I read an interesting review published in Physiology that examined how sperm from different species are especially evolved to help promote fertilization, which if successful, results in the creation of an offspring. It is simply amazing to think of the multitude of ways this process occurs in nature.
Take marine invertebrates and fish for example. These animals release eggs and sperm into the water. In contrast, fertilization happens internally in mammals and birds where males release sperm into a female. In both situations, the sperm have to locate the egg for fertilization to occur. Not surprisingly, sperm can come in a variety of shapes. Typically, sperm have a head (which contains the DNA), midpiece, and a tail (flagellum) that helps the sperm swim (example above).
Could you guess what species produces the largest sperm relative to its body size?
It turns out the answer is a fruit fly (Drosophila bifruca). The sperm is actually 20 x longer than an adult! Weird!
Some plants have sperm with more than one tail and sperm from Mastotermes darwiniensis termites have up to 100 tails – not sure how they all agree on which way to go. Still other sperm don’t have tails at all. Even the shape of the sperm head varies between species from round, oval, elongated, and hooked, to a myriad of other shapes. These variations in sperm likely arose from the various environments they have to function in, not to mention their need to compete with other sperm.
To reach an egg, sperm must be able to swim. For many species the tail is what propels the sperm forward, a process that requires cellular energy in the form of ATP (produced by mitochondria). There are a variety of signals that sperm detect to find an egg including chemical signals (chemotaxis), temperature (thermotaxis), and fluid movement (rheotaxis). Once the sperm reaches the egg, it must break through its outer layer to fertilize it. Reportedly, most sperm from animals complete this task with the help of a specialized structure in the head called the acrosome. The acrosome releases an acidic enzyme substance that helps break through the egg’s ‘shell’.
Interestingly, there are a variety of specialized vesicles in the body of males (in the epididymis and prostate) and females (in the genital tract) that could potentially alter sperm by transferring proteins, RNA, as well as metabolites to the sperm as they migrate to the egg. Although it is currently not clear how these secretions alter sperm motility or function.
A Darszon, T Nishigaki, I Lopez-Gonzalez, PE Visconti, CL Trevino. Differences and Similarities: The Richness of Comparative Sperm Physiology. Physiology. 35(3): 196-208, 2020. https://doi.org/10.1152/physiol.00033.2019
For more on the fruit fly sperm, see: