Chromosome profile of cross-bred bulls in Kerala

Abstract

Cytogenetic study was conducted on 53 young cross-bred (Bos Taurus x B. indicus) bulls stationed at the farm at Dhoni, belonging to Kerala Livestock Development Board. Young bulls included those selected for superior semen quality and others just started producing semen. The bulls were classified into Jersey cross, Holstein-Friesian cross and Brown-Swiss cross based on the paternal line. The semen quality and related attributes of the bulls were recorded, and the association between these traits and the karyological parameters were determined. Comparative chromosome study were performed in the three genetic groups. Metaphase spreads for staining and G-banding were obtained by peripheral leucocyte culture technique. The basal medium used for culturing was RPMI 1640 and mitosis was initiated in lymphocytes by a combination of phytohemagglutinin and pokeweed mitogen. The G-banding was done by incubating the chromosome spreads in 2 x SSC containing trypsin solution for 45 minutes at 660C. Karyological parameters such as chromosome number, morphology, relative length, arm ratio and centromeric index were studied. The nature, number and position of G bands were also examined. The reproductive attributes recorded included age at first semen collection age at freezable semen production, volume of semen, sperm concentration, total sperm output in first ten collections, number of ejaculates accepted for freezing and total freezable sperm output in first ten collections, and the morphological abnormalities of sperms. All the bulls except one, exhibited a diploid chromosome complement (2n=60, XY) in their cells. There were 29 pairs of acrocentric autosomes and a sub-metacentric x chromosome. The Y chromosome was sub-metacentric in Holstein-Friesian cross, and apparently metacentric in other two genetic groups. In one bull diploid/tetraploid mosaicism was observed with 6.67 per cent of lymphocytes carrying 120 chromosomes. The mean relative length of longest and shortest autosomes were 6.0174 ± 0.0273 and 1.6186 ± 0.0101 respectively. The X and Y chromosomes had a mean relative length of 5.5918 ± 0.0401 and 1.9636 ± 0.0396 respectively. In the X chromosome the arm ratio was 2.47 ± 0.04 and the centromeric index was 28.74 ± 0.33. A total of 405 bands were identified in the karyotype of the bulls. The G-banding pattern of cross-bred bulls in Kerala was not previously investigated, and hence the banding pattern observed in the study would be useful for cytogenetic screening of bulls in the state. On Analysing the semen quality and related attributes of the bulls it was found that one of the bull was oligospermic. The semen of this bull exhibited a high frequency of loose sperm heads. Semen of another bull was found to contain abnormal percentage of sperms with persistant proximal cytoplasmic droplet. A third bull produced semen in which the frequency of sperms with knobbed acrosome defect was very high. All the three bulls had produced ejaculates which were found unsuitable for freezing. The incidence of diploid/tetraploid mosaicism was detected in the bull producing sperms with knobbed acrosome defect. None of the ejaculates of this bull was suitable for freezing. However, further study was essential to conclude on the association between mixoploidy and knobbed acrosome defect or its influence on semen freezability. The other two bulls with seminal abnormalities exhibited cytogenetic profile similar as that of bulls producing normal semen. The effect of genetic group on the morphometry of sex chromosomes was found to be insignificant. However, the Y chromosome morphology was observed to be a suitable marker for identifying Holstein-Friesian crosses among the cross-bred bulls used for breeding in Kerala.