Chimeric embryos—potential mechanism of avian offspring sex manipulation

Abstract

Environmental and social effects on offspring sex ratio bias in birds are among the most studied topics in evolutionary biology. Although it is established that offspring sex is determined at the stage of the first meiotic division, there is no direct evidence for the common belief that the bias happens via nonrandom sex chromosome segregation. In this paper, we suggest an alternative mechanism based on the failure of polar body emission, followed by fertilization of multiple haploid nuclei through polyspermy and subsequent competition of Z and W bearing cells within chimeric embryos. Although the occurrence of failure of polar body extrusion is obvious from the observations of chimeric birds, the idea that it could be the first step of offspring sex ratio bias has been entirely overlooked. We review the evidence in support of that idea, demonstrate that it is consistent with the observations of biased offspring sex ratio reported so far, and suggest a way to verify it.

Secondary sex ratio of nestlings of the blackbird (Turdus merula)

“…Therefore, the potential ability to control offspring sex could have strong fitness consequences (Godfray, Werren 1996;Tagirov, Rutkowska 2013). In birds, proportion of males in a clutch rose correspondingly with their fathers' sexual attractiveness in the collared flycatcher Ficedula albicollis as found by Ellergren et.…”

Climatically driven changes in population composition and offspring sex‐morph ratio in a polymorphic species

“…Therefore, results suggest that mechanisms allowing control of offspring sex-morph ratios are not less effective in TFs than in WFs, despite the fact that TFs are homozygous for morphdetermining chromosome 2 and thus unable to control the morph of eggs that matured or were fertilized. Instead, control of offspring sex-morph ratio could occur via chimeric embryos (Tagirov and Rutkowska 2013), reabsorption of zygotes, or egg dumping (Komdeur et al 2002, Pike and Petrie 2003, Navara 2013. In any case, genetic architecture with respect to the female's genotype for chromosome 2 does not appear to constrain adjustment of sex-morph ratio, suggesting flexibility of the mechanisms through which females adjust brood composition.…”

“…Therefore, results suggest that mechanisms allowing control of offspring sex‐morph ratios are not less effective in TFs than in WFs, despite the fact that TFs are homozygous for morph‐determining chromosome 2 and thus unable to control the morph of eggs that matured or were fertilized. Instead, control of offspring sex‐morph ratio could occur via chimeric embryos (Tagirov and Rutkowska ), reabsorption of zygotes, or egg dumping (Komdeur et al. , Pike and Petrie , Navara ).…”

“…, Navara ). However, because female birds are not necessarily heterozygous for morph‐determining chromosomes or genes, additional mechanisms would be needed to explain adjustment of brood morph ratios in birds, potentially including favoring certain sperm, reabsorbing zygotes, or chimeric embryos (Navara , Tagirov and Rutkowska ). Whether adaptive adjustment of brood sex‐morph ratios occurs in species with genetic polymorphisms remains unknown.…”

“…Some have suggested that female birds have stronger control over offspring sex than other species with chromosomal sex determination because they are heterozygous for the W and Z sex chromosomes (Komdeur et al 2002, Navara 2013). However, because female birds are not necessarily heterozygous for morph-determining chromosomes or genes, additional mechanisms would be needed to explain adjustment of brood morph ratios in birds, potentially including favoring certain sperm, reabsorbing zygotes, or chimeric embryos (Navara 2013, Tagirov andRutkowska 2013). Whether adaptive adjustment of brood sex-morph ratios occurs in species with genetic polymorphisms remains unknown.…”