Tuesday, July 22, 2008

Hybridization or Contamination - Update

I mentioned in a post a week or so ago that I was looking for evidence of hybridization in two species that aren't supposed to hybridize. Well, the first round of cloning is over, and all the individuals so far that came up as hybrids in the single-gene assay are genotyping as one of the species, not hybrids, and not a mix of species.

I'm in the process of sequencing more clones from these individuals, but it highlights an issue in the search for evidence of hybridization. It will be difficult to show hybridization is occurring without additional markers. With this in mind, I have started a round of cloning and sequencing of the mitochondrial gene that codes for subunit 1 of an enzyme called cytochrome c oxidase (aka CO1) for the hybrids. That's right, the Bar Code gene.

Here's a computer representation of the enzyme, showing how its structure.

The barcode gene doesn't work for everything, and yes, it does sort of trivialize the process of phylogenetically distinguishing taxa, but I like the concept of the barcode because it simplifies it for the public to the extent that they can "get it" and learn more if they want. You'll notice I said "mitochondrial". Reach back to high school biology and remember that mitochondria are in the cytoplasm of the cell, not the nucleus, so they are present in the egg cell and passed along, intact, to the fertilized egg.

That means CO1 is a maternally inherited marker, and using it provides insight into another level or taxonomic organization. For example, I can't use it for the species complex of mosquitoes that I work with (of which one is involved in this hybridization), because this sequence is basically the same in all of them. However, if I'm trying to tell purported hybrids of two good species apart, this kind of marker will hopefully tell me at least who their mamas were.

Will that do it? No, I'll have to look at additional markers as well, probably something like my panel of microsatellites, which rely on differences in noncoding (therefore selectively neutral) nuclear variation. Only then will we be able to adequately characterize these critters and say whether they represent interspecific hybrids (ooh! another publication!) or sloppy benchwork.

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Hi, sorry to make the humans do an extra step.