Grampa Hagfish: say hello to your greatest uncle

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Today is Hagfish Day! Who knew?

What is a hagfish?

Hagfish are primitive eel-like chordates make famous for their relative unattractiveness*, profuse production of slime, and charismatic ability to tie themselves in knots. They are perhaps the only ‘fish’ that possesses a skull, but no vertebral column. But the question “What is a hagfish?” goes much deeper than that and it’s answer is fundamental to the evolution of vertebrates and, ultimately, us.

The National Academy of Sciences must have known today is Hagfish Day too, because yesterday afternoon they published online microRNAs reveal the interrelationships of hagfish, lampreys, and gnathostomes and the nature of the ancestral vertebrate. You see, hagfish have a problem, and it’s much bigger than too much slime blocking the gills. For most of their history, no one knew quite what to do with them.

Two competing views of hagfish evolution (from Heimberg et al. 2010)

There are two opposing views of hagfish evolution. On one side, molecular evidence suggests that hagfish are most closely related to lampreys, a fair enough assumption and one that fits comfortably into our framework of vertebrate evolution. On the other side, morphology suggests that hagfish and lampreys were paraphyletic – that is, they aren’t most closely related to each other – and that lampreys and jawed vertebrates were BPF (best phylogenetic friends). The problem with the second option is that it would mean that the convergent evolution of hagfish and lampreys, or the subsequent degeneration of both groups into their current complementary forms would represent the single most exceptional event in all of evolution. We’d basically be talking about the transition from tunicate to vermiform ‘fish’ happening twice.

So Heimberg et al. set out to solve this puzzle using microRNA’s. microRNA’s are small bits of RNA that control gene expression in certain parts of an organism. They can essentially be treated much like morphological traits – the more microRNA’s two species have in common, the more closely related they are. By building the most parsimonious tree of microRNA acquisition and loss, they reconstructed a basic picture of early vertebrate evolution.

Not surprisingly this new analysis agreed with the morphology. Hagfish and lampreys are more closely related to each other than to anything else. This leaves us with a new problem though, because we still do not have a good picture of the common ancestor of all vertebrates, the lineage that unites hagfish, lampreys, and Homo sapiens into a single taxon. Somewhere between Tunicata and Cyclostomata we acquired huge numbers of regulatory genes and experienced a massive expansion of our genome. The mystery of cyclostome monophyly may be solved, but the transition to vertebrate body plan remains unresolved.

~Southern Fried Scientist

For a more, see Wired Science: Hagfish Analysis Opens Major Gaps in Tree of Life

*I think they’re beautiful

Update: Tree!

Heimberg, A., Cowper-Sal{middle dot}lari, R., Semon, M., Donoghue, P., & Peterson, K. (2010). microRNAs reveal the interrelationships of hagfish, lampreys, and gnathostomes and the nature of the ancestral vertebrate Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1010350107

October 20, 2010 • 8:00 am