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Fathers can influence the sex of their offspring

A new study in wild mice shows that fathers can and do influence the sex of their babies...


It is taught that in mammals only mothers are able to influence the sex of their offspring. But, a new study conducted out of Oxford University"s Department of Zoology, with wild mice, shows that fathers do influence sex ratios amongst their children. The paper is published in the Proceedings of the Royal Society B and involves researchers from the UK, Spain and the USA.
These results offer a potential explanation for sex ratio bias — such as found in humans.

"In mammals, theory predicts that offspring sex ratios can only be determined by the mother, as fathers have always been thought to inseminate an equal proportion of X and Y sperm, having a random effect on offspring sex that they could not shift from equality — or 50:50. Also, mothers can influence their offspring in a number of ways from copulation to birth, whereas fathers have control over sperm only. This gives mothers more scope to alter the sex ratio of their offspring. The physical costs of gestation are obviously higher for the mother, so it"s in her own interests from an evolutionary point of view, to invest her resources wisely in terms of the sex, size and quality of her offspring," explains Aurelio Malo MRes PhD in the department of Zoology at the University of Oxford in Oxford, United Kingdom.
Inbreeding, or having children by a relative, increases the chances of receiving a damaging trait inherited from both mother and father.

Researchers used the white footed mouse to model the relationship between a father's genes and the number of sons and daughters he produces. It turns out the size of nuclei in the sperm 'head' reflects the proportion of X to Y sperms. Fathers' sperm with smaller 'head' nuclei — which occurs more often in Y sperm — produced more sons.

The research also provides an explanation for why it is in the father's interest to alter the probability of having sons or daughters. According to Dr Malo, males of lower genetic quality minimise the cost of having sons — which are more susceptible to inheriting negative effects of inbreeding as they have only Y chromosomes — by shifting the sex ratio to daughters who are more resilient to negative traits having both Y and X chromosomes, so more variability.

Researchers used the white footed mouse to model the relationship between a father's genes and the number of sons and daughters he produces. It turns out the size of nuclei in the sperm 'head' reflects the proportion of X to Y sperms. Fathers' sperm with smaller 'head' nuclei — which occurs more often in Y sperm — produced more sons.
The study gives evidence that dads as well as moms can alter the sex of their offspring. This ability may have evolved through natural selection, to protect animals from inheriting negative traits passed on by the inbreeding within small communities.

Abstract
Sex ratio allocation has important fitness consequences, and theory predicts that parents should adjust offspring sex ratio in cases where the fitness returns of producing male and female offspring vary. The ability of fathers to bias offspring sex ratios has traditionally been dismissed given the expectation of an equal proportion of X- and Y-chromosome-bearing sperm (CBS) in ejaculates due to segregation of sex chromosomes at meiosis. This expectation has been recently refuted. Here we used Peromyscus leucopus to demonstrate that sex ratio is explained by an exclusive effect of the father, and suggest a likely mechanism by which male-driven sex-ratio bias is attained. We identified a male sperm morphological marker that is associated with the mechanism leading to sex ratio bias; differences among males in the sperm nucleus area (a proxy for the sex chromosome that the sperm contains) explain 22% variation in litter sex ratio. We further show the role played by the sperm nucleus area as a mediator in the relationship between individual genetic variation and sex-ratio bias. Fathers with high levels of genetic variation had ejaculates with a higher proportion of sperm with small nuclei area. This, in turn, led to siring a higher proportion of sons (25% increase in sons per 0.1 decrease in the inbreeding coefficient). Our results reveal a plausible mechanism underlying unexplored male-driven sex-ratio biases. We also discuss why this pattern of paternal bias can be adaptive. This research puts to rest the idea that father contribution to sex ratio variation should be disregarded in vertebrates, and will stimulate research on evolutionary constraints to sex ratios—for example, whether fathers and mothers have divergent, coinciding, or neutral sex allocation interests. Finally, these results offer a potential explanation for those intriguing cases in which there are sex ratio biases, such as in humans.

Authors of the study: Aurelio F. Malo, Felipe Martinez-Pastor, Francisco Garcia-Gonzalez, Julián Garde, Jonathan D. Ballou, Robert C. Lacy.


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Sep 13, 2017   Fetal Timeline   Maternal Timeline   News   News Archive




Differences among males in sperm nucleus size may explain the
22% variation in sex ratio between males and females in mouse litters.
Image credit: Wikipedia annotated.



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