The study compares the genetic structure of two vastly different flowering plants to see whether differences exist in the set of circuits that create each species’ flower. Researchers examined the genetic circuitry of Arabidopsis thaliana, a small flowering plant commonly used as a model organism in plant genetics research, and the avocado tree Persea americana, which belongs to an older lineage of so-called basal angiosperms.
“What we found is that the flower of Persea is a genetic fossil, still carrying genetic instructions that would have allowed for the transformation of cones into flowers,” Chanderbali said.
Advanced angiosperms have four organ types: female organs (carpels), male organs (stamens), petals (typically colorful) and sepals (typically green). Basal angiosperms have three: carpels, stamens and tepals, which are typically petal-like structures. The researchers expected each type of organ found in Persea’s flowers would have a unique set of genetic instructions. Instead they found the genetic instructions showed significant overlap among the three organ types.
“Although the organs are developing to ultimately become different things, from a genetic developmental perspective, they share much more than you would expect,” Chanderbali said. “As you go back in time, the borders fade to a blur.”
“With these facts established, we can now think about the vast space open to natural selection to establish ever more rigid borders,” said Virginia Walbot, a biology professor at Stanford University who is familiar with the research. The selection process arrived at a “narrow solution in terms of four discrete organs but with fantastic diversity of organ numbers, shapes and colors that provide the defining phenotypes of each flowering plant species.”
Researchers don’t know exactly which gymnosperms gave rise to flowering plants, but previous research suggests some genetic program in the gymnosperms was modified to make the first flower, Soltis said. A pine tree produces pinecones that are either male or female, unlike flowers, which contain both male and female parts. But a male pinecone has almost everything that a flower has in terms of its genetic wiring.
Douglas Soltis, chair of the University of Florida botany department, emphasized the study highlights the importance of studying primitive flowering plants such as the avocado to gain insight into the early history of the flower. Survivors of ancient lineages represent a crucial link to the first flowers that cannot be obtained by studying highly derived models such as Arabidopsis.
The study, “Transcriptional signatures of ancient floral developmental genetics in avocado (Persea americana; Lauraceae),” is available free of charge at PNAS.