How Does A Cape Cod Mummichog Differ From A South Carolina Mummichog?

After talking to Kris today for about an hour, I realize that we can specifically answer that question -- looking at about a quarter of the cytochrome-b gene in the mitochondrion, they differ at exactly 5 DNA sequence points, i.e. having a T instead of a C, etc. The fish in our research from Maine, Cape Cod and Nantucket all share these 5 differences while fish from Chincoteague and Virginia Beach, VA; Charleston, SC; and Sapelo Island, GA, have something different. Since DNA typically codes for an amino acid sequence composing a protein, we looked at these five sequence differences and none of them code for a different amino acid in the protein. So they're all apparently neutral mutations, not surprising since the protein cytochrome-b is pretty conserved among vertebrates. The interesting thing about these shared mutations is how old they must be. New England was mostly covered with ice until about 12,000 years ago, at which point coastal fishes like mummichogs were able to recolonize the ice-free coast. But Nantucket was cut off from the mainland by rising sea level about 8,000 years ago, blocking mummichog gene flow with the mainland. The easiest explanation is that the New England fish share a common ancestor after New England was recolonized, but only before sea level rose. So these five mutations date to between 12 and 8 thousand years ago. The furthest north mummichog populations at the time of glacial retreat available to recolonize the northern Atlantic coast would have been at the mouth of what's now Chesapeake Bay, and the two DNA sequences we have from that area don't share any of these five mutations. So, we seem to be looking at some kind of founder effect underlying current mummichog populations in New England, where all existing individuals share ancestry from a very small number of fish sometime between glacial retreat and significant sea level rise. I'm thrilled enough at this as a good story that I'm tempted to say, "That's cool, that's trash" but really I should say, "That's cool, that's real biology." So I just did.

And of course it's Darwin's 200th birthday this Thursday, Feb. 12. I should offer drinks all around, but this medium doesn't support that...