Selection of breeding stock in pigs favours 6PGD heterozygotes.

Associations between genetic variation at four enzyme polymorphisms and growth rate, feed conversion and economic index were studied in commercial lines of pigs. These lines were also tested for the fit of observed genotypic frequencies to expected genotypic frequencies at the four polymorphic loci. Genotypic distribution fit Hardy-Weinberg expectations in four pure-breeding lines but crosses among lines produced excesses in heterozygosity at the locus coding for 6-phosphogluconate dehydrogenase (6PGD) due to differences in allelic frequencies among lines. Selection of breeding individuals further enhanced the heterozygosity at 6PGD to 73-93 per cent but it had no impact on a linked locus that was sometimes in linkage disequilibrium with 6PGD. The relationship between feed conversion and individual heterozygosity was examined in one population sample; feed conversion increased with individual heterozygosity.

Selection for body weight at eight weeks of age. 19. Influences of heterozygosity and dwarfism on early egg production and associated traits.

Body weight, body components, and reproductive traits were compared during the first 60 days after onset of lay in pullets from lines of White Plymouth Rocks selected for high or low juvenile body weight, their reciprocal F1 crosses, an F2 cross, and dwarfs from the parental lines. Absolute and relative body weight gains were less for dwarf than for nondwarf pullets in the high weight line. In the low weight line, there were differences in absolute but not relative weight gains. Among nondwarfs, both relative and absolute changes in body weight during the first 60 days of lay were similar for crosses and the high weight parental line, and considerably less for the low weight parental line. Numbers of ovulation were the same for the crosses and the high weight parental line, however, more normal eggs were produced by F1 crosses than either parental line. Rate of production was influenced mainly by variation in body weight at onset of lay in the high weight nondwarfs and low weight dwarfs, and by age at first egg in low weight nondwarf pullets. None of the independent variables had a significant association with normal egg production in crosses and high weight dwarf populations.

Selection for body weight at eight weeks of age. 18. Comparisons between mature and immature pullets at the same live weight and age.

Body component and compositional traits were measured at the same age and live body weight in sexually mature and immature pullets from several genetic stocks. Populations included parental lines of White Plymouth Rocks selected for 25 generations for high and low juvenile body weight, their reciprocal F1 crosses, and an F2, as well as dwarfs from the parental lines. Although matched for age and body weight, percent liver weight and percent breast weight were greater for mature than immature pullets. Population by physiological stage interactions for percentages of abdominal fat and carcass lipid resulted from differences between mature and immature pullets, which were greater in the parental than in the cross and dwarf populations. Moreover, the pattern was reversed in the parental populations. The data supported the hypothesis that compositional requirements, independent of age and body weight, are necessary for the onset of egg production, and that these requirements vary among populations. They also demonstrated physiological buffering properties of heterozygosity and of the sex-linked dwarfing allele.

Ovum formation and multiple ovulation in lines of white Plymouth Rocks and their crosses.

The production of double-yolked eggs and the duration of the rapid growth phase of yolks were measured in parental lines of White Plymouth Rock pullets and their crosses over 30 d, commencing with the day of first egg. Significant differences were found between mating combinations in the incidence of multiple-yolked eggs, but not in the period of rapid yolk growth. Heterosis and recombination effects for multiple ovulation were respectively -9% and -66% of the mean, while the corresponding values for the period required for rapid yolk growth were -1% and -5%. Three double-yolked eggs were observed containing yolks which differed by 3 d in their periods of rapid growth. Hypotheses are were presented for the origin of double-yolked eggs.

Inheritance of traits associated with sexual maturity when populations of chickens reach 50% lay.

