Resolving Systematic Relationships with G-Bands: A Study of Five Genera of South American Cricetine Rodents

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Abstract
G- and C-banded chromosomes of Holochilus brasiliensis, Nectomys squamipes, Neacomys guianae, Sigmodon hispidus and 11 spp. of Oryzomys [O. capito, Oryzomys sp., O. palustris, O. caliginosus, O. couesi, O. caudatus, O. concolor, O. delicatus, O. bicolor, O. melanotis and O. alfaroi] are compared and cladistically analyzed to document direction, type and magnitude of chromosomal evolution in these species. Direction of chromosomal evolution was determined from the proposed primitive G-band sequences for cricetid rodents. Numbers and types of chromosomal rearrangements identified in the largest 12 chromosomes of Oryzomys were 13 Robertsonian translocations (9 of which are polymorphic), 21 other translocations, 8 pericentric inversions, 1 paracentric inversion and 12 rearrangements whose nature could not be identified from banding sequences. In 11 spp. of Oryzomys there are .apprx. 2.5 times as many euchromatic rearrangements as found in 18 spp. of Peromyscus. The early radiation of the peromyscine-neotomine-oryzomyine lineages was accompanied by little chromosomal evolution. Chromosomal evolution in cricetine rodents does not appear to fit the predictions of the canalization model. None of the currently accepted broadly-based models of chromosomal evolution appears adequate to explain the pattern of variation observed in the cricetines (especially the variation within and between Oryzomys and Peromyscus). Nondifferentially stained chromosomes generally will not be a useful predictor of phylogenetic relationships. For some clades, G-banded sequences provide synapomorphies for the development of systematic hypotheses; several branching events cannot be resolved with the quality of G-bands available. Some of the phylogenetic hypotheses are as follows: of the 5 genera of South American cricetids examined, Oryzomys and Holochilus appear to be the most closely related (based on morphology, Holochilus was thought to be more closely related to Sigmodon); that Neacomys, Holochilus and species of Oryzomys examined shared a common ancestry after diverging from the Nectomys lineage; and chromosomal characters identified failed to document that Sigmodon is more closely related to the other South American genera (Oryzomys, Holochilus, Neacomys and Nectomys) than it is to the North American cricetids (e.g., Neotoma and Peromyscus). One example of lack of congruence documents convergence in chromosomal rearrangements.