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Comparative Study
. 2009 Feb 23;5(1):47-50.
doi: 10.1098/rsbl.2008.0491.

Palaeogenomics of Pterosaurs and the Evolution of Small Genome Size in Flying Vertebrates

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Free PMC article
Comparative Study

Palaeogenomics of Pterosaurs and the Evolution of Small Genome Size in Flying Vertebrates

Chris L Organ et al. Biol Lett. .
Free PMC article

Abstract

The two living groups of flying vertebrates, birds and bats, both have constricted genome sizes compared with their close relatives. But nothing is known about the genomic characteristics of pterosaurs, which took to the air over 70 Myr before birds and were the first group of vertebrates to evolve powered flight. Here, we estimate genome size for four species of pterosaurs and seven species of basal archosauromorphs using a Bayesian comparative approach. Our results suggest that small genomes commonly associated with flight in bats and birds also evolved in pterosaurs, and that the rate of genome-size evolution is proportional to genome size within amniotes, with the fastest rates occurring in lineages with the largest genomes. We examine the role that drift may have played in the evolution of genome size within tetrapods by testing for correlated evolution between genome size and body size, but find no support for this hypothesis. By contrast, we find evidence suggesting that a combination of adaptation and phylogenetic inertia best explains the correlated evolution of flight and genome-size contraction. These results suggest that small genome/cell size evolved prior to or concurrently with flight in pterosaurs. We predict that, similar to the pattern seen in theropod dinosaurs, genome-size contraction preceded flight in pterosaurs and bats.

Figures

Figure 1
Figure 1
Genome size regression models. (a) The phylogenetic generalized least-squares (PGLS) regression line relating genome size to bone cell size in 38 extant tetrapod species is drawn from the average of a Bayesian posterior distribution of regression models and takes the form (ln genome size)=−0.81+0.35(ln cell size), r2=0.43 (p<0.0001, H0: β=0). The axes denote the distributions of the x and y data, with labels marking the minimum, 25 per cent quartile, median, 75 per cent quartile, and the maximum. (b) The Bayesian posterior distribution of β, the regression coefficient relating genome size to cell size (mean=0.35, σ=0.07). (c) The maximum-likelihood PGLS line relating genome size to body mass in 87 extant tetrapods takes the form (ln genome size)=1.64+0.00003(ln body size), r2=0.0 (p=0.94, H0: β=0). (d) The regression line relating estimated node values to the amount of change in immediate descendants (standardized contrasts) in 130 tetrapods. It takes the form ln(standardized contrasts)=−4.93+1.53 ln(estimated node values in genome size), r2=0.32 (p<0.001, H0: β=0). Coloured markers in (a,c) denote amphibians (blue), mammals (brown), non-avian reptiles (green), and birds (red).
Figure 2
Figure 2
Genome-size evolution in birds, bats and pterosaurs. (a) Phylogeny and bar graph of average genome size in picograms including the inferred genome size of non-avian dinosaurs, basal archosaurs and pterosaurs (green bars), as well as birds (red bars) and bats (brown bars). Estimates of genome size for extinct dinosaurs are re-estimates of Organ et al. (2007). (b) Posterior distributions of ancestral genome size in picograms for (i) pterosaurs (median=2.08, σ=0.40), (ii) birds (Aves; median=1.72, σ=0.38) and (iii) bats (Chiroptera; median=2.85, σ=0.54). The axes in both panels are labelled with the minimum, 25 per cent quartile, median, 75 per cent quartile, and the maximum of the distributions.

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