Thursday, October 22, 2009

Mendel is REALLY not enough

Back in 1927, creationist pioneer Byron C. Nelson published a book called "After its Kind", in which he detailed a theory of species origins based on the science of Mendelian genetics. The basic idea is that the isolation of particular genetic alleles in different populations explains the origin of "new" traits. This idea became quite popular in creationism. For example, Will Tinkle (one of the founders of the Creation Research Society) recycled it as the "theory of heterozygous creation" for his book Heredity: A Study in Science and the Bible. Though statements of this theory have gotten more sophisticated over the years, they're still basically nineteenth century genetics.

Modern comparative genetics and genomics continue to show that Mendelian genetics simply does not explain the changes that occur when a new species originates. I've mentioned this before (Mendel is not enough), and I proposed at the 2003 ICC that genomic rearrangements lead to the evolution of new traits and the emergence of new species. This week's Nature has three articles that illuminate our understanding the origin of species. I think it's the best evidence yet that genomic rearrangements are linked to the phenotypic changes that underly the origin of species.

In the first paper, Kitano et al. report on a new sex chromosome in threespine sticklebacks from Japan. Sticklebacks from the Japan Sea have a fusion between the Y chromosome and an autosome. This fused chromosome is not found in sticklebacks from the Pacific Ocean. Though there is a hybrid zone between the two forms, reproductive isolation is generally maintained through behavioral differences and hybrid male sterility. Here's the neat thing: some of the traits that contribute to reproductive isolation of these forms are found on the novel Y chromosome of the sticklebacks from the Japan Sea.

The second paper by Naito et al. looked at an expansion of DNA transposons in a strain of rice called EG4. These transposons are not found in Nipponbare rice, which has its genome completely sequenced. Naito et al. used sequencing techniques to identify the insertion location of 1664 transposons in EG4 and found that they generally inserted into the upstream regions of genes, and either upregulated them or had no transcriptional effect. Here's the cool part, straight from the abstract:
Furthermore, we document the generation of new regulatory networks by a subset of mPing insertions that render adjacent genes stress inducible. As such, this study provides evidence for models first proposed previously for the involvement of transposable elements and other repetitive sequences in genome restructuring and gene regulation.

In the final paper, Martin et al. examined sex determination in melons. In plants, some flowers have both male and female parts, but some plants plants separate male and female parts into different flowers (as in maize) or on entirely different plants. In melons, Martin et al. found that the condition known as gynoecy, the development of an entirely female plant, was linked to the presence of a transposon that modified the expression of a gene used in the development of the female flower parts.

In each of these cases, genomes have been altered in ways that Mendel never imagined, and now we're finally beginning to see how those genomic alterations can lead to new characteristics (in rice and melon) and possibly even new species (in sticklebacks). This is an exciting time to be a creationist!

Kitano et al. 2009. A role for a neo-sex chromosome in stickleback speciation. Nature

Naito et al. 2009. Unexpected consequences of a sudden and massive transposon amplification on rice gene expression. Nature 461:1130-1134.

Martin et al. 2009. A transposon-induced epigenetic change leads to sex determination in melon. Nature 461:1135-1138.