The Tasmanian devil (Sarcophilus harrisii) is an endangered carnivorous marsupial found only on the island of Tasmania, Australia. The species has garnered global attention due to its unusual genetic makeup and the devastating disease that threatens its survival. Examining the devil’s specific chromosomal organization reveals a story of genetic simplicity with complex consequences.
The Chromosome Count and Definition
The Tasmanian devil has 7 homologous pairs of chromosomes. This represents a total of 14 chromosomes in every somatic (body) cell, designated as its diploid number (2n=14). This count is one of the lowest among all mammals.
A homologous pair consists of two chromosomes of approximately the same length and centromere position. One chromosome is inherited from the mother and the other from the father. These pairs carry the same genes in the same order, though they may possess different versions, or alleles, of those genes.
Unique Structure of the Tasmanian Devil Karyotype
The karyotype, which is the complete set of chromosomes arranged in order of size, reveals more about this small count. The devil’s 7 pairs are categorized as six pairs of autosomes and one pair of sex chromosomes. The chromosomes are generally small and characterized by having their centromeres positioned near one end (acrocentric or telocentric).
The sex chromosomes follow the typical mammalian XX/XY system, with females possessing two X chromosomes and males possessing one X and one Y chromosome. The devil’s Y chromosome is noticeably smaller than its X chromosome. Marsupial sex chromosomes, in general, are considered to represent an ancestral state compared to placental mammals, typically lacking the large pseudoautosomal region where X and Y chromosomes exchange genetic material during meiosis.
Genetic Diversity and Disease
The small number of chromosomes reflects extremely low genetic variation within the species. The devil population has experienced historical “bottlenecks,” periods where the population size dramatically decreased, which reduced the overall gene pool. This lack of diversity is particularly noticeable in the Major Histocompatibility Complex (MHC) genes, which are responsible for the immune system’s ability to recognize foreign cells.
This genetic uniformity is directly linked to the spread of Devil Facial Tumor Disease (DFTD), a fatal, transmissible cancer. The tumor cells are transferred when devils bite each other, acting as an allograft between genetically similar individuals. The initial hypothesis was that the host devil’s immune system failed to recognize the tumor cells as foreign due to the low MHC diversity.
However, more recent research has shown a more nuanced picture. Devils can reject skin grafts from unrelated devils, suggesting their immune system is functional. Instead, the tumor itself appears to have evolved a mechanism to escape detection by actively down-regulating the expression of MHC molecules on its surface. This allows the tumor to successfully evade the immune response, making the devil’s low genetic variation a contributing factor to the disease’s rapid spread and the species’ ongoing struggle for survival.