非破壊変形に対する選抜による卵殻強度の遺伝的改良に関する研究

Broken and cracked eggshells cause major economic losses for egg producers. Genetic improvement of eggshell strength for egg laying hens is thus needed. A nondestructive deformation is an indicator of egg shell quality, and can be measured without breaking the eggs. This study aimed to clarify the usefulness of nondestructive deformation as a selection criterion for the genetic improvement of eggshell strength. 1. A White Leghorn population was used for two-way selection to increase (weak line) or decrease (strong line) nondestructive deformation over 10 generations. The differences in nondestructive deformation between both lines became larger due to selection. Nondestructive deformation from the first to the 10th generation was changed from 64.7±11.8 to 100.6±13.4 μm/kg in the weak line and from 59.9±10.4 to 51.9±6.2 μm/kg in the strong line. The selection response in the strong line was smaller than in the weak line. An asymmetrical response between the two lines was observed. Breaking strength was thought to be the most reliable indicator of eggshell strength. Breaking strength was changed from 2.77±0.45 to 2.17±0.34kg in the weak line and from 2.95±0.45 to 3.75±0.46kg in the strong line. This indicated that the selection to decrease nondestructive deformation improved eggshell strength. Realized heritabilities for nondestructive deformation were asymmetrical with respect to the direction of selection, and were 0.16 in the strong line and 0.38 in the weak line. 2. Egg weight declined in both lines as the selection proceeded, and was smaller in the weak line than in the strong one. Shell thickness in the strong line became thick but declined in the weak line due to selection. Shell weight in the strong line generally continued to be flat, and to decrease linearly in the weak line as the selection proceeded. Percentage shell increased in the strong line and decreased in the weak line due to selection, and indicated a similar tendency to shell thickness. Age at first egg in the strong line tended to be later and in the weak line to be earlier as the selection proceeded. The rate of lay over the short term in the weak line generally continued to be flat and to decline in a linear fashion in the strong line as the selection proceeded. The rate of lay over the long term indicated a similar tendency to the rate of lay over the short term. These results demonstrated that the selection to decrease nondestructive deformation increased egg thickness and percentage shell but reduced egg weight and the rate of lay. In the weak line, the percentages of broken eggs and soft shell eggs increased. This indicated that eggshell strength in the weak line declined. 3. Heritability estimates for eggshell traits were 0.22～0.48, and those for nondestructive deformation, breaking strength and percentage shell were somewhat higher in the weak line than in the strong one. Heritability estimates for egg productivity traits were 0.19～0.41, and were higher in the strong line than in the weak line. Heritability estimates for egg weight and shape index were high (0.5～0.6) in both lines. Genetic correlations between nondestructive deformation and the other egg shell traits were negative and large, and those of nondestructive deformation with percentage shell and shell thickness were especially larger. This showed that nondestructive deformation had a genetic relationship with percentage shell and shell thickness. The genetic correlation between nondestructive deformation and rate of lay was positive and higher in the strong line than in the weak one. The genetic correlations