Fertility Investigation in F1 Hybrid and Backcross Progeny of Cattle (Bos taurus) and Yak (B. gruniens) in Mongolia. : II. Little variation in gene products studied in male sterile and fertile animals.
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Crosses between cattle and yak produce and F1 hybrid, the khainag, which is fertile in the females but sterile in the males. Female khainag can be mated with either cattle or yak to produce a B1 backcross animal, the ortoom, which is also female fertile but male sterile. Further backcrossing of ortoom females to cattle or yak bulls will vield a B2 backcross animal, the usanguzee. Male usanguzees can be fertile, the male sterility in hybrids have been diluted by three consecutive backcrosses. We have shown that recovery of male fertility is associated with recovery of homologous pairing during meiosis (Tumennasan et al., 1997). We have speculated that the loss of pairing and recombination or the related molecular events during meiotic prophase are resnonsible for lack of sperm in khainag and ortoom but not in the normal yak and cattle and usanguzee with recovered fertility. We have Isolated several meiosis specific genes from lily meiotic cells (Kobayashi et al., 1994) and the antibodies for such gene products have been prepared (Morohashi et al., 2000). This paper reports the results obtained by using those to compare the meiosis specinc gene expression in fertile and sterile animals. The expression of genes tested so far showed the absence of any noticeable difference between fertile and sterile animals but the plausible mechanism of recovery of fertility in consecutive backcrosses was discussed. In Mongolian mountain areas, both yak and cattle are important economic animals and the cross products, khainag and ortoom, are also useful animals for farmers. However, F1 hybrid and F2 backcross males are sterile while females are fertile with yak and cattle (Fig.1). The karyo-types of the domestic cattle Bos taurus (2n=60) and yak Bos grunniens (2n=60) are identical in terms of both autosomes and X and Y sex chromosomes. Most of the interspecific hybrids are sterile in both sexes despite inheriting closely related or even identical chromosome sets from the two parents (reviewed by Hale et al. 1993). In the hybrids structurally different chromosome sets exhibit major pairing abnormalities at meiotic prophase and this is associated with meiotic failure leading to sterility. In the case of cross between yak and cattle reported here only the male becomes sterile and the sterility starts to disappear after two sequential backcrosses. We suspected that the sterility may come from inactivation of some genes during meiotic prophase due to the one to one mixture of two sets of chromosomes in the male. Changing this mixing ratio the suppression may be reduced and the fertility increases. However, no meiosis specific product from gene(s) is known these animals and extremely difficult to obtain because of the cost. In the present report, we describe the use of antibodies raised against the selected gene products, LIM9, LIM10, LIM13, LIM14, LIM15, LIM16 and LIM18 which are all the products of meiosis specific gene, to detect the presence of such protein in testicular cells from male sterile and male fertile animals by means of Western blotting and immuno fluorescent microscopy (FISH method). Although these are the products of cDNAs isolated from lily meiotic cells, we applied for the fertile and sterile animals because of the well known facts that the presence of highly conserved nature of meiotic processes throughout eukaryotes (Hotta et al. 1997). The comparison of the expression of these genes in meiotic cells in testes from sterile and [figure] Fig.1 Hybrid formation between yak and cattle and their backrross Mating either yak or cattle males with cow or female yak, repectively, produces male sterile and female fertile animals called khainag. Backcrossing of female khainag with male yak or ox produce F2 hybrids called ortoom which are also male sterile and female fertile. Backcrosseing of a female ortoom with male yak or ox will produces usanguzees which are both male and female fertile, although the male shows reduced fertility. fertile animal were carried out by in situ hybridization and Northern hybridization but no significant differnce was found. The aim of these studies was to show the presence of phylogenic similarities in the meiosis specific gene-products and cellular distribution despite fertile or sterile and yak or cattle. Then, the possible mechanism of recovery of fertility in backcrossed male is discussed.