http://www.eurekalert.org/pub_releases/2007-10/uu-gft100507.php
Genes from the father facilitate the formation of new species
Public release date: 5-Oct-2007
Contact: Anna Qvarnström
Uppsala University
The two closely related bird species, the collared flycatcher and the
pied flycatcher, can reproduce with each other, but the females are
more strongly attracted to a male of their own species. This has been
shown by an international research team directed by Anna Qvarnström at
Uppsala University in today’s Net edition of Science. They demonstrate
that the gene for this sexual preference is found on the sex chromosome
that is inherited from the father and that only females have a copy of.
The discovery sheds new light on how new species are formed.
The formation of new species takes millions of years. It often happens
when a population (group of individuals) is divided and separated
geographically and then adapts to disparate environments over thousands
of generations. For instance, the earth’s ice ages have led to many
such population splits. But divided populations often come into
contact with each other before they have had time to become entirely
sexually isolated from each other. When individuals mate with each
other from such split populations that have not quite become separate
species, their offspring (so-called hybrids) often have limited
viability.
Anna Qvarnström’s research team from Uppsala University in Sweden,
working with scientists from Norway, the Czech Republic, the US, and
Holland, have studied natural hybridization between two closely related
bird species, the collared flycatcher and the pied flycatcher. The two
flycatcher species (or “quasi species”) have come into contact with
each other after having been separated during the last ice age. The
question they addressed was whether the flycatchers will conclude the
species formation that is under way and become entirely sexually
isolated from each other or, instead, if they will meld into the same
species again.
“We found that females in the hybrid zone develop a sexual preference
for males belonging to their own species and that this preference is
determined by genes located on the sex chromosome,” says Anna
Qvarnström.
In birds, in contrast with most other animals, it is the females that
are the so-called heterogametic sex. Their sex chromosomes are called
ZW and correspond to XY in humans. In birds, it is the female that is
ZW and the male ZZ, but in humans men are XY and women XX. The results
show that the preference for their own species is sited on the sex
chromosome that the females inherit from their father. The same
chromosome also houses the genes that govern the development of the
species-specific plumage.
“When genes regulate species-specific features and the preference for
these are located close to each other in the DNA, in this case on the
same chromosome, species formation is favored. Therefore, the
probability of these two flycatcher species merging into the same
species again is small,” says Anna Qvarnström.
It is possible that this will prove to be a general pattern that can
explain how new species can continue to exist even if they occasionally
hybridize with each other.
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