The comparison of three species pairs leads to some surprising findings.
In 2005, Thomas Turner and his colleagues reported on “genomic speciation islands” in the African malaria mosquito (Anopheles gambiae). In their PLoS Biology paper, the authors described how some genomic regions remain differentiated despite considerable gene flow, and they speculated that these regions might contain the genes responsible for reproductive isolation. However, further studies on other organisms, such as Heliconius butterflies and Ficedula flycatchers, indicated that the term “speciation islands” was a bit premature. Other evolutionary processes can give rise to differentiated genomic islands. To understand how these genomic islands can arise, we must first take a closer look at the popular summary statistic Fst.
The fixation index (Fst) is a measure of population differentiation due to genetic structure. It is important to realize that Fst is a relative measure because it compares the genetic diversity between populations while taking into account the genetic diversity within each population (you can nicely see this in the formula below, where π is genetic diversity). Hence, you can get a peak in Fst at a certain genomic region when one population has low genetic diversity at this location. This reduction in genetic diversity can be the outcome of genetic drift or a selective sweep, and might thus be unrelated to reproductive isolation. This issue with Fst can be resolved by calculating another summary statistic (Dxy) which is not influenced by genetic diversity within populations. The relationship between Fst and Dxy can be very insightful: Fst peaks that result from locally reduced gene flow are predicted to have elevated Dxy, while Fst peaks resulting from lower genetic diversity in a population are not.
Hummingbirds and Chromosomes
With this knowledge in mind, evolutionary biologists try to understand how genetic differentiation accumulates in the genome during speciation. Are peaks in Fst related to reproductive isolation or are they the outcome of reduced genetic diversity? Because it is mostly not feasible to document the entire speciation process (which takes at least thousands of years), researchers compare closely related species pairs at different stages of divergence. A recent study in the journal BMC Evolutionary Biology focused on three pairs of hummingbirds that diverged at different times, namely:
- Anna’s (Calypte anna) and Costa’s hummingbird (C. costae) – 2.5 million years
- Black-chinned (Archilochus alexandri) and Ruby-throated hummingbird (A. colubris) – 1.5 million years
- Allen’s (Selasphorus sasin) and Rufous hummingbirds (S. rufus) – 0.93 million years
The researchers – Elisa Henderson and Alan Brelsford – were mainly interested in the role of recombination in the build-up of genetic differentiation. Low recombination rates are predicted to lead to reduced genetic diversity because selection on one genetic variant will affect large genomic regions that are linked to this variant. If recombination rate is high, however, the genetic variant under selection will be confined to a smaller genomic region and the reduction in genetic diversity will be more localized. Given that large chromosomes have lower recombination rates, we can expect bigger reductions in genetic diversity and consequently more peaks in Fst. In other words, larger chromosomes will diverge faster compared to smaller chromosomes. In addition, sex chromosomes (Z and W for birds) also show reduced recombination and can thus accumulate genetic differentiation faster than autosomes.
The genomic analyses resulted in some interesting results. The authors found that “speciation seems to progress at different rates based on chromosome type, with the sex chromosome diverging first, the microchromosomes diverging next, and divergence only appearing on the macrochromosomes in late stages of reproductive isolation.” The finding that sex chromosomes diverge first is logical. These chromosomes show reduced rates of recombination and are known to accumulate incompatible alleles that can contribute to reproductive isolation (see for example this blog post on the Reunion grey white-eye, Zosterops borbonicus).
Given the predictions outlined above, the result that microchromosomes diverge before macrochromosomes is quite surprising. Given the lower recombination rate on larger chromosomes, we would have expected the opposite pattern. The authors suspect that the early accumulation of Fst peaks on microchromosomes may be due to certain characteristics of these small chromosomes. For example, microchromosomes have a high gene density which might provide more targets for selection, leading to lower genetic diversity and consequently peaks in Fst. Or perhaps these small chromosomes might harbor specific genes that contribute to reproductive isolation? More research is needed to pinpoint the exact mechanisms.
Apart from Fst, the researchers also calculated Dxy. As explained above, Fst peaks that result from locally reduced gene flow are predicted to have elevated Dxy, while Fst peaks resulting from lower genetic diversity in a population are not. In this case, there was a negative correlation between Fst and Dxy, suggesting that most differentiated regions are the outcome of lower genetic diversity in one population (due to genetic drift or selection). There might be some genomic regions that are involved in reproductive isolation, but more detailed analyses are needed to find these.
This study shows how we can gain insights into the process of speciation by comparing species pairs at different stages of divergence. There is, however, an important issue to take into account when performing these kinds of analyses. Namely, species-specific differences in natural history and morphology can lead to different genetic signatures during the speciation process. The authors nicely formulated this caveat at the end of their paper.
These differences across the species used in this study highlight that each species pair is subject to its own evolutionary trajectory leading to a unique speciation event. While this is a general caveat of using independent species pairs as a proxy for the speciation continuum, we believe that the differences we observe among chromosome types can inform the ongoing debate about the roles of selection and recombination in the genetics of speciation.
Henderson, E. C., & Brelsford, A. (2020). Genomic differentiation across the speciation continuum in three hummingbird species pairs. BMC Evolutionary Biology, 20(1), 1-11.
Featured image: Ruby-throated hummingbird (A. colubris) © JeffreyW | Wikimedia Commons
The paper has been added to the Apodiformes page.