The assumption of simple fusion in a group showing a constant number of chromosome arms was tested by comparison of the G-band patterns of chromosomes of three Phyllotis species. The karyotypes, each of which has 40 chromosome arms, have a 2n of 38, 38, and 40 and are made up of mostly metacentric chromosomes. Operational concepts describing the amount of matching in G-band patterns are proposed, separating chromosomes or arms into those with total correspondence, partial correspondence, or unique cases. Seven chromosomes and 21 arms out of the total were identical in the three species, denoting a conservation of whole-arm band sequences in this group. A greater number of identical arms than of identical chromosomes was observed, giving some support to the simple fusion hypothesis. An unexpected chromosomal divergence was detected, including chromosomal variation in the C-banded sex chromosomes.
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