Natural Acts: A Sidelong View of Science and Nature - David Quammen (1996)
ELOQUENT PRACTICES, NATURAL ACTS
Is Sex Necessary?
BIRDS DO IT, BEES DO IT, or so goes the tune. But the songsters, as usual, would mislead us with drastic oversimplifications. The full biological truth happens to be more eccentrically nonlibidinous. Sometimes they don’t do it, those very creatures, and get the reproductive results anyway. Bees of all species, for instance, are notable for their ability to produce offspring while doing without. Birds mostly do mate, yes, but at least one variety—the Beltsville Small White Turkey, a domestic breed out of Beltsville, Maryland—has achieved scientific renown for a similar feat. What we’re talking about here is celibate motherhood, procreation without copulation, a phenomenon that goes by the name parthenogenesis. Translated from the Greek roots: virgin birth.
And you don’t have to be Catholic to believe in this one.
Miraculous as it may seem, parthenogenesis is actually rather common throughout nature, practiced regularly or intermittently by at least some species within almost every group of animals except (for reasons still unknown) dragonflies and mammals. Reproduction by virgin females has been discovered among fishes, amphibians, birds, reptiles,* crustaceans, mollusks, ticks, the jellyfish clan, flatworms, roundworms, segmented worms; and among insects (notwithstanding those unrelentingly sexy dragonflies) it is especially favored. The order Hymenoptera, including all bees and wasps, is uniformly parthenogenetic in the manner by which males are produced: Every male honeybee is born without any genetic contribution from a father. Among the beetles, there are thirty-five different forms of parthenogenetic weevil. The African weaver ant employs parthenogenesis, as do twenty-three species of fruit fly and at least one kind of roach. Gall midges of the species Miastor metraloas are notorious for the exceptionally bizarre and grisly scenario that allows their fatherless young to see daylight: M. metraloas daughters cannibalize the mother from inside, with ruthless impatience, until her hollowed skin splits open like the door of an overcrowded nursery. But the foremost practitioners of virgin birth—their elaborate and versatile proficiency unmatched in the animal kingdom—are undoubtedly the aphids.
Now no sensible reader, not even one who has chosen this book, can be expected to care much, I realize, about aphid biology qua aphid biology. But there’s a larger reason for dragging you into the subject. The life cycle of these nebbishy insects, the very same that infest rosebushes and houseplants, exemplifies not only how parthenogenesis works but also, very clearly, why evolution has devised such a reproductive shortcut.
First the basics. A typical aphid, which feeds entirely on plant juices tapped from the vascular system of young leaves, spends winter as an egg, dormant and protected. The egg is attached near a bud site on the new growth of, say, a poplar tree. In March, when the tree sap has begun to rise and the buds have begun to burgeon, the egg opens and an aphid hatchling appears, promptly plugging its sharp snout into the tree’s tender plumbing. This solitary individual aphid will be, necessarily, a wingless female. If she is lucky, she will become sole founder of a vast aphid population. Having sucked enough poplar sap to reach maturity, she produces (by live birth now, not egg-laying, and without benefit of a mate) daughters identical to herself. These wingless daughters also plug into the tree’s flow of sap, and they also produce wingless daughters—whose daughters produce more daughters, geometrically more, generation following generation until sometime in late spring, when crowding becomes an issue and that particular branch of that particular tree can support no more thirsty aphids. Suddenly there is a change: The next generation of daughters are born with wings. They fly off in search of a better situation.
One such aviatrix lands on a herbaceous plant—a young climbing bean, say, in someone’s garden—and the pattern repeats. She plugs into the sap ducts on the underside of a new leaf, commences feasting, robbing the plant of its vital juices, and then delivers by parthenogenesis a great brood of wingless daughters. The daughters beget more daughters, those daughters beget still more, and so on, until the poor bean plant is encrusted with a dense population of these fat little sisters. Then again, neatly triggered by the crowded conditions, a generation of daughters are born with wings. Away they fly, looking for prospects, and one of them lights on, say, a sugar beet. (The switch from bean to beet is possible for our species of typical aphid, because it is not a dietary specialist committed to only one plant.) The sugar beet before long is covered, sucked upon mercilessly, victimized by a horde of mothers and nieces and granddaughters. Still not a single male aphid has appeared anywhere in the lineage.
The lurching from one plant to another continues; the alternation between wingless and winged daughters continues. Then, in September, with fresh and tender plant growth increasingly hard to find, there comes another change.
