Baldwin's Lab:

In the Mojave Desert, the ephemeral tarweed Layia glandulosa blooms in spring. It's a wildflower no taller than your knee, and its white-rayed bloom resembles a miniature sunflower.

Two thousand miles away on the island of Maui, another plant is also preparing to bloom. The Haleakala silversword--whose aptly named silvery, sword-shaped leaves are arranged in a rosette--sends a five-foot stalk into the air, adorned with hundreds of flower heads and reminiscent, in shape, of a narrow Christmas tree. To an untrained eye, the two plants look nothing alike. But in the mid 20th Century, some evolutionary biologists suggested that the Hawaiian silverswords, of which there are about 30 species, are direct descendants of the California tarweeds. Others said that was impossible: the Pacific Ocean was too great a barrier to species dispersal from North America.

And then came Bruce Baldwin, a graduate student at UC Davis in the late 1980s. Armed with the relatively new tools of chloroplast DNA restriction site mapping (prior to development of PCR techniques), he settled the debate, once and for all: the Hawaiian silverswords radiated from a colonization event by a tarweed from North America.  Now, several other Hawaiian plant groups are known to have originated from North American colonists, based in part on work in Baldwin's lab, and long-distance dispersal has been shown to be far more important--and frequent--than suspected in the 1980s.

Unraveling mysteries of the past in order to understand today's plant diversity is what drives Baldwin's research. "I'm intrigued by questions of biogeography and diversification," he says.  But when Baldwin finished graduate school, resolving questions about plant evolution meant relying largely on chloroplast DNA, which evolves differently than nuclear DNA, and can result in misleading interpretations. "You can end up being really misled about what is going on with the history of the nuclear genome," Baldwin explains.

So he tried to produce more reliable information about plants' evolutionary histories, using transcribed spacer regions of nuclear ribosomal genes. These sections of non-coding DNA repeat throughout the genome, can be amplified and sequenced without much difficulty, and provide surprisingly useful information about a plant's evolutionary past. Now, plant scientists around the world follow Baldwin's lead in using internal and external transcribed spacer (ITS and ETS) regions to estimate plant phylogenies.

"Many people have been able to easily generate phylogenetic trees from ITS and ETS sequences that are highly reliable, and that provide a lot of resolution about relationships among different species," he says.  Baldwin emphasizes that no single gene region can be trusted to provide a comprehensive view of plant relationships, but that the transcribed spacers of rRNA genes (ITS and ETS) have allowed for rapid progress toward phylogenetic understanding of hundreds of recently diversified plant lineages.

Though perhaps best known as a molecular systematist, Baldwin considers himself a botanist who cut his teeth doing plant surveys in the Mojave Desert. He became fascinated by the California flora as a kid growing up in San Luis Obispo County, and he's most at home in California. After a stint teaching at Duke, he now curates the University's Jepson Herbarium, a major center for the study of California's native plants, and is the convening editor for the second edition of The Jepson Manual--the most comprehensive guide to native and naturalized plants of California.

Land managers use The Jepson Manual and a growing body of Jepson online resources to assess the impacts of new development projects in fast-growing California, as do scientists trying to understand the impacts of climate change on plant distribution. But they're operating with outdated information now, which is why Baldwin feels a responsibility to get the new edition out as soon as possible: in the 15 years since the first edition of the Manual was published, scientists have described over 100 new plant species and subspecies from the state--in part because, following in Baldwin's footsteps, many biosystematists have used ITS sequences to refine understanding of  plant phylogeny at fine-scale levels of diversity.

"It's important to get this information out to scientists and society so they recognize that that diversity exists," he says, "and doesn't get overlooked."

Describing species accurately is not just a matter of good recordkeeping--it also has a direct impact on conservation. A dwindling population of an "overlooked" group that has been mistakenly lumped in with another species may go extinct without anyone realizing it ever existed. But once a scientist describes that overlooked group as a separate species or subspecies, it is tracked by plant conservationists and is eligible for legal protection under the Endangered Species Act.

In addition to curating the herbarium, Baldwin continues researching, teaching, and advising graduate students. Currently, he's collaborating with colleagues at other universities to understand the evolution of mating systems in the genus Collinsia, also known as the Blue-eyed Mary group--part of the snapdragon family, a recent interest for Baldwin, whose specialty is the sunflower family. Like many plants, the Blue-eyed Marys can reproduce sexually in two ways: they can receive pollen from another plant, via an insect pollinator, or they can self-pollinate. The latter, says Baldwin, is a good strategy in the short term if pollinators are scarce or unreliable, but results in decreased genetic variation--when a plant self-pollinates, there is no chance for new genes to recombine and the diversity of genes declines with repeated selfing until there is little variation left for natural selection to act upon.

"It may be a kind of Faustian bargain for the plants," he explains. "They're not as reliant on pollinators anymore, and in an unreliable climate or unpredictable conditions it's a safe bet to reproduce--but the raw material for evolution is really reduced."

To test the long-held idea that "selfing" is an evolutionary dead-end, Baldwin and his collaborators are studying reproduction in large- and small-flowered Blue-eyed Mary species and reconstructing their evolutionary pasts. Smaller flowers generally rely on selfing to a greater degree and are thought to be less likely to successfully receive pollen from another plant, an assumption that is being tested for Blue-eyed Marys with genetic estimates of outcrossing rates. If the dead-end hypothesis holds, these small-flowered, putatively selfing plants should be among the youngest species of Blue-eyed Marys and should not have given rise to much new diversity. But according to Baldwin's phylogenetic trees, that result is not consistently found: in some instances, large-flowered species appear to have descended from small-flowered species. Figuring out how that's happened, and why, is Baldwin's and his collaborators' next step.

Meanwhile, he has not forgotten the tarweeds and their Hawaiian descendants that were the focus of his early career and continue to be subjects of detailed study in his lab. Lately, biologists and plant lovers have noticed that some species of the silversword alliance on Kauai are not reproducing. Baldwin and a colleague are trying to figure out why. He spent several months at the National Tropical Botanical Garden in Kauai, working to find out whether the recruitment problem has to do with pollen limitation. Most silverswords cannot self--which facilitates maintenance of genetic diversity in island populations, but "now, with population sizes being reduced dramatically by all of the herbivore pressure and competition with non-native plants, there may be a real downside to obligate outcrossing," Baldwin says.  "Some populations are now so small that species may lack enough genetic diversity to be able to successfully mate with neighboring plants, and loss of insect pollinators is an even more immediate threat for some species." 

Baldwin teaches plant systematics, and says he is gratified by the evolution of students' ability to recognize plant families and appreciate plant diversity. "We tend to see the world as a green blur," he says. "Little kids can identify major animal groups, but even adults have trouble identifying major plant groups. The nice thing about learning plant diversity is that the world's not such a blur anymore. You become a lot more aware of the rich diversity of life around you."


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