Taking a closer look at gene expression in the testis.
Every student of speciation should be familiar with the Bateson-Dobzhansky-Muller (BDM) model of genetic incompatibilities. Most evolutionary biologists can probably explain the rationale behind this model, but not everyone will know its interesting history (and why I chose to list these three names). The model was formulated by Dobzhansky (1934) and further developed by Muller (1942). However, Bateson (1909) already published an essentially identical model, apparently unknown to Dobzhansky and Muller, to explain the “secret of interracial sterility”. The BDM-model is very intuitive. Here is the short version from my PhD thesis:
Consider two allopatric populations diverging independently, with the same ancestral genotype AABB in both populations. In one population, a mutation (A -> a) appears and goes to fixation, resulting in aaBB, which is fertile and viable. In the other population, another mutation (B -> b) appears and goes to fixation, resulting in AAbb, which is also fertile and viable. When these populations meet and interbreed, this will result in the genotype AaBb. Alleles a and b have never “met” each other and it is possible that allele a has a deleterious effect that becomes apparent when allele b is present, or vice versa. Over evolutionary time, numerous of these incompatibilities may arise, each possibly contributing to hybrid sterility or unviability.
This model has been mostly applied to mutations in protein-coding genes, but could be extended to the regulation of gene expression. Regulatory regions come in two main types: cis-regulatory elements that are linked to nearby genes and trans-regulatory elements that affect distant genes (millions of DNA-letters apart). Interacting cis- and trans-regulatory elements often evolve in concert, and a mutation in one element can be compensated by a mutation in the other element. When species have experienced different compensatory mutations and interbreed, the gene expression in hybrids might be disturbed, leading to sterility or unviability.
A recent study in the journal Genome Research applied this reasoning to hybrids between Pied Flycatcher (Ficedula hypoleuca) and Collared Flycatcher (F. albicollis). These two species diverged about one million years ago and interbreed in several locations, including the Swedish island of Öland (where the group of Anna Qvarnström has been monitoring the breeding populations for numerous years). Previous work showed that male hybrids are infertile due to the production of abnormal sperm cells. Could male sterility be the result of disrupted gene expression due to mismatches between cis- and trans-regulatory elements? To answer this question, the researchers took a closer look at gene expression patterns in five Pied Flycatchers, five Collared Flycatchers and three natural hybrids.
The analyses focused on misexpression in hybrids, which can be detected by gene expression levels in hybrids that are either higher or lower than any of the parental species. The researchers reported “evidence for abundant hybrid misexpression in heart, kidney, and liver but not in brain or testis.” In addition, more detailed analyses of genes involved in spermatogenesis did not reveal misexpression in hybrids. All in all, this study could not provide evidence that disrupted gene expression in the testis causes sterility in hybrid males. However, the high levels of misexpression in other tissues could contribute to lower hybrid fitness in other ways.
Evolution at Two Levels
Although the testis showed no clear signs of misexpression in hybrids, this tissue did experience the highest level of divergence in gene expression between Pied and Collared Flycatcher. More research will be needed to unravel the exact changes in gene expression and their contribution to male sterility, but it seems unlikely that mismatches between cis- and trans-regulatory elements play a major role.
Despite the “negative” result, this study nicely highlights the potential involvement of regulatory changes in evolution and the formation of new species. In 1975, Mary-Claire King and A. C. Wilson already drew attention to the contrast between evolution at the sequence level and changes in patterns of gene expression. Focusing on human evolution, they noted that “a relatively small number of genetic changes in systems controlling the expression of genes may account for the major organismal differences between humans and chimpanzee.” At the time, we did not have the methods to explore how regulatory changes shape evolutionary trajectories. The development of new techniques, such as RNAseq, provide exciting opportunities to understand how changes in gene expression contribute to the origin of new species. What a wonderful time to be an evolutionary biologist.
Mugal, C.F., Wang, M., Backström, N., Wheatcroft, D., Ålund, M., Sémon, M., McFarlane, S.E., Dutoit, L., Qvarnström, A. & Ellegren, H. (2020). Tissue-specific patterns of regulatory changes underlying gene expression differences among Ficedula flycatchers and their naturally occurring F1 hybrids. Genome Research, 30(12), 1727-1739.
Featured image: Collared Flycatcher (Ficedula albicollis) © Andrej Chudy | Wikimedia Commons