三种微生物学级别SSB小鼠主要脏器的解剖学比较

本文对普通，清洁，无菌近交系ＳＳＢ小鼠的主要脏器进行解剖学比较，发现该三种微生物学级别ＳＳＢ小鼠胸腺、脾脏和肺的重量与体重的比值均为ＣＶ＞ＣＬ＞ＣＧＦ。ＧＦ小鼠心脏和肝脏极显著小于ＣＶ，ＧＦ小鼠消化道变化最明显的是盲肠肥大。     

肠道菌群变化对实验小鼠肠黏膜免疫的影响

目的探讨肠道菌群变化对肠黏膜相关淋巴组织的影响。方法通过变性梯度凝胶电泳（Denatu-ring gradient gel electrophoresis,DGGE）法研究了三种不同级别实验小鼠即清洁级小鼠、SPF小鼠和普通小鼠肠道菌群的组成,并用免疫组织化学（immunohistochemistry,IHC）方法研究了此三种不同级别的实验小鼠肠黏膜相关淋巴组织sIgA阳性细胞分布情况。结果普通小鼠肠道细菌种类最多,其sIgA阳性细胞分布最多,肠道不同部位之间sIgA分布情况差异有显著性（P〈0.05）,小肠和大肠之间的阳性细胞分布差异极显著（P〈0.01）;其次是清洁级小鼠,其肠道不同部位之间菌种组成差异无显著性,小肠和大肠之间的阳性细胞分布差异有显著性（P〈0.05）;SPF小鼠肠道细菌种类最少,故其sIgA阳性细胞分布最少,且其肠道不同部位之间菌种组成差异无显著性,小肠和大肠之间的阳性细胞分布差异无显著性（P〉0.05）。结论随着动物微生物控制级别的增高,肠道微生物多样性递减;sIgA阳性细胞与肠道细菌种类正相关。     

昆明种稀毛鼠与正常鼠生长繁殖的比较

目的探讨昆明（KM）种稀毛鼠与正常鼠的生长繁殖是否有差异。方法对KM种稀毛鼠与正常鼠1～8胎进行平均胎间隔、受胎率、产仔数、初生体重、仔鼠离乳成活率等繁殖力指标及1～12周龄平均体重增长等生长指标作对比实验,将实验结果进行统计学处理。结果从第5胎起稀毛鼠的受胎率、平均胎间隔、仔鼠离乳成活率三项繁殖指标均低于正常鼠。但两者的产仔数、仔鼠初生体重及育成鼠周平均增重却无明显差异。结论平均胎间隔、平均受胎率、仔鼠离乳成活率是影响稀毛鼠正常生产的关键。     

无菌级C3H/OrlSlac小鼠生长繁殖、主要脏器参数以及血液生理生化指标的测定分析

目的了解无菌级C3H/OdSlac小鼠的生长、繁殖性能;测定无菌级C3H/OrlSlac小鼠主要脏器重量以及血液生理、生化正常值并进行分析比较.方法 ①统计无菌级C3H/OrlSlae小鼠的1～3胎的繁殖指标数据,包括:平均每胎产子数、离乳率、怀孕率、胎间隔和生产指数;②分别称取60只(雌雄各半)0～112 d的无菌级...     

Nrf2基因敲除小鼠的繁育及生长发育的生物学特性

目的了解ICR-Nrf2小鼠的生物学特性。方法用原种繁殖扩群,观察繁殖性能及仔鼠的生长发育。结果ICR-Nrf2--/-与ICR-Nrf2-＋/＋比,总的产仔数与离乳数减少（P〈0.01或P〈0.05）;体重在出生后第3周时差异有显著性（P〈0.01）,第5周至第25周时同性别间差异非常显著（P〈0.01）。结论Nrf2基因缺失小鼠的产仔数、离乳数均少于野生型小鼠,仔鼠的生长速度也较野生型小鼠慢。     

