Tracking Down the Genes Behind Speciation

The 'species' concept is one of the most important in biology, providing the framework within which we organize our knowledge of this planet's living diversity. Despite this, its precise definition remains elusive, a continual source of debate. The reproductive species concept — the idea that a species is a group of organisms which can reproduce with each other but not with others — is perhaps the most common among biologists. A natural consequence of this idea is that reproductive barriers determine the boundaries of each species. These barriers might initially be physical obstacles — a river or a mountain range are commonly-used examples — or life-history based, such as the ability to exploit a new food source. Over time, these external barriers can become internalized; genetic differences between the two populations accumulate until they can no longer interbreed, thus giving rise to new species.

Identifying the genetic factors underlying these reproductive barriers would be a major boon to efforts to understand speciation. To track them down, researchers use hybrids between subspecies that haven't fully separated. The hybrids are often less fertile than individuals of either subspecies — an incipient reproductive barrier — and genetic approaches can be used to find out which parts of the genome are linked to the reduced fertility. For example, studies have shown that the X chromosome somehow contributes to reduced male fertility in hybrid mice. The problem is that these approaches are relatively low resolution. Hybrid sterility is probably caused by many small genetic changes, not a few big ones, so genetic approaches end up linking it to large genomic regions containing many candidate genes, making it hard to track down precisely which changes actually contribute.

To get a higher resolution view, Leslie Turner and Bettina Harr carried out a genome-wide assocaition study (GWAS) on the offspring of wild-caught hybrid mice. This gave them a snapshot of variation across the entire genome at once, which they linked to two hybrd-infertility traits, testis weight and gene expression in the testes. While they still found many regions linked with hybrid infertility, this approach enabled them to reduce the size of each region, resulting in a shorter list of candidate genes for follow-up studies to examine.

While tracking down the specific genes involved will probbaly lead to some interesting insights, the study's real value may be in providing a genome-scale view of hybrid infertility. This allows the researchers to study questions like how many different sites are involved, how much impact each has, and how they interact. Only time — and more studies like this — will tell, but factors such as these seem more likely to be shared between species than the specific incompatibility genes. Identifying the genes will help us understand the reproductive barriers between a particular species pair, but understanding hybrid infertility at a genomic scale may prove key in understanding reproductive barriers themselves — that is, the origin of species.

Refs
Phifer-Rixley, M. Searching for the genes that separate species. eLife 2014;3:e05377. (2014) DOI: 10.7554/eLife.05377
Turner, LM and Harr, B. Genome-wide mapping in a house mouse hybrid zone reveals hybrid sterility loci and Dobzhansky-Muller interactions. eLife 2014;3:e02504. (2014) DOI: 10.7554/eLife.02504

Image credits
The mouse image is a public domain image produced by the NIH.