By Jennifer R. Mandel
listen to Dr. Mandel discuss her recent paper
Making up one in every ten flowering plants on Earth, members of the daisy, or sunflower, family have fascinated scientists for hundreds of years. Many species of plants that you might not expect belong to this family: Gerber Daisies and the iconic sunflower of course, but lettuce, artichoke, wormwood (think absinthe), and dandelions. Sunflower relatives aren't all herbaceous either--there are trees, vines, and shrubs. The oldest fossil of a sunflower-like flower head is roughly 40 million years (MY) old and was discovered in Southern South America in Patagonia. From 40 MY onward, the fossil record for sunflowers picks up with evidence showing up around the globe. In large part due to these findings, the sunflower family was thought to have originated in the Eocene (56 to 33.9 MY) by most scientists making it a (relatively) young family in evolution's time frame.
However, with the advent of molecular dating techniques and the rapid advances made in sequencing technologies and genomics, the data began to hint that the family could be older--possibly even twice as old. Two small studies in the family described a sunflower scenario with a much older origin that was prior to the mass extinction event that corresponded to the extinction of dinosaurs about 66 MY. Adding to this, a fossil pollen grain ascribed to the family was discovered in Antarctica and dated to 72 MY in the very late Cretaceous. The Antarctica of today was not the Antarctica of the Cretaceous, the climate was cool temperate, and no ice existed. At this time, Southern South America, Antarctica, and Australia were much closer to one another and possibly even connected. This land connection describes how many of the plant and animal distributions that are now separated may have originated including the closest family relatives of sunflowers.
The Mandel Lab along with collaborators at the Smithsonian Institute and Oklahoma State University have been investigating the origins of the family in both space and time making use of the new sequencing technologies and fossil evidence. We sequenced nearly 1000 genes in 250 species and used molecular dating, biogeographical analyses, and diversification tests to ask where, how, and when did the sunflower family originate. Our data show that the family probably originated in Southern South America though Antarctica cannot be ruled out (my money would be on the latter). And we further confirm a pre-mass extinction origin for daisies when dinosaurs still walked the Earth! We also showed that the family tremendously increased in species number during the Eocene connecting our work with the extensive fossil record from that time. Finally, we link this explosion of diversity with a global change in climate that likely transformed many of the habitats to those that were most suitable for sunflowers and their relatives to thrive. This study is only the beginning of our investigations into understanding how sunflowers have become the largest and most diverse family of flowering plants on Earth. We aim to use this new information to continue to understand how climate, habitats, and genomic diversity influence plant biodiversity and distributions.
The full story published June 2019 in PNAS can be read here.