To better understand the organization of the genome of bats, we examined by in situ hybridization, the number of ribosomal DNA sites in 50 species of bats representing both suborders, 7 families, and 38 genera. Number of sites ranged from one to four pairs (average, 1.76) in bats, whereas the number of sites in 40 species of rodents ranged from two to ten pairs (average, 4.19). The possible relationship of a reduced number of sites to a smaller amount of DNA in the genome of bats is explored. We find little evidence to support the hypothesis that bats are retaining a fixed primitive condition of a low number of sites and we conclude that the most probable explanation is that bats, like other groups of mammals, have mechanisms that tend to increase the number of sites. However, the balance between mechanisms that increase and those that reduce the number of sites is more strongly in favor of reduction of sites than is characteristic of other mammals such as rodents.

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