Meet the Bee With a Body That’s Half Male, Half Female
So-called gynandromorphs are rare, but they can teach us a lot about development and evolution
The face of a sweat bee (Megalopta amoena) that is half female (viewer’s left, bee’s right) and half male (viewer’s right, bee’s left) (Chelsey Ritner / Utah State University)
n the spring of 2018, Erin Krichilsky stumbled upon the most baffling bee she’d ever seen.
While the right side of its face sported a stout, rugged jawline trimmed with teeny teeth—characteristics normally found on a female—the left half of the insect’s mug had the delicate, wispy features of a male. A quick skim of the rest of the bee’s body revealed much of the same: a she on the right, a he on the left. It was as if someone had cleaved a male bee and a female bee in two and stitched half of each together.
Peering into the microscope at the 4-millimeter-long insect, Krichilsky—then a research assistant at the Smithsonian Tropical Research Institute (STRI) in Panama—realized she’d found something extraordinary. “It was this super cool individual that wasn’t anything like what I was used to seeing,” she recalls. “It was a very exciting day.”
This mysterious insect, described recently in the Journal of Hymenoptera Research, was an exceedingly rare gynandromorph—an animal that is anatomically half male, half female—the first one ever identified in the species Megalopta amoena, a type of sweat bee found in Central and South America.
Unlike hermaphrodites, which often outwardly appear male or female but have the reproductive organs of both, gynandromorphs boast entire bodies that are sexual mosaics. Because of their rarity in nature, these sex-split individuals are poorly understood. Still, researchers have documented gynandromorphs in creatures ranging from butterflies and crustaceans to reptiles and birds—all with literally mixed results.
One other Megalopta gynandromorph has shown its face to scientists before: a male-female hybrid in a closely related sweat bee species called Megalopta genalis, identified in 1999. In the two decades since, STRI researchers have collected and analyzed tens of thousands of other bees without uncovering a second example—making the new Megalopta amoena specimen’s recent and serendipitous appearance a welcome encore act, says Krichilsky.
The team didn’t analyze the bee’s genes to confirm its gynandromorph status. But the insect’s asymmetrical anatomy was kind of a dead giveaway, says study author Adam Smith, a biologist at George Washington University.
Generally speaking, bees, wasps and ants—which belong to the group Hymenoptera—live in matriarchal societies in which females “do all the things that make bees interesting,” Smith says. “They collect pollen, build nests, take care of the kids.” As such, evolution has equipped these ladies with traits compatible with their endless list of chores: strong jaws capable of digging into wood; thick, hairy hindlegs that can snare and transport pollen; and a sharp-tipped stinger for defense.
Males, however, “do nothing useful except mate,” Smith says, and have the feeble physique to match.
Though the researchers aren’t sure how exactly this bizarre bee came to be, studies in similar insects might provide some hints. Several years ago, another team of scientists led by University of Sydney bee expert Benjamin Oldroyd analyzed the genes of several honeybee gynandromorphs and found that the male-female hybrids were likely the result of a developmental mishap.
In humans, biological sex is determined by two sex chromosomes—one from mom and one from dad. Inheriting two X’s yields a female, while an X paired with a Y creates a male. But bees do things a little differently. All fertilized eggs, which carry genetic material from a mother and a father, hatch female bees. Unfertilized eggs, however, can still yield offspring: fatherless males that carry only one set of chromosomes from their mothers—half of what’s found in females. Sex, in other words, is determined by the quantity of genetic information in a bee’s cells.
A sweat bee (Megalopta amoena) that is half female (right side of the body) and half male (left side of the body). Females of this species have bigger jaws, thicker and hairier legs and stingers. (Chelsey Ritner / Utah State University)
On very, very rare occasions, a second sperm can sneak its way into an already-fertilized egg—a would-be female—and start copying itself, Oldroyd explains. This creates two asymmetrical lineages that each populate their own half of the growing embryo: One arising from the union of the egg and the first sperm that develops as female, and another, born out solely from the second, freewheeling sperm. Because this second sperm never partners up with its own egg, the chromosome count in its lineage stays low, creating only male cells.
These double fertilization events seem to explain at least some honeybee gynandromorphs, though male-female hybrids in other species can manifest in other ways. Another explanation might involve a cell in a typical female embryo making a mistake while copying itself, generating one female cell and one male cell instead of two female cells. Those new cells would then go on dividing independently, yielding two sexually divergent lines.
Either or neither of these scenarios may have played out in the new Megalopta bee, which has since been immortalized in the STRI’s collections. Without dissecting the specimen and analyzing its genome, researchers can’t tell.
Before the Megalopta bee died, though, Krichilsky and her colleagues decided to perform a different sort of test: tracking its daily sleep cycle. When they plopped the insect in an activity monitor, they found it woke up a little earlier to forage for food than typical males and females of its species.
With only one specimen to study, the team can’t draw firm conclusions about this behavioral quirk. “Maybe it’s weird because it’s a gynandromorph,” Smith says. “Or it’s just weird because it’s weird.”
Still, the team’s findings are notable simply because they include any behavioral data at all, says Sydney Cameron, an entomologist at the University of Illinois at Urbana-Champaign who wasn’t involved in the study. Most gynandromorphs are discovered only after they’ve died and been stowed in museum collections, making it impossible to know how they navigated their surroundings and social relationships in life. Though this specimen can’t speak for all gynandromorphs, Cameron says, its actions are interesting to document.
Ehab Abouheif, a biologist at McGill University who wasn’t involved in the study, stresses the importance of not dismissing gynandromorphs as “freaks of nature.” Species can only survive and persist if their populations are diverse. In this light, unusual individuals aren’t errors to be written off—they’re fodder for adaptation.
Many, if not most, gynandromorphs are likely infertile, and probably aren’t founding new species themselves. But developmental changes that blur the anatomical lines between sexes can still drive evolution in other contexts, Smith says. In some parasitic bees, for instance, females have lost many of the usual traits that feminize other species, and can appear almost male.
This sort of sexual fluidity “probably happens more often than we’re aware of” in nature, Krichilsky says. “There are some niches occupied by a more typically female or male. Maybe [some individuals] can occupy something in between, or both—or become a whole new organism.”
Unusual though they are, gynandromorphs “are still bees, just like other bees,” she says. “And we can learn a lot from them.”
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