Genetic study uncovers gene flow between several Catharus thrushes.
“Hybridization is not always limited to two species; often multiple species are interbreeding.” This is the first sentence of my recent Avian Research review on multispecies hybridization in birds. In that paper, I argue that hybridization between multiple bird species is probably a common phenomenon, but that we do not know how important it is from an evolutionary point of view. However, before we can assess the evolutionary importance of multispecies hybridization, we first need to know which species are hybridizing. A recent study in the journal Molecular Phylogenetics and Evolution provided some insights for thrushes of the genus Catharus.
A Veery (Catharus fuscescens) © Cephas | Wikimedia Commons
Twelve Species
The Catharus thrushes are small passerines that have been important in understanding the genomic basis of migration (see here). However, the evolutionary relationships between the 12 species in this genus remain contentious. Therefore, Kathryn Everson and her colleagues used a set of ultraconserved elements (UCEs, you can read more about these molecular markers here) to delve into the speciation history of these birds.
Analyses of over 2000 UCEs resulted in well-resolved species tree in which the position of Swainson’s Thrush (C. ustulatus) was rather surprising. In contrast to previous studies, this species clustered with the Veery (C. fuscescens), the Grey-cheeked Thrush (C. minimus) and the Bicknell’s Thrush (C. bicknelli).
A Swainson’s Thrush (Catharus ustulatus) © VJAnderson | Wikimedia Commons
Hybridization
However, a closer look at individual gene trees revealed extensive conflict between several genes. In other words, different genes tell different evolutionary stories. Moreover, the species tree (based on UCEs) did not match the mitochondrial tree. These results suggest that hybridization might have influenced the evolution of these thrushes.
Indeed, testing explicitly for introgressive hybridization (using the D-statistic, see here for more details about this test) showed extensive gene flow between several species. The phylogenetic tree below shows the different hybrid interactions between the thrushes. Clearly, multispecies hybridization.
Phylogenetic tree of the genus Catharus. The arrows indicate gene flow between different species. Solid arrows show strong support (p<0.05), while dashed arrows show weaker support (p<0.1). From: Everson et al. (2019) Molecular Phylogenetics and Evolution
Mitochondrial Capture
The disagreement between UCEs and mtDNA can be explained by mitochondrial capture. The researchers suspect that an ancient hybridization event between Swainson’s thrush and the ancestor of the Ruddy-capped Nightingale-thrush (C. frantzii) and the Black-billed Nightingale-thrush (C. gracilirostris) might have resulted in the exchange of mtDNA.
A Black-billed Nightingale-thrush (Catharus gracilirostris) © Jerry Oldenettel | Flickr
Heteropatric Speciation
The results of this study are not only of interest to the question of multispecies hybridization, they are also relevant for the heteropatric speciation scenario. This model applies to populations that occur in the same area at some times during the year (when they can hybridize), but are geographically separated at other times. Migratory species, such as some of the Catharus thrushes, are an excellent study system to explore this speciation model. And the results from this study do fit the proposed scenario. Indeed, the authors write that “seasonal sympatry could promote hybridization and result in reticulate or networked genetic evolution among congeners.”
References
Everson, K. M., McLaughlin, J. F., Cato, I. A., Evans, M. M., Gastaldi, A. R., Mills, K. K., Shink, K. G., Wilbur, S. M. & Winker, K. (2019). Speciation, gene flow, and seasonal migration in Catharus thrushes (Aves: Turdidae). Molecular Phylogenetics and Evolution, 139, 106564.
Ottenburghs, J. (2019). Multispecies hybridization in birds. Avian Research, 10(1), 20.
Thanks to Kevin Winker for sending me this paper, which has been added to the Turdidae page.
Avian Hybrids 