This author synthesized data from several different sources including his own previous work to examine possible correlations between genome size, developmental rate, and metabolism predicted by nucleotypic theory. The nucleotype effect is defined here as the variations in genome size that are the result of selective pressures for morphological and functional characteristics of cell and organism that favour adaptations to given environments or lifestyles.
Reptiles are phenotypically intermediate in many ways between anamniotes (amphibians) and homeothermic amniotes (birds and mammals). The range of genome sizes of reptiles is similar to the range in mammals, reptile DNA-methylation patterns are similar to birds and mammals, and reptile AT-content patterns (isochores) are similar to those of amphibians. Many reptiles are capable of generating significant endogenous heat, and can maintain body temperatures above ambient for “fairly long periods”; in other words, there is not a strict division between pure endothermy and pure poikilothermy, as the middle ground is occupied by various reptiles. Given the paraphyletic status of the class Reptilia, these intermediates are not surprising.
The paucity of data for all three parameters for each species led to an analysis at higher taxonomic scales, including family, suborder, and order. Several correlations emerged at these higher scales: metabolic rate was not significantly correlated with genome size at the species level, but was significant at the order level. However, this correlation, as illustrated in figure 3, appears to be largely driven by one order, the Tuatara or Sphenodon. Similarly, the correlation between metabolic rate and genome size for 9 families of lizards (for which consistent data were available from a single study of metabolism) appears to be driven by the single family with the largest genome size; all other data points appear in figure 5 to be approximately on 5 pg genome size, with considerable variation in metabolic rate among those 5 pg families.
The discussion section includes a long consideration of the models of genome size evolution of Hartl and Petrov (various references these authors, between about 2000 and 2002). Under their model of variations in “indel spectra” with mass of noncoding DNA (nc-DNA), a runaway process of increasing genome size may occur, and may have occurred in chelonians (turtles); this may explain the generally larger genomes of chelonians compared to squamates. A role for recombination frequency and the transition areas between R (gene rich isochores) and G (gene poor isochores) bands is also described; chelonians appear to have more suitable sites for TE insertion than do either squamates or homeotherms.
This author reiterates the point that the nucleotypic effect provides thresholds, between which phenotypes such as genome size may vary independently of other phenotypes. Thus, some differences between taxa may be driven more by other, unexamined factors than by genome size or (e.g.) metabolic rate per se; large-scale comparisons are most useful for revealing the role of the nucleotypic effect in genomic evolution.
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