Rates of Mammalian Karyotype Evolution by the Karyograph Method

Abstract

Using the karyograph method devised by Imai and Crozier (1980), we attempted to estimate the rate of mammalian karyotype evolution. Denoting the chromosome number and arm number of the mean haploid karyotype (K̄i) in a given mammalian taxon by n̄i and ĀN̄i, respectively, the karyotype can be represented at a point Ki(ĀN̄i, n̄i) on a two-dimensional lattice (karyograph), where the arm number is plotted against the chromosome number. If we assume that Ko(ANo, no) is the karyotype of primordial mammals, it is possible to define three types of karyological distance between the Ko and Ki. Namely, the distance defined by the difference in the arm numbers (D̄AN = ĀN̄i - ANo), the distance by the chromosome numbers (D̄n = ni - no), and the karyonumber distance (D̄KN= D̄AN + D̄n). Imai and Crozier (1980) 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 (Ā) to metacentrics (M̄) (p.i.(ĀM̄)). The distances D̄n and D̄AN are, therefore, the total numbers of centric fissions and of p.i.(ĀM̄) accumulated in the K̄i since the karyotype diverged from the Ko. We assumed that mammalian karyotype evolution began from the karyotype Ko = 3M̄, 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. Now, the mean rate of arm number change (V̄AN), chromosome number change (V̄), and karyonumber change (V̄KN) is given as V̄AN = D̄AN/t, V̄n = D̄n/t, and V̄KN = V̄AN + V̄n. We estimated the rates of karyotype evolution using 1,070 mammalian species, and obtained V̄n = 0.19, V̄AN = 0.27, and V̄KN = 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 one in every 5.3, 3.7, and 2.2 million yr.