Birds, Bees, and STDs

Natural History, Feb, 1999 by Yvonne Baskin

Even flowers have sexually transmitted infections.

At the Mountain Lake Biological Station in Virginia's Allegheny Mountains, the roadsides are dotted with white campion (Silene alba), a weedy invader from Europe. Botanist Janis Antonovics had spotted the gangly plant on visits to the station and recognized it from his childhood in England. He also knew why many of its white, three-quarter-inch flowers sported smutty black centers instead of the usual yellow ones. These plants were infected by Ustilago violacea, a fungus, or smut, that also plagues white campion in Europe. This smut essentially commandeers the plants, forcing both male and female flowers to produce fungal spores instead of pollen or ovules.

Because infected plants are easy to spot, Antonovics, a professor at the University of Virginia in Charlottesville, thought they would make ideal subjects for probing the seldom-studied interactions between a pathogen and its natural host population. At his suggestion, Helen Alexander, now an associate professor at the University of Kansas in Lawrence, became the first of many of his colleagues and students to scrutinize these smutted flowers over the past decade. Her results soon convinced Antonovics that U. violacea plays by fundamentally different rules than do most infectious diseases. The reason: it's sexually transmitted.

Bees, moths, flies, and other visitors to white campion transfer pollen from male to female flowers. When they visit infected flowers, however, they carry away only fungal spores to spread to the next flower. Although all this may seem a world apart from human syphilis or gonorrhea, Antonovics and his colleagues have recently begun to see general patterns in the way all sexually transmitted diseases (STDs) operate, whether they infect plants, animals, or humans. They have also shown that STDs--a small subset of the infectious disease world and long considered minor curiosities--are present wherever you find sexual contact, whether in earwigs, frogs, koalas, or humans.

One of the most fundamental distinctions between STDs and other infectious diseases is that a healthy individual's chances of becoming infected depend more on the proportion of diseased to healthy individuals in the population than on their density. Antonovics explains this with a nightclub analogy: If 10 percent of the people in a nightclub have a cold, your chances of catching it are much greater if there are one thousand people in the club rather than one hundred, because you will be crowded in with one hundred sneezing sufferers instead of ten. Imagine instead that 10 percent of the patrons have a venereal disease. If you single out someone to have sex with, the chances are one in ten that the person will be infected, whether you choose from one hundred prospects or one thousand. Likewise, for a healthy campion plant, the probability that an arriving pollinator previously visited a diseased plant depends on the fraction of the population infected.

One consequence of this dynamic is that while other infectious diseases require some minimal density of hosts in order to spread, STDs can persist even in sparse populations, as long as there are enough hosts left to mate. Antonovics says this advantage may make up for "the cost of being an STD," which is the reduced host pool that a disease carrier can infect sexually. Put another way, this means that an individual generally chooses sex partners from only half the population (male or female), whereas he or she can potentially transmit an influenza virus to anyone.

Realizing that STDs had never been systematically tallied, Antonovics and his colleagues surveyed the literature for animal diseases transmitted to some degree through sexual contact (many diseases have multiple routes of transmission). They did not include plant STDs because of the complex array of infections that might qualify, such as pollen-transmitted viruses or rust fungi that force asexual hosts to produce "pseudoflowers" and attract pollinators that carry out the rust's sexual cycle. The team halted its search after finding two hundred STDs, because that seemed adequate to make their point that the diseases are "legion and widespread." Most of those on their list infect humans, livestock, or crop pests; few studies have looked at diseases in wildlife. The pathogens involved include viruses, bacteria, nematodes, spirochetes, prions, and at least one trypanosome (which causes dourine in horses).

Despite their diversity, animal STDs seem to share a number of traits that set them apart from other infectious diseases: They tend to be less severe, in that they kill fewer of their hosts, but they frequently cause sterilization; they live longer in their hosts and remain infectious longer; and each STD targets a narrower range of hosts. Although similar generalizations are not available for plant STDs, studies of smuts like Ustilago violacea have revealed many parallels with pathogens causing STDs in animals. The fungus overwinters in the rosette, or basal leaves, forcing the plant to produce smutted flowers each summer; yet it seldom shortens the plant's life span. Also, once the fungus systemically infects a plant, it sterilizes it.