CYTOGENETIC STUDIES OF THE HORSE, EQUUS CABALLUS (L.), THE DONKEY, EQUUS ASINUS (L.) (PERISSODACTYLA: EQUIDAE) AND THE MULE
Comparative cytogenetic studies using conventional stains, as well as different banding techniques, have been performed on three diploid fibroblast cell lines derived from horse (Equus caballus, 2n = 64), donkey (Equus asinus, 2n = 62) and mule (2n = 63).^ Conventional staining and banding techniques were utilized to morphologically identify and characterize individual chromosomes of horse, donkey and mule in metaphase chromosome preparations.^ Studies performed on horse metaphases yielded results similar to those obtained in previously published reports. Similar studies performed on donkey metaphase preparations have clearly established that out of 31 chromosome pairs, 19 pairs of autosomes were metacentric or submetacentric, and 11 paris of autosomes were acrocentric with the donkey X chromosome classified as the fourth largest submetacentric chromosome pair in the donkey diploid complement.^ The cytogenetic studies performed on horse and donkey chromosome preparations facilitated the ultimate identification of the horse- and donkey-derived chromosomes in the mule diploid chromosome complement. A system of nomenclature has now been established as a result of this study to clearly identify individual mule chromosomes. Within the mule diploid chromosome complement 32 chromosomes have been identified as being derived from the female horse parent, while 31 mule chromosomes have been identified as being derived from the male donkey parent.^ The mule diploid chromosome complement displayed a great deal of conservatism with respect to the comparative banding patterns of horse and donkey. This may indicate a similarity to horse and donkey in the arrangement of gene sequences on the mule chromosomes. These gene sequences in mule are probably transcribed differently than in the horse and donkey. Some differences could be observed in mule chromosomes banding patterns when compared to donkey and horse banding patterns. However these variations could be due to chromosome condensation differences that naturally arise in different metaphase spreads of all three cell lines.^ Heterogeneity was observed between homologous horse and donkey chromosomes with respect to the degree of heterochromatic staining obtained by the C banding technique. This phenomenon has been observed in many different organisms and it may be involved in the karyotypic evolution of some organisms by initiating karyotypic changes before gross phenotypic changes are observed.^ Ag-As staining of NOR's which codes specifically for rRNA was performed on all three cell lines. In horse, an average of 8 chromosomes exhibited telomeric NOR's along with 58-60 chromosomes which exhibited centromeric NOR's. In donkey an average of 8 chromosomes in each metaphase exhibited telomeric NOR's, and a range of 3-26 chromosomes exhibited some centromeric NOR's. In mule, however, only 4 chromosomes exhibited telomeric NOR's which was substantially lower than expected based on the number of NOR's observed in the horse and donkey. This low number of observable NOR's in the interspecific mule hybrid indicates some type of suppression mechanism is operating in the mule. The precise nature of this suppression mechanism is unknown, but may be operating at the level of transcription of one parent species rDNA.^
CANNIZZARO, LINDA ANN, "CYTOGENETIC STUDIES OF THE HORSE, EQUUS CABALLUS (L.), THE DONKEY, EQUUS ASINUS (L.) (PERISSODACTYLA: EQUIDAE) AND THE MULE" (1981). ETD Collection for Fordham University. AAI8124276.