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Many bird species are monogamous. However, genetic studies have shown that the social partner is often not the genetic father of all offspring. Some studies found biased sex ratios: more males than females among extra-pair fledglings. This has been interpreted as evidence of adaptive sex allocation by females: if an extra-pair mate is of high quality and this quality has a genetic basis, fitness can be optimized if offspring with the extra-pair mate's "good" genes are predominantly male. However, there is no known mechanism by which such a shift in sex ratio is achieved.
Neuhäuser et al. (2025) now propose an alternative, non-adaptive explanation. Some molecular complexes required for cellular respiration are composed of proteins, some of which are encoded by nuclear genes on the sex chromosome Z and others by mitochondrial genes. These proteins must be compatible; incompatibility can impair viability. Female birds are more vulnerable than males because female birds have only one Z chromosome (from the father), and mitochondrial genes are inherited through the maternal line as usual. Males have two Z chromosomes, thus they receive both a Z chromosome and the mitochondria from their mother. If an extra-pair father is less compatible with a female than the social partner, relatively greater mortality among extra-pair daughters can bias the sex ratio. This is plausible in various situations.
Using data from the northern house wren (Troglodytes aedon), a North American songbird, where extra-pair young have been shown to be more likely to be male, the mitonuclear incompatibility hypothesis was retrospectively tested. Consistent with the hypothesis, there were more unhatched eggs in broods with extra-pair young and significant differences in the variance and skewness of the distributions between extra-pair and intra-pair sex ratios.
Participating institutions:
Koblenz University of Applied Sciences, Deutschland
Towson University, Maryland, USA
Illinois State University, USA
Koblenz University of Applied Sciences, RheinAhrCampus, Dept. of Mathematics, Informatics and Technology
neuhaeuser@rheinahrcampus.de
Neuhäuser, M., Johnson L.S., Master, B.S., Sakaluk, S.K. & Thompson C.F. (2025): Mito-nuclear incompatibility as an alternative hypothesis for male-biased offspring sex ratios arising from extra-pair matings. Journal of Ornithology (https://link.springer.com/article/10.1007/s10336-025-02284-2)
https://doi.org/10.1007/s10336-025-02284-2
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