Flying daughters are born who have a different destiny: They wing back to the poplar tree, where they give birth to a crop of wingless females unlike any so far. These latest girls know the meaning of sex! Meanwhile, at long last, the starving survivors back on that final bedraggled sugar beet have brought forth a generation of males. The males too have wings. They take to the air in search of poplar trees and first love. Et voilà. The mated females lay eggs that will wait out the winter near bud sites on that poplar tree, and the circle is closed. One single aphid hatchling—call her the matriarch—in this way can give rise in the course of a year, from her own ovaries exclusively, to roughly a zillion aphids.
Good for her, you say. But what’s the point of it?
The point, for aphids as for most other parthenogenetic animals, is 1) exceptionally fast reproduction that allows 2) maximal exploitation of temporary resource abundance and unstable environmental conditions, while 3) facilitating the successful colonization of unfamiliar habitats. In other words, the aphid, like the gall midge and the weaver ant and the rest of their fellow parthenogens, is by its evolved character a hasty opportunist.
This is a term of science, not of abuse. Population ecologists make an illuminating distinction between what they label “equilibrium species” and “opportunistic species.” According to William Birky and John Gilbert, from a paper in the journal American Zoologist: “Equilibrium species, exemplified by many vertebrates, maintain relatively constant population sizes, in part by being adapted to reproduce, at least slowly, in most of the environmental conditions which they meet. Opportunistic species, on the other hand, show extreme population fluctuations; they are adapted to reproduce only in a relatively narrow range of conditions, but make up for this by reproducing extremely rapidly in favorable circumstances. At least in some cases, opportunistic organisms can also be categorized as colonizing organisms.” Birky and Gilbert emphasize that the potential for such rapid reproduction is “the essential evolutionary ticket for entry into the opportunistic life style.”
And parthenogenesis, in turn, is the greatest time-saving trick in the history of animal reproduction. No hours or days are wasted while a female looks for a mate; no minutes lost to the act of mating itself. The female aphid attains sexual maturity and, bang, she becomes automatically pregnant. No waiting, no courtship, no fooling around. She delivers her brood of daughters, they grow to puberty and, zap, another generation immediately. The time saved by a parthenogenetic species may seem trivial, but it is not. It adds up dizzyingly: In the same duration required for a sexually reproducing insect to complete three generations for a total of 1,200 off-spring, an aphid can progress through six generations (assuming the same maturation rate and the same number of progeny per litter) to yield an extended family of 318,000,000.
Even this isn’t speedy enough for some restless opportunists. That matricidal gall midge Miastor metraloas, whose larvae feed on fleeting eruptions of fungus under the bark of trees, has developed a startling way to cut further time from the cycle of procreation. Far from waiting for a mate, M. metraloas does not even wait for maturity. When food is abundant, it is the larva, not the adult female fly, who is eaten alive from inside by her own daughters. And as those voracious daughters burst free of the husk that was their mother, each of them already contains further larval daughters taking shape ominously within its own ovaries. While the food lasts, while opportunity endures, no Miastor metraloas female can live to adulthood without dying of motherhood.
The implicit principle behind all this nonsexual reproduction, all this hurry, is simple: Don’t pause to fix what isn’t broken. Don’t tinker with a genetic blueprint that works. Unmated female aphids, and gall midges, pass on their own genotypes virtually unaltered (except for the occasional mutation) to their daughters. Sexual reproduction, on the other hand, exists to allow genetic change. The whole purpose of joining sperm with egg is to shuffle the genes of both parents and come up with a new combination that might perhaps be more advantageous. Give the kid some potent new mix of possibilities, based on a fortuitous selection from what Mom and Pop individually had. Parthenogenetic species, during their hurried phases at least, dispense with this genetic shuffle. They stick stubbornly to the genotype that seems to be working. They produce (with certain complicated exceptions) natural clones of themselves.
But what they gain thereby in reproductive rate, in great explosions of population, they lose in flexibility. They minimize their genetic variability—that is, their options. They lessen their chances of adapting to unforeseen changes of circumstance.
Which is why more than one biologist has drawn the same conclusion as M.J.D. White: “Parthenogenetic forms seem to be frequently successful in the particular ecological niche which they occupy, but sooner or later the inherent disadvantages of their genetic systems must be expected to lead to a lack of adaptability, followed by eventual extinction, or perhaps in some cases by a return to sexuality.”
So it is necessary, even for aphids, this thing called sex. At least intermittently. A hedge against change and oblivion. As you and I knew it must be. Otherwise, surely by now we mammals and dragonflies would have come up with something more dignified.