An answer to the mystery of sperm evolution

Sperm are extremely variable in size, ranging from an amazing six centimeters in fruit flies to .002 millimeters in tiny rotifers. In fact, sperm cells are the most variable known cell type in animals. The diversity of sperm sizes has been a mystery because sperm all perform the same role – fertilization of eggs. 

Depending on the animal, sperm meet their fate in different ways. In corals or sea urchins for instance, the sex cells meet eggs outside the body. In mammals, sperm meet eggs within a female body. 

A new study from Stockholm University has found that the meeting place for sperm and egg may impact the evolution of size in sperm. The research shows that evolution of sperm got a boost, only when sperm swim inside females. 

“Researchers usually try to explain sperm diversity by focusing on how sperm compete to fertilize eggs or how females choose which sperm fertilize their eggs. But it turns out that there is a missing piece of the puzzle – the location where sperm and eggs meet can also influence sperm size,” explained study co-author Ariel Kahrl.

The researchers examined the sperm size of 3,200 animals and analyzed where these sex cells meet with the egg. They confirmed that meeting place is important in the evolution of sperm size. 

In animals with internal fertilization such as mammals, sperm were six times longer and increased in size at a faster rate than in animals such as coral with external fertilization. 

The study’s findings make a lot of sense. There’s a trade off when creating large sperm – you can’t create as many. The opposite is true – when creating a large amount of sperm, it usually necessitates making smaller sperm. 

Animals with external fertilization, such as corals, have selection pressure to make a lot of sperm. With coral reproduction in particular it’s a game of chance – sperm and egg are just cast into the water around the sessile animals to fertilize when they come into contact. It’s obvious why so many sperm need to be produced. 

In animals with internal fertilization, the pressure pushes in the opposite direction – to create larger and fewer (relatively speaking) sperm.    

“Our results clearly show that interactions between sperm and females help generate the tremendous diversity in sperm size we see in animals today. The greater the potential for interactions between sperm and females, the faster sperm evolve,” said study co-author John Fitzpatrick.

The study is published in the journal Nature Ecology & Evolution.

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By Zach Fitzner, Earth.com Staff Writer