Written by David Templeton
A strawberry plant reveals evolution in the making
Like some queen of green, the bold little Fragaria virginiana thrives inside a campus greenhouse. Five white petals guard her yellow pistils, all surrounded by a sentinel of jagged-edged leaves. To the untrained eye, this strawberry plant does not stand out as anything special. But she is a floral diva, worthy of the throne-size black flowerpot in which she sits in the biology department in Langley Hall.
More than a decade ago, Pitt biologist Tia-Lynn Ashman created this sprouting domain, where two long lines of strawberry plants in small black pots stand at attention like soldierly green minions before the queen plant. Early in her Pitt career, Ashman began searching for clues to a puzzle that has intrigued biologists for more than a century. What happened in the evolutionary process to create separate male and female biology?
Surprisingly, the Virginian wild strawberry plant, F. virginiana, is providing some intriguing insights into the mysteries of gender evolution.
Ashman, who studied behavioral ecology and evolution as an undergraduate at the University of California, San Diego, earned a doctoral degree in ecology from UC Davis. She became fascinated by questions like: Why are some flowers fragrant dazzlers, but others plain and without perfume? Why do some blossoms thrive for months, while others survive only a few hours?
Another mystery intrigued her, too. Some plants have coexisting, functioning genes for both male and female development. Could these combined-gender plants, known as hermaphrodites, reveal keys to the evolution of separate male and female development?
Not long after arriving at Pitt in 1994, Ashman began searching for a plant species in which the evolution into separate sexes was not yet complete. She found a contender at the University’s Pymatuning Laboratory of Ecology in Crawford County, where she discovered wild strawberry plants growing alongside railroad tracks. The plants were F. virginiana, and they had an interesting ratio of far more hermaphrodites and females than males. Soon, she and her research team, with help from several hundred Pitt undergraduates, were growing the plants on campus at Langley Hall and examining their propagation.
Ashman discovered that this species of strawberry sits at an opportune time in evolutionary history—smack in the middle of transitioning from hermaphrodites into the separate male and female sexes. Hermaphroditic plants, with both male and female genetics, are self-breeding.
Significantly, Ashman’s research has revealed that F. virginiana sometimes sprouts “neuter” plants, which are unable to propagate sexually, as well as male plants that never produce fruit. It turns out that F. virginiana hermaphrodites have two interesting gene positions on a single sex chromosome: One gene location controls male sterility and fertility while the other controls female sterility and fertility. Through the process of genetic propagation, offspring might inherit both fertility types and become hermaphrodites, or inherit one fertility and one sterility type to become either male or female, or inherit both sterility types to become completely sterile and unable to reproduce. The fully sterile plants die out.
Ashman’s work shows that, through the confluence of genetics and evolution, the frequency of single-sex offspring increases: Male and female plants become more abundant, and hermaphrodites recede. Further, her findings elucidate how evolution—Darwin’s “survival of the fittest” principle—favors separate-sex divergence. Two separate plants, male and female, result in better plant efficiency, says Ashman, because a single plant doesn’t need to develop both reproductive organs (pistils and anthers). Also, the existence of separate male and female plants improves healthy genetic diversity. Separate-sex characteristics allow more plants to take better advantage of different soils and environments, favoring more abundant reproduction. Meanwhile, a self-breeding hermaphroditic plant is more likely to produce offspring expressing detrimental mutations, leading to reproductive inefficiencies.
By studying the progeny of key plants, Ashman is gaining a better understanding of genetic characteristics and how separate sexes evolve from hermaphroditism. She also is comparing F. virginiana’s development to other plants farther along the evolutionary cycle and gleaning new insights into sex chromosome development.
Soon her 200 plants will multiply to 600 inside Langley Hall, where each is providing new evidence to advance her studies. She’s using computer analysis to track genetic makeup and to create a detailed genetic map of the offspring produced by her throne room full of F. virginiana.
A few years ago, Ashman published related research in the prestigious journal Science, noting that animals and flowering plants use similar strategies to increase successful propagation. Yet, many questions remain. What else affects or favors the biology of male and female divergence? Harsh climates, as Darwin suggested in his theory of evolution? What about the effects of pathogens or the destruction of certain pollinators? Or the ratio of the sexes in a given location? What role might a habitat change play? What causes genetic mutations that affect sexual morphing?
Ashman’s greenhouse kingdom is sprouting fresh evidence in the ongoing quest to understand gender divergence and diversity.