Report on a recent UST Biology publication: “Can resource costs of polyploidy provide an advantage to sex?” by Adam Kay

Phosphorus and the evolution of sexual reproduction 

Mating Potamopyrgus antipodarum snails (photo Bart Zijlstra)

Why do animals reproduce sexually? The answer may seem obvious until you consider that some animal species actually reproduce by making identical clones without input from a partner. Cloning, also referred to as asexual reproduction, has advantages. Most importantly, it results in parents transferring their full complement of genetic information to their offspring; in contrast, sexual parents only transfer half of their genes (the other half comes from the other parent). This advantage is enormous, making it paradoxical that sex is so common in the animal kingdom.

So why is sex so common? Most hypotheses have focused on the benefits of genetic recombination or of producing genetically distinct offspring. One idea – the Red Queen Hypothesis – suggests that sex might be advantageous because it allows parents to produce genetically distinct offspring that are less susceptible to infection by co-evolving parasites. In contrast, asexual groups could have parasites that, over time, evolve specializations that allow them to successfully attack the asexual genotype. The Red Queen Hypothesis has received considerable support. However, it’s unlikely that parasites are the only ecological factor favoring sexual reproduction, and the extent to which other factors influence the success of sexual reproduction is poorly understood.

In our recently published paper in the journal Heredity, my collaborators Dr. Maurine Neiman (from the University of Iowa),  Dr. Amy Krist (from the University of Wyoming), and I develop a new hypothesis about how the availability of food resources could influence the success of sexual reproduction. Our focus is on the role of phosphorus in food and in organisms themselves. We start with the fact that most asexual animals tend to have higher ploidy (=# of chromosome sets) than closely related sexual groups. For example, the New Zealand freshwater snail Potamopyrgus antipodarum consists of both diploid sexual individuals and asexual individuals that are either triploid or tetraploid. The key part of our argument is that higher ploidy should generally increase an individual’s sensitivity to scarcity of dietary phosphorus. The reason for this is that, all else being equal, higher ploidy individuals should have more nucleic acids (DNA and RNA) in their bodies than do individuals with lower ploidy. Because nucleic acids contain more phosphorus (about 9% by dry mass) than other major biomolecules and can make up a substantial fraction of an animal’s body mass, higher ploidy animals will generally contain more phosphorus. We argue that this higher phosphorus content should increase demand for dietary phosphorus and in turn make organisms more vulnerable to phosphorus scarcity in the environment. In our paper, we explain how environmental phosphorus availability should influence competitive interactions between sexual and asexual groups that differ in ploidy. Finally, we discuss whether our hypothesis is consistent with some well-documented patterns of where sexual and asexual groups are found.

Hopefully our paper will be a step toward a more comprehensive understanding about how environmental factors explain why sex is so common in animals.