Even if two populations or species overcome these barriers, some genetic hurdles might stand in the way of reproducing. Sometimes distantly related species (horses and donkeys) can mate and reproduce, but the resulting offspring will often be sterile (the mules). This hybrid sterility might be caused by differing chromosome numbers or incompatibilities between genetic loci in the hybrid organism. In a cool paper recently published in Science, Malik and Bayes report that highly repetitive satellite DNA might be a major player in driving this kind of genetic reproductive isolation.
Female Drosophila similans, one of the two fruit fly species studied by Malik and Bayes.
The model organisms that Malik and Bayes used in this study are Drosophila simulans and Drosophila mauritiana. These closely related fruit fly species are able to mate with one another, producing fertile females and sterile males. This property makes them a favorite subject for research on speciation and reproductive isolation. The central protein in this paper is OdsH, which is able to bind DNA via its homeodomain. Since it was known that OdsH is involved male sterility in hybrid Drosophila, researchers suspected that it bound to promoter regions of genes and that hybrid sterility could be explained by transcriptional defects. However, ablation of the gene in drosophila did not affect male sterility much, so Malik and Bayes hypothesized that OdsH binds to satellite DNA instead and that hybrid sterility arises due to incorrect packing and condensation of heterochromatin.
Using clever fluorescent tagging of the different OdsH proteins from simulans and mauritiana (OdsHsim and OdsHmau, respectively), the researchers were able to pinpoint to which stretches of DNA OdsH binds in both normal and hybrid genetic backgrounds. They did a lot of different localization experiments, whereas I’m only showing some of the essential ones here due to space constraints. In figure 1H and 1G you can see what happens when OdsHsim and OdsHmau are both expressed in a Drosophila simulans genome. In female cells, the two different OdsH proteins bind to the centromeric repetetive regions of the same chromosomes (4 and X). In male cells however, OdsHmau binds to the Y-chromosome whereas OdsHsim does not! If you look carefully, you might notice that it is difficult to distinguish chromosome 4 and Y, due to the decondensation of both chromosomes, hinting at the possible mechanism behind the male sterility.
The localization of OdsHsim and OdsHmau in male and female neuroblast cells.
In addition, the researchers investigated where OdsHsim and OdsHmau localize in mauritiana and sechellia (a sister species). The results of these experiments are shown in figure 2B below. The localization of the OdsH proteins to these homologous chromosomes is remarkably different in all species, suggesting massive changes in OdsH binding sites over the past 250,000 years (when the different lineages diverged). This is also reflected in the high number of non-synonymous mutations between OdsHsim and OdsHmau.
Summary of all the localization studies that have been performed with the OdsH proteins.
Using introgressed sterile Drosophila simulans carrying the OdsHmau gene, Malik and Bayes have shown that OdsHmau accumulates and is active longer in spermatocytes than OdsHsim. It is likely that this adverse activity of OdsHmau affects the normal development of sperm cells, leading to the male hybrid sterility. Malik says that the decondensation of the Y-chromosome might be responsible for chromosome segregation defects or that altered chromatin modifications, causing male sterility. All in all, changes in the repetetive satellite DNA seem to be of major importance for reproductive isolation and evolution. I really like this paper because it uncovers evolutionary properties of a type of DNA which I previously thought was a bit boring (in my defense: ‘repetetive’ really does sound a bit boring!).
Bayes, J., & Malik, H. (2009). Altered Heterochromatin Binding by a Hybrid Sterility Protein in Drosophila Sibling Species Science, 326 (5959), 1538-1541 DOI: 10.1126/science.1181756
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Interesting post! I remember working with fruit flies; we didn’t use any microscopes, and it was very difficult to tell the difference between the males and the females (males smaller with a darker abdomen). I’m pretty sure that at least some of my breeding set-ups didn’t work due to imcompatible sexes!