Variations in Karyotypic Characteristics of Different Breed Groups of Water Buffaloes ( Bubalus bubalis ) * *

Karyotype analysis was carried out on blood samples of 30 water buffaloes belonging to different breed groups (i.e. Philippine Carabao (PC), Indian Murrah (IM), Bulgarian Murrah (BM), “F1 50% IM-50% PC”, “F1 50% BM-50% PC” and “75% IM25% PC”), using the modified Leucocyte Culture Technique. The modal chromosome numbers of the PC, “F1 50% IM-50% PC”, “F1 50% BM-50% PC”, IM, BM and “75% IM-25% PC” were 2n=48, 49, 49, 50, 50 and 50, respectively. The water buffalo chromosomes are mostly acrocentric (79.67%) and the remainder submetacentric (20.33%). Results of the ordinary least square analysis showed significant breed effects (p<0.01) on other karyotypic characteristics (i.e. relative length, arm ratio and centromeric index). Significant correlation between karyotypic characteristics and some animal performance traits were also found. The significant correlation values imply that karyotypic characteristics can be used as important criteria to select potentially productive young water buffaloes. In the future, more production and reproduction traits from non-institutional herds should be included in the analysis to reveal meaningful correlations with various karyotypic characteristics. (Asian-Aust. J. Anim. Sci. 2002. Vol 15, No. 3 : 321-325)

While chromosome numbers and morphology are important to determine their relationships with incidence of reduced fertility in crossbred buffaloes, evaluation of their variability between breed groups can be especially useful in improving buffalo productivity.For example, indirect selection for overall productivity based on the karyotypic characteristics can be done at a much earlier age, thereby reducing generation interval considerably.Farmers could also have their animals tested at an early age in order to identify and cull those that are going to be unproductive and infertile.Karyotype analysis can be used to predict the purity or grade of animals when breeding history and records are not available to the farmer.
This study aimed to determine the variability of karyotypic characteristics (i.e.modal chromosome number, percent chromosome type based on centromeric location, centromeric index, arm ratio, and relative length) between 3 purebreds and 3 crossbreeds of water buffaloes.Relationships of the karyotypic characteristics with animal performance traits (i.e.body weight at 6 months, 1, 2 and 3 years old; heart girth, body length, and wither height, milk production, and semen characteristics) were also determined by correlation analysis.
Using the modified Leucocyte Culture Technique (Bongso and Hilmi, 1982), blood samples were cultured in the laboratory to induce the leukocytes to grow at metaphase stage.Photomicrographs of the metaphase spreads were taken at 400x total magnification from a microscope using black and white film.At least 5 pictures from 3 slide preparations from each buffalo were selected, scanned and stored in microcomputer files.The photomicrographs were further enhanced, magnified (4 to 5 times), and printed.Chromosome pairs were manually cut and pasted to form the photokaryotype.
The chromosomes were identified as pairs according to their length and position of the centromere.In the photokaryotype, the identified pairs of autosomes were arranged in decreasing order of length in each category, namely submetacentric, metacentric and acrocentric.The sex chromosome pair was placed at the last in the karyotype.The X chromosome was the largest acrocentric pair in the karyotype while the Y chromosome was one of the smallest acrocentric chromosomes.Examples of karyotype pictures in the PC, IM and 50% IM-50% PC are shown in figure 1.
Analysis of variance by ordinary least squares method was used to determine differences among breed groups in terms of karyotypic characteristics such as the modal chromosome number based on the highest chromosome frequency of 2n=47, 48, 49 or 50 percent chromosome type based on centromeric location, relative length, arm ratio, and centromeric index.Taking the total length of all chromosomes in the genome as 100, the relative length was computed as percentage of the total length.Centromeric index was computed as percentage of the short arm to the total length of the chromosome.The arm ratio was the length of the long arm divided by the length of the short arm.
Individual performance records of the experimental pure breed and crossbred buffaloes were also gathered from the PCC-UPLB.The production and reproduction records included body weight at 6 months, 1, 2 and 3 years old; heart girth, body length, and wither height, horn type, lactation yield, lactation length, and age at first calving.Pearson correlation analysis was then used to determine relationships between karyotypic characteristics and individual performance traits.

