-derived hematopoietic tissues as was previously described. All somatic tissue types sampled were found to be chimeric. Notably, chimerism was demonstrated to be present in germ-line tissues, an event never before documented as naturally occurring in a primate. In fact, we found that chimeric marmosets often transmit sibling alleles acquired in utero to their own offspring. Thus, an individual that contributes gametes to an offspring is not necessarily the genetic parent of that offspring. The presence of somatic and germ-line chimerism may have influenced the evolution of the extensive paternal and alloparental care system of this taxon. Although the exact mechanisms of sociobiological change associated with chimerism have not been fully explored, we show here that chimerism alters relatedness between twins and may alter the perceived relatedness between family members, thus influencing the allocation of parental care. Consistent with this prediction, we found a significant correlation between paternal care effort and the presence of epithelial chimerism, with males carrying chimeric infants more often than nonchimeric infants. Therefore, we propose that the presence of placental chorionic fusion and the exchange of cell lines between embryos may represent a unique adaptation affecting the evolution of cooperative care in this group of primates.

Translation: According to the seminar I went to today, what this all means is that Marmosets and their relatives almost always produce fraternal twins, and the embryos grow in close proximity, with out the usual membranes separating them. The two developing embryos can actually have blood vessels in common, meaning that blood born cells can move from one embryo to the other. And stay there. And develop. So when the little monkeys are born and grow up, they can be riddled with cells that are genetically part of their sibling. This is what we call a chimera, when one individual has cells that are of different genetic lineages.
So then one of the chimeric monkeys mates. But some of his germ line cells (the ones that make sperm) are genetically his brother. So he's doing the mating, but the young could be genetically his nephews. Weird, I know. And one outcome of all this is that marmosets put a lot more energy into taking care of their nieces and nephews than would otherwise be expected. Ain't evolution weird and wonderful.

Cartoon explanation:

Duetting Alone

I attended a seminar yesterday given by a former lab mate of mine, discussing her work to understand the duetting call of the California Towhee.

(An article in the Berkeley Alumni Magazine discusses the work and links to a sound file of the call.)

She described how this squeal-call is almost never given by just one bird, and is almost always a duet by the pairs who mate for life (and cheat on each other like mad).
It has been called the "reunion squeal" because the pair always fly towards each other while duetting, and end the duet side by side.

So this morning I was in my neighbor's backyard and noticed a pile of feathers on the ground. They have four outside cats, so I was not surprised. I bent over to examine the feathers and see who the victim was. Drab brown feathers. The wing and tail feathers were too small for a robin or a jay, but too big for most of the sparrows and finches around here. Then I noticed the clump of orange feathers, just the color of a towhee's undertail coverts. A California Towhee, one of the most common birds in the neighborhood, had bit it.
As I stood up, I heard the squeal. A rather half hearted squeal I thought. Then I looked up to see the towhee, sitting on the fence, squealing. It wasn't a half hearted squeal, it was that this bird was doing its half of the duet alone. It was initiating the reunion squeal, but couldn't reunite, because its mate wouldn't squeal back. It squealed twice more in the next several minutes, far more often than is common among pairs.
The cats sat on their lawn chairs and looked smug.