xI“CE of the week

Device Sees More inside Live Cells The delicate touch of visible light al-

lows scientists to peer into living cells without disrupting them. However, to dis- cern the subtlest details, optical micro- scopes have long been considered too crude a tool. Instead, researchers have developed techniques such as electron microscopy to make out the finest fea- tures, but only in dead specimens.

Enter the stimulated emission deple- tion microscope. This novel optical de- vice harmlessly resolves fluorescently la- beled bits of living cells that are smaller than the so-called diffraction limit, say its developers at the Max Planck Institute for Biophysical Chemistry in Gottingen, Ger- many. Because light bends, or diffracts, around the edges of objects, ordinary o p

While the immediate focus is biology, “this beautiful story is not going to end here,” he adds, predicting possible appli- cations in lithography, electro-optics, magnetmptics, and other areas.

Like other so-called confocal micro- scopes, the novel instrument works by focusing a laser inside cells tagged with nontoxic fluorescent molecules. By scan- ning the light across horizontal levels in the cell and then assembling the slices in a computer, researchers can generate a three-dimensional fluorescent image of the cell’s interior.

Typically, an oblong laser spot created by a confocal microscope has a diffrac- tion-limited size of about 600 nm in depth and 200 nm in girth, says team leader Ste- fan w. Hell. What enables the new micro- scope to both beat the diffraction limit and reduce distortion from its oblong

spot, he explains, is a one-two combina- tion of laser pulses.

The device generates a 0.2-picosecond pulse of green light, which creates the typical oblong illumination. Immediately afterward, it fires an oddly shaped and much longer-lasting red pulse into the area excited by the green pulse. The red squelches fluorescence from the green light anywhere the pulses overlap.

Because the second pulse is doughnut- shaped, with blobs of light above and be- low its doughnut hole, the red light con- fines fluorescence to a roughly spherical spot 100 nm across. Comparing the green oblong to an American football, Hell says that the final truncated spot resembles a tennis ball. Thanks to that smaller spot size, he adds, the microscope can scan with a finer point and discern unprece- dentedly small features. -P Weiss

Elongated region (leh) where laser would excite fluorescence shrinks and becomes more spherical (right) when canceling pulse switches on. Blue is lowest intensity, red is highest.

tical microscopes can discern features no closer than a half-wavelength apart, a dis- tance of 200 nanometers (nm) or so.

Beating the diffraction limit, the new prototype instrument can resolve depth to one-sixth of what the best convention- al optical microscopes can achieve. Its horizontal resolution comes in under one-half the other instruments’ limit, re- port Thomas A. Klar and his colleagues in the July 18 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES.

Considering all three dimensions, the device can distinguish structures only oneeighteenth the minimum volume dis- cernable by diffraction-limited equip- ment, the researchers say.

“The work by [the Gottingen team] has the potential to transform the fluores- cence microscopy ‘Renaissance’ we are currently experiencing into an ‘Enlighten- ment Millennium,”’ says Shimon Weiss of Lawrence Berkeley (Calif.) National Labe ratory in a commentary in the same jour- nal issue.

Flowers, not flirting, make sexes differ Let’s not get so obsessed with attract-

ing the opposite sex, cautions a hum- mingbird research team.

Sex appeal may seem like all that makes the world go round, especially to anyone reading recent scientific studies about why males look different from fe- males, remarks Ethan J. Temeles of Amherst (Mass.) College.

Evolutionary pressure to charm and fight for a mate, or sexual selection, is usually easier to test for than natural se- lection, or the bottom-line pressure for survival, says Temeles. Yet he and his colleagues have found a Caribbean is- land where they say they can distinguish between the two.

Among purple-throated Caribs, Eu- lampis jugularis, the largest hummingbird on St. Lucia, it’s food and not flirtation that’s driven males and females to devel- op bills in his-and-hers models, the re- searchers argue.

“This is the first really unambiguous example of ecology playing a role in the morphological differences between the sexes,” Temeles says.

To be fair, he points out, biologists never claimed sexual selection explained all gender differences. Darwin himself proposed that specialized diets led to bill differences in a New Zealand bird, the huia. Males’ stubby bills allowed them to drill into trees for insects, while the fe- males’ long, curved bills pried insects out of crevices. Verifying Darwin’s theory has been difficult since the bird went ex- tinct more than a century ago.

Evidence has been thin to verify any claim of ecological causes for sex differ-

ences. Some studies have suggested that food preferences may foster gender-relat- ed size differences in snakes, weasels, and predatory birds. Based on anatomi- cal studies, scientists have argued that in certain mosquitoes, male mouth parts look ideal for sipping nectar while the fe- males’ counterparts look better for suck- ing blood.

On St. Lucia, “you couldn’t design a bet- ter system” for studying food contribu- tions to sex differences, Temeles crows. During breeding season, male humming- birds sip nectar almost exclusively from Heliconia caribaea, whereas females es- chew that flower for Heliconia bihai, the researchers report in the July 21 SCIENCE.

Measuring each gender’s bills revealed that their curvature and length best fit the preferred flower. Indeed, females fed in just three-quarters the time when sip ping from their preferred bloom instead of the males’, Temeles reports.

Hummingbird expert Larry L. Wolf of Syracuse (N.Y.) University considers Temeles’ explanation for the sex differ- ences “quite reasonable.” Elsewhere, he notes, hummers have developed species- specific bills to fit their bloomin’ diet. Caribs are just extending that bill special- ization to gender, he observes.

Their preferred flowers are also excit- ing, says Heliconia specialist W. John Kress of the Smithsonian Institution’s Na- tional Museum of Natural History in Washington, D.C. He marvels that Carib females prefer nectar from green flowers when many hummingbird-pollinated flow- ers blaze red or orange. In fact, the Heli- conia genus-related to the bird-of-para-

52 SCIENCE NEWS, VOL. 158 JULY 22,2000