Summer flings: firefly courtship, sex, and death

Natural History, July-August, 2003 by Sara Adler, James E. Lloyd

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Females, meanwhile, remain perched low in the vegetation; in Photinus species the females rarely fly. Females respond to male advertisements with a single pulse (or in a few species, multiple pulses). After a male sees a female response, he drops out of the air to continue his search "on foot." The flash dialogues continue, often lasting more than an hour, and the ongoing conversation acts as a magnet for other males. By the time courtship flight ends, several males can often be found scrambling up and down blades of grass, searching for the stationary female.

Biologists have long pondered how fireflies generate their precisely timed flashes. Work done recently by an interdisciplinary team of cell biologists, physiologists, and ecologists from Tufts University and Harvard Medical School has provided a key piece to the puzzle.

In fireflies light is produced in a chemical reaction, which can occur only in the presence of oxygen, between the compound luciferin and the enzyme luciferase. In the firefly lantern, thousands of specialized cells called photocytes sequester luciferin and luciferase deep within their interiors. Densely packed around the photocyte margins are mitochondria, the oxygen-consuming power plants that occur in nearly all eukaryotic cells. The team found that the firefly nervous system does not control the photocytes directly; instead, the flash-triggering nerve impulse arrives in the lantern at nonluminescent cells adjacent to the photocytes.

The nerve signals trigger the production of nitric oxide (NO). That discovery offered significant insight into firefly flash control. The NO molecule is a ubiquitous intercellular messenger that has an astonishing array of biological functions. In people, it controls blood pressure, regulates penile erection, and mediates learning and memory. In the firefly lantern, NO switches the flash on by temporarily shutting down the oxygen consumption of the photocyte's mitochondria. Oxygen can then diffuse farther into the interior of the photocyte, where it triggers the light-producing reaction between luciferin and luciferase. The flash turns off as NO quickly degrades and mitochondrial oxygen consumption is restored.

The highly visible courtship signals among fireflies, and their short adult lifespans, make fireflies particularly amenable to studies of sexual selection. Darwin coined the term "sexual selection" to refer to differences among a species' males and how successful they are at gaining access to females. According to Darwin, the reproductive advantage goes to males that can prevail over rivals or that can more effectively attract females. Later biologists have realized that mere copulation is not sufficient to ensure a male's reproductive success; in many animals females mate with multiple males, and both male sperm competition and female sperm choice can create differences among mating males in the number of offspring they sire. In fireflies, a male's reproductive success depends not only on his courtship ability, but also on his postcopulatory ability to fertilize his mate's eggs.