Fmr1基因敲除对小鼠生理发育和繁殖性能的影响

目的对清洁级FVB.KO和FVB的生理发育指标和繁殖性能进行分析比较,探讨Fmr1基因敲除对小鼠生理发育和繁殖性能的影响。方法挑选10周龄清洁级FVB.KO和FVB各20只（雌雄各半）,采取1∶1同居,全部同胞兄妹近交繁殖,测定新生仔鼠生理发育指标和品系繁殖性能。结果 FVB.KO在耳廓分离、体毛长出、门齿萌发、眼睑开裂等生理发育指标上和FVB比较接近,在平均每窝产仔数和离乳率等方面偏低,在雌雄比例上有显著差异。结论 Fmr1基因敲除对小鼠生理发育和繁殖性能影响较小,对后代雌雄比例可能有一定的影响。     

C57BL/6J-HBV转基因小鼠的繁育与检测

目的　摸清C57BL 6J -HBV转基因小鼠的繁育规律 ,建立可靠的HBsAg表达检测方法。方法　对C57BL 6J-HBV转基因小鼠两种不同交配方式的繁殖性能和HBsAg的表达情况进行比较研究 ;应用免疫组化的方法证实血清学诊断HBsAg的准确性并用激光扫描共聚焦显微镜确定HBsAg在肝细胞中的表达部位。结果与结论　从繁殖性能来看 ,两种交配方式窝产仔数差异不显著 (P >0 .0 5 ) ,而离乳数差异具有显著性 ( 0 .0 1

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食物营养成分变化对卡氏小鼠和黄毛鼠摄食消化的影响

动物从环境中获取足够营养和能量的能力决定着其生存和繁殖。为探讨同域分布不同体型的鼠类对不同营养成分食物的摄食消化对策,以三种不同营养成分食物分别饲喂卡氏小鼠和黄毛鼠,在第10天和20天,以食物平衡法测定日摄食量、日粪便量、净摄入量和表观消化率。结果发现,在整个驯化过程中,两种鼠的不同食物组间体重无显著差异,表明均能够适应食物营养的变化维持收支平衡。在驯化第10天时,高蛋白组卡氏小鼠的日摄食量显著高于高淀粉组,但两组与普通组均无差异;高蛋白组卡氏小鼠的日粪便量显著高于高淀粉组和普通组;而净摄入量无组间差异;高淀粉组和普通组的表观消化率显著高于高蛋白组。至第20天时,三组动物日摄食量无差异;高蛋白组的日粪便量高于高淀粉组和普通组。黄毛鼠仅在第10天时高淀粉组和普通组表观消化率高于高蛋白组,三组黄毛鼠其他消化参数在第10天和第20天均无显著组间差异。结果表明,体型不同的鼠类因消化冗余能力的差异,面对相同的食物营养成分变化,会以不同的摄食消化策略调节适应。     

无菌小鼠的人工哺乳培育

采用无菌子宫摘取术,在隔离器中剥离子宫取仔１６只,生存１５只,在我国首次采用每２４小时插胃管灌胃５～６次的人工哺乳方法,哺乳１５只,离乳６只。粪便、垫料、棉拭子经无菌检查,均无细菌生长。同时对人工乳成分进行了分析比较,比较了人工乳灭菌温度与时间对各营养成分破坏情况,比较了人工乳、豚鼠母乳与小鼠母乳之间的差异     

