In the process of speciation, when two new species go their separate evolutionary ways, a critical step is reproductive isolation. This happens when groups of individuals acquire differences that prevent them from successfully interbreeding. At the genetic level, the most widely considered cause of this isolation is a change in the function of a key gene in one of the diverging groups, rendering the two populations sexually incompatible. A recent study now provides evidence for an alternative route to speciation in which the jumping of a gene that is essential for fertility to a new location in the genome is the cause.

John Masly and colleagues studied a hybrid Drosophila line in which all of the genome was derived from Drosophila melanogaster apart from the tiny fourth chromosome, which came from the closely related species Drosophila simulans. All females from this line are fertile, as are males that carry one copy of the fourth chromosome from each species. However, males that are homozygous for the D. simulans fourth chromosome are sterile as a result of sperm immotility. This provides a model of hybrid sterility, which is one route to reproductive isolation.

The authors mapped the genetic cause of this sterility to the D. melanogaster fourth chromosome gene JYAlpha , which encodes a transmembrane ion-exchange protein. Using P-element transposition, they created a JYAlpha allele that encodes a truncated protein. Male flies in which one copy of the fourth chromosome carried this null allele and the other copy came from D. simulans were sterile, confirming the role of JYAlpha in hybrid male infertility.

So have the JYAlpha genes in D. melanogaster and D. simulans undergone a functional divergence that causes this incompatibility? When they came to mapping and sequencing the D. simulans version to make the necessary comparison, Masly and colleagues found that in this species the gene is located in a completely different genomic location — on the third chromosome. This provides a simple explanation of hybrid incompatibility in this model: males that are homozygous for the D. simulans fourth chromosome in an otherwise D. melanogaster background entirely lack JYAlpha and are sterile as a result.

As the authors point out, this example does not provide an exact model of how hybrid sterility leads to reproductive isolation: F1 hybrids between D. melanogaster and D. simulans are sterile or inviable for other reasons and only a fraction of the F2 generation is affected by JYAlpha. However, it is clear that in other situations a change in the location of a gene could lead to a more complete isolation of two populations. For example, this would occur if a Y chromosome gene that is essential for fertility relocated to the X chromosome.

Previous studies that have looked for divergences of gene function as a cause for reproductive isolation have so far found few examples. This study suggests that searching for changes in the genomic location of genes could prove a fruitful approach for future studies that aim to understand the genetic basis of speciation.