The Chromosomes of the Horse (Equus caballus) (Chromosome studies in domestic mammals, I)

Abstract

The chromosomes of the horse were investigated in the following two distinct breeds, the Percheron and the Ryukyu Pony. So far as the number and other general morphological characters of chromosomes are concerned, the two breeds under consideration show no remarkable and significant difference. The number of chromosomes in the horse is not so small as described by previous authors. The spermatogonium contains 66 chromosomes in which are contained at least 12 atelomitic V-shaped chromosomes along with the remaining rod-shaped elements ranging in length from long rod to short one. The primary spermatocyte metaphase shows 33 tetrads. A heteromorphic tetrad of XY-complex is always found lying at the periphery of the equatorial plate. Both elements of the XY-bivalent seem to be of telomitic nature. The X-element assumes a characteristc tripartite structure, being divided by two distinct constrictions into three consecutive segments. There are distinguishable two different garnitures of chromosomes in the secondary spermatocyte as regards the X and Y chromosomes, both being 33 in number. The one is X-class cell consisting of 32 autosome dyads (6 atelomitic and 26 telomitic) and an X-element of single V-shape. The other is Y-class and contains 6 atelomitic and 27 telomitic dyads in which the Y-element is included. The X-element bears a characteristic structure distinguishable from the other autosomes in the metaphase, whereas the Y is difficult to be discriminated among the others. © 1943, Japan Mendel Society, International Society of Cytology. All rights reserved.