Evolutionary dynamics of intron size, genome size, and physiological correlates in archosaurs.

It has been proposed that intron and genome sizes in birds are reduced in comparison with mammals because of the metabolic demands of flight. To test this hypothesis, we examined the sizes of 14 introns in a nonflying relative of birds, the American alligator (Alligator mississippiensis), and in 19 flighted and flightless birds in 12 taxonomic orders. Our results indicate that a substantial fraction (66%) of the reduction in intron size as well as in genome size had already occurred in nonflying archosaurs. Using phylogenetically independent contrasts, we found that the proposed inverse correlation of genome size and basal metabolic rate (BMR) is significant among amniotes and archosaurs, whereas intron and genome size variation within birds showed no significant correlation with BMR. We show statistically that the distribution of genome sizes in birds and mammals is underdispersed compared with the Brownian motion model and consistent with strong stabilizing selection; that genome size differences between vertebrate clades are overdispersed and punctuational; and that evolution of BMR and avian intron size is consistent with Brownian motion. These results suggest that the contrast between genome size/BMR and intron size/BMR correlations may be a consequence of different intensities of selection for these traits and that we should not expect changes in intron size to be significantly associated with metabolically costly behaviors such as flight.