The large genome constraint hypothesis: evolution, ecology and phenotype.

BACKGROUND AND AIMS: If large genomes are truly saturated with unnecessary 'junk' DNA, it would seem natural that there would be costs associated ith accumulation and replication of this excess DNA. Here we examine the available evidence to support this hypothesis, which we term the 'large genome constraint'. We examine the large genome constraint at three scales: evolution, ecology, and the plant phenotype. SCOPE: In evolution, we tested the hypothesis that plant lineages with large genomes are diversifying more slowly. We found that genera with large genomes are less likely to be highly specious -- suggesting a large genome constraint on speciation. In ecology, we found that species with large genomes are under-represented in extreme environments -- again suggesting a large genome constraint for the distribution and abundance of species. Ultimately, if these ecological and evolutionary constraints are real, the genome size effect must be expressed in the phenotype and confer selective disadvantages. Therefore, in phenotype, we review data on the physiological correlates of genome size, and present new analyses involving maximum photosynthetic rate and specific leaf area. Most notably, we found that species with large genomes have reduced maximum photosynthetic rates - again suggesting a large genome constraint on plant performance. Finally, we discuss whether these phenotypic correlations may help explain why species with large genomes are trimmed from the evolutionary tree and have restricted ecological distributions.
CONCLUSION: Our review tentatively supports the large genome constraint hypothesis.