Let’s talk about mouse sperm: how the environment influences reproduction
As you may know, mice are some of the most studied animals in modern science. But did you know there’s a world of insight to be found in mouse testicles? AJM’s Environmental Scientist, Misha Lavoie, explains!
Before starting at AJM, I left my home in Alberta and went to the University of Western Australia to study Zoology. My final thesis was about the secrets found in mouse sperm, focusing on the father’s influence in the sex of his offspring.
What determines the sex of offspring?
An offspring’s sex is determined by their sex chromosomes; in mammals, females have two X chromosomes and males have one X and one Y. The father then contributes the gamete that determines the sex of his offspring: a Y-chromosome bearing sperm (Y-sperm) will produce a son, and an X-chromosome bearing sperm (X-sperm) will produce a daughter. However, the study of how animals use resources for male vs. female offspring (sex allocation) has overwhelmingly focused on female influences. Females have the “final say” when it comes to the sex of their offspring, as fertilization occurs within the female. It has been well established that female mammals can manipulate offspring sex ratios in relation to their own fitness or status, and in response to local conditions; for example, female possums have been shown to alter the sex ratio of their offspring by up to 2:1 (sons: daughters) based on den availability. However, the paternal influence on sex allocation has largely been negated.
For fathers, the assumption has been that their sperm sex ratio (i.e., their ratio of X-sperm to Y-sperm) is determined by meiosis during sperm production. This assumption would lead to a ratio of exactly 50:50 with no ability to be changed. Our study set out to determine whether the ratio of X-sperm: Y-sperm is adaptive based on social conditions or fixed at 50:50.
My Research
The house mice in our experiment were reared in one of two social environments: in the first environment, we surrounded males with other males (high male density) to create the perception that there was strong competition for a mate; and in the second environment, males were exposed only to females (high female density), so male competition was negligible.
Sperm samples were taken from our mice, from which DNA was extracted. The DNA samples were run using quantitative PCR to determine the proportions of X-sperm to Y-sperm. Sperm quality traits were examined, such as sperm number, sperm velocity, and sperm motility.
We found that males that grew up in a competitive environment had a small but significant increase in their proportion of Y-sperm, compared to males that were reared in an environment without substantial male competition (Figure 1). Interestingly, males that were born into litters with a higher percentage of brothers also produced a higher proportion of Y-sperm, compared to mice that had more sisters (Figure 1).
Figure 1. Greater proportions of Y-sperm were produced in males that were raised with high male density (A) compared to males raised with high female density (B). An increasing proportion of Y-sperm was produced as the proportion of males in their litter increased (A and B). The litter sex ratio indicates what percentage of the litter was male; a low litter sex ratio (0.3) indicates a high proportion of sisters and a litter sex ratio of 1.0 means the male had only brothers.
Males that were reared in environments with lots of other males also grew to a significantly larger size than males that grew up in non-competitive environments. This increase in body size may lead to greater success when fighting off competition for mates. We also found that males that had grown up in a competitive social environment produced more and better-quality sperm. This indicates that they may increase investment in traits which increase their chance at successfully fertilizing an egg when surrounded by male competition.
This research shows that the sperm sex ratio is significantly affected by both how many brothers and sisters the male was born with, as well as their social environment following weaning. This indicates that the sperm sex ratio is adaptive and responds to both pre- and post-natal conditions. This was the first study to demonstrate that that the sperm sex ratio is adaptive based on a father’s social environment and not fixed at 50:50.
What does mouse sperm have to do with environmental consulting?
At first glance, there may not seem to be much of a link. But when you evaluate the greater context this research represents, the influence that the environment can have on wildlife and their reproduction can be expanded to more than just house mice.
Our environmental team brings a variety of knowledge, education, and expertise to the field. At AJM, we share a common thirst for knowledge and having such a diverse team of scientists makes it easy to look at problems from different perspectives.
About the Author
Prior to starting at AJM as an Environmental Scientist, Misha Lavoie conducted this research for a Master’s degree in Zoology at the University of Western Australia. It has since been published in Evolution Letters. Please contact info@ajmenv.com with any questions.