C1型尼曼-匹克病小鼠生长繁殖及血液生理生化指标的测定分析

目的测定SPF级C1型尼曼-匹克病小鼠(Npc1~(-/-)小鼠)的生长繁殖及血液生理生化指标,为开展NPC1病人的研究提供理论性的基础资料。方法 (1)选取0~77日龄Npc1~(-/-)、Npc1~(+/-)和Npc1+/+小鼠雌雄各40只,定期称重并绘制生长曲线;(2)Npc1~(-/-)小鼠由Npc1~(+/-)小鼠交配繁殖产生,统计Npc1~(+/-)小鼠连续4代的繁殖数据;(3)检测60日龄Npc1~(-/-)和Npc1+/+小鼠的血液生理生化指标。结果 (1)Npc1~(-/-)小鼠的体重在7周前随着日龄的增加而增加,7周后随着日龄的增加而减少,直至11周左右死亡。Npc1+/+和Npc1~(+/-)小鼠的体重均随着日龄的增长而逐渐增加,两者之间并没有显著差别。4周后雄性比雌性体重增加明显比雌性小鼠快;(2)Npc1~(+/-)小鼠不同代内交配分娩间隔、窝产仔数、离乳仔数、离乳仔数中雌雄数量及阳性数量差异无显著性(P0.05),而第2代的离乳率明显大于第1代(P0.05);(3)Npc1~(-/-)和Npc1+/+小鼠血液生理指标中平均红细胞血红蛋白含量(MCH)和平均过氧化物酶指数(MPXI)差异有显著性(P0.05),其余差异无显著性(P0.05)。生化指标中尿素(UREA)差异有极显著性(P0.01),而天门冬氨酸氨基转移酶(AST)、葡萄糖(GLU)、乳酸脱氢酶(LDH)、钾(K)和铜(Cu)等差异有显著性(P0.05),其余差异无显著性(P0.05)。结论 (1)Npc1~(-/-)、Npc1~(+/-)和Npc1+/+小鼠的生长曲线因基因型和性别的不同而存在差异;(2)Npc1~(+/-)小鼠不同代的繁殖能力差异无显著性;(3)Npc1~(-/-)和Npc1+/+小鼠的部分血液生理生化指标差异有显著性。     

Body weight loss during lactation and its influence on weaning-to-service interval and ovulation rate in Landrace and Yorkshire sows in the tropical environment of Thailand

The aim of this study was to investigate the ovulation rate and the weaning-to-service interval (WSI) of sows in relation to their body weight loss during lactation in tropical climatic conditions. Effect of lactation length (LL), number of total born piglets, number of live born piglets, litter birth weight, average piglet birth weight, number of pigs weaned, litter weaning weight and average pig weaned weight on sow weight loss during lactation were also studied. This study was conducted in two commercial purebred sow herds (A, B) in the central part of Thailand from August to December 1997. The herds had both Landrace (L) and Yorkshire (Y) sows. The 123 sows (55 L and 68 Y) in herd A and 153 sows (95 L and 58 Y) in herd B, parity 1-4, were weighed within 4 days after farrowing and at weaning. Lactation length, litter size at birth and at weaning, litter weight at birth and at weaning, and WSI were recorded for each of these sows. In herd A, 52 sows (20 L and 32 Y) were examined once by laparoscopy between days 8 and 14 after AI-service. These sows had farrowed at least seven piglets in the previous parturition. The numbers of corpora lutea (CL) in both ovaries were counted, and were assumed to equal the ovulation rate. L-sows had significantly (P < 0.05) higher relative weight loss during lactation (RWL) than Y-sows. The RWL increased by 0.7% for each extra pig weaned. When LL increased by 1 day, within the interval of 17-34 days, RWL decreased by 0.6%. Sows with a high weight loss had significantly (P < 0.05) longer WSI than sows with medium or low weight loss. Weight loss had a significant (P < 0.05) effect on WSI in parity 1 and 2 sows. Y-sows had more CL than L-sows (15.7 versus 14.0) (P < 0.05). RWL, parity and regression on lactation length had no significant effect on number of CL. In conclusion, sows with higher number of pigs weaned lose more weight. Under the restricted feeding regime applied, high weight loss during lactation prolongs WSI in parity 1 and 2 sows, but has no influence on the ovulation rate at first oestrus after weaning. The ovulation rate is higher in Yorkshire than in Landrace sows. The ovulation rate is independent of parity.     

