Sperm were shown to cooperate and form groups while driving toward the egg, and they ended up reaching the egg faster than sperm cells that swam alone. The speed of swimming was not the factor that made the difference but the sperm that swam in the group, compared to individual sperm cells, got to their destination faster because they went in a straight line.
The study was carried out by scientists in the Department of Organismic and Evolutionary Biology, the Department of Molecular and Cellular Biology, and the School of Engineering and Applied Sciences at Harvard University. The work was done in the lab of Hopi Koekstra, who is a Howard Hughes investigator. The study was published in the journal Proceedings of the National Academy of Sciences.
The researchers created both an experimental model and mathematical model to analyze how sperm swim to get to an egg. Two species of mice were compared in this analysis. One species, Peromyscus maniculatus, is known to be promiscuous and the breed produces sperm that clump together; that is, sperm from the same male group together in the mix. The other species of mice, Peromyscus polionotus, is monogamous and sperm tend to travel alone.
The results from the analysis showed that when sperm formed groups of an optimal size, they got to the egg faster than individual sperm that travel on their own. This was not because the group of sperm was traveling faster but because, when traveling in a group, they traveled closer to a straight line. It is well known that the shortest distance between one point and another is in a straight line.
This study was actually the latest in a series of studies performed to answer the question about why sperm clump together. It was the use of a mathematical model that assisted the scientists in solving this problem and provided an explanation for this observation of cooperation among sperm cells. According to traditional ideas on natural selection, the sperm cells should compete as individual sperm cells, scurrying as fast as possible toward the egg. Various predictions with the mathematical model, however, were confirmed by experimental observation and individual sperm competition did not win out as the best strategy but cooperation to the extent of forming an optimal group size was best.
The authors of the study report suggested that more work needs to be done on this topic. They have not answered the final question but have sharpened the question with this study. They have placed conceptual notions of competition and cooperation in measurable physical and physiological variables, which can be used to test predictions.
The understanding of how fertilization of an egg occurs has gained much ground in the last few decades with the development of microscopes that can provide higher-powered visualization. The actual event of fertilization has now been observed many times over. Whereas, a common idea has been that sperm cells fight against all odds to reach the egg and then pound on the egg cell’s membrane to push their way in, an alternative view has been offered. Some scientists have said that, rather than the winning sperm penetrating the egg, it seems more that the egg chooses a sperm and allows it to enter the egg. This study showed a role for cooperation and forming groups in sperm cells that are swimming toward the egg, and it offers another eye opening view on the very interesting phenomenon of fertilization.
By Margaret Lutze