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Wallabies Rock the Basis of Speciation

Credit: Henry Cook

Credit: Henry Cook

By Sally Potter & Mark Eldridge

Six rock-wallaby species in Queensland have different numbers of chromosomes, yet gene flow somehow occurs between them. What does this tell us about how new species form?

Six closely related species of rock-wallaby from north-east Queensland have long been considered a classic example of chromosomal speciation. Each of these species is similar in almost every way, yet each differs in the shape and number of their chromosomes.

These chromosome differences should make gene flow between these species almost impossible, since hybrids would have reduced fertility due to their chromosome differences.

However, contrary to expectations, we have found no relationship between the degree of chromosome differences and the amount of gene flow among these species. For example, we found relatively high levels of gene flow between some species that differ by multiple chromosome changes, and alternatively we found low gene flow between species with similar chromosomes.

This indicates that the mechanisms driving species formation are much more complex than just the incompatibilities caused by chromosome rearrangements. It appears then that complex interactions between the way genetic material is packaged into chromosomes and how the chromosomes differ in their shape, number and arrangement is driving species formation.

Chromosomes are the long molecules of deoxyribonucleic acid (DNA) found in each cell that contain all the genetic information each organism requires to function. For example, each human cell contains about 2 metres of DNA, separated into 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 formation has long been debated. Several different sorts of chromosome rearrangements have been identified, including fusions, fissions 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 chromosomes.

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 chromosomes 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 difficulties 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 races in which the chromosome number varies from 16 to 22, rock-wallabies provide an excellent 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 flies, 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 reproduction 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 flow 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 flow that occurs when they come into contact, we examined both their mitochondrial 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 influenced 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-specific clustering, 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 flow 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 individuals are able to successfully complete meiosis and produce viable gametes even though up to five separate chromosomes must first come together in a highly complex pairing arrangement and then separate to form balanced gametes. How this remarkable feat is achieved remains unknown, yet it must occur as otherwise no gene flow would have been detected.

Another theory has been proposed for chromosomal speciation: that segments of the chromosome that have changed have mutated genes that better adapt the individual to its environment. Since this mutation is within the altered chromosome segment, it becomes fixed over time within the species and the region surrounding the mutation also becomes linked to the mutation. This results in a chunk of the chromosome that is different and cannot move between species, while the rest of the chromosome 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 chromosome 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 find segments that are different,this could indicate that these regions have different selection pressures and could explain why we now have separate 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.