A recent study uses captive breeding records to assess hybrid fertility in capuchino seedeaters.
If you explore this website (which I encourage you to do), you will notice that many avian hybrids have been documented. The latest estimate suggests that about 16% of bird species has hybridized with at least one other species. But are these hybrids also fertile? In many cases, we don’t have a direct measure of hybrid fertility and viability. A recent study in PLoS One tries to assess the fertility of Sporophila hybrids using captive breeding experiments.
The genus Sporophila contains 11 species of capuchino seedeaters that evolved in the last million years. The species differ in their male plumage, as illustrated by the excellent drawings by Jillian Ditner below. Based on abnormal plumage patterns, some hybrids have been documented in the wild. But nothing is known about their fertility. Leonardo Campagna and his colleagues compiled breeding data between 2006 and 2016 to explore hybrid fertility.
The results showed that hybrid crosses had a higher hatching success than the pure ones. This might seem counter-intuitive, but the lower hatching success of conspecific pairs is probably a consequence of inbreeding in captivity. Nevertheless, the data show that in general Sporophila hybrids are fertile.
Further analyses revealed that hybrid crosses produced an excess of males and that some hybrid females were infertile. This observation is in line with Haldane’s Rule, which states that in hybrids, the sex with two different sex chromosomes (in birds, the females with ZW) will be the first to suffer from infertility or inviability.
Plenty of Plumage Patterns
Let’s focus on the plumage of the hybrids. In short, there was a lot of variation: some hybrids looked like their parents, some were intermediate and others showed completely new plumage patterns. This explains why it is so difficult to observe hybrid seedeaters in the wild. Some hybrids are just indistinguishable from the pure species.
A Final Lesson
The dataset in this study was not ideal to explore hybrid fertility in these birds. The authors write that ‘breeding records [were] originally compiled by hobbyist aviculturalists during efforts to establish lines of captive-born capuchinos that would alleviate the trapping pressure of wild birds from illegal pet trade.’ But I think they used the data to its full potential.
It does provide us with an important lesson: if possible, collaborate with bird breeders. These people have years of experience and often produced a wealth of data waiting to be analysed. During my PhD, I worked with waterfowl breeders to obtain blood samples from different goose species. I could not have finished my PhD without them!
Campagna, L., Rodriguez, P. & Mazzulla, J.C. (2018) Transgressive phenotypes and evidence of weak postzygotic isolation in F1 hybrids between closely related capuchino seedeaters. PLoS One, 13(6):e0199113.
This paper has been added to the Thraupidae page.
Solution to F1 hybrid pictures:
- Left = S. palustris (zelichi morph) x S. hypoxantha
- Right = S. ruficollis x S. cinnamomoea