DNA DNA HYBRIDIZATION STUDIES OF CARNIVOROUS MARSUPIALS .5. A FURTHERESTIMATE OF RELATIONSHIPS AMONG OPOSSUMS (MARSUPIALIA, DIDELPHIDAE)/

We present DNA-hybridization comparisons among representatives of all genera of didelphine opossums plus two species of outgroup caluromyine s (Caluromys philander and Glironia venusta). Data generated with six tracers used in an earlier study were combined with those from ten new ly labeled species; a complete table amenable to FITCH analysis was pr oduced by reflection of single missing values from their reciprocals a nd estimation of pairs of reciprocal comparisons by an ultrametric pro cedure, both after symmetrization. Additional comparisons were made am ong species and subspecies of Thylamys, using four more labels. The re sults replicate and extend our earlier conclusions by showing that Mar mosops and Gracilinanus are a sister-pair, jointly the nearest relativ es of Thylamys and Lestodelphys. However, Marmosops may be paraphyleti c, as M. dorothea is almost indistinguishable from Gracilinanus agilis , while M. parvidens is several degrees distant from both. This clade of four small-bodied, 2n = 14 opossum genera (Gracilinanus, Marmosops, Thylamys, Lestodelphys) appears to be closer to the group of large-bo died didelphines (the 2n = 22 genera Didelphis, Philander, Lutreolina, and Chironectes, with the karyotypically plesiomorphic Metachirus the ir sister-taxon), than either is to a third group comprising Monodelph is, Marmosa s.s., and Micoureus, with the latter two paired. Our phylo geny shows that the 2n = 18 karyotype of Monodelphis is neither a redu ction from 2n = 22 nor an intermediate step in the derivation of 2n = 22 from a plesiomorphic 2n = 14 ancestor, because all but one known sp ecies among the small-bodied forms (a Marmosa) have the 2n = 14 karyot ype, and Monodelphis is more closely related to some of these than to the large-bodied, 2n = 22 genera. The topology is supported by bootstr apping, jackknifing, and varying input-orders of the taxa to FITCH. Th e implied classification of Didelphidae is shown to differ markedly fr om another recent taxonomic study of opossums.