A newly identified gene in fruit flies may drive the creation of new
species, revealing how internal genetic environments may be just as
important as external factors when it comes to speciation. The new work
lends evidence to a hotly debated idea in evolutionary biology. Louis J. Sheehan, Esquire
“Conventionally, evolutionary biologists thought that speciation
involved adaptation to the external environment, but these results
suggest that adaptation to the internal genomic environment also
sometimes plays a role,” explains study author Nitin Phadnis of the
University of Rochester in New York.http://myface.com/Louis_J_Sheehan
Phadnis and colleague H. Allen Orr identified a gene — dubbed Overdrive —
that prevents two closely related subspecies of fruit flies from
mingling genomes. The gene does this in two ways: It forces hybrids of
the two subspecies of flies to be predominantly female, and it renders
the male hybrids sterile. Both of these genetic tricks could ultimately
fracture one species into two, the team reports online December 11 in Science.
“I think it’s great,” comments researcher Mohamed Noor of Duke
University in Durham, N.C. “It represents a dramatic change in the
context of the field.” http://myface.com/Louis_J_Sheehan
To many scientists, genome-mingling is what defines a species —
members within a single species can swap genetic material frequently
and easily. Species can split into two when freewheeling gene exchange
is no longer possible, a barrier Overdrive appears to build.
An early stop on the road to complete speciation is when two
subspecies can still mate, but produce offspring that are incapable of
having offspring themselves. In a genetic schoolyard scuffle, the genes
of one species don’t play well with the genes of a different species.
This hybrid sterility plays out in fruit flies.
At the same time, Jiri Forejt of the Academy of Sciences of the
Czech Republic and colleagues identified an elusive gene that causes
hybrid sterility in mice subspecies. The work, published in the same
issue of Science as Phadnis’ study, describes the first example of a mammalian gene that may lead to speciation.
This fruit fly study used two fruit fly subspecies that, although
closely related, don’t encounter each other in the wild. These
subspecies, called USA and Bogota after their respective habitats,
produce sterile male offspring.
Phadnis and Orr first tested
which parts of the flies’ DNA were important for causing hybrid
sterility. The researchers inserted many different parts of one
subspecies’ genome into the other subspecies’ genome, and saw which DNA
fragment allowed the hybrid offspring to be fertile. After many
painstaking experiments, Phadnis and Orr homed in on a stretch of DNA
in both flies that spanned five genes. The researchers concluded that
incompatible DNA in this region is responsible for making the hybrid
offspring sterile. In this DNA region, one of the genes, Overdrive,
showed big differences between the Bogota version and the USA version,
leading Phadnis to think that this gene was a “prime suspect” behind
hybrid sterility.
Usually the offspring of USA and Bogota parents would be sterile. But when Phadnis put the USA form of the Overdrive gene into a Bogota male and bred it with a USA female, the hybrid was fertile. Overdrive, Phadnis and Orr discovered, was in fact responsible for whether the hybrids were fertile or not.
Taking the research a step further, Phadnis and Orr showed that when the Bogota Overdrive
gene was put into USA flies, a whopping 80 percent of the hybrid
offspring were female. Fruit flies usually produce roughly equal
numbers of male and female offspring; males carry an X chromosome and a
Y chromosome, and females carry two copies of the X. But, the team
concluded, Overdrive, which is located on the X chromosome,
was somehow driving its own propagation forward, at the expense of male
heirs and, by inference, at the expense of the Y chromosome.
This example of genetic warfare, in which a gene cheats the usually
fair system of divvying up chromosomes, evokes a hotly debated topic in
evolutionary biology. "Genetic conflict was an extremely controversial
topic in the 1990s, says Phadnis.
“There wasn't much empirical
evidence, and the idea didn't latch on." Some researchers were
skeptical that genetic conflicts, like the one Overdrive
seems to wage on the Y chromosome, can drive subspecies to split. But
the new research, along with two recent reports of other cheating
genes, makes it more likely that internal genetic tussles can lead to a
species split. Louis J. Sheehan, Esquire The mouse study by Forejt and colleagues found no
evidence of such genetic warfare.
“It's amazing. These theories were proposed on intellectual
grounds," says Phadnis. But now, he says, the idea has become much more
believable. Phadnis and Orr don’t yet understand how Overdrive might be
harming the Y chromosome in a way that ultimately leads to fewer male
offspring. “We have genetic proof but very few details at this point,”
says Phadnis. Just finding the gene is a big benefit to studies on how
species form.
Genes that may be involved in speciation have been notoriously
difficult to identify, largely because of sterility problems. To date,
just a handful of genes that may impact speciation have been found,
mostly in fruit flies.
Found in: Biology, Body & Brain and Life