The "Got Milk?" slogan has to be one of the most often mimicked ads of all time. I did a quick search, and found the figure above, apparently compiled by the milk folks themselves.
So, how did animals "Get Milk" in the first place? In other words, how did this novelty originate during evolution?
A new paper published by Lemay et al in Genome Biology has taken advantage of the recently completed genome sequence of the bovine, Bos taurus, and has begun to address this very question.
Although I am not a mammalian biologist (meaning I don't study mammals, despite being one myself), this is the third mammalian novelty I have highlighted here (see also placenta and hair). Mammalian genome biology is ahead of other animal groups, for obvious reasons. All my mammalian novelty posts tell a common story: the building blocks of complex biological features pre-date the origin of the integrated traits themselves. What I wrote for hair, can also apply to milk and mammary glands:
"Just ten years ago, results ... clarifying the molecular components of trait evolution were rare, but they have become common now that genome sequences are available for many species. Before we had some idea of gene function, and before genome sequencing, scientists could only examine one level of biological organization – the trait (hair [milk] in this case). And that could only get science so far. In the case of hair [milk], it mainly got science as far as Figure 1, which leads to the inference that hair [milk] evolved a bit before the common ancestor of living mammals. But “hair [milk]” is not one thing. It is a complex of building blocks, including structural genes (like [casein]) and developmental processes. Today, scientists can decompose a trait, like hair [milk], into its components and study the evolutionary history of each part separately, tracing the parts through various genomes."
Figure 1 is here, again, replace hair with "milk".
I learned many amazing things about the genomics of milk and mammary glands from the Lamay et al paper.
- Some 6000 genes are considered "mammary related" and are found on all the bovine chromosomes. There are 197 unique milk protein genes!
- Compared to non-mammary genes, mammary genes are more commonly present in all mammal genomes studied. This indicates that mammary genes are evolving more slowly and may be lost less often than other genes. This could indicate purifying selection owing to the importance of lactation for mammalian life history.
- Milk does vary a lot among species - some babies need more fat or a bigger immunity boost, depending on the lifestyle of the species. Variation tends to be caused by variation in number of gene duplicates, but not in the sequence of the milk proteins themselves. One explanation for milk variation could be the levels of expression of different genes (regulatory variation).
- Mammary genes are found together in the genome. Also milk proteins are found along with mammary genes in the genome. This could indicate that these clusters are expressed/regulated together as groups.
- The genes expressed in milk fat globule secretion have similarity with other secretory organs.
This final result suggests that mammary glands might be considered "duplicates" (paralogs in molecular evolution parlance) of other secretory organs. It reminds us that traits do not come from nothing. Some designer did not shoot a lightening bolt into the first mammal, imparting it with mammary glands and milk. Furthermore, natural selection did not modify lactation genes to perfection, thereby erasing their history. These traits, like all other traits, evolved from existing building blocks, duplicating and recombining them to form something new. The evidence for common descent is strong and it is deep.
As the authors wrote about lactation:
"the ontogeny of the mammary gland [may have] occurred by co-opting existing structures and developmental pathways. Lactation may be less than 200 million years old, but its biological roots are far more ancient."
Lemay, D., Lynn, D., Martin, W., Neville, M., Casey, T., Rincon, G., Kriventseva, E., Barris, W., Hinrichs, A., Molenaar, A., Pollard, K., Maqbool, N., Singh, K., Murney, R., Zdobnov, E., Tellam, R., Medrano, J., German, J., & Rijnkels, M. (2009). The bovine lactation genome: insights into the evolution of mammalian milk Genome Biology, 10 (4) DOI: 10.1186/gb-2009-10-4-r43