O-44: Characterisation of Monotreme CaseinsReveals Lineage Specific Expansion of an AncestralCasein Locus in Mammals

Background: One important reproductive characteristic of Mammals is the production of milk to nurse the neonate. In order to better understand the evolution of milk we have investigated gene expression in milk cells from monotremes which are the most ancient representative of the mammalian lineage. Materials and Methods: Using a milk cell cDNA sequencing approach we characterise milk protein sequences from two monotreme species, platypus (Ornithorhynchus anatinus) and echidna (Tachyglossus aculeatus) and exhibit a full set of caseins and casein variants. Results: The genomic organization of the platypus casein locus is compared to other mammalian genomes, including the marsupial opossum and several eutherians. Physical linkage of casein genes has been seen in the casein loci of all mammalian genomes examined and we confirm that this is also observed in platypus. However, we show that a recent duplication of beta-casein occurred in the monotreme lineage, as opposed to more ancient duplications of alpha-casein in the eutherian lineage, while marsupials possess only single copies of alpha and beta caseins. Despite this variability, the close proximity of the main alpha and beta casein genes in an inverted tail-tail orientation and the relative orientation of the more distant kappa casein genes are similar in allmammalian genome sequences so far available. Conclusion: Overall the conservation of the genomic organisation of the caseins indicates the early, premonotreme development of the fundamental role of caseins during lactation. In contrast, the lineage specific gene duplications that have occurred within the casein locus of monotremes and eutherians but not marsupials, which may have lost part of the ancestral casein locus, emphases the independent selection on milk provision strategies to the young, most likely linked to different developmental strategies. The monotremes therefore provide insight into the ancestral drivers for lactation and how these have adapted in different lineages, including our own.