Evolution of threat response-related polymorphisms at the SLC6A4 locus in callitrichid primates

Variation in an upstream repetitive region at the SLC6A4 locus, which
encodes the serotonin transporter, is associated with anxiety-related
behaviour in a few primate species, including humans and rhesus macaques,
and has been suggested to be related to ecological adaptability among
macaques. In this study we investigate evolution of SLC6A4 polymorphisms
associated with anxiety-related behaviour in common marmosets (Callithrix
jacchus). Assaying variation in the SLC6A4 repeat region across 14 species
in 8 genera of callitrichid primates (marmosets and tamarins) we find
large interspecific variation in the number of repeats present (24-43).
The black tufted-ear marmoset (C. penicillata) has sequence polymorphisms
similar to those found in the common marmoset, which is its sister
species, and no other species has intraspecific variation at these sites.
We conclude that, similar to humans and macaques, the functional
polymorphism at SLC6A4 in common marmosets has a recent evolutionary
origin, and that the anxiety-related allele is evolutionarily derived.
Common/black tufted-ear marmosets and rhesus/bonnet macaques share high
ecological adaptability and behavioural flexibility that we propose may be
related to the maintenance of the polymorphism.