Evolution on Tasmainian Devils Essay
The world’s largest surviving carnivorous marsupial, the Tasmanian devil has a thick-set, squat build, with a relatively large, broad head and short, thick tail. The fur is wholly black, but white markings often occur on the rump and chest. Body size also varies greatly, depending on the diet and habitat. Adult males are usually larger than adult females. Devils once occurred on mainland Australia, but have been confined to Tasmania since pre-European times.
The survival of Tasmanian Devils is threatened by Devil Facial Tumour Disease (DFTD), and the species is now listed as Endangered. Devil facial tumour disease emerged in 1996 and causes tumours around in and around the mouth, face and neck and sometimes other parts of the Devils. The disease develops rapidly and is fatal: affected animals die within six months of the lesions first appearing. DFTD is contagious – the cancer cells are spread by biting during feeding and mating. It has spread across approximately 60% of Tasmania and has caused a rapid decline in wild Devil populations.
Characteristics of Tasmanian devil cells have fourteen chromosomes, while the oldest-known strain of the tumour cells contains thirteen chromosomes, nine of which are recognizable and four of which are mutated. More recently-evolved strains have an additional mutant marker chromosome, for a total of fourteen chromosomes. The karyotype anomalies of DFTD cells are similar to those of cancer cells from canine transmissible venereal tumour (CTVT), a cancer of dogs that is transmitted between canines by physical contact.
DFTD cells are not only genetically identical to each other, but also genetically distinct from their hosts, and from all known Tasmanian devils. The cancer originated in a single female individual and spread, rather than arising separately within each individual. Later the devil developed tumours from lesions caused by infected devil’s bites, confirming that the disease is spread by allograft, and that the normal methods of transmission include biting, scratching, and aggressive sexual activity between individuals.
Infectious facial cancer may be able to spread because of low diversity in devil immune genes. The same genes are also found in the tumours, so the devil’s immune system does not recognise the tumour cells as foreign. There are at least four or more strains of the cancer, showing that it is evolving, and may become more virulent. The strains may also complicate attempts to develop a vaccine, and the mutation of the cancer may mean that it could spread to other related species. Devil facial tumour disease was originated in the Schwann cells, of a single devil, most likely a female.
Schwann cells are found in the peripheral nervous system, and produce myelin and other proteins essential for the functions of nerve cells in the peripheral nervous system. The researchers sampled 25 tumours and found that the tumours were genetically identical. Several specific markers were identified which may enable veterinarians to more easily distinguish DFTD from other types of cancer, and might eventually help identify a genetic pathway that can be targeted to treat it.
Due to the decreased life expectancy of the devils due to DFTD, they have begun breeding at younger ages in the wild, with reports that many only live to participate in one breeding cycle. Tasmanian devils have changed their breeding habits in response to the disease. Females previously started breeding at the age of two, then annually for about three more years until dying normally. Now they commonly breed at the age of one, and die of tumours shortly thereafter. It is speculated that the disease is spread by devils biting each other during the mating season.
Social interactions have been seen spreading DFTD. It is one of three known contagious cancers. Wild Tasmanian devil populations are being monitored to track the spread of the disease and to identify changes in disease prevalence. To determine where the presence of the disease is and determine the number of affected animals. Areas are visited repeatedly to characterise the spread of the disease over time. Field workers are also testing the effectiveness by trapping and removing diseased devils.
It is hoped that the removal of diseased devils from wild populations should decrease disease prevalence and allow more devils to survive beyond their young years and breed. At present with the population reduced by 60% since 1996, if a cure is not found then scientists predict they will become extinct by 2035. Scientists and vets are working to find out how the disease can be stopped. Meanwhile, healthy pairs of Tasmanian devils have been moved to the mainland to breed in captivity. When Tasmania is once again disease free, those devils will be released into the wild.
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