Supplementation of dextrose to the diet during the weaning to estrus interval affects subsequent variation in within-litter piglet birth weight

Effects of supplementation of dextrose to the diet of sows during the weaning-to-estrus interval (WEI) on subsequent litter size and within-litter variation were investigated. After weaning, 223 sows (first to fifth parity) were fed 3.5 kg/d. Half of the sows additionally received 150 g of dextrose per day as topdressing on the feed. WEI and estrus duration were determined as well as subsequent pregnancy rate and litter size. Piglets were weighed individually at birth and at weaning (day 26.4; S.D.: 2.5). Supplementation of dextrose to the diet during the WEI did not affect WEI (106 h), pregnancy rate (88.2%), farrowing rate (84.2%), subsequent litter size (total born: 13.70), or birth weight (1599 g). The within-litter variation in birth weight was lower in sows on the dextrose treatment (CV: 17.5% versus 21.2% for the dextrose and control group, respectively, P=0.03). From this experiment, we concluded that addition of dextrose during the weaning to estrus interval did not increase litter size, but seems to affect the uniformity in birth weight of the litter.     

LH receptor induction and ovulation rate in mice selected for litter size and body weight

Female mice from lines which had undergone long-term single trait and antagonistic index selection for litter size and body weight were analysed for ovulation rate and LH receptor induction. Compared to randomly selected controls, selection for large litter size increased ovulation rate (60%; P less than 0.001) and decreased LH receptor induction per microgram ovarian DNA (87%; P less than 0.01). Selection for large body weight increased ovulation rate (18%; P less than 0.001), but did not lead to a significant correlated response in LH receptor induction. Index selection for large litter size and small body weight increased ovulation rate (14%; P less than 0.01) and decreased LH receptor induction (72%; P less than 0.01), while index selection for small litter size and large body weight did not significantly alter either ovulation rate or LH receptor induction. LH receptor quantities in testes of males from the 5 lines did not exhibit the among-line profile which was observed in ovaries of females. These results confirm the role of ovulation rate in mediation of the positive genetic correlation between litter size and body weight in mice. Increased ovulation rate in mice selected for large litter size may be due to mechanisms associated with LH receptors as well as factors related to growth. In contrast, increased ovulation rate in mice selected for large body weight may be due exclusively to factors related to growth.     

Coprophagy in the germfree mouse

Coprophagy was observed in germfree (GF) ICR mice of both sexes, and the results were compared with those of conventional mice. Frequency of coprophagy per animal per day in GF mice was 5.1 in males and 5.8 in females. In conventional (CV) mice, the frequencies were 6.2 in males and 5.3 in females (data from Zoological Science 2:249-255, 1985), with no significant differences compared with GF mice. Coprophagy in CV mice was frequently observed during 6-8 hr after lighting, whereas such close time relationships tended to weaken in GF animals. In a comparison of levels of constituents per unit weight between feces and diet, fecal crude protein and crude fat exhibited lower values than those in the diet. Levels of fecal crude ash and crude fiber were higher than those in the diet, and nitrogen-free extract was almost equal to that in the diet. No essential difference in these tendencies was found compared with CV mice. Levels of fecal vitamin B1, B2, B12 and folic acid were lower than those in the diet. In CV mice, except for vitamin B1, these vitamins exhibited either almost equal or much higher levels compared with those in the diet (data from Experimental Animals 35: 381-386, 1986). From the fact that coprophagy was observed in GF mice, it is suggested that the behavior is inherent in the mouse.     

Ovulation rate, embryo survival and ovarian sensitivity to gonadotrophins in mice selected for litter size and body weight

Single trait selection of mice for either large body size or large litter size resulted in an increased ovulation rate because of possible enhanced ovarian sensitivity to gonadotrophins. There was no difference in pre-implantation embryonic survival in either of the selected lines when compared to control mice. Selection for body weight did not alter post-implantation embryo survival, but fewer fetuses were lost after implantation in the litter size line compared to the control line. Index selection for large body size and small litter size did not change ovulation rate but increased pre- and post-implantation embryonic mortality. Selection for small body size and large litter size increased ovulation rate and decreased early embryonic death.     