between the other eggshell traits and rate of lay were negative and larger in the weak line than in the strong line. However, the genetic correlations of egg weight with nondestructive deformation and breaking strength showed a different tendency between lines. When breaking strength was improved without decline of egg weight and rate of lay by the selection index method using an eggshell trait, percentage shell was estimated to be the most efficient eggshell trait, followed by nondestructive deformation and shell thickness. However, those traits were inefficient compared to breaking strength. 4. Hatchability for fertile eggs in the strong line became increasingly higher as the selection for nondestructive deformation proceeded, but remained at the 70% level in the weak line. The difference in hatchability between the strong and weak lines became gradually larger as the selection proceeded. This suggested that the selection to decrease nondestructive deformation had a good effect on hatchability. In the strong line, the percentages of early and late embryo mortality decreased with selection. Selection to decrease nondestructive deformation was effective in improving the hatchability of fertile eggs stored for more than two weeks. However, there was no definite relationship between the magnitude of nondestructive deformation and hatchability. The effect of plastic packaging on the hatchability of fertile eggs was investigated in order to clarify the relationship between nondestructive deformation and the hatchability of fertile eggs stored for more than two weeks. The hatchabilities of fertile eggs stored for 3～5weeks were improved. There were chicks hatched from fertile eggs stored for 6 weeks. The eggs in plastic bags did not lose weight. Therefore, it was suggested that the reduction in egg weight loss improved hatchability and was thought to contribute to the increasing hatchability in the strong line. Since strong eggshells are thought to be good culture vessels, an experiment was carried out to culture early embryos of Japanese quail to hatch in eggshells of the strong line. The culture was conducted using three methods. All methods had some hatching success. The rates of hatching of the cultured embryos were 11.4～17.6%. This showed that it was possible to culture early embryos of Japanese quail to hatch using chicken eggshells as the culture vessels. 5. An experiment was carried out to investigate the effect of selection for nondestructive deformation on egg formation. The interval from oviposition to entry of the next ovum into the uterus in the strong line was slightly shorter than in the weak line. Time spent by the ovum in the uterus in the strong line was longer than in the weak line. This suggests an association between eggshell strength and time spent by the ovum in the uterus. Oviduct length in the weak line was longer than in the strong line. But the proportion of the uterus in which the eggshell was formed to oviduct length in the strong line was larger than in the weak line. The results suggested that the selection for nondestructive deformation affected the time spent by the ovum in the uterus and the proportion of uterus to oviduct length, and that there was a strong relation between the eggshell formations in the uterus and eggshell strength. The results are summarized as follows. Large (weak line) and small (strong line) nondestructive deformation lines were founded to increase or decrease nondestructive deformation by the two-way selection, and it showed the efficacy of selection. Breaking strength was increased by the selection to decrease