A genetic analysis of reproduction in growth selected lines of chickens

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Virginia Polytechnic Institute and State University


Genetic and physiological aspects of egg production patterns and their relationships with growth parameters were studied utilizing S₁₅, S₁₆(R₃) and S₁₇(R₄) generation pullets from lines selected for high (HW) and low (LW) 8-week body weight. HW pullets laid significantly more defective eggs than LW pullets at each of the physiological ages measured from the date of first egg to 160 days of lay. Also, the pattern of egg production differed among lines. Arhythmic ovulation patterns were associated with physiological age, and delaying sexual maturity by feed restriction did not alter the patterns. Relaxing selection had a correlated effect on yolk synthesis and production of certain egg types. Increasing photoperiods beyond 14 hours in a 24-hour cycle did not have a consistent effect on growth reproduction.

The average heritability estimates for percentage hen-day production of normal eggs and yolks were similar and ranged from 0.19 to 0.41 indicating common genetic factors controlling these traits. Heritabilities of the composit trait, total defective eggs, ranged from 0.26 to 0.48. Genetic correlations were generally higher than the phenotypic correlations and genotype-environment interactions were evident in the relationships between age at maturity and egg production traits. Environmental forces that delayed sexual maturity had no effect on the production of defective eggs. The genetic correlations between normal eggs and total yolks ranged from 0.88 to 0.93, while the environmental correlations between normal and defective eggs ranged from -0.34 to -0.46, suggesting that a genetic approach to the problem of defective eggs is realistic.

The dw gene was introduced into HW and LW lines and the specific selected inheritance was increased by repeated backcrossing. Data from the B₃ and B₄ generations enabled comparisons among Dw and dw alleles in specific genetic backgrounds. The depressive effect of the dwarfism was more severe in the LW than HW line and for body weight than for skeletal growth. In the B₃ and B₄ generations, the HW dwarfs matured 5 days earlier, while the LW dwarfs matured 6 days later than their normal sibs. On a percentage hen-day basis, HW dwarfs produced fewer yolks but more normal eggs than their nondwarf sibs. This supports the hypothesis that dw has a synchronizing effect on ovarian and oviducal functions in populations with excess yolk synthesis and arhythmic ovulation patterns.

A qualitative analysis, based on ultrastructural studies of adenohypophyseal gonadotrophs revealed higher synthetic activity and autodigestion of secretory products in the HW than in the LW pullets.

The frequencies of chromosomal abnormalities in random samples of embryos were 14.5% and 6.2% in the HW and LW lines, respectively. The primary cause of this difference was the higher incidence of euploid mosaics in the HW line. It was hypothesized that greater need for nutrients by fast growing embryos may induce mosaicism through polyspermy and development of supernumerary nuclei.