PATTERNS OF PARENTAL CARE AND PARENTAL INVESTMENT IN MARSUPIALS

I. Information on growth, development and care of young has been assembled for 62 species of marsupial. 2. During gestation, development of the marsupial embryo proceeds only so far as to allow the neonate to make its way from the urogenital opening to the mammary area on the abdomen of the female where it attaches to a teat. Specific structural adaptations keep the neonate firmly attached to the teat for at least the first month after birth. 3. Six types of pouch are distinguished ranging from lateral folds of skin, which do not cover the mammary area or enclose the developing young, to a fold of skin that covers the mammary area and forms a deep pouch, completely enclosing the developing young. 4. Although the young is very small at birth and birth is rapid, specific changes in the behaviour of females occur around the time of birth, and a specific birth position is adopted. 5. The time at which marsupial young leave the pouch cannot be equated with birth in eutherians, because of the considerable variations in the type of pouch and in patterns of parental care. From a consideration of the functional development of the young in the pouch, it is suggested that the nearest equivalent to eutherian birth is the time at which the marsupial young achieves homeostasis, when it is well furred and endothermic. 6. Maternal behaviour is influenced by the type of pouch. In all species, the mother keeps the young and the pouch clean by licking, especially when the young are wholly within the pouch or attached to the abdomen. In species with reduced pouches where young are left in a nest at an early stage of development, maternal behaviour includes nest building, defence, and retrieving and carrying the young. These functions are performed by the pouch itself in species with large deep pouches in which the young is carried for a much larger part of its development, and other specific maternal behaviours are infrequent. 7. The patterns of parental care are reviewed over all families of marsupial. Not all members of a family have the same pattern of parental care, which appears to be influenced by many factors including body size, type of pouch, diet, litter size and other aspects of life history strategy. 8. Three patterns of parental care are distinguished: (A) As soon as young begin to release the teat they are no longer carried by the mother, and are left in a nest when still barely furred, ectothermic and before the eyes open. This pattern is found in species with large litter size and a pouch reduced or absent, e.g. some Dasyuridae and some Didelphidae. (B) Young remain in the pouch after they begin to release the teat but are left in a nest, at a later stage of development than in A, when well furred, endothermic and with eyes open. After first pouch exit, there is generally a period when young return to the pouch from time to time. This pattern is found in species with well developed pouches and litters of I or > 1 e.g. Peramelidae, some Didelphidae. (C) Young remain in the pouch after they begin to release the teat. At first pouch exit, the young is well furred and endothermic, and leaves the pouch only for brief periods, gradually spending more time out until permanent pouch exit. It is not usually left in a nest. This pattern is found in species with well developed pouches and litters of one, e.g. Macropodidae. 9. Pattern A is seen particularly in the smaller species in any family, where large litter size means that by the time young release the teat, the litter is about 50% of maternal body weight and a considerable burden. In such species, young are left in a nest as soon as possible. In larger species with patterns B or C, litter size is smaller, and by the time they are no longer carried by the female, the litter is still only 20% of maternal weight. 10. Whatever the pattern of parental care, mortality from birth to permanent pouch exit is not unusually high in marsupials in comparison with eutherians. 11. I suggest that the presence of the pouch and the associated patterns of parental care are important determinants of social organization in marsupials. For much of the period of dependence, the young is small, attached to a teat or in a pouch. The male can make little contribution to parental care, and there is little room for improvement in the care of young in complex social groups. In most species, the female on her own is sufficient caretaker. The male is most likely to increase his own biological fitness by going off to mate again and leaving the female to raise his offspring. 