18 | APRIL 2016
46 chromosomes. This genetic material is passed from parent
to offspring, providing the instructions for cells and organs to
function. In most species, chromosomes are usually too thin
to visualise, but whenever a cell divides they shorten and thicken,
becoming so tightly packaged that they actually become visible
under a light microscope.
The study of chromosomes has revealed that most species vary
in the shape and number of their chromosomes, but whether
these differences are the cause or consequence of species forma-
tion has long been debated. Several different sorts of chromo-
some rearrangements have been identiied, including fusions,
issions and inversions (Fig. 1).- Inversions involve a segment of the chromosome reversing
in position so that the DNA is in the opposite order but in
the same location on the chromosome. - A fusion involves two separate chromosomes coming together
and joining to form one larger chromosome. - Fission is where a larger chromosome splits into two chro-
mosomes.
In addition, every chromosome has a centromere that links
the two strands of the chromosome together. The centromere
can also shift its position along the chromosome and be in the
middle or towards the end, changing the overall shape of the
chromosome.
Most sexually reproducing organisms have two copies of
each chromosome, one derived from the female parent and one
from the male parent. Changes in the structure of chromo-
somes are important: during meiosis, the process of cell division
that produces gametes (Fig. 2), the maternal and paternal copy
of each chromosome needs to pair up so that each gamete has
one copy of all necessary genetic material. If the chromosomes
that are trying to pair during this process have rearranged, then
diiculties will arise in the cell division process, leading to
gametes that are missing or have extra genetic information and
are therefore not viable.
Simple differences in chromosomes may still enable the
pairing process to be completed normally. However, genetic
theory predicts that the process will break down when complex
chromosome changes take place, and offspring will not be viable
or will be infertile.
The rock-wallabies in north-east Queensland seemed to
follow this pattern. When the different species were crossed
in captivity, male offspring were sterile, suggesting that their
chromosome differences led to reproductive isolation. While
most female hybrids were also sterile, some were sub-fertile and
produced fewer offspring.
While the chromosomes of most marsupials have changed
little in their number and shape over tens of millions of years,
Australia’s unique rock-wallabies are famous for their highly
variable chromosomes. With 17 species and 23 chromosomal
Figure 1. Chromosome rearrangements. Fusion is where two
chromosomes fuse together to form one larger chromosome;
fission is where a chromosome splits into two chromosomes; and
inversion is where a segment of a chromosome is reversed.Figure 2. Meiosis is a process of cell division where the parent
cells split into daughter cells with half as many chromosomes
(gametes). The chromosomes replicate, pair and align, but if there
is a rearrangement in the chromosomes it can cause errors as a
result of mispairing, and as a consequence uneven gametes are
formed.Figure 3. A map of the six north-east Queensland rock-wallaby
species and their associated karyotypes (shape and number of
chromosomes). Highlighted are fusions, including chromosomes
5, 6, 9 and 10.