(Mini review) Use of the Syrian hamster, Mesocricetus auratus, in selection for fertility in animal breeding 1. Physiology of reproduction for selection experiments Masahiro SATOH Genetic Diversity Department, National Institute of Agrobiological Sciences 1. Introduction reproductive performance of cattle, sheep, and pig The Syrian hamster, Mesocricetus auratus, has been populations with particular attention to variations in used as a laboratory animal for three-quarters of a century. ovulation rate, litter size, and embryo survival. Rydhmer It is the most commonly used of several related species that (2000) discussed the possibility of improving different sow include the Chinese hamster (Cricetulus griseus) and, more reproduction traits, such as age at puberty, estrous recently, the grey hamster (Cricetulus migratorius), which symptoms, and ability to become pregnant. At the are bred in captivity and used in biological and medical University of Nebraska in the U.S.A., litter size at birth in research laboratories (Fulton 1968). Recently, the Syrian pigs and its components have been selected, and direct and hamster has been used as a pilot animal for improving correlated responses such as ovulation rate, uterine fertility in domestic livestock (Furukawa et al. 1988; Satoh dimensions, and uterine capacity have been investigated et al. 1997, 1998c; Ishii et at. 2003). (Gama & Johnson 1993; Ruiz-Flores & Johnson 2001; So far animal breeders have selected mainly for Mesa et al. 2003; Petry & Johnson 2004; Petry et at. 2004). litter size at birth in pigs and sheep, litter weight at weaning Testicular characteristics have been studied for their genetic and number of teats in pigs, multiple births in sheep, and relationship with female reproductive traits in pigs weaning weight in beef cattle in studies of reproductive (Schinckel et al. 1983; Toelle & Robison 1985; Young et performance of livestock. They have also studied age at at. 1986; Johnson et al. 1994) and in sheep (Land & Can puberty, age at first service, and age at first calving in 1975; Hanrahan & Quirke 1977). studies of sexual maturity (Holm et at. 2004b; Serenius et Traits for selection in livestock should be not only at. 2004b); percentage of live births and prenatal and pre- economically important, but also easy to measure. In pilot weaning survival in studies of productivity of the dam tests of selection methods, laboratory animals offer (Lund et at. 2002; Leenhouwers et at. 2003); return rate advantages over livestock in having a much shorter lifespan after parturition, duration of parturition, the need for birth and traits that are easy to measure. Kent (1968) reviewed assistance, and lactation length in studies of fertility for the existing knowledge of reproductive physiology of the management of the dam (Holm et at. 2004a, 2005); Syrian hamster. My group has explored the reproductive gestation length, interval from weaning to first service, and physiology of the Syrian hamster for selection experiments number of parturitions in a lifetime in studies of long-term since the 1980s. The objectives of this paper are to reproduction (Moeller et at. 2004; Serenius et at. 2004a; summarize current knowledge and to introduce our results Holm et at. 2005); and testicular growth in studies of male of recent experiments in fertility of Syrian hamsters. reproductive efficiency (Wilson et at. 1977; Neely & Robison 1983). Response to selection could be improved if some characteristics could be used as a predictors of the Correspondence: Masahiro Satoh, Genetic Diversity Department, reproductive efficiency of genetically related selected traits. National Institute of Agrobiological Sciences, Tsukuba 305-8602, Hanrahan (1982) reviewed evidence for genetic variation in Japan (e-mail: hereford@ affrc.go.jp) Journal of Animal Genetics (2005) 32(2), 55-64 55 M. Satoh 2. General reproductive capacity (Satoh et al. 1999a). These differences indicate that the Reproductive features generation interval is affected by genetics. It also depends The lifespan of the Syrian hamster varies between 1 on environmental factors such as nutrition and housing. In and 3 years, though over 2 years is uncommon. Females long-term selection experiments, 4 generations per year are tend to mature about 2 weeks earlier than males, which desirable. apparently reach maturity at between 40 and 60 days of age (Magalhaes 1968a). Females can breed at about 6 weeks of Diet for optimum reproduction age, but they should be bred first when they are between 8 Commercial pelleted diets for hamsters are and 14 weeks of age because of small litter size at birth and available, similar to those used for other laboratory animals weaning before 8 weeks and low conception rate after 14 such as mice and rats, catering to the Syrian hamster's weeks. Sexually active females have a 4-day estrous cycle specific