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Background  Kangaroos and wallabies have specialised limbs that allow for their hopping mode of locomotion. The hindlimbs differentiate much later in development but become much larger than the forelimbs. The hindlimb autopod has only four digits, the fourth of which is greatly elongated, while digits two and three are syndactylous. We investigated the expression of two genes,  HOXA13 and HOXD13 , that are crucial for digit patterning in mice during formation of the limbs of the tammar wallaby.    Results  We describe the development of the tammar limbs at key stages before birth. There was marked heterochrony and the hindlimb developed more slowly than the forelimb. Both tammar  HOXA13  and  HOXD13  have two exons as in humans, mice and chickens.  HOXA13  had an early and distal mRNA distribution in the tammar limb bud as in the mouse, but forelimb expression preceded that in the hindlimb.  HOXD13  mRNA was expressed earlier in the forelimb than the hindlimb and was predominantly detected in the interdigital tissues of the forelimb. In contrast, the hindlimb had a more restricted expression pattern that appeared to be expressed at discrete points at both posterior and anterior margins of the limb bud, and was unlike expression seen in the mouse and the chicken.    Conclusions  This is the first examination of  HOXA  and  HOXD  gene expression in a marsupial. The gene structure and predicted proteins were highly conserved with their eutherian orthologues. Interestingly, despite the morphological differences in hindlimb patterning, there were no modifications to the polyalanine tract of either  HOXA13  or  HOXD13  when compared to those of the mouse and bat but there was a marked difference between the tammar and the other mammals in the region of the first polyserine tract of  HOXD13 . There were also altered expression domains for both genes in the developing tammar limbs compared to the chicken and mouse. Together these findings suggest that the timing of  HOX  gene expression may contribute to the heterochrony of the forelimb and hindlimb and that alteration to  HOX  domains may influence phenotypic differences that lead to the development of marsupial syndactylous digits.

HOXA13 and HOXD13 expression during development of the syndactylous digits in the marsupial Macropus eugenii