Che Figata! New studies on the ecology and evolution of a hybrid species, the Italian Sparrow

The Italian Sparrow (Passer italiae) is a curious little bird. The plumage of this sparrow, which occurs in Italy (what a surprise!) and several Mediterranean islands, seems to be a mixture of a House Sparrow (P. domesticus) and a Spanish Sparrow (P. hispaniolensis). Biologists speculated that the Italian Sparrow is the result of hybridization between these species. A recent study in Science Advances provides a detailed look into the genome of this hybrid species.

 

An Italian Sparrow (from http://www.gobirding.eu)

 

Genetic Mixture

As expected, the genome of the Italian Sparrow is a mixture of the two parental species. Some parts of the DNA resemble those of House Sparrow, while others are closer to Spanish Sparrow. In general, however, the Italian Sparrow is genetically most similar to the House Sparrow. It could be that there were just more House Sparrows around during the first wave of hybridization with Spanish Sparrows, leading to an excess of the former species DNA.

But it is also possible that House Sparrow genes provide a selective advantage. This hypothesis is supported by the similar ecology of House Sparrow and Italian Sparrow. Both species live in cities and villages where they follow a diet of grains, rice and insects. The Spanish Sparrow, which mostly occurs in dry areas, has a different diet, consisting primarily of insects. A previous study in the journal Heredity already showed that these differences in diet lead to divergence in beak morphology. The new study supports this conclusion with genetic data: genes that influence beak morphology are significantly different between Italian Sparrow and Spanish Sparrow.

An additional discovery of this genetic study was the presence of parental mitochondrial DNA (mtDNA) in some individuals. Normally, mtDNA is only passed on to the offspring through the maternal line. The mtDNA from the father is destroyed when the sperm cell reached the egg. Other studies have shown that this mechanism sometimes fails in hybrids. This might explain why some Italian Sparrows inherited mtDNA from their fathers.

 

A Spanish Sparrow (from http://www.hbw.com)

 

Competition

Another recent study, published in the Proceedings of the Royal Society B, focused on the interaction between the Italian Sparrow and one of its parents, the Spanish Sparrow. In 2013, Spanish Sparrows invaded a population of Italian Sparrows in the nature reserve station of Lago Salso (Italy). The arrival of the Spanish ‘invaders’ led to a change in feeding behavior. Before 2013, Italian Sparrows would feed on cereal fields close to the nature reserve station. By 2015, the Spanish Sparrows has almost completely monopolized this field, forcing their Italian cousins to feed in other locations. This shift in habitat use by Italian Sparrows led to a significant drop in body condition, suggesting a negative impact of competition with Spanish Sparrows.

The changes in habitat use and diet did not result in morphological changes, but were nonetheless noticeable on a genetic level. There was an clear increase in genetic divergence between the two species following the arrival of Spanish Sparrows. Interestingly, four highly divergent genes are known to be involved in cellular processes linked to learning and brain development. The precise mechanism behind these patterns remains to be investigated. But one thing is certain, the Italian Sparrow is a goldmine for geneticists!

 

 

References

Elgvin, T. O., C. N. Trier, O. K. Tørresen, I. J. Hagen, S. Lien, A. J. Nederbragt, M. Ravinet, H. Jensen and G.-P. Sætre (2017). The genomic mosaicism of hybrid speciation. Science Advances 3(6): e1602996.

Eroukhmanoff, F., J. S. Hermansen, R. I. Bailey, S. A. Saether and G. P. Saetre (2013). Local adaptation within a hybrid species. Heredity 111(4): 286-292.

Saetre, G. P., A. Cuevas, J. S. Hermansen, T. O. Elgvin, L. P. Fernandez, S. A. Saether, C. L. Cascio Saetre and F. Eroukhmanoff (2017). Rapid polygenic response to secondary contact in a hybrid species. Proc Biol Sci 284(1853).

 

These papers have been added to the Passeridae page.

Splitting Buntings in the Sahara

The House Bunting complex houses two distinct species, but might have had a history of hybridization.

In taxonomy, you have lumpers and splitters. Lumpers like to keep everything together, while splitters don’t mind dividing a species into two. The Saharo-Arabian desert belt is a splitters dream. This region extends from the Sahara in North Africa over the Arabian Peninsula into Pakistan and India. Several bird species have distinct populations in the eastern and western part of this arid environment. Distinct populations? Geographically separated, you say? Let’s start splitting!

Indeed, in recent years, it has been proposed to elevate some populations to species status. For example, the Houbara Bustard (Chlamydotis undulata) and MacQueen’s Bustard (C. macqueenii), and the African Desert Warbler (Sylvia deserti) and the Asian Desert Warbler (S. nana). In a new study, Manual Schweizer and colleagues focus on another species complex in the Saharo-Arabian desert belt: the House Bunting (Emberiza striolata) complex.