12. Patterns of energy expenditure on offspring by female marsupials were assessed throughout the development of young. Investment before birth was assessed by weight of the neonate, during development by growth rate and the time for which the young was carried (pouch life), and total investment by weight of young at weaning and time from birth to weaning. Regression of measures of investment against maternal body weight allowed comparison of investment in animals of different size. 13. Investment in young before birth is very small. Neonatal marsupials range in size from 0·01 to 1 g, and the largest is less than 0·2 % of the size of the mother. Larger mothers produce larger young which are smaller relative to the mother than are the young of smaller species. Individual young in the family Dasyuridae are particularly small. 14. Growth rates in g/d were calculated over the period from permanent pouch exit to weaning. There is a very close correlation between growth rate and maternal body weight - that of litters increases as the 0·78 power of body weight. During this period the growth rate of individuals is comparable with that of eutherian young during lactation, and in litters it is higher still, suggesting that the difference in patterns of growth are not due to the lower metabolic rate of marsupials. As in eutherians there is considerable individual variation in growth rate; it is very high in litters of small dasyurids, which have individual rates comparable to those of larger species. Young of the family Peramelidae grow and develop rapidly; those of the arboreal folivore Phascolarctos do both slowly. I 5. Pouch life, the period for which the young is carried by the mother, increases with body size; as expected, species with pattern A parental care have shorter pouch lives than species of the same size with patterns B or C, reflecting the early stage of development at which young are left in the nest in pattern A. 16. Time from birth to weaning is also longer in larger species. There is a close relationship of age at weaning with maternal weight, with some significant exceptions. For their size, the family Peramelidae have a very short time from birth to weaning, and the time from pouch exit to weaning is particularly short. Many arboreal species have longer periods of dependence than expected from their size. 17. The weight at weaning of individual young is closely related to MBW^0·71, but the weight of one young relative to maternal body weight shows no trend with size, and ranges from 25–61 %, with a mean of 42 %. 18. Parental Investment, as measured by the function Wt. of litter at weaning × 100/MBW, decreases with increasing size of mother as MBW^0·28. The highest levels of investment are found in very small species. In many small species of the family Dasyuridae, a litter at weaning is > 300% MBW. By contrast, investment in the family Peramelidae is low - at weaning a litter of three is about 50% MBW, comparable with investment in a single young of the family Macropodidae. 19. The evolution of patterns of parental care and investment appears to follow three main lines: (1) Species with large litter size, high levels of investment in litters and in individual young. Investment is directed to growth and not to carrying the young in the pouch, since young are left in a nest at an early stage. Typical of this group is the family Dasyuridae, in which many species make few reproductive attempts per year. (2) Species with litters of more than one, low levels of investment in litters and in individuals, but rapid growth and development of young. Because of the small relative size of young they are carried in the pouch for a large part of the period from birth to weaning. This pattern is shown by the family Peramelidae, and seen as an adaptation to rapid and repeated reproduction in an environment with an extended favourable season. (3) Species with small litter size, lower total investment, but investment in individual young is not low, and investment in carrying young to an advanced stage of development is high. Patterns of this type are found in the Diprotodonta, with extreme development in the Macropodidae. 20. Many of the measures of investment have been expressed as a power function of maternal body weight. The exponents of body weight in these functions are such as to suggest that an important underlying variable is metabolic rate. 21. It has been suggested elsewhere that the marsupial mode of reproduction evolved as an adaptation to environmental uncertainty, in that it allows a reproductive attempt to be abandoned at any time much more readily than in eutherians, thereby increasing the likelihood that a female will survive to reproduce again. I consider this suggestion in the light of patterns of parental investment. For small, short-lived species, any reproductive attempt represents a substantial part of its lifetime reproductive output. Investment in any one reproductive attempt is high, and the cost of replacing an abandoned attempt is so high that it seems unlikely that the desertion of offspring would be an important reproductive strategy in small ancestral marsupials, although it may be an important response to environmental uncertainty in certain large modern macropodids.     

