家鶏におけるヘテローシスの研究

Abstract

Within-breed and between-breed matings were using 16 sires and 141 dams of White Leghorns (WL) and 16 sires and 119 dams of Barred Plymouth Rocks (BPR). Average coe. JJicients of inbreeding of parental purebreds were 3.4 and 9.2% in WL and BPR, respectively. Mating systems were such that these purebreds and crossbreds were half-sisters. A mortality analysis of data given in a previous report, exclusive of those on the T2 group (kawahara, 1957) and birds which died accidentally, was performed.Data were collected from 615 purebred and 508 crossbred females. The birds were maintafined, without any conscious culling, up to 120 days after they laid their first egg. Mortality was recorded in two periods, i. e. the growing (up to 18 weeks of age) and the maturhlg (from 18 weeks of age to 120 days after the first egg was laid). Total mortality data with the significance test based on chi-square are summarized in Table 1. The diseases observed at death were classified into ten general types shown in Fig. 1. Statistical analysis of the data gave the follow- ing results.1) Total mortality in the growing period was 5.7% lower among crossbred chicks than purebreds. This difference was statistically significant at the 1% level. Total mortalitY was 3.2% higher in BPR than in WL (11.5% vs. 8.3%). Mortality among F1 hybrids was l.7% hlgher in BPR_??_×WL_??_ cross than in its reciprocal (4.9% vs. 3.2%). These differences were not statistically significant.2) Total mortality in the maturing period among F1 hybrid chicks was high than that among purebreds (11.8% in WL, 20.9% in BPR, 15.6% in WL _??_×BPR_??_, and 21.5% in BPR_??_ ×WL_??_), and was only a little higher than that in the maternal purebred half-sister group. The differences in total mortality between the purebreds and the maternal crossbreds however were not statistically significant.3) It is obvious that the main cause of difference in total mortality between the growing and the maturing period was loss from visceral-form leucosis. Fig. 1 shows that a larger part of the difference in mortality between various groups was also due to leucosis. Deaths from leucosis among hybrids were actually more than those among their purebred maternal half-sisters (2.5% in WL, 9.9% in BPR, 6.4 % in WL_??_×BPR_??_, and 11.0% in BPR_??_×WL_??_). In this case mortality from this disease was as high as 8.5% in the crossbreds and 5.5% in the purebreds, the difference apProaching the significance level (x2=3.79, .10>P> .05). The difference of 6.2% in mortality between the maternal groups (4.2% in the WL maternal groups and 10.4% in the BPR maternal groups) was statistica11y significant at the 1% level.Resistance and susceptibility to visceral-form leucosis were controlled, in larger part, by maternal factors and genotypes.