Reproduction is one of the most important processes in biology: it ensures the persistence of species and, on a smaller scale, the transmission of an individual’s genetic material to the next generation. It isn’t surprising then that populations have evolved various traits to improve their chances of reproducing.
Male peacocks maintain showy tails, while male scorpionflies present dead insects to their mates as gifts. For some roundworms in the genus Caenorhabditis, males with giant sperm do best in reproduction.
According to a study by University of Toronto researchers Asher Cutter and Jeremy Gray and their colleagues at the University of Nice, sperm gigantism has evolved independently in multiple roundworm lineages. This suggests that giant sperm are adaptive. Individuals with giant sperm produce more offspring than those with smaller sperm.
These sperm cells have pseudopods or ‘false feet’ that allow them to move along the female reproductive tract. The bigger the sperm, the faster they move, and the more likely they are to fertilize eggs in a competitive setting. In some species, a sperm cell can comprise up to a quarter of a worm’s body.
Cutter, an Associate Professor in Ecology and Evolutionary Biology, explained, “We think that the differences in sperm size among species is the outcome of a special type of natural selection in which competition between the sperm cells of different males has led to the evolution of cell size differently in different species. These species differences are likely to reflect [differences] in the details of the ‘rules of mating’ in the species.”
These ‘rules of mating’ have given rise to two distinct strategies: produce a few large sperm or many smaller sperm cells. The relative importance of sperm size and number might depend on whether male reproduction is constrained by mate availability or by fertilization success.
“The idea is that in large populations of millions of animals… males can maximize their reproductive success by mating with as many females as possible, which means making as many sperm as fast as possible, even if they are a bit small,” said Cutter. “In those tiny isolated populations, however, this strategy won’t work: there just aren’t enough females. So we think that in these small mating groups, what gives a male a reproductive advantage is maximizing the fertilization success in a given female, which would lead to especially strong sperm competition. Larger amoeboid sperm of these species are more competitive and better able to fertilize an egg compared to smaller sperm.”
Remarkably, sperm size is highly variable within an individual. “We are less sure of what is the cause, but one hypothesis is that developmental control of size is noisy as the gonad tries to both make large sperm and to make them fast,” said Cutter.
Caenorhabditis worms in particular are great model organisms for sperm competition. Their skin is transparent, allowing researchers to fluorescently label sperm and egg cells and watch them interact in real time.
“As an evolutionary biologist, I am fascinated by how different organisms can accomplish that same goal to make babies in so many different ways with so many different kinds of flair,” said Cutter.
He added, “You can also make the practical argument that understanding how gamete cells interact is another way that cells communicate. Understanding cell communication is really a crucial and a fundamental part of the signalling between many kinds of cells, the breakdown of which is key in human problems like infertility and more generally in many cancers and autoimmune disorders.”
So is bigger always better? As in many problems in biology, it all depends on context.