R. Alan Harris, Suzette D. Tardif, Tomas Vinar, Derek E. Wildman, Julienne N. Rutherford, Jeffrey Rogers, Kim C. Worley, Kjersti M. Aagaard. Evolutionary genetics and implications of small size and twinning incallitrichine primates. Proceedings of the National Academy of Sciences of the United States of America, 111(4):1467-1472. 2014.

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New World monkeys (NWMs) are characterized by an extensive size range, with
clawed NWMs (subfamily Callitrichinae, or callitrichines) such as the common
marmoset manifesting diminutive size and unique reproductive adaptations. Perhaps
the most notable of these adaptations is their propensity toward multiple
gestations (i.e., dichorionic twins and trichorionic triplets). Indeed, with the 
exception of Goeldi's monkey (Callimico), callitrichine singleton pregnancies
rarely occur. Multiple gestations seem to have coevolved with a suite of
reproductive adaptations, including hematopoetic chimerism of siblings,
suppression of reproduction in nondominant females, and cooperative
alloparenting. The sequencing of the common marmoset (Callithrix jacchus) genome 
offers the opportunity to explore the genetic basis of these unusual traits
within this primate lineage. In this study, we hypothesized that genetic changes 
arising during callitrichine evolution resulted in multiple ovulated ova with
each cycle, and that these changes triggered adaptations that minimized
complications common to multiple gestations in other primates, including humans. 
Callitrichine-specific nonsynonymous substitutions were identified in GDF9,
BMP15, BMP4, and WFIKKN1. WFIKKN1, a multidomain protease inhibitor that binds
growth factors and bone morphogenetic proteins, has nonsynonymous changes found
exclusively in common marmosets and other tested callitrichine species that twin.
In the one callitrichine species that does not produce twins (Callimico), this
change has reverted back to the ancestral (nontwinning) primate sequence.
Polymorphisms in GDF9 occur among human cohorts with a propensity for dizygotic
twins, and polymorphisms in GDF9 and BMP15 are associated with twinning in sheep.
We postulate that positive selection affected NWM growth patterns, with
callitrichine miniaturization coevolving with a series of reproductive