Influence of maternal dietary calcium levels on milk zinc, calcium and phosphorus contents and milk production in rats

The aim of this study was to analyze zinc (Zn), calcium (Ca) and phosphorus (P) contents in milk and the lactational performance in rats fed different Ca levels. Female Wistar rats were fed during pregnancy and lactation with experimental diets containing 20% protein and high (0.90%, HCa), normal (0.60%, NCa) or low (0.20%, LCa) Ca levels. Milk samples were collected after 15 days to determine the milk mineral composition. Pup weight was recorded from birth to weaning (litter size: 6-8 pups) to determine weight gain and calculate milk production. At delivery there were no significant differences in the body weight of the pups between the groups, but at day 15, the LCa group showed lower values than both NCa and HCa groups (p<0.05). The weight gain of the LCa group was significantly lower than of the HCa and NCa groups, between delivery and day 5 (p<0.05). This reduced rate of weight gain led to the LCa group reaching weaning weight later than the other groups. Milk production (g/pup/day) was significantly lower when dams were fed the LCa than the NCa and HCa diets (p<0.05). There were no significant differences among the groups in milk Ca, P and Zn levels and Ca/P ratio. The body mineral composition of the pups at birth did not differ between the groups; at weaning, however, both LCa and HCa groups had lower element contents than the NCa group (p<0.05). In conclusion, dams fed with a diet containing low Ca levels produced smaller volumes of milk and their pups reached weaning weights later than the other groups. As the milk mineral composition was not affected, it can be hypothesized that in dams fed low dietary Ca, the smaller milk yield might have been a way of maintaining milk quality. High Ca levels affected neither pregnancy outcome nor lactational performance.     

Crossbreeding effects after long-term selection for purebred performance: a model experiment with mice

Summary Results are presented from two replicated three-breed cross diallels that were conducted after 20 generations of selection for purebred performance in mice. The selection criteria for the different lines were: litter size at birth (LS), weaning weight at 4 weeks (WW), weight gain from week 4 to week 6 (WG), and body fat content at week 6 (FT). Additionally, a random-mating control line (C) was kept. Significant maternal heterosis was found in litter size and weaning weight. Estimates of maternal heterosis in litter size were very high, ranging from 17 to 50% of the mean of the corresponding single crosses. Maternal heterosis in weaning weight usually was negative and ranged from +9 to -11%. Significant maternal heterosis in feed efficiency and weaning weight could only be found in a few cases. Total performance of three-breed crosses was highly superior to that of single crosses and purebreds. Means of the corresponding purebreds or single crosses were of little use in predicting three-breed cross performance.     

A reproductive study of Weiser-Maples guinea pigs]

The Weiser-Maples (WE) guinea pig strain was introduced by Backshire Co., Ltd. (USA) in 1977. We have been breeding WE strain guinea pigs for skin melanization research. The WE guinea pig colony produced 1271 pups in 417 litters from May 1978 through December 1983. Breeding date are shown below. The mean litter size was 3.05, the stillborn rate was 15.2%, the weaning rate for live-born pups was 93.5% and the sex ratio was 1.01. The average age at first vaginal membrane rupture was 31.4 days at which time body weight was 290.5g. The mean length of the first 7 estrous cycles was about 17 days, with no cyclical variation in length. The mean duration of gestation was 67.9 days. Duration of pregnancy varied with litter size. There was an inverse relationship between litter size and duration of pregnancy. Most of the pups were delivered alive in mid-pregnancy with a parturition range of 56 to 76 days. The probability of pup death depends on gestational length: the lowest incidence of mortality was seen in litters born at 70 days. The mortalities were related to litter size but not to parity. There was an inverse relationship between birth weight and litter size. In WE guinea pigs, the mean weight for a litter of 1 was 120 g; for a litter of 5, the mean body weight was 58g. Male body weights were slightly heavier than female at birth and at weaning age. The mean body weights are shown below, date of birth: female 88.3g, male 93.3g, weaning age (2 weeks): female 181.1g, male 198.8g and 30 weeks: female 758.7g, male 1018.0g. These date for WE guinea pigs are comparable to those of other strains.