Abstract
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.
Highlights
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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
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
Summary
The Harvard community has made this article openly available. Please share how this access benefits you. Even if their data indicating that chicken introns are smaller than their human homologues were strong, this two-species comparison alone cannot be used to suggest that such decreases occurred directly or indirectly because of flight This is because many nonflying ancestors with diverse physiologies, behaviors, and ecologies that could modulate genome size separate humans from chickens along the phylogenetic tree for vertebrates (Garland and Adolph 1994). Corroboration of the hypothesis of a link between decreases in intron size and increases in metabolic rate or flight requires the coincident evolution of these traits in the common ancestor of birds or within birds, patterns that are not yet established Other studies, such as Vinogradov (1997), make use of comparative methods but analyze small numbers of taxa in very restricted avian clades given available information (Bennett and Harvey 1987; Tiersch and Wachtel 1991). Vinogradov’s latest study (1999) is large but, with respect to birds, is another two-species comparison and does not employ comparative methods
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