Weird Animal Genomes & Reproduction

Comparisons between the genomes of distantly related species provide powerful ways to interpret genome organization, function and evolution. There are three groups of extant mammals, two confined to Australasia. Marsupials (kangaroos and their relatives) and monotremes (echidna and the fabulous platypus) have been evolving independently of placental mammals for most of mammalian history. Their genomes are particularly valuable because these “alternative mammals” fill a phylogenetic gap in vertebrate species lined up for exhaustive genomic study. Human and mice (~70MY) are too close to distinguish signal, whereas mammal/bird comparisons (~310MY) are too distant to allow alignment. Kangaroos (180 MY) and platypus (210 MY) are just right. Sequence has diverged sufficiently for stringent detection of homologies that can reveal coding regions and regulatory signals. Importantly, marsupials and monotremes share with humans many mammal-specific developmental pathways and regulatory systems such as sex determination and lactation.

We are characterizing the genomes of marsupials and monotreme mammals in order to make many such comparisons possible. We are developing detailed physical and linkage maps of the genome to complement sequencing, and are preparing arrayed cDNAs for functional studies, especially of reproduction and development. Complete sequence of the Brazilian short-tailed opossum (Monodelphis domestica) is available, and the platypus genome has also been sequenced; 2X sequence of our model kangaroo (the tammar wallaby Macropus eugenii) is available and is in assembly. We have isolated and characterized many BACs and cDNAs containing kangaroo and platypus genes of interest, and demonstrate the value of comparisons to reveal conserved genome organization and function, and new insights and surprises in the evolution of the mammalian genome, particularly the origin and differentiation of mammalian sex chromosomes.