Abstract
Although the fighting behaviour in gamecocks has evolved because of artificial selection, it is unknown whether the selection for aggressiveness affects neurotransmitter levels in the avian central nervous system. We sought to identify the source and origin of this trait. We collected the brain samples from 6 female Shamo gamecocks and 5 Shaver Brown chickens (control; bred for egg production). The midbrain levels of norepinephrine (NE) were significantly higher in Shamo gamecocks (P = 0.0087) than in the controls. Moreover, alleles encoding adrenergic receptors differed between the breeds in terms of response to NE. Gene mutations specific to Shamo and potentially associated with fighting behaviour were in sites T440N of ADRα1D; V296I of ADRα2A; and T44I, Q232R, and T277M of ADRβ2. The evolutionary analysis indicated that the ADRβ2 (T44I and Q232R) mutations were heritable in all Galliformes, whereas the T440N mutation of ADRα1D and V296I mutations of ADRα2A were unique to Shamo and originated by artificial selection. A high NE level may confer a selective advantage by enabling gamecocks to be aggressive and pain tolerant. Therefore, the strong fighting behaviour of Shamo has resulted from a combination of naturally inherited and mutant genes derived by artificial selection.
Highlights
The fighting behaviour in gamecocks has evolved because of artificial selection, it is unknown whether the selection for aggressiveness affects neurotransmitter levels in the avian central nervous system
There were no significant differences between Shamo and Shaver Brown in terms of the levels of the other monoamines in the St, Cx, and Mid
We analysed 10 brain neurotransmitters and the analysed the receptor genes (ADR) genes that may either regulate aggression in Shamo used for cockfights or determine calmness of Shaver Brown bred for egg production under high population densities
Summary
The fighting behaviour in gamecocks has evolved because of artificial selection, it is unknown whether the selection for aggressiveness affects neurotransmitter levels in the avian central nervous system. Seventeen varieties of domesticated chicken (Gallus gallus domesticus) known as Japanese ornamental chickens were developed by artificial selection over a long period for their cultural entertainment value They are characterised by different body colours and shapes[1,2,3,4,5]. The aim was to examine the effects of neurotransmitters and their receptors in the brain on Shamo strength and aggression under high-stress conditions used for artificial selection. This information will increase our understanding of the mechanism underlying the aggressive behaviour of Shamo gamecocks It may help elucidate the roles of neurotransmitters in the human brain and under stress-related conditions such as panic disorder, depression, syncope, and anxiety[8,9,10,11,12]
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