AbstractBackgroundKangaroos 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.ResultsWe 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 tammarHOXA13andHOXD13have two exons as in humans, mice and chickens.HOXA13had an early and distal mRNA distribution in the tammar limb bud as in the mouse, but forelimb expression preceded that in the hindlimb.HOXD13mRNA 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.ConclusionsThis is the first examination ofHOXAandHOXDgene 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 eitherHOXA13orHOXD13when 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 ofHOXD13. 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 ofHOXgene expression may contribute to the heterochrony of the forelimb and hindlimb and that alteration toHOXdomains may influence phenotypic differences that lead to the development of marsupial syndactylous digits.