nondestructive deformation. This showed that selection to decrease nondestructive deformation exhibited the effect of genetic improvement on eggshell strength. Selection to decrease nondestructive deformation reduced egg weight and the rate of lay. Nondestructive deformation was as effective as the other eggshell traits for genetic improvement of eggshell strength, but that it was inefficient compared to breaking strength. Selection to decrease nondestructive deformation was effective in improving the hatchability of fertile eggs stored for more than two weeks. Selection for nondestructive deformation affected the time spent by the ovum in the uterus and the proportion of uterus to oviduct length. The above findings clarified that nondestructive deformation as a selection trait was useful for genetic improvement of eggshell strength.

卵用鶏では卵殻強度の低下による破卵の増加が問題になっており, 卵殻強度の遺伝的改良が望まれている。非破壊変形は卵殻形質の1つで, 卵を非破壊で測定できるという利点がある。そこで本研究では, 卵殻強度の遺伝的改良における, 非破壊変形に対する選抜の有用性を明らかにすることを目的とした。1. 白色レグホーンを用いて, 非破壊変形に対する大 (弱系) 及び小 (強系) 方向への2方向選抜実験を10世代にわたって行った。選抜形質である非破壊変形は, 強系, 弱系ともに選抜に伴い選抜反応が認められ, 第1世代では強系が 59.9±10.4μm/kg, 弱系が 64.7±11.8μm/kg だったが, 第10世代では強系が 51.9±6.2μm/kg, 弱系が 100.6±13.4μm/kg となった。選抜反応は強系の方が小さく, 強弱2系統間で非対称な反応を示した。破壊強度は, 第1世代では強系が 2.95±0.45kg, 弱系が 2.77±0.45kg だったが, 第10世代では強系が 3.75±0.46kg, 弱系が 2.17±0.34kg となった。このことから非破壊変形を指標とした選抜によって, 卵殻強度の改良が可能であることが示された。また非破壊変形の実現遺伝率は, 強系では 0.16, 弱系では 0.38 と推定され, 強弱2系統間で非対称な値を示した。2. 他の卵殻諸形質及び産卵能力における間接選抜反応は以下の通りであった。卵重は強系, 弱系ともに選抜に伴い減少し, 弱系の方がその傾向が著しかった。卵殻厚は, 選抜によって強系は卵殻が厚く, 弱系は卵殻が薄くなった。卵殻重は, 強系ではほぼ一定の値で推移したが, 弱系では直線的に減少した。卵殻卵重比は, 強系では増加する傾向を示したが, 弱系では減少する傾向を示し, 卵殻厚とよく似た推移を示した。初産日齢は, 強系が遅くなる傾向を示したが, 弱系は早くなる傾向を示した。短期産卵率は, 弱系では一定の水準で推移したが, 強系ではほぼ直線的に低下した。長期産卵率は, 短期産卵率とよく似た推移を示した。したがって非破壊変形の小方向への選抜により, 卵殻厚や卵殻卵重比は増加し, 卵重や産卵率は低下することが明らかとなった。また弱系では破卵の出現率が高くなり, また軟卵の割合が高く, 卵殻強度の低下が確認された。3. 卵殻諸形質の遺伝率は 0.22～0.48 と推定され, 非破壊変形と破壊強度, 卵殻卵重比は弱系の方がやや高かった。また産卵能力形質では, 0.19～0.41 と推定され, 強系の方が高い値が推定された。卵重と卵形係数では両系とも 0.5～0.6 の高い値が推定された。非破壊変形と卵殻諸形質と間には, 高い負の遺伝相関が推定され, 特に卵殻卵重比と卵殻厚との相関が高く, 非破壊変形は遺伝的には卵殻卵重比や卵殻厚との関係が大きいことが示された。非破壊変形と産卵率との遺伝相関は, 強系が弱系よりも高い正の値が推定された。また他の卵殻諸形質と産卵率との間には, 負の遺伝相関が推定され, 強系の方が弱系よりも高かった。一方, 非破壊変形及び破壊強度と卵重との遺伝相関には系統間で差がみられ, 卵殻強度と卵重との相関関係において強系と弱系で違いがあると推定された。選抜指数法を用いて卵殻形質の情報を取り入れて卵重や産卵率を低下させずに破壊強度を改良する場合, 取り入れる卵殻形質としては卵殻卵重比が最も効率が良く, 次いで非破壊変形, 卵殻厚の順と推定されたが, 破壊強度を用いる場合の効率には及ばなかった。4. 対受精卵孵化率は世代の推移に伴い, 強系がしだいに高くなるのに対し弱系は70%付近で推移した。したがって非破壊変形の小方向への選抜が, 孵化率を向上させる効果があることが示された。また強系の発生中止卵の出現率が, 孵卵初期および孵化直前の両方の時期で選抜に伴い低下し, 特に孵化直前時期に選抜の影響が強く現れていた。非破壊変形の小方向への選抜は2週間以上の長期保存卵の孵化率を改善する効果があった。しかし個々の受精卵の非破壊変形の大きさと孵化率には一定の傾向は認められなかった。長期保存した受精卵の孵化率に非破壊変形が関係することから, 受精卵のパッキングによる長期保存の孵化率に及ぼす影響を調べたところ, 3～5週間保存における孵化率の向上がみられ, 6週間保存でも孵化例が得られた。パッキング保存では卵重は減少しなかった。したがって保存中の卵重の減少を抑えることが孵化率を改善することが示唆され, このことが強系の孵化率の向上の一因と考えられた。このように卵殻は胚の培養器として重要なものであり, また卵殻強度の強い系統の卵の方が保存容器としても培養器としても優れていることが示された。そこでこうした卵殻の特性を利用して, ウズラ初期胚を強系のニワトリの卵殻を用いて培養し孵化させることを試みた。3つの方法で培養したところ, 3つの方法とも孵化例が得られ孵化率は11.4～17.6%であった。これによってニワトリ卵殻を用いてウズラ胚を放卵直後の発生初期の段階から孵化まで培養できること, また孵化において卵殻が重要であることを示した。5. 卵形成において子宮部への移行時間は強系の方がわずかに早かった。また子宮部滞留時間は強系の方が長かった。したがって卵殻強度と子宮部滞留時間の間に関連のあることが示唆された。卵管の長さは弱系の方が長かったが, 卵殻を形成する子宮部の割合は強系が弱系よりも大きかった。これらの結果から, 非破壊変形に対する選抜が, 卵形成における子宮部滞留時間と卵管における子宮部の割合に影響を与えることが明らかとなり, 子宮部における卵殻形成が卵殻強度と大きく関連することが示された。以上の結果をまとめると, 非破壊変形に対する大小2方向への選抜実験によって, 非破壊変形の大系統 (弱系) と小系統 (強系) を作出することに成功し, 選抜の効果を確認した。そして破壊強度における間接選抜反応から, 非破壊変形の小方向への選抜は卵殻強度を遺伝的に改良することを明らかにした。また非破壊変形の小方向への選抜が, 卵重や産卵率を低下させることが明らかとなった。また非破壊変形は, 破壊強度よりは劣るものの, その他の卵殻形質と同程度の改良効率であることを明らかにした。さらに非破壊変形の小方向への選抜は長期保存卵の孵化率を改善することを明らかにした。非破壊変形に対する選抜は, 卵形成における子宮部滞留時間と, 卵管における子宮部の割合に影響を与えることが明らかとなった。以上のことから, 卵殻強度の遺伝的改良において, 非破壊変形は有用な形質であることが明らかとなった。