Let me take a selfie: Scientists discover hybridization in a selfing fish

Newly discovered site in the Bahamas houses a hybrid between two groups of mangrove rivulus fish.

Self-fertilization is extremely rare in vertebrates. The mangrove rivulus is the only known vertebrate that routinely reproduces by selfing (teenagers taking selfies do not count). Given this peculiar mode of reproduction, hybridization should be rare. Or so you would think…

 

Three Groups

The mangrove rivulus currently comprises two species (Kryptolebias marmoratus and K. hermaphroditus), but can be divided into three main groups:

  1. Northern group in Florida and the Caribbean that corresponds to K. marmoratus
  2. Southern group in Brazil that corresponds to K. hermaphroditus
  3. Central group in the Caribbean that probably corresponds to a third species
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A selfing vertebrate, the mangrove rivulus (from: http://www.wikipedia.com/)

 

Natural Hybrids

A laboratory experiment already showed that hybridization between fish from the central group and K. hermaphroditus is possible. But does it also occur in nature? A recent study in the journal Biology Letters answers this question: yes!

Andrey Tatarenkov and his colleagues discovered a site in San Salvador (the Bahamas) where members from the central group and K. marmoratus coexist. This finding is already quite special because the three groups of rivulus were not known to co-occur. Genetic characterization of this population revealed a hybrid individual: a cross between a male K. marmoratus and a hermaphrodite of the Central group. Further analyses suggested that this hybrid has been selfing for two generations.

selfing hybrid.jpg

The hybrid (OY9 in the red circle) is genetically intermediate between K. marmoratus (left) and the central group (right). Pink squares are individuals from the newly discovered site in the Bahamas (from: Tatarenkov et al. 2018 Biology Letters)

How common hybridization is in this population and if there is gene flow between the different species/groups remains to be investigated.

 

References

Tatarenkov, A., Earley, R.L., Taylor, D.S., Davis, W.P. & Avise, J.C. (2018) Natural hybridization between divergent lineages in a selfing hermaphroditic fish. Biology Letters, 14:20180118.

 

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Coal tits exchange genes in central Europe

Genetic study of European coal tits reveals gene flow across a continental contact zone.

Hybridization is often studied in the context of hybrid zones. After a period of geographical isolation, populations might come into secondary contact and interbreed. In Europe, this scenario has been documented for numerous animal and plant taxa. The classical papers by Godfrey Hewitt (see for example herehere and here) describe an intriguing story of how populations were pushed in southern refugia by expanding ice sheets during the Quaternary (the geological period starting about 2.5 million years ago). Possible refugia included the Iberian peninsula, Italy and the Balkans. When the climate warmed and the ice retreated, these populations expanded from their refugia and recolonized the European continent. Organisms from different refugia met each other in different contact zones (see figure below).

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Four examples of how hybrid zones (HZ) can be established as populations expand out of their southern refugia: (a) meadow grasshopper, (b) European hedgehog, (c) brown bear, and (d) common chub. From: Hewitt (2004).

 

German Coal Tits 

The establishment of a secondary contact zone has several possible outcomes: (1) the formation of a narrow hybrid zone, (2) a broad zone of intergradation, or (3) merging of both lineages. A recent study in the Biological Journal of the Linnean Society explored which of these outcomes best described the situation of the coal tit (Periparus ater).

The genetic analysis of this small passerine revealed that all German populations were a mixture of southern and northern populations. This amalgamation of coal tit genes was not restricted to a narrow contact zone, but expanded across a wide area of intergradation. This genetic pattern is supported by morphological data from previous studies that show how different subspecies gradually change from north to south.

1200px-Coal_tit_UK09

A Coal Tit (from http://www.wikipedia.com/)

 

Isolated Island Populations

In addition, island populations of the coal tit were genetically distinct from the mainland birds. The population on Cyprus (subspecies cypriotes) is a genetically and morphologically distinctive form. It probably dates back to an ancient colonization event. The genetic differences between the populations on Corsica and Sardinia (subspecies sardus) and the mainland are more subtle. These islands might have received – or are still receiving – gene flow from continental birds.

 

References

Tritsch, C., Stuckas, H., Martens, J., Pentzold, S., Kvist, L., Lo Valvo, M., Giacalone, G., Tietze, D.T., Nazarenko, A.A. & Päckert, M. (2018) Gene flow in the European coal tit, Periparus ater (Aves: Passeriformes): low among Mediterranean populations but high in a continental contact zone. Biological Journal of the Linnean Society, 124(3):319-338.

 

The paper has been added to the Paridae page.

 

 

 

OMG (Owls, Migration and Genetics)! A Heteropatric Speciation Model for Northern Saw-whet Owls

Two owl subspecies might be the outcome of a peculiar speciation model.

Open a standard textbook on the origin of new species and you will probably come across three main speciation models: allopatric, sympatric and parapatric speciation. In short, allopatric speciation occurs when two populations become geographically isolated leading to a sudden break in gene flow. Over time these populations diverge genetically into different species. Alternatively, during sympatric speciation, populations diverge even though they live in the  same habitat. Parapatric speciation, finally, entails the differentiation of two neighbouring populations that still exchange some genes in the process.

 

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Three different speciation models in a standard textbook.

 

Mixing the Models

Besides this Mayrian triumvirate (as German biologist Ernst Mayr popularized these models), several other modes of speciation are possible. One of these is heteropatric speciation, in which populations occur in the same area at some times during the year, but are geographically separated at other times (in a sense a hybrid between allopatric and sympatric speciation). This model nicely applies to birds where one population is migratory while the other is sedentary. You can read Kevin Winkers’ excellent review on this process in Ornithological Monographs.

 

The Origin of Owl Subspecies

Two subspecies of the Saw-whet Owl (Aegolius acadicus) might have diverged in a heteropatric fashion. One subspecies brooksi remains sedentary on the island of Haida Gwaii (British Columbia) while the other subspecies acadicus is migratory. During fall and winter both subspecies co-occur, but during the rest of the year they are far apart. Are they still interbreeding and exchanging genes when they meet?

To figure this out, Jack Withrow, Spencer Sealy and – not surprisingly – Kevin Winker sequenced mitochondrial and nuclear DNA (AFLPs) from both subspecies. The analyses revealed that the subspecies are genetically differentiated with extremely low levels of gene flow. In fact, it seems likely that there is no gene flow at all (a genomic approach is necessary to be sure).

 

saw-whet owl.jpg

A Northern Saw-whet Owl (from http://www.hwb.com)

 

Ice Ages

Nonetheless, the most plausible scenario is that an ancestral acadicus population colonized the Haida Gwaii about 16,000 years ago when this island was a refugium during the ice ages. This population became sedentary (and who could blame them, wouldn’t you like to live on an island?) and started diverging from the migratory mainland population. A nice example of heteropatric speciation?

 

References

Withrow, J. J., S. G. Sealy and K. Winker (2014). Genetics of divergence in the Northern Saw-whet Owl (Aegolius acadicus). The Auk 131(1): 73-85.

 

The paper paper has been added to the Strigiformes page.