The genetic improvement of the water buffalo ( Bubalus bubalis ) : the contribution of the cytogenetics

Although water buffalo is only 1/ 9 of the cattle population, it interests a larger human population, especially in southeastern countries. The DNA-sequence of the entire genome has been performed for several domestic species but it is sti ll lacking in river buffalo. However, high resolution Q-, G-, and R-banded standard karyotypes are available and cytogenetic maps at the low resolution density have been constructed for river buffalo. I n addition, cytogenetic analyses on females with reproductive problems have established that about 20% of these animals are steri le for serious damages to internal sex adducts due to the presence of sex chromosome abnormalities. I n this study I summarize the contribution of the cytogenetics in the genetic improvement of the water buffaloes concerning the following aspects: (a) chromosome characterization by high resolution chromosome banding; (b) chromosome evolution within Bovidae fami ly by using both comparative chromosome banding and gene order (FI SH-mapping technique) approaches; (c) clinical cytogenetics applied to reproduction; (d) molecular cytogenetics ( the most advanced cytogenetic maps) . Chromosome characterization Two main types of buffaloes are present in the world: the Asiatic water buffalo (Bubalus bubalis) and the African buffalo (Syncerus caffer) . The water buffalo has two subspecies or types: the river and the swamp buffalo which differ in their chromosome number (2n=50 in river type and 2n=48 in swamp type). Crosses between these two species are possible and also the hybrids (2n=49) are ferti le. The reason of this is due to a conservation of chromosome arms (and gene pool) between the two species. I ndeed, the large swamp buffalo chromosome 1 originated by tandem fusion translocation between river buffalo chromosomes 5p and 9 (Di Berardino and I annuzzi, 1981). During this chromosome rearrangement the nucleolus organizer regions (NOR) present in river buffalo 5p were lost, as well as large portions of the constitutive heterochromatin present in the centromeric regions of river buffalo chromosome 9 (Di Berardino and I annuzzi, 1981). I n particular, DNA SAT I was conserved and DNA SAT I I was lost (Tanaka et al., 1999; I annuzzi et al., 2009). Actually, a crossbreeding program has been performing in Phi l ippines to improve swamp buffalo mi lk production by crossing river x swamp buffalo (the milk production increased three times in F1 and four time in F2 crossings, depending by the food avai labi li ty) . Having the two species a di fferent chromosome number, the cyogenetic control could be another useful tool to check the various crosses. The African buffalos also have two subspecies: the Syncerus caffer caffer and the S. caffer nanus which differ in their diploid number being 2n=52 in S. caffer caffer and 2n=54 in Syncerus caffer nanus. Also these two subspecies are ferti le since they conserved the same chromosome arms (and genes). The advent of chromosome banding techniques al lowed to not iceably improve our knowledge on animal chromosomes permitt in g to construct banded karyotypes with high resolution banding patterns (450-500 band for haploid set) and, more importantly, standard banded karyotypes which have been the point of reference for clinical ( identification of chromosome involved in chromosome abnormalities) , evolutionary (relationships among related and unrelated species) and molecular (construct ion of detai led cytogenetic maps) studies. Figure 1 shows a bull RBG-banded karyotype arranged accordi ng to this standard at the 450 band level.