Hidas A.
Institute for Small Animal Research,
H-2101 Gödöllő P.O.B. 417. Hungary
INTRODUCTION
European domestic goose breeds are originated from the Anser anser, while the Asian and African ones were bred from the Anser cygnoides. Following conventional Giemsa staining, the karyotypic difference is obvious on the 4th pair, which is submetacentric in case of European breeds (Hammar 1966), while metacentric in the others (Bhatnagar 1968; Belterman and DeBoer 1984). The difference was earlier supposed to be due to a pericentric inversion, according to G-banding observations (Silversides et al., 1988).
MATERIALS AND METHODS
A small stock of a the Kuban goose breed (derived from Russia) from the Goose Research Station (Babat, Gödöllő Univ. Agric. Sci.) was used to study the heteromorphic chromosome 4. This breed was established from the two types of goose breeds. Fibroblast cultures were set up from body wall of 10 days old embryos for chromosome analysis. CBG staining was carried out following Sumner (1972). For restriction enzyme banding, Alu I enzyme (1U/µl) digestion was used (Kaelbling et al., 1984) on fresh slides under coverslips at 37 oC for 1-4 hours, then stained with Giemsa (5 % in phosphate buffer pH 6.8).
RESULTS
Embryos were found to carry the metacentric chromosome 4 of A. cygnoides type in both heterozygous and homozygous conditions. Following CBG staining, a heterochromatic segment was revealed inserted into intercalaric region of the metacentric chromosome 4. Treating the metaphase chromosomes with restriction endonuclease Alu I showed only W chromosome of A. anser karyotype to be resistant to digestion. In the karyotype containing chromosome 4 from the A. cygnoides not only W chromosome but also intercalaric heterochromatin of the metacentric chromosome 4 was stainable following Alu I treatment.
Giemsa stained goose metaphase chromosomes (female) with a heteromorphic chromosome 4. Note the structural difference of the two types of chromosome 4. 4(A): chromosome 4 of A. anser type, 4(C): chromosome 4 of A. cygnoides type
C-banded goose metaphase chromosomes (female) with a heteromorphic chromosome 4.
Alu I induced staining of goose metaphase chromosomes (female) with a heteromorphic chromosome 4. Note the positively stained W chromosome and the heterochromatin of the heteromorphic chromosome 4.
DISCUSSION
These findings show that the difference between the two types of chromosome 4 is unlikely to be resulted from a simple inversion, since an inserted heterochromatic block is present. Interestingly, the intercalaric heterochromatin present on chromosome 4 of A. cygnoides was Alu I resistant as well as chromosome W, while all of the other heterochromatic regions were digested. Restriction endonucleases are suitable for demonstration of heterochromatin heterogeneity in different species, such as human (Babu and Verma 1990), pig, cattle, horse (Hidas and Gustavsson 1992). Therefore, behaviour of the two positively stained heterochromatin in A. cygnoides may reflect similarity in their composition as well. The origin of the extra heterochromatin is uncertain, since there is no other heterochromatic region in the A. anser karyotype, which is resistant to Alu I, except for W chromosome. Both possible explanations may be worth doing further studies: - transposition of W chromosome material to chromosome 4 - amplification of a new or existing, but cytologically undetectable Alu I resistant repetitive sequences forming heterochromatic segment on the chromosome 4. Although, this karyotypical alteration must have occurred before separation of the two species, it should be quite new from evolutionary point of view, since they can produce a fertile hybrid.
REFERENCE:
Hidas A. (1993): Cytogenetic studies on a species hybrid goose breed. Proc. 8th North American Colloqium on Domestic Animal Cytogenetics and Gene Mapping, Guelph, Canada, 153-155.