Rates of Mammalian Karyotype Evolution by the Karyograph Method

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Abstract
Using the karyograph method devised by Imai and Crozier, the rate of mammalian karotype evolution was estimated. Denoting the chromosome number and arm number of the mean haploid karyotype (.hivin.Ki) in a given mammalian taxon by .hivin.ni and .hivin.A.hivin.Ni, respectively, the karyotype can be represented at a point Ki(.hivin.A.hivin.Ni, .hivin.ni) on a 2-dimensional lattice (karyograph), where the arm number is plotted against the chromosome number. Assuming that Ko(ANo,no) is the karyotype of primordial mammals, it is possible to define 3 types of karyological distance between the Ko and Ki. Namely, the distance defined by the difference in the arm numbers (.hivin.DAN = .hivin.A.hivin.Ni, .sbd. ANo), the distance by the chromosome numbers (.hivin.Dn = ni .sbd. no), and the karyonumber distance (.hivin.DKN = .hivin.DAN + .hivin.Dn). Imai and Crozier found that mammalian karyotypes tend to evolve, in a statistical sense, orthodirectionally toward increasing chromosome number and/or arm number mainly by centric fission and by pericentric inversion changing acrocentrics (.hivin.A) to metacentrics (.hivin.M) (p.i. (.hivin.A.hivin.M)). The distances .hivin.Dn and DAN are, therefore, the total numbers of centric fissions and of p.i. (.hivin.A.hivin.M) accumulated in the .hivin.Ki since the karyotype diverged from the Ko. Mammalian karyotype evolution presumably began from the karyotype Ko = 3.hivin.M, which is the lowest-numbered karyotype among extant mammals, about 100 million yr ago; i.e., the common ancestor has Ko(ANo = 6, no = 3) and the divergence time (t) is 100 million yr. The mean rate of arm number change (.hivin.VAN), chromosome number change (.hivin.V), and karyonumber change (.hivin.VKN) is given as .hivin.VAN = .hivin.DAN/t, .hivin.Vn = .hivin.Dn/t, and .hivin.VKN = .hivin.VAN + .hivin.Vn. The rates of karyotype evolution were estimated using 1070 mammalian species, and obtained .hivin.Vn = 0.19, .hivin.VAN = 0.27, and .hivin.VKN = 0.46. In other words, the chromosome number (n), arm number (AN), and karyonumber (KN = n + AN) in mammals change, respectively, at a rate of about 1 in every 5.3, 3.7, and 2.2 million yr.

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