to trial their reintroduction into the UK. Genetic history A 2003 study of mitochondrial DNA indicated four distinct maternal lineages in the tribe Bovini: Taurine cattle and zebu Wisent American bison and yak Banteng, gaur, and gayal Y chromosome analysis associated wisent and American bison. An earlier study, using amplified fragment-length polymorphism fingerprinting, showed a close association of wisent and American bison and probably with yak. It noted the interbreeding of Bovini species made determining relationships problematic. European bison can crossbreed with American bison. This hybrid is known in Poland as a żubrobizon. The products of a German interbreeding programme were destroyed after the Second World War. This programme was related to the impulse which created the Heck cattle. The cross-bred individuals created at other zoos were eliminated from breed books by the 1950s. A Russian back-breeding programme resulted in a wild herd of hybrid animals, which presently lives in the Caucasian Biosphere Reserve (550 animals in 1999). Wisent-cattle hybrids also occur, similarly to the North American beefalo. Cattle and European bison hybridise fairly readily. First-generation males are infertile. In 1847, a herd of wisent-cattle hybrids named żubroń () was created by Leopold Walicki. The animals were intended to become durable and cheap alternatives to cattle. The experiment was continued by researchers from the Polish Academy of Sciences until the late 1980s. Although the program resulted in a quite successful animal that was both hardy and could be bred in marginal grazing lands, it was eventually discontinued. Currently, the only surviving żubroń herd consists of just a few animals in Białowieża Forest, Poland and Belarus. In 2016, the first whole genome sequencing data from two European bison bulls from the Białowieża Forest revealed that the bison and bovine species diverged from about 1.7 to 0.85 Mya, through a speciation process involving limited gene flow. These data further support the occurrence of more recent secondary contacts, posterior to the divergence between Bos primigenius primigenius and B. p. namadicus (ca. 150,000 years ago), between the wisent and (European) taurine cattle lineages. An independent study of mitochondrial DNA and autosomal markers confirmed these secondary contacts (with an estimate of up to 10% of bovine ancestry in the modern wisent genome) leading the authors to go further in their conclusions by proposing the wisent to be a hybrid between steppe bison and aurochs with a hybridisation event originating before 120,000 years ago. However, other