digestive system (e.g., Matsumoto 1955; Hoover et (Table 1). al. 1969), especially forage digestive ability (Manila & The breeding span of females is about 10 to 13 Takano 1976; Manila & Gotoh 1976; Shinohara et al. months, and pregnancy rate decreases after 9 months 1989). However, few reliable investigations of the (Okuki 1972). The use of over-age females is sometimes the hamster's feed requirements for reproduction have been cause of breeding failures. Pregnancy rate may decrease made. Our group reported that Syrian hamsters raised on after the third parity. However, males have sired litters at fibrous feed before mating age gave optimal reproductive about 2 years of age. Females of inbred strains over 12 performance (Satoh & Obata 1995). In general, diets must months of age rarely breed, but we have found difference supply sufficient vitamins B1, C, and D, and fat females among lines. In general, synthetic strains have a longer should be avoided for optimal reproduction. breeding span than inbred strains. The generation interval depends on the age of sexual 3. Estrous cycle and mating maturity, pregnancy rate, and parturition interval. Under a Estrus cycle harem-mating system, the average generation intervals from Nakano (1960) observed the first corpora luteal at weaning of 3 inbred lines were 65.5, 78.4, and 85.3 days 30 days. But Kupperman et al. (1944) reported that luteal Table 1. Breeding data for Syrian hamsters 1) Festing (1972) . 2) Author. Journal of Animal Genetics (2005) 32(2), 55-64 56 Use of Syrian hamster in animal breeding 1. Physiology of reproduction formation and cyclic changes in the vagina were not On the morning following ovulation, a creamy white, thick, initiated until the 33rd to the 38th day. The first opaque, stringy, viscous, mucous discharge with a lingering spontaneous estrus is about 5 weeks, but it depends on pungent odor fills the vagina. This avoids the need for tools genetics and nutrition. such as a microscope. If this characteristic postovulatory The Syrian hamster exhibits regular ovarian cycles discharge does not ooze out of the vaginal orifice while the with ovulation every 4 days. The cycle is classified into 4 animal is being handled, gentle pressure at the sides of the stages: proestrus, estrus, metestrus, and diestrus (Figure 1). orifice will cause it to exude. The discharge occurs from the Typical durations of proestrus, estrus, and metestrus are end of estrus through metestrus. The external signs continue approximately 3, 12, and 4 hours, respectively. The onset of in the morning. psychic estrus occurs approximately 8 hours before At this stage, the number of ova can be determined ovulation in any single animal, and the time of onset is by flushing out the ampulla of the oviduct with saline therefore a guide to the approximate time of ovulation. containing 0.5 % hyaluronidase and counting under a Most Syrian hamsters ovulate 8 to 9 hours after the onset of stereomicroscope. On the next day following the sign, the psychic estrus. Orsini (1961) reported that ovulation was number of fertilized (2 cells or more) and unfertilized ova under way approximately 7 hours after the onset of can be counted by this method if the female was mated at darkness. It is a common observation that psychic estrus the previous estrus. The number of ova can be also counted commences at dusk or within an hour or 2 thereafter. by observing the corpus luteum. About 3 hours after ovulation, the liberated ova have Our group reported means of 10.4 total and 9.7 reached the ampulla of the oviduct. The diameter of the normal ova in 247 females of the CBN inbred strain (Satoh vitellus of the unfertilized ovum is about 60,um. The et al. 1998a). We found evidence of strain differences in the dimensions of the unfertilized ovum change slowly but numbers of total and normal ova, averaging 8.4 and 7.7 per steadily after ovulation. The maximum fertilizable life of litter in line AC and 9.9 and 9.1 in line CBN (Satoh et al. ova is about 13 hours. At 6 hours after ovulation, only about 1999a). Numbers of total and normal ova from the right half of the ova remain fertilizable, and coatings after ovary were 0.6 and 0.5 greater than those from left ovary. daybreak are less likely to produce young (Kent 1968). They also found that the number of ova was affected by age Fertilization rate is higher in morning matings than in and feed condition. We reported a mean of 12.6 normal ova evening matings. The ova reach the uterus during the third from 915 females of a closed herd of a synthetic strain day after ovulation. It remains free in the uterus for (Satoh et al. 2001a). The number of ova from the right approximately 24 hours. ovary was 0.6 greater than that from the left ovary. These External signs indicate certain stages of the cycle. lifferences between right and left ovaries agree with those Fig.