Modal chromosome number
Table 1 shows that the modal chromosome number based on the highest chromosome frequency, for the PC is 2n=48.IM, BM and "75% IM-25% PC" had 2n=50 chromosomes, with chromosome frequency of 69.75%, 80.79% and 64.22% respectively.The diploid chromosome number of the F 1 crosses (i.e."50% IM-50% PC" and "50% BM-50% PC" was 49, with chromosome frequency of 68.58% and 76.34% respectively.The chromosome complement of 2n=49 in the F 1 hybrid was consistent with the results reported by previous workers (e.g., Fischer and Ulbrich, 1968;Songsri and Ramirez, 1979;Bongso et al., 1984).The features of chromosomes without partners in "50% IM-50% PC" and "50% BM-50% PC" conform to the tandem fusion between chromosome numbers 4 and 9 of the river type described previously by Bongso and Hilmi (1982).Such information was commonly suggested to be used to predict the grade of animals when breeding history and records are not available to farmers.

Chromosome type based on centromeric location
Contrary to early published reports (e.g., Songsri and Ramirez, 1979), no metacentric chromosomes were found in all water buffalo breed groups.On the average, 20.33% of all chromosomes were submetacentric and 79.67% were acrocentric (table 2).
In the Philippine Carabao, 10 out of 48 chromosomes (20.8%) were submetacentric.The rest of the chromosomes were acrocentric.The type of chromosome based on the location of the centromere was similar for the BM, IM and 75% "IM-25% PC" whose modal chromosome number is 2n=50.The chromosome type was also similar for "F 1 50% IM-50% PC" and "F 1 50% BM-50% PC", both with chromosome number 2n=49.

Breed effects on karyotypic characteristics
Results of the analysis of variance revealed significant effects of breed on chromosome frequency (p<0.01) and chromosome location (p=0.08).Highly significant breed effects (p<0.01) were also found for relative length, arm ratio, and centromeric index of chromosome 4A and 4B (table 3).The relative length of chromosomes 1B, 2A, 2B, 3B, 5B, 8A, 8B, 16B, 22A, 22B and 23A were also different between breed groups of water buffaloes.The significant variations in some karyotypic characteristics between breed groups provide extra information to previous reports (e.g., Azmi et al., 1990) showing trends mostly in chromosome numbers that were usually associated with differences in growth rates among crossbred water buffaloes.

Relationships between karyotypic characteristics and animal performance
Table 4 shows the list of correlation coefficients (r) between karyotypic characteristics and some animal performance traits.Significant correlations were found to be more common between animal performance and relative length than with animal performance and chromosome frequency, arm ratio or centromeric index.Other important findings were as follows: 1.Some measures of body weight appeared to be related with arm ratio of chromosomes 3A and 4B, and with centromeric index of chromosome 2B.2. Heart girth was mostly negatively correlated to relative length of chromosomes 3B, 13B, 14A, 14B, and 23A. 3. Withers height was significantly correlated with relative length of chromosomes 3B, 5B, 22A, and 23A. 4. Lactation yield and lactation length seemed to be associated with chromosome frequency (2n=50) and with relative length, arm ratio, and centromeric index of chromosome 4A and 4B.
The significant correlation values may imply that some karyotypic characteristics can be used as important criteria to select potentially productive young water buffaloes.Hence, indirect selection for overall productivity based on the karyotypic characteristics of breeding animals can be done at a much earlier age, thereby reducing generation interval considerably.The physiological mechanisms that determine the correlation between karyotypic characteristics and some performance traits however, need to be elucidated further.

Table 2 .
Chromosome type (in percent) based on centromeric location

Table 4 .
Significant correlations (p<0.05)betweenkaryotypic characteristics and some animal performance traits