 

House Bunting (from http://www.hbw.com)

 

Recent taxonomic studies have culminated in the proposal to recognize two species in this complex: the House Bunting (E. sahari) in the west and the Striolated Bunting (E. striolata) in the east. In addition, morphologically intermediate birds have been found in Sudan and Chad. Might these be hybrids?

The east-west divergence is supported by two mitochondrial markers, suggesting that the split is justified. The intermediate birds, however, reveal a striking pattern. Morphologically, these specimens resemble the eastern species (E. striolata), but genetically they are closer to the western species (E. sahari). Incongruence between mtDNA and morphology has been reported in other studies as well and can be explained by incomplete lineage sorting or hybridization. The researchers speculate that ‘introgressive hybridization in a secondary contact zone is the most likely explanation.’ But to test this hypothesis, more genetic data is needed.

Luckily for the splitters, these results do not impinge (a wonderful word the authors used in the abstract) on the decision to recognize two species.

 

Two Striolated Buntings (from http://www.birdforum.net)

 

References

Schweizer, M., H. Shirihai, H. Schmaljohann and G. M. Kirwan (2017). Phylogeography of the House Bunting complex: discordance between species limits and genetic markers. Journal of Ornithology: 1-15.

 

This paper has been added to the Emberizidae page.

More Species Than Meets the Eye: A Genetic Look at the Red-eyed Vireo Complex

The Red-eyed Vireo complex houses more than the five species that are currently recognized. And of course, there has been some hybridization.

The taxonomic world of birds is full of species complexes, groups of closely related species that are so similar in appearance that the boundaries between them are fuzzy. And often these ‘species’ are also interbreeding, rendering the situation even more complex. Some examples include Redpolls (genus Acanthis) and Bean Geese (Anser fabalis, see here). These species complexes provide great opportunities to study hybridization and speciation in birds.

In a recent study, published in Molecular Phylogenetics and Evolution, C.J. Battey and John Klicka focus on the Red-eyed Vireo (Vireo olivaceus) species complex. This group of passerine birds is currently divided into five species:

• Red-eyed Vireo (V. olivaceus)
• Noronha Vireo (V. gracilirostris)
• Yellow-green Vireo (V. flavoviridis)
• Black-whiskered Vireo (V. altiloquus)
• Yucatan Vireo (V. magister)

The researchers obtained material of all species, except for the Noronha Vireo, a species endemic to the Noronha Island (Brazil). A DNA analysis of the other four species led to some interesting findings. Let’s have look!

 

Cryptic Species

The genetic data revealed that there are more species than meets the eye. The Red-eyed Vireo consists of Northern and Southern hemisphere populations, whereas the Yellow-green Vireo is structured into Eastern and Western populations. For the Red-eyed Vireo this distinction is quite obvious and there does not seem to be any gene flow between the populations. The situation for the Yellow-green Vireo, however, is less clear. It appears that this species is currently stuck in the “species/subspecies conundrum.” More data are warranted here.

 

Red-eyed Vireo (from: http://www.wikipedia.com/)

 

Ancient Gene Flow

Two species pairs show evidence for gene flow. The Northern population of the Red-eyed Vireo has exchanged genes with the Western population of the Yellow-green Vireo. And the Southern population of the Red-eyed Vireo has interbred with the Black-whiskered Vireo. The very low levels of introgression suggest that it probably concerns historic gene flow.

 

Yellow-green Vireo (from: http://www.hbw.com/)

 

Recent Hybrids?

Interestingly, populations of Red-eyed Vireo on the island of Trinidad seem to be hybrids between Red-eyed Vireo and Black-whiskered Vireo. However, more extensive sampling is needed to check whether there is a hybrid zone on this island.

Black-whiskered Vireo (from http://www.birdspix.com/)

 

Speciation via Migration Loss

Finally, the Yucatan Vireo might represent an example of speciation via loss of migration. This speciation model states that a group of individuals ‘decides’ to stop migrating and settle down, while the remainder of the population continues its migratory habits. Over time, the resident and migratory populations diverge until they can be considered two separate species. Two observations support this scenario for the Yucatan Vireo: (1) its ancestors are mainly migratory and (2) its current range is close to a stopover site for the Black-whiskered Vireo, which is its closest relative.

 

Yucatan Vireo (from http://www.hbw.com/)

 

References

Battey, C. J. and J. Klicka (2017). Cryptic speciation and gene flow in a migratory songbird Species Complex: Insights from the Red-Eyed Vireo (Vireo olivaceus). Molecular Phylogenetics and Evolution 113: 67-75

 

This paper has been added to the brand-new Vireonidae page

Mottled Mallards on the Western Gulf Coast

The Mallard (Anas platyrhynchos) is one of the most active species when it comes to hybridization. This familiar duck species interbreeds with numerous other duck species, including the Mottled Duck (A. fulvigula).

