The Journal of Heredity 2001:92(3)
© 2001 The American Genetic Association 92:221-225
Current Status of the River Buffalo (Bubalus bubalis L.) Gene Map
From the Department of Cell Biology, National Research Center, Cairo, Egypt (El Nahas and de Hondt) and Department of Veterinary Pathobiology, College of Veterinary Medicine, Texas A&M University, Hwy. 60, College Station, TX 77843 (Womack).
Address correspondence to J. E. Womack at the address above or e-mail: jwomack{at}cvm.tamu.edu.
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Ninety-nine loci have been assigned to river buffalo chromosomes, 67 of which are coding genes and 32 of which are anonymous DNA segments (microsatellites). Sixty-seven assignments were based on cosegregation of cellular markers in somatic cell hybrids (synteny), whereas 39 were based on in situ hybridization of fixed metaphase chromosomes with labeled DNA probes. Seven loci were assigned by both methods. Of the 67 assignments in somatic cell hybrids, 38 were based on polymerase chain reaction (PCR), 11 on isozyme electrophoresis, 10 on restriction endonuclease digestion of DNA, 4 on immunofluorescence, and 4 on chromosomal identification. A genetic marker or syntenic group has been assigned to each arm of the five submetacentric buffalo chromosomes as well as to the 19 acrocentric autosomes, and the X and Y chromosomes. These same markers map to the 29 cattle autosomes and the X and Y chromosomes, and without exception, cattle markers map to the buffalo chromosome or chromosomal region predicted from chromosome banding similarity.
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Introduction |
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The river buffalo is an economically important livestock species in many Asian and Mediterranean countries, and its genetic improvement, especially in reproductive performance and quantity of meat and milk production, ranks high among agricultural research needs of these countries. The majority of important traits in buffalo, as in other farm animals, are polygenic in nature and therefore difficult to isolate and identify at the genome level. However, recent success in mapping quantitative trait loci (QTL) in cattle, pigs, and other livestock species has demonstrated new opportunities for analyzing, and eventually controlling, these continuously distributed traits (Andersson et al. 1994; Georges et al. 1995).
Animal improvement by the genomic approach has been targeted by establishing physical and linkage maps as tools for developing more efficient breeding strategies. Physical mapping includes the use of somatic cell hybrids and in situ hybridization to determine syntenic relationships between loci and to assign loci to specific regions of their respective chromosomes. The establishment of a physical map at the chromosomal level of resolution will facilitate the development of a genetic map, which defines the linear relationship of the loci on the chromosome and estimates the distances between them by frequency of meiotic recombination. Genetic maps will be used to study economically important trait loci. A saturated map of markers, applied to reference families segregating economic traits, will reveal the existence of linkage associations between some of these trait loci and microsatellite markers. Microsatellite markers are easy to identify and often highly polymorphic, and will thus permit marker-assisted selection (MAS) of desirable traits to which they are linked. Some of these marker-trait associations have already been discovered in cattle (Georges et al. 1995).
The close chromosomal relationship between buffalo and cattle is useful in constructing the buffalo genome map. Extensive chromosome arm homology between cattle and river buffalo has been established (Report of the Committee for the Standardization of Banded Karyotypes of the River Buffalo 1994). While the cattle genome consists of 29 acrocentric autosomes plus the X and Y chromosomes, the buffalo genome has 5 biarmed and 19 acrocentric autosomes. Thus the arm number is identical between the species and every arm of the five biarmed buffalo chromosomes has banding similarity to a cattle acrocentric chromosome. Comparative cytogenetics and physical gene mapping have shown that chromosome band identity between closely related species is a good indicator of genetic homology (Nash and O'Brien 1982) and therefore chromosome banding conservation is predictive of conservation of genetic content. Because of the extensive chromosome conservation between cattle and river buffalo, the cattle physical map can potentially function as a template for developing the buffalo gene map.
The cattle genome is well characterized, with synteny and linkage groups assigned to each chromosome (Masabanda et al. 1996; Mezzelani et al. 1994). Based on banding similarity, the five submetacentric pairs of the buffalo are assumed to originate from fusion of 10 cattle acrocentrics (Report of the Committee for the Standardization of Banded Karyotypes of the River Buffalo 1994). There is no obvious rearrangement of banding pattern in any of the autosomal arms, although the X chromosomes reveal morphologic differences, including the location of the centromere. So far 67 coding genes and 32 DNA segments (microsatellites) have been assigned to river buffalo chromosomes (Figure 1, Table 1). Thirty-nine loci were assigned directly to chromosomes using in situ hybridization, whereas 67 loci were assigned to syntenic groups and to chromosomes using somatic cell hybrids (7 loci were assigned by both methods). These assignments were made in a panel of 37 hybrids resulting from a fusion of the Chinese hamster cell line wg3h with river buffalo lymphocytes (El Nahas et al. 1996b). The investigated markers cover 29 cattle autosomes and the X and Y chromosomes. A marker or a syntenic group was assigned to each arm of the five buffalo submetacentrics plus the 19 acrocentric autosomes and the X and Y chromosomes. Comparative synteny mapping has confirmed the hypothesis that the five submetacentric chromosome pairs of the river buffalo arose by centric fusion of 10 cattle acrocentric pairs, and has also contributed to identifying the nature of the biarmed buffalo chromosome (de Hondt et al. 2000; El Nahas et al. 1997, 1999; Othman and El Nahas 1999). Up to now, no syntenic discrepancies have been found between cattle and buffalo. It should be pointed out here that in 1996, the Texas nomenclature reversed the position of cattle chromosomes 4 and 6 in the Reading Conference (1980) and ISCNDA 89 (1990) nomenclature, taking chromosome 6 as 4 and vice versa. As a result, U13, previously assigned to BTA 4 (Neibergs et al. 1993) and consequently to BBU 7 (El Nahas et al. 1996a) is now assigned to BTA 6 and BBU 8.
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A linkage map of microsatellites or other highly polymorphic markers has not yet been established for river buffalo. It appears, however, that cattle microsatellites are conserved in the buffalo genome and map to the chromosomes predicted from similarity of banding patterns. If these conserved microsatellites are polymorphic in river buffalo, genome scans for buffalo QTL can be performed with microsatellites derived from cattle and organized on cattle linkage maps. Studies to determine the relative polymorphic information content (PIC) of cattle microsatellites in buffalo must be done to facilitate the approach. It is also likely that recombination rates between colinear markers will vary in cattle and buffalo gametogenesis. The potential for variation in recombination may be enhanced around the centromeres of the biarmed buffalo chrosomes.
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Footnotes |
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Corresponding Editor: Ann Bowling
Received January 20, 2000
Accepted January 15, 2001
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