Sex, eye size, and the rate of myopic eye growth due to form deprivation in outbred White Leghorn chickens

PURPOSE. There is considerable variation in the degree of formdeprivation myopia (FDM) induced in chickens by a uniform treatment regimen. Sex and pretreatment eye size have been found to be predictive of the rate of FD-induced eye growth. Therefore, this study was undertaken to test whether the greater rate of myopic eye growth in males is a consequence of their larger eyes or of some other aspect of their sex. METHODS. Monocular FDM was induced in 4-day-old White Leghorn chicks for 4 days. Changes in ocular component dimensions and refractive error were assessed by A-scan ultrasonography and retinoscopy, respectively. Sex identification of chicks was performed by DNA test. Relationships between traits were assessed by multiple regression. RESULTS. FD produced (mean ± SD) 13.47 ± 3.12 D of myopia and 0.47 ± 0.14 mm of vitreous chamber elongation. The level of induced myopia was not significantly different between the sexes, but the males had larger eyes initially and showed greater myopic eye growth than did the females. In multiple linear regression analysis, the partial correlation between sex and the degree of induced eye growth remained significant (P = 0.008) after adjustment for eye size, whereas the partial correlation between initial eye size and the degree of induced eye growth was no longer significant after adjustment for sex (P = 0.11). After adjustment for other factors, the chicks' sex accounted for 6.4% of the variation in FD-induced vitreous chamber elongation. CONCLUSIONS. The sex of the chick influences the rate of experimentally induced myopic eye growth, independent of its effects on eye size.