In January 2017, I wrote about Mottled Ducks in North America (see here). This blogpost focused on the genetic divergence between two Mottled Duck populations in Florida and the Western Gulf Coast. I briefly mentioned hybridization with Mallards.

In a recent study, published in The Condor, Robert Ford (Louisiana State University) and colleagues investigate the hybridization dynamics between these duck species in the Western Gulf Coast populations. Using microsatellites, they estimate levels of hybridization between 5 and 8%, which is lower compared to Florida (about 9%).

A pair of Mottled Ducks (from http://www.birdzilla.com)

 

Limited Interactions

Why are these levels of interbreeding so low? Probably because Mallards and Mottled Ducks do not meet that often. Mottled Ducks are not migratory and remain all year at the Western Gulf Coast, whearas Mallards do migrate. In addition, Mottled Ducks form pairs in fall, starting as early as August. When the Mallards arrive, most Mottled Ducks have already found a partner. But despite these limited opportunities for hybridization, hybrids do occur. This can probably be explained by such behavioural processes as interspecific nest parasitism and extra pair copulations (see here for an overview in geese). Hybridization might also be due to feral Mallards that have escaped from game breeders.

 

No Conservation Issue (Yet?)

For now, it is unclear whether duck hybridization in the Western Gulf Coast will be a conservation issue in the future. The most important factor seems to be habitat loss, which could drive Mottled Ducks into urban areas where feral Mallards could welcome them with open arms (or wings)…

 

References

Ford, R. J., Selman, W. & Taylor, S. S. (2017). Hybridization between Mottled Ducks (Anas fulvigula maculosa) and Mallards (A. platyrhynchos) in the western Gulf Coast region. The Condor 119, 683-696.

 

Thanks to Robert Ford for sending me a copy of the paper. This study has been added to the Anseriformes page.

 

Hybridization in Primates (and yes, that includes you!)

A recent paper in Current Opinion in Genetics and Development gives a concise overview of hybridization in primates, including humans.

Let’s start with some numbers. The genetic contribution of one primate species to the genome of another by means of ancient gene flow:

• Tibetan Macaque genomes contain 1-8% DNA from Rhesus Macaques
• Chimpanzee genomes harbor about 1% Bonobo DNA
• Non-African humans have 1 to 5% of Neanderthal DNA (Africans never came into contact with Neanderthals and hence to not have their DNA)
• Modern-day people from Oceania have 4-6% Denisovian (an extinct group of archaic humans) ancestry in their genomes

These figures clearly indicate that gene flow has occurred during primate evolution. The obvious question is: did it matter? Did ancient hybridization influence the evolutionary history of primates? For Neanderthal genes it has been shown that they contribute to depression and the immune system. For non-human primates, the question remains unanswered for now. But the advent of more genomic data hold promise to tackle this issue.

A Tibetan Macaque. Understandably irresistible to ancient Rhesus Macaques.

Hybrid Fertility

An interesting observation by the authors (Jenny Tung and Luis Barreiro) is absence of sterility or low viability of primate hybrids. Most hybrids, including those involving modern humans are alive and kicking (i.e. fertile). Some exceptions are hybrids in captivity, such as a Baboon x Rhesus Macaque cross, and hybrids between Black Howler Monkeys and Mantled Howler Monkeys in Mexico. In the latter case, only female hybrids have been reported, suggesting that male offspring are not viable. This is in line with the predictions of Haldane’s Rule.

A Black Howler Monkey. Hybrids between this species and Mantled Howler Monkeys are always female.

Evolutionary Ghosts

Another striking discovery is the existence of so-called ‘ghost lineages’. By sequencing ancient DNA researchers have found indications of gene flow in Africa between anatomically modern humans and older lineages that are now extinct. A similar patters was uncovered for African Baboons of the genus Papio. This seems to suggest that the African continent is home to several primate ghosts lineages. Scary! But seriously, this ancient DNA approach could lead to the discovery of unknown extinct species.

As if this guy isn’t scary enough with his long canines. His ancestors might have interacted with a ‘ghost lineage’.

Neanderthal DNA

The popular press has focused most on ancient gene flow between humans and Neanderthals. For some reason, intercourse between these two closely related species captivates the attention of the general public. Commercial companies that offer personal genome sequencing, such as 23andMe, even provide you with an estimate of Neanderthal ancestry in your own genome. I had my genome sequences this year. The result: 3.0% Neanderthal!

A Neanderthal kindly posing for a picture.

References

Tung, J. and L. B. Barreiro (2017). The contribution of admixture to primate evolution. Current Opinion in Genetics & Development 47: 61-68.