Mutation speeds up sperm of zebra finches
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. This was discovered by scientists of the Max Planck Institute for Ornithology in Seewiesen. 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.