Female Forever
"An Evolutionary Scandal"
Birds do it. Bees do it. Bdelloid rotifers, it seems, eschew
it. Not love, that is, but sex, which these multi-celled freshwater
invertebrates have not had in perhaps 80 million years, say Cabot
professor of the natural sciences Matthew Meselson and post-doctoral
fellow David Mark Welch, Ph.D. '99, who report the evidence in Science
magazine.
The discovery is more than just a sexual oddity--biologists call
it an evolutionary scandal. Creatures that can reproduce without
sex have arisen sporadically throughout history, but the fossil
record and other evidence show that within several thousand years
they become extinct. "It's clear that creatures originate every
once in a while that can reproduce without fertilization," says
Meselson. "The female's eggs just hatch. This has happened in many
insects, and in some fishes, snakes, and lizards." But once males
disappear, even though the species may do better than its sexual
progenitors for a while, it eventually becomes extinct. Mark Welch's
and Meselson's new evidence for bdelloid (pronounced like "deltoid"
without the "t") rotifers' multimillion-year abstinence raises anew
the questions, Why is sex necessary for most life, and why not for
the bdelloids?
Meselson and Mark Welch studied four genes in four species of
bdelloids. (There are about 360 bdelloid species, all reproducing
asexually, and all thought to be descended from a common female
ancestor.) Sexual organisms carry two copies of any given gene,
each supplied by one parent. But in asexual organisms, there is
only one parent, which supplies its offspring with both of
its own copies. In each subsequent generation, the two copies of
the gene are reproduced. Over time, tiny mistakes in the copying
process can independently accumulate in each gene (if the mutations
are not fatal). After many millions of years, the difference betwen
the two copies will have become far greater than it is in sexually-reproducing
species. This is exactly what Mark Welch and Meselson found. Using
standard rates of mutation in other animals, they estimate that
the common female ancestor of all bdelloid rotifers lived 50 million
to 100 million years ago.
That's a long time to go without sex, which biologists define
not as copulation, but as DNA exchange between organisms. As early
as 1887, August Weismann recognized the cost of sex, and hypothesized
that sex must therefore confer important advantages on its practitioners.
"If we bear in mind that in sexual propagation twice as many individuals
are required to produce any number of descendants, and if we further
remember the important morphological differentiations which must
take place in order to render sexual propagation possible," Weissman
wrote in the journal Nature, "we are led to the conviction
that sexual propagation must confer immense benefits upon organic
life. I believe that...sexual propagation may be regarded as a source
of individual variability, furnishing material for the operation
of natural selection." To this day, Weissman's argument for why
sex is costly, and his hypothesis about what benefits sex confers,
remain popular.
 |
| Cabot professor of the natural sciences Matthew Meselson
and David Mark Welch, Ph.D. '99, have found evidence that bdelloid
rotifers, a class of multicelled animals, have survived for
50 million to 100 million years without sexual reproduction.
Biologists call it an evolutionary scandal. |
|
Photograph by Stu Rosner
|
An alternative hypothesis is that of the "Red Queen." In Lewis
Carroll's Through the Looking- Glass,
the Red Queen tells Alice, "Now here, you see, it takes
all the running you can do, to keep in the same place." To biologists,
this idea of running in place aptly describes a parasite-host relationship.
The parasite adapts to attack its host--and the host adapts to resist
being eaten by the parasite--in an ongoing, inherently tense dynamic
of constant keeping-up, a striving to avoid the loss of fitness,
in both organisms. Sex increases the rate of genetic variation,
which is, from the host's perspective, an advantage in fending off
parasites--and an advantage to parasites in remaining able to attack
the host.
A different type of hypothesis to account for the predominance
of sex, one that Meselson's laboratory is currently exploring, holds
that sex somehow purges the genome of deleterious mutations. But
many biologists who have pondered the problem, says Meselson, would
agree that we do not yet know why sex exists.
"What good is sex doing for 99.9 percent of living creatures?"
Meselson asks. "Or, put differently, what is it that goes wrong
with most asexual species, causing their early extinction? It's
very strange that there is this one huge exception--the bdelloid
rotifers--to what otherwise appears to be a universal rule. The
answers to such fundamental questions," he says, "must lie very
deep in the bedrock of biology."
~Jonathan Shaw
 |
 |
 |
 |
|
There are no males among the 360 described species
of bdelloid rotifers, and no evidence of sexual reproduction
has ever been found. Above, four species of bdelloid rotifers.
The female at top left is eating algae; an egg is visible
in the rotifer on the upper right.
|
|
Photographs by the Meselson Laboratory
|
