Effect of eumelanic phenotypes on the expression of amelanosis in the Smyth chicken.

The Smyth line is characterized by an autoimmune loss of melanin in the feather and eye in association with a hypermelanizing melanocyte, which presumably triggers immune system intervention. Inheritance appears to be multigenic. The present study was designed to determine if eumelanin-enhancing modifiers influence the incidence and severity of the line-associated amelanosis. Smyth chicks with dark brown (eb) down had a higher incidence of amelanosis (P less than .01) than did their hatchmates with light brown down. Furthermore, parents with dark down at hatch produced a higher incidence of amelanotic progeny than parents with light down. Reciprocal crosses of the Smyth line to a highly eumelanized (eb/eb) Recessive Black (RB) line produced F1 amelanosis. However, sires from the Smyth line produced significantly more amelanotics than did RB males (P less than .01). The influence of dark down on amelanotic development was also apparent in the Smyth-RBF1. The use of amelanotic F1 parents produced a significantly higher incidence of affected F2 offspring than did the use of unaffected parents. A backcross to the Smyth line produced an incidence of 67.6% amelanosis, whereas only one chick (2.04%) developed amelanosis from an F1 x RB mating. The finding that dark-downed Smyth chicks exhibit, and subsequently produce, a significantly higher incidence of amelanosis supports ultrastructural observations that associate the Smyth line amelanosis with a hyperactive melanocyte. The unusually high expression of amelanosis (22.7%) in the Smyth-RB F1 suggests that the two lines share one or more common eumelanogenic genetic factors.