APRIL 2016|| 19races in which the chromosome number varies
from 16 to 22, rock-wallabies provide an excel-
lent model system for scientists to explore
how chromosome differences create genetic
incompatibilities and drive speciation.
Unlike other model systems studied
around the world, such as mice, shrews and
vinegar lies, rock-wallabies live in small semi-
isolated populations and reproduce slowly.
These differences in their biology provide a
valuable and unique comparison when
looking at mechanisms that inhibit repro-
duction between species, to see if similar
processes drive the same outcomes.
The six species of rock-wallaby found in
north-east Queensland are all very closely
related and only diverged from each other
0.44–1.58 million years ago. They appear very
similar, and are almost impossible to tell apart
using standard morphological and genetic
tests.
However, they all differ chromosomally. Among them are
various chromosome rearrangements, ranging from one or two
differences to up to six changes (Fig. 3). This range of differences
provided us with an opportunity to test a long-held theory that
more complex chromosome differences were associated with less
gene low between species than those with simple chromosome
differences due to mispairing during meiosis.
Although each species lives in a geographically discrete region
of Queensland, they come into contact where their ranges abut.
Although most species contact one or two others, in one case
three species come into contact and have the opportunity to
hybridise.
To assess the genetic differences between the rock-wallaby
species and to evaluate the amount of gene low that occurs
when they come into contact, we examined both their mito-
chondrial DNA and their microsatellite DNA, which are
random repeats of DNA in the genome. Both of these evolve
rapidly. Based on these genetic markers, we compared the
genetic diversity within and between species to see how the
packaging of the genetic material inluenced speciation.
The results gave some evidence of genetic separation between
species, but this was not always the case. The mitochondrial
DNA results in particular showed no species-speciic clus-
tering, with most of the diversity being shared among species
rather than within species, as is usually the case. Where the
three rock-wallaby species come into contact, we found the
opposite to what we predicted, with greater gene low between
the two species with more complex chromosomal differences
than between the species with simple differences.
Remarkably, our results demonstrate that even several
complex rearrangements are not resulting in complete hybrid
sterility, as we expected. This means that some hybrid indi-
viduals are able to successfully complete meiosis and produce
viable gametes even though up to ive separate chromosomes
must irst come together in a highly complex pairing arrange-
ment and then separate to form balanced gametes. How this
remarkable feat is achieved remains unknown, yet it must occur
as otherwise no gene low would have been detected.
Another theory has been proposed for chromosomal specia-
tion: that segments of the chromosome that have changed have
mutated genes that better adapt the individual to its environ-
ment. Since this mutation is within the altered chromosome
segment, it becomes ixed over time within the species and the
region surrounding the mutation also becomes linked to the muta-
tion. This results in a chunk of the chromosome that is different
and cannot move between species, while the rest of the chromo-
some is free to mix. It is this region that could provide the clues
to reproductive isolation between two different species.
Our next step in exploring the process of speciation in these
rock-wallabies is to try and see if segments involved in the chro-
mosome changes between the six north-east Queensland species
are different, while the remainder of the genetic material along
the chromosomes is more similar. If we ind segments that are
different,this could indicate that these regions have different
selection pressures and could explain why we now have sepa-
rate rock-wallaby species.
Sally Potter is a postdoctoral researcher at The Australian National University’s Department
of Evolution, Ecology and Genetics, and a research affiliate at the Australian Museum
Research Institute, where Mark Eldridge is Principal Research Scientist.Credit: Henry CookRemarkably, our results demonstrate that
even several complex rearrangements are
not resulting in complete hybrid sterility...