Single supergene controls sperm size and swimming speed

The size and swimming speed of sperm are controlled by a single supergene in birds, reveals a new study by the University of Sheffield and Macquarie University . 

Published in Nature Ecology and Evolution, the study used zebra finches to understand the genetics of sperm. 

Previous studies have shown that sperm shape and speed are inherited; fathers with long, fast sperm have sons with long, fast sperm. However, it was not known which genes were responsible for sperm characteristics being passed from one generation to the next.

The new research shows that the shape, size and swimming speed of sperm is due to something called a supergene. A supergene is a number of genes that are next to each other on a chromosome and inherited together as one unit. The idea that supergenes were important was first proposed in the 1930s, but evidence for them was lacking until recently.

“Sperm is an interesting trait in sexually reproducing organisms because it is very intimately associated with fitness — males that produce the fastest swimming sperm are likely to fertilise more eggs than those producing slower sperm,” said study co-author Professor Simon Griffith, from Macquarie University’s Department of Biological Sciences.

Strong selection should reduce the variation in the type of sperm in a population, and yet that is often not the case. 

Professor Jon Slate, from the Department of Animal and Plant Sciences at the University of Sheffield, said, “Like humans, birds have sex chromosomes; males have two Z chromosomes and females have a Z and a W. The zebra finch supergene that affects sperm is on the Z chromosome and has arisen because parts of the Z chromosome have been flipped around. Three different orientations of the Z chromosome (named A, B and C) now exist and they have been evolving independently of one another for thousands, possibly millions, of years.

“Because males have two copies of the Z chromosome, they can either have two identical (eg, AA) or two different (eg, AB) copies of the supergene. The males with two different versions of the supergene have the best sperm, with long midpieces, long tails, fast swimming speed and a higher fertilisation success in sperm competition experiments. Geneticists call this phenomenon heterozygote advantage.”

Scientists believe that a better understanding of how the shape and size of sperm influences fertilisation success in non-human animals such as the zebra finch may point us in new directions for human fertility research.

Interestingly, scientists from the Max Planck Institute for Ornithology  in Seewiesen used a different approach but arrived at the same result while studying the fitness consequences of the Z chromosome inversion in zebra finches. The scientists have also published their own study on how mutation speeds up the sperm of zebra finches in Nature Ecology and Evolution. They discovered that in zebra finches, sperm velocity and morphology and hence reproductive success strongly depend on a specific mutation (an inversion) on one of the sex chromosomes, called Z.

“Males always carry two copies of the Z chromosome, but those that possess two different versions of it, one regular and one inverted, benefit from an optimal sperm design. Even under the strongest selection, in which only the fastest sperm will successfully fertilize the ovum, the genetically inferior types cannot go to extinction. This is because for the ideal morphology a combination of both forms is needed (heterozygotes), which is not possible without also producing the suboptimal types (homozygotes). According to the scientists, this effect can partly explain infertility in zebra finch males,” said the Max Planck Institute for Ornithology in a statement.