(Sperm beetle presents a variety of shapes and structures. Image: Dawn Higginson / University of Arizona)

American researchers have studied the sperm and female morphology of 42 species of beetles. These data have identified large variations in the structure of sperm-related developmental changes in the genitalia of females.

“Both sperm morphology and female reproductive tracts have experienced many diversifications in aquatic beetles,” explains to SINC Dawn Higginson, author of the study and researcher at the University of Arizona (USA).

“The dimensions of the sperm and the presence of conjugates (clusters formed in the male sperm that are transferred in copulation) are correlated significantly with the dimensions of the attributes of females,” says Higginson.

In some species found thousands of spermatozoa bound in clusters as a feather badminton, or forming a fairly rigid rod-like worm that moves inside the female. In other species, the sperm heads were connected with a kind of glue.

The authors wanted to investigate what evolutionary forces had led to this diversity. “We found that when female morphology undergoes evolutionary change, the sperm is compensated,” says Higginson. “We can not say that males are following the evolution of the female, but seems to be an arms race.”

As explained by Scott Pitnick of Syracuse University, New York (USA), “to observe the intricate morphology of the reproductive tracts is inevitable that sperm need a swiss army knife to penetrate them. Females make him very difficult. “

The study challenges the paradigm that the development of sexual characteristics will be determined by competition between males, and instead places the change in the variations of the female. 

“Evolution is crazy”

 According to the authors, the sperm does not vary much in species where the female leaves the eggs before fertilizing, as in many of the fish. “But when there is internal fertilization, something happens and evolution is crazy,” says Scott Pitnick of Syracuse University, New York (USA). “In almost all animals found there is internal fertilization, sperm has been transformed in many different ways. “

To analyze these many evolutionary changes, the researchers measured parameters such as total length, the heads and tails of spermatozoa, the shape of the head, the length of the female ducts and the presence of clusters of sperm.

Then they used statistical tools to analyze the evolutionary relationships between different traits. “We use statistical models for sequence evolution and find changes in the dimensions of the feminine attributes that would lead to changes in sperm morphology,” says Higginson.

Coevolutionary process

In most cases of sexual selection, there is a coevolution caused by female preference for a male trait, such as striking tails of peacocks. “Females prefer individuals with colorful tails and males therefore want to have them as well, so there is a coevolutionary circle.”

“We can not ensure that this is exactly what happens here, but it is possible that the diversity we observe in the semen is the equivalent of gaudy feathers of peacocks, and female reproductive tracts have been developed that favor one or the other “analyzes Higginson.

According to the researcher, it could be something else that influenced this evolutionary change of females. “You may need to be large enough to handle the egg properly, or something. But the point is that female sexual drive the evolution. Sperm only keeping up, “he concludes.