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Why These Tiny Monkeys Have So Many Twins

Why These Tiny Monkeys Have So Many Twins

Researchers find genes involved in body size and twin births

Deep in the jungles of Central and South America, members of the primate group known as the New World Monkeys roam.  Among them the diminutive callitrichines, with the world’s smallest known primate, the pygmy marmoset (or Cebuella pygmaea). Despite their size, there’s something else that sets these small tree-dwellers apart from their close evolutionary cousins (including us humans).

Pygmy marmosets tend to give birth to twins and triplets, unlike other primate lineages, where so-called singleton pregnancies are the norm. Along with this unusual trait of producing multiplets, callitrichines show a suite of other reproductive adaptations, such as hematopoetic chimerism of siblings (meaning that twins in the womb share blood cells), the suppression of reproduction in non-dominant females and the cooperative upbringing of young monkeys in the social group.

The recent sequencing of the common marmoset’s (Callithrix jacchus) genome now allows researchers to begin unraveling the genetic roots of this peculiar twinning trend. Based on previous studies, investigating genes that are thought to play a role in the occurrence of twins or the regulation of growth and body size, the scientists identified 63 genes to scrutinize further. Four of these showed specific changes in the callitrichine group of New World Monkeys. Those genes go by the names of GDF9, BMP15, BMP4 and WFIKKN1.

The genes have been linked to twin births in different ways. Both GDF9 and BMP15 have been implicated in the occurrence of twin births in sheep. Groups of humans with a tendency to produce twins also show changes in the GDF9 gene. BMP4 has been shown to influences sheep’s ovulation number. And, lastly, WFIKKN1, a gene thought to be involved in bone growth and ovulation, showed several changes that occur exclusively among the callitrichines. Its implication in twinning among this group of tiny monkeys is strengthened by the discovery that the only callitrichine that does not twin (Goeldi’s monkey, or Callimico goeldii) possesses a version of WFIKKN1 that corresponds to the one in other non-twinning primates.

Evolutionary advantage

Backed by their genetic detective work, the authors propose a complex refined model of callitrichine evolution. In this model the exploitation of new niches upon arrival in the Americas, reduction in body size, and the proclivity to give birth to twins and triplets all interact, together with altered other traits related to reproduction and the raising of young.

In a wider context, the researchers point to the potential relevance of their findings for human medicine. It might provide a guideline in the search for the genetic basis of twinning in humans. This could prove to be important in the further development of, and the safety of conception through, assisted reproductive technology. Moreover, it paves the way for an investigation of how to reduce the complications that often accompany twin (or other multiplet) births in non-callitrichines, which might, one day, save human lives.

Harris RA, Tardif SD, Vinar T, Wildman DE, Rutherford JN, Rogers J, Worley KC, & Aagaard KM (2013). Evolutionary genetics and implications of small size and twinning in callitrichine primates. Proceedings of the National Academy of Sciences of the United States of America PMID: 24379383

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