Cytogenetic investigation in Saanen and Alpine artificial insemination bucks. Identification of a Robertsonian translocation

Summary - The cytogenetic study of 224 AI Saanen and Alpine he-goats revealed the presence of a Saanen animal carrying a Robertsonian translocation. The chromosomes involved in this translocation were determined using G (GTG) and C (CBG) banding techniques. The chromosomes in question were identified as chromosomes 6 and 15.

(Received 21 June 1991; accepted 23 July 1991) Summary -The cytogenetic study of 224 AI Saanen and Alpine he-goats revealed the presence of a Saanen animal carrying a Robertsonian translocation. The chromosomes involved in this translocation were determined using G (GTG) and C (CBG) banding techniques. The chromosomes in question were identified as chromosomes 6 and 15. goat / chromosome / Robertsonian translocation INTRODUCTION In domesticated species, numerical chromosome aberrations are readily eliminated by natural selection, whereas structural abnormalities may give rise to polymorphic systems whose deleterious effects on fertility have been established in cattle (Gustavsson, 1969;Refsdal, 1976;Kovacks and Csulky, 1980). In goats, the most widespread structural aberrations are Robertsonian translocations (Cribiu and Lherm, 1986). Since these translocations are widespread in the Saanen breed (Cribiu and Lherm, 1986), a cytogenetic survey of AI goats was conducted in two French AI centres.

MATERIALS AND METHODS
The cytogenetic investigation was carried out in three groups : i) 224 (Grouchy et al, 1964), incubated at 37°C for 68 h. Cells were spread on glass slides, flame-dried, and either stored unstained at room temperature, or stained for 10 min with a 4% Giemsa solution. Unstained slides were treated for G-banding (GTG) using the method of Seabright (1971) and C-banding (CBG) following the method of Sumner (1972).
The chromosomes were identified, paired and arranged according to the recommendations of the Reading Conference (1976) and ISCNDA (1989).

RESULTS
Out of a total of 224 AI bucks examined, one Saanen male has been found to carry a structural chromosome abnormality, whereas no numerical aberration has been detected. The frequency of abnormal carriers among the 98 AI Saanen bucks examined was 1.02%. The G-banding technique (GTG) made it possible to identify the chromosomal pairs involved in this translocation as pairs 6 and 15, respectively  2). Among the parents of the translocated buck, the sire had a normal karyotype and the dam was heterozygous for the 6;15 Robertsonian translocation. The ancestors of the translocated dam were imported from Great Britain, Germany and Switzerland.
Among the 62 daughters of the heterozygous buck, 32 were found to be carriers of the 6;15 Robertsonian translocation in the heterozygous state and 30 had a normal karyotype.

DISCUSSION AND CONCLUSION
Robertsonian translocations are the most frequently reported anomalies in domesticated bovidae. These translocations, also known as centric fusions, are named after Robertson (1916), who reported these rearrangements in the chromosomes of grasshoppers. Fifty years later, Padeh et al (1965) reported an unusual number of chromosomes (2n = 59) in a hermaphrodite Saanen goat, and among the autosomes, a large submetacentric chromosome was noted. Further studies on Saanen goats reported a Robertsonian translocation similar to that reported by Padeh et al (1965) (Soller et al, 1966;Hulot, 1969;Padeh et al, 1971;Popescu, 1972;Sohrab et al, 1973). Later cytogenetic investigations have permitted identification of the chromosomes involved; the submetacentric chromosome resulted from the fusion of autosomes 5 and 15 (Evans et al, 1973;Jorge, 1987), 6 and 17 (Elminger and Stranzinger, 1982) or 6 and 15 (Burguete et al, 1987;Yang et al, 1991). The comparison of the banding pattern of the 5;15, 6;17 and 6;15 translocations with that of the present paper permitted the conclusion that it is the same translocation. This translocation occurs at frequencies as high as 25% in some herds of Saanen goats in Brazil (Jorge, 1987). It has not been found in other breeds. Three other Robertsonian translocations have been detected in different breeds : a 3;7 translocation in Toggenburg (Dolf and Hediger, 1984), a 10;12 translocation in Malaga (Moreno and Franganillo, 1988) and a unidentified autosomal translocation in Nlurciana-Granadina (Burguete, 1991). Robertsonian translocations result from the fusion of two acrocentric chromosomes. Three different mechanisms have been suggested for the formation of Robertsonian translocations from two acrocentric chromosomes, depending where the breakpoints are located. In the first case, one of the chromosomal breakpoints involves the short arms of one chromosome and the other is on the long arms of the second chromosome near the centromeric region. In the second case the breakpoints occur within the centromeres. In these two cases, the fusion gives rise to a monocentric metaor submetacentric chromosome and a minute fragment containing the centromere which is lost during the subsequent cell divisions. In the third case, if the breakpoints involve only the short arms of both chromosomes in the centromeric region, the fusion leads to the formation of a dicentric metaor submetacentric chromosome and loss of two acentric fragments. The use of the C-banding technique made it possible to suggest the mechanism by which this translocation arose: the 6;15 chromosome with two constitutive heterochromatin blocks in the pericentromeric region could be dicentric.
On the basis that structural chromosome anomalies, unlike recurrent genetic mutations, are unique events (White, 1968), this translocation seems to be transmitted as a Mendelian co-dominant trait. The origin of the translocation is uncertain, since the ancestors of the translocated he-goat come from Great Britain, Germany and Switzerland. Most cytogeneticists believe that the 6;15 translocation originated from Switzerland since the Saanen herds in which it was found have been constituted from animals coming from Switzerland (Hulot, 1969;Padeh, 1965;Popescu, 1972;Elminger and Stranzinger, 1982;Jorge, 1987).
As with a majority of Robertsonian translocations found in animal populations, the 6;15 translocation does not seem to be associated with phenotypic characteristics (Cribiu and Lherm, 1986). In the absence of fertility records, a reduced fecundity in heterozygotes resulting from anaphase I nondisjunction and/or changes in the pattern of recombination in such individuals, can not be excluded. An increased frequency of anaphase I nondisjunction has been reported in the laboratory mouse (Gropp and Winking, 1981) and the most widely studied chromosome translocation in cattle, the 1 ;29 Robertsonian translocation, showed reduced fertility in daughters of carrier bulls (Gustavsson, 1969;Refsdal, 1976).