The 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 (
ligator 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 underdis-
persed 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 evo-
lution of BMR and avian intron size is consistent with Brownian mo-
tion. 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.

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Last updated on 09/22/2016