Characterization of the intronic VNTR polymorphisms found in a paralog of chicken serotonin transporter gene.

Polymorphisms in the neurotransmitter-related genes can be a major source of behavioral variations. We searched for polymorphic sites in chicken neurotransmitter-related genes and identified two variable number of tandem repeat (VNTR) loci encompassing the paralog of chicken serotonin transporter gene (5-HTT). Both intronic VNTR were highly polymorphic across chicken breeds and the other Galliformes species, even though predominant alleles were considerably different among breeds. One VNTR locus contained sequences complementary to a conserved motif of CCCTC-binding factor (CTCF) within each repetitive unit, indicating that transcription of chicken 5-HTT paralog may be regulated by the CTCF protein. It is of great interest to contrast these results with previous knowledge on the human 5-HTT that also has CTCF binding sites in the repetitive units of intronic VNTR. Additionally, we measured the degree of impulsiveness in domestic chicks for their preference of immediate/small to large/delayed rewards. A significant difference in the impulsiveness score was detected between two chicken breeds (White Leghorn vs. Boris Brown; P < 0.01), as well as between White Leghorn chicks with different 5-HTT genotypes. These findings imply the possibility that 5-HTT VNTR genotypes may have some impact on chicks' impulsive choice by modifying the serotonergic neurotransmission.