The bovine lactation genome: insights into the evolution of mammalian milk

Abstract

Background: The newly assembled Bos taurus genome sequence enables the linkage of bovine milk and lactation data with other mammalian genomes.

The evolution of milk secretion and its ancient origins

“…The proposal that mammary secretion has an ancient origin and long evolutionary history (Oftedal, 2002a and2002b) is increasingly accepted, especially in the face of supportive molecular evidence (Kawasaki and Weiss, 2003;Vorbach et al, 2006;Brawand et al, 2008;McClellan et al, 2008;Capuco and Akers, 2009;Lemay et al, 2009;Lefevre et al, 2010;Kawasaki et al, 2011). It is now possible to formulate a more detailed scenario by which the secreted fluid came to resemble in form and function what we now know as milk.…”

“…From an evolutionary perspective, it would be very interesting to compare the array of genes expressed by developing and secreting apocrine glands with those expressed during mammary gland development, milk secretion, and mammary involution. For example, nearly 200 milk protein genes and more than 6000 other genes have been identified as expressed in the mammary glands in virgin, pregnant, lactating, involuting, and mastitic cows (Lemay et al, 2009), but how many of these genes are expressed in apocrine glands is unknown. It would also be instructive to compare gene expression of both apocrine and mammary glands with that occurring in amphibian skin glands to document similarities and differences that may indicate ancestral and derived conditions.…”

“…The secretory products of the mammary gland represent the expression of a large number of genes that are upregulated during lactation, but many of these products remain unknown (except as genes) or their functionalities are poorly understood (Smolenski et al, 2007;Lemay et al, 2009). Comparisons of monotreme, marsupial and eutherian genomes suggest that milk and mammary genes tend to be conserved, that is, have not diverged as much as other genes (Lemay et al, 2009).…”

“…Comparisons of monotreme, marsupial and eutherian genomes suggest that milk and mammary genes tend to be conserved, that is, have not diverged as much as other genes (Lemay et al, 2009). Nonetheless, there are major constituents of milk in these three lineages for which comparative data provide evidence of an evolutionary origin, especially when milk proteins are compared with closely related gene products.…”

“…The detailed signaling crosstalk among epithelial and underlying mesenchyme cells that is required for the differentiation, ductal branching, and proliferation of mammary tissue is only now being unraveled (Watson and Khaled, 2008), and the functional significance and patterns of expression of thousands of mammary genes are under investigation (Lemay et al, 2009). Secreted milk is extremely varied in composition -from trace levels of fat in rhinos to more than 60% fat in some ice-breeding seals (Oftedal and Iverson, 1995) -and contains unique proteins (a-, b-, and k-caseins, b-lactoglobulin, a-lactalbumin, whey acidic protein (WAP)), membrane-enclosed lipid droplets, and sugars (lactose, milk oligosaccharides) that are not found elsewhere in nature.…”