A new analysis of the sunflower family tree reveals that flower symmetry has evolved multiple times independently among the members of this large plant family through a process called convergent evolution. This research, led by a Penn State biologist, provides insight into the evolution of the sunflower family, which includes asters, daisies, and food crops like lettuce and artichokes. The findings, published in the journal Plant Communication, may help identify useful traits for selectively breeding plants with more desirable characteristics.
The sunflower head is composed of multiple smaller flowers that can have different forms of symmetry, such as radial symmetry or bilateral symmetry. According to the study, bilateral symmetry has evolved and been lost multiple times independently in sunflowers over evolutionary history. This convergent evolution is likely related to changes in the number of copies and expression patterns of the floral regulatory gene, CYC2. Understanding the evolution of flower symmetry in sunflowers is crucial for determining how closely related different species are.
Researchers have built family trees for related species using transcriptomes, which are genetic sequences of all expressed genes in a species. Transcriptomes are more accessible than high-quality whole-genome sequences but require fresh plant samples and can be costly to prepare. In this study, the team used low-coverage genome sequences, produced through genome skimming, to increase the number of species in their analysis. This allowed them to resolve more branches on the sunflower family tree and reconstruct the evolution of traits like flower symmetry.
The research team analyzed 706 species from 16 subfamilies, 41 tribes, and 144 subtribe-level groups in the sunflower family using a combination of transcriptomes and skimmed genomes. By increasing the sample size, the team was able to resolve more branches in the sunflower family tree, providing a higher resolution view of the evolutionary relationships within the family. They were able to reconstruct the evolution of traits like flower symmetry and determine that bilateral symmetry evolved independently multiple times.
The molecular evolution of genes involved in flower development among sunflowers was also studied, with a focus on the CYC2 gene. This gene, found in multiple copies in each species’ genome, was activated in species with bilaterally symmetric flowers, indicating its role in the convergent evolution of this trait. Further experiments quantified the expression of CYC2 in species with different symmetries, revealing a clear relationship between gene expression and flower symmetry. Understanding the molecular basis of flower symmetry evolution in sunflowers can help identify useful traits for breeding purposes.
The sunflower family is one of the largest families of flowering plants, containing over 28,000 species, including many economically important agricultural and horticultural species. By understanding the evolutionary relationships within this family, researchers can determine how and when various traits evolved. This knowledge could be used to selectively breed domesticated species with desirable traits from closely related wild species. The research team included scientists from Penn State, China, and the University of Texas, Austin, with funding from various institutions supporting the study.
