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Chromosomal relationships among the native rodents (Cricetidae: Oryzomyini) of the Galápagos Islands, Ecuador

Although the Galápagos Islands are recognized for their contribution to our understanding of evolutionary theory and have received the attention of scientists for over 185 years, our understanding of the native rodents there has been minimal relative to many other groups of organisms.  Much of what we knew through most of the 20th century was based solely on species descriptions.  Chromosome data has been limited to only Nesoryzomys narboroughi (2n = 32, FN (number of autosomal arms) = 50) and Aegialomys galapagoensis (2n = 56; FN = 58).  We present the karyotypes of the only remaining extant species in the genus, N. swarthi (2n = 56; FN = 54) and N. fernandinae (2n = 44; FN = 54).  Chromosomal banding reveals that extensive rearrangement has occurred within Nesoryzomys, including Robertsonian fusion and tandem fusion events but these alone cannot account for the diverse diploid numbers found within the genus.  We propose that 1) N. swarthi represents the ancestral karyotype for the genus, similar to A. galapagoensis, 2) N. swarthi and N. fernandinae share the same fundamental number, suggesting divergence by Robertsonian fusions, and 3) N. narboroughi has the most derived karyotype, based on banding morphology and low diploid number.