The success of flowering plants, a group that includes everything from orchids and tulips to grasses and wheat, represents a long-standing puzzle for biologists. In an 1879 letter to the renowned botanist Joseph Dalton Hooker, Charles Darwin called it an “abominable mystery.” Terrestrial plants first appeared nearly half a billion years ago, but flowering plants arose only in the past 100 million years, beginning in the Cretaceous period. Yet, once angiosperms emerged, their structural and functional diversity exploded — far outpacing the diversification and spread of the other major plant groups, the gymnosperms (including conifers) and ferns.Perhaps the answer has been so elusive because scientists have usually focused on the physiological traits that set the angiosperms apart from their relatives. Kevin Simonin, a plant biologist at San Francisco State University, and Adam Roddy, at Yale University, argue that it’s the genome sizes underlying those individual adaptations that really matter.The emergence of angiosperms was marked by many events in which lineages of plants duplicated their whole genome. This process opened a door for greater diversification because the extra copies of genes could evolve and take on new functions. But because carrying so much genetic material can also be physiologically taxing, natural selection typically followed up these duplication events by aggressively pruning unneeded sequences. This “genome downsizing” often left flowering plants with less DNA than their parent species had.Less DNA made it possible for the flowering plants to build their leaves from smaller cells, which in turn allowed them to pack more of certain cell types into the same volume. They could therefore have a higher density of stomata — the pores that facilitate the intake of carbon dioxide from the air and the release of water vapor — and a higher density of veins to provide enough water to keep those pores open. And the flowering plants did not have to sacrifice a high density of photosynthetic cells to achieve these benefits.As a result, flowering plants could turn sunlight into sugars through photosynthesis much more efficiently. The rise of their superior capacity for hydration and gas exchange also coincided with falling levels of atmospheric carbon dioxide during the Cretaceous period, which contributed further to the angiosperms’ competitive edge over their green-plant peers.Although flowering-plant genomes skew small, downsizing did not make that compulsory; rather, it expanded the range of cell and genome sizes available to the plants. Angiosperms exhibit an enormous, nearly 2,400-fold difference between their smallest and largest genomes. By comparison, ferns have a 196-fold difference and gymnosperms a 16-fold one. The flowering plants’ greater range “means they can better fine-tune their physiology to the environment, and potentially live in more diverse habitats,” Roddy said.It suggests that the ability of flowering plants to have more dynamic genomes puts angiosperms in a place where they have the evolutionary flexibility that other lineages seem to be constrained by. Simonin, Roddy, and colleagues aim to understand how genome size influences physiological and evolutionary success, including which DNA parts are lost during genome downsizing, why angiosperms tend to minimize their genomes unlike other plants, and the specific advantages linked to genome size versus other factors.Question 5Simonin and Roddy believe genome size is key to the success of flowering plants because:Genome size is directly related to the efficiency of photosynthesis and range of habitats.Downsizing of genomes has constrained the evolutionary flexibility of angiosperms.Smaller genomes allow for more efficient photosynthesis and a wider range of habitats.Genome downsizing leads to a decrease in the density of stomata and photosynthetic cells.