BRAINERD, E.L.*; MURRAY, S.S.: Patterns of genome size evolution in tetraodontiform fishes

We used flow cytometry to measure genome size (pg of DNA per diploid cell) in 15 species belonging to 7 tetraodontiform families and subfamilies. Previous workers have found that members of Tetraodontidae (smooth puffers) have the smallest genome size of any vertebrate measured to date (0.8 pg vs. a mean of 2.0 pg for all teleosts). We found that this tiny genome is present in members of both tetraodontid subfamilies indicating that the primitive condition for Tetraodontidae was a genome size of 0.8�0.9 pg. In contrast, Diodontidae (spiny puffers, sister group to Tetraodontidae), possess a genome which is about two times larger (1.60 pg). Mola mola, a member of the sister group to Diodontidae+Tetraodontidae, also does not have a particularly small genome (1.70 pg). The most parsimonious explanation for this pattern is that the primitive condition for Molidae+(Diodontidae+Tetraodontidae) is a genome size of 1.60�1.70 pg, indicating that the tiny genome size of smooth puffers is a derived character unique to tetraodontids. Previous workers have proposed that the loss of anatomical parts in puffers (no ribs, no pelvis, few vertebrae) might be related to the loss of genetic material in these fishes. Our results do not support this hypothesis because molids and diodontids are missing just as many bits and pieces as smooth puffers, yet they do not have particularly small genomes. We propose that the smooth puffers are a good model system in which to study the evolutionary mechanisms of genome size reduction because: (1) they have an exceptionally small genome; (2) their closest outgroups have much larger genomes; and (3) a smoothpuffer, Fugu rubripes, is being used as a model organism for gene identification and molecular tools emerging from this work will be useful in studies of genome size evolution in these fishes.