Biological Journal zyxwvuts of the Linnean Society, 9: 191-216. zyxwvu With 5 plates June 1977 zyxwvutsrqp The chromosomes of the Didelphidae (Marsupialia) and their evolutionary signiiicance OSVALDO A. REIG, F.L.S. zyxwvu Departamento de Estudios Ambientales, Divisibn de Ciencias Biolbgicas, Universidad Sirn6n Bolivar, Apartado 80,659, Caracas 108, Venezuela ALFRED L. GARDNER National Fish and Wildlife Laboratory, Fish and Wildlife Service, National Museum o f Natural History, Washington, D.C., U.S.A. 20560 NESTOR 0. BIANCHI Instituto Multidisciplinario de Biologia Celular (IMBICE), Calle 526 entre I0 y 1 I, La Pluta, Argentina AND JAMES L. PATTON Museum of Vertebrate Zoology, University o f California, Berkeley, California, U.S.A. 94720 Accepted for publication September I976 One hundred and seventy-seven specimens of American didelphids, representing 9 genera and 22 species have been studied for their chromosomal constitution. Didelphids are very conservative in chromosomal complements. All of the studied species can be sorted into one of three kinds of karyotypes: 2n zyxwvuts = zyxwvut 14 (three species of Didelphis, one of Lutreolina, two of Philander, and one of Chironectes), 2n = 14 (eight species of Marmosa. one of Metachims, three of Caluromys, and one of Dromiciops), and 2n = 18 (three species of Monodelphis). These karyotypes are stable, showing only minor variations within each basic pattern. It is concluded that chromosornals evolution in the Didelphidae proceededs from low numbers to higher numbers by a process of centromeric fissioning complemented by some pericentric inversions and/or translocations. The pattern of karyotypic stability is consistent with bradytely at the organismic level of evolution. This zyxwvu is explained by a low rate of regulatory genetic evolution promoted by epistatic selection favouring the retention of chromosomal arrangements highly advantageous for overall adaptation. 191