In order to study the mode of inheritance of traits associated with the onset of sexual maturity, data from stocks differing greatly in genetic background were analyzed when populations achieved 50% lay. Chickens used in this experiment consisted of three parental lines and five crosses. Parental lines were White Plymouth Rocks bidirectionally selected for high (HH) and low (LL) 56-day body weight, and a closed population of Jersey Black Giants (JJ). Crosses were produced from reciprocal matings of the White Rock lines (HL and LH), their F2 (HL X HL), and Jersey males mated to weight-line females (JH, JL). When 50% of the pullets within a population had laid, various traits were measured for each individual in that population. Retrospective analyses showed that sexually mature pullets were heavier at 91 and 126 days of age than those which had not commenced lay. Body weight dimorphism between layers and nonlayers was disproportionately greater in LL and F2 pullets than in those of other populations, resulting in a population by physiological stage interaction. Fat depots were heavier and percentages of fat were higher in laying than nonlaying pullets. Evidence was obtained suggesting that an absolute quantity of breast tissue was necessary before lean tissue percentage became meaningful as a threshold trait for the onset of egg production. Modes of inheritance for body weight and compositional characteristics were different in sexually immature and mature pullets, suggesting that muscle functioned as a nutrient reserve in laying pullets and a nutrient store in those that had not reached this physiological stage.

Growth to sexual maturity of dwarf and nondwarf White Rock chickens divergently selected for juvenile body weight.

Growth from hatching to the onset of lay (sexual maturity) was studied in White Plymouth Rock pullets from lines selected for high (HN) and low (LN) 56-day body weight, their reciprocal F1 crosses, an F2 cross, and two dwarf populations originating from the HN and LN parental lines. The highest R(2)s for describing growth for all populations except HN were obtained when body weight was expressed relative to body weight at sexual maturity. Modes of inheritance differed depending on whether patterns of growth were expressed in a chronological time frame or on a physiological basis. Heterosis observed for age at sexual maturity and for body weight after the age when selection was made was not in evidence when ages and weights were expressed as a proportion of those at sexual maturity.

Ontogeny of aggressive-sexual behavior between males in lines of Japanese quail (Coturnix coturnix japonica) selected for male mating frequency.

Japanese quail from lines bidirectionally selected for high and low male mating frequency and from the random-bred base population were observed in an ontogenetic study of aggressive-sexual behavior. Quail were reared in sex-intermingled flocks until 28 days of age at which time half of the males from each line were housed as all-male flocks and half were transferred to individual cages. At 41 days of age, individually caged males exhibited mounting behavior to win encounters with other males. By 45 days, males from the high and control lines exhibited significantly more aggressive-sexual behavior than those from the low lines. Line x Rearing Experience interactions for mounting behavior at 45 and 56 days of age were due to the individually caged high- and control-line males' winning encounters by mounts; those maintained in flocks rarely mounted. Although some low-line males won encounters by mounts, occurrence of this behavior was infrequent. Males maintained in flocks were placed in individual cages at 57 days of age. When these males were compared with those housed in individual cages from 28 days of age, at 84 days of age and older, the Line x Rearing Experience interaction observed previously disappeared, and only differences among genetic lines were evident. Genetic and rearing experience effects are discussed as influencing agonistic and sexual behaviors.

Skin breaking strength in chickens: comparisons among genetic combinations.

Skin breaking strength was measured in lines of chickens divergently selected for high (H) and low (L) juvenile body weight, their F1 reciprocal crosses (HL and LH), and an F2 generation derived from HL X HL matings. In both lines, skin covering the breast had higher breaking strength than that covering the thigh. Skin of chicks from the LL line had the lowest breaking strength, that from HH and LH matings the greatest, and that from HL and F2 matings was intermediate. Percentage heterosis for breaking strength was significantly positive while percentage recombination was not significant.

Body weight and composition of Japanese quail (Coturnix coturnix japonica) at sexual maturity.

The influence of dietary protein (26%, 24%, 20%, 16%, and 14%) and restricted feeding (100%, 80%, and 60% of ad libitum) on body weight and body composition of Japanese quail (Coturnix coturnix japonica) at sexual maturity was examined in two similar experiments. Although less variable than chronological age, body weight at sexual maturity also differed significantly. In Experiment 1, there were no significant differences in breast weight, tibia length or carcass fat due to diet. In Experiment 2, differences in body composition were apparent as age at the onset of lay was increased by dietary manipulation. Early maturing quail tended to enter lay when a minimum chronological age or critical body lipid level was reached; late maturing quail entered lay when a critical lean body weight or skeletal size was reached. It is proposed that multiple threshold traits associated with growth and body composition are critical for the onset of sexual maturity.