Deanna M. Soper, Ph.D.
Assistant Professor of Biology, University of Dallas, Irving, Texas
In the Northern Hemisphere, the month of January means colder temperatures and shorter days, while in the Southern Hemisphere, where the Taxon of the Month is native, it is summer. Potamopyrgus antipodarum is a small (2–5mm), freshwater snail species native to New Zealand and can be quite abundant in lakes and streams on both the North and South Islands of New Zealand. This species is being used across the world to answer a wide variety of different biological questions including: the evolution of sexual reproduction, evolutionary genetics, ecotoxicology, invasive biology, and reproductive behavioral evolution. Its current range includes not only New Zealand, but also invasive populations across Europe and North America. Potamopyrgus antipodarum, sometimes referred to as the New Zealand Mud Snail, was first described by John Edward Gray in 1843 and later characterized by Michael Winterbourn (1970, 1972). In 1974, Michael Winterbourn documented infection of the snails by several sterilizing trematode parasites (worms), but one genus, Microphallus, is particularly common in lake populations (Lively, 1987). This sterilizing trematode first infects snail hosts, where the parasite develops into cercariae. Infected snails are then eaten by ducks where the parasite develops into the adult worm stage. The worms undergo sexual reproduction whereby eggs are produced and then released with the duck’s feces. This gives the snails an opportunity to eat the eggs and become infected with the parasite starting the life cycle over again.
Reading about this snail, the famed evolutionary biologist John Maynard Smith (1978) proposed that this species could serve as a model system to solve a long-standing riddle: the evolution of sex. Why sexual reproduction evolves and persists in populations has been a question since Charles Darwin’s time. In 1859, Darwin published the first edition of On the Origin of Species and in it expressed doubt that long-standing asexual lineages existed when he said that “Finally then, we may conclude that in many organic beings, a cross between two individuals is an obvious necessity for each birth; in many others it occurs perhaps only at long intervals; but in none, as I suspect, can self-fertilisation go on for perpetuity.” (Darwin, 1859, page 101). Sexual females are required to produce males, which means that they have a reduced growth rate compared to asexual females because males cannot produce offspring (see figure 1). This means that all else being equal, asexual females should take over sexual females in the same population quickly. And yet, sexuality is ubiquitous among all higher order plants and animals.
This snail species provides an ideal opportunity to provide answers to the question of sex because many endemic populations contain ecologically non-distinct sexual and asexual lineages. Consequently, Curtis Lively (1987) launched P. antipodarum as system for use in the field of evolutionary biology when he sought to answer the question of why sexual reproduction evolves and is maintained in populations over time. In the early 1990’s it was discovered that some snails contain two copies of each chromosome (diploid) and are sexual – females produce on average 50% female, 50% male. While other snails contain three copies of each chromosome (triploid) and are asexual – females produce mostly all females (Wallace, 1992). Since then, snail populations containing up to six copies of each chromosome have been discovered (Neiman et al., 2011).
Research on the relationship between the snail and the sterilizing trematode parasite has found a strong positive association between presence of parasites and presence of males, which is a measurement of the percent of snails in a population that are sexual (Lively, 1987; Jokela, 2009; King et al., 2011). This means that where there are parasites that coevolve with their hosts, there is sexual reproduction. Sexuality is favored in environments with coevolving parasites because sexual lineages generate genetic diversity in each generation, while asexual females produce offspring that are 100% related. The sexual portions of the population are a “moving target” and the asexual lineages are a “static target” resulting in parasites more easily evolving the ability to infect asexual lineages. Parasites keep the asexual populations “in check” allowing the sexuals to remain in the population.
Although some biological and ecological characteristics of the snail have been documented, other aspects remain a mystery. For example, it is not known what determines that a snail embryo will develop into a male vs a female adult snail. The sexual life of this species is unique because unlike most snails that lay eggs, P. antipodarum female snails undergo “pregnancy” (internal gestation) and give live birth (see Fig. 3 of baby snail just born). Occasionally, baby snails can be born inside their gestational sac (see video below). Males can be identified by external genitalia that they use to fertilize sexual females (Fig. 1). A male can initiate mating by crawling on top of a female at which time the female can reject the male mating attempt by vigorously shaking her shell back and forth to knock the male off. Recent research has found that females can be choosey, but what females pay attention to during mating attempts and what causes females to prefer particular males also remain unanswered questions. This little snail has provided important insights to evolutionary biology, ecology, and reproductive biology, but still holds the answer to many questions that need to be explored.
Potamopyrgus antipodarum born in gestational sac. Video Credit: Deanna Soper
Deanna Soper is an Assistant Professor of Biology at the University of Dallas where she uses P. antipodarum to understand how parasitic selective pressure and host/parasite coevolution influences the evolution of reproductive behaviors. Her lab website can be found here.
Darwin, C. R. On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. London: John Murray. (1st edition).
Jokela, J., M. F. Dybdahl and C. M. Lively. 2009. The Maintenance of Sex, Clonal Dynamics, and Host-Parasite Coevolution in a Mixed Population of Sexual and Asexual Snails. 174: S43–S53.
King, K. C., L. F. Delph, J. Jokela and C. M. Lively. 2011. Coevolutionary hotspots and coldspots for host sex and parasite local adaptation in a snail-trematode interaction. Oikos. 120: 1335–1340.
Lively, C. M. 1987. Evidence from a New Zealand snail for the maintenance of sex by parasitism. Nature. 328: 519–521.
Maynard Smith, J. 1978. The Evolution of Sex. Cambridge University Press. Cambridge.
Neiman, M., D. Paczesniak, D. M. Soper, A. T. Baldwin and G. Hehman. 2011. Wide Variation in Ploidy level and Genome Size in a New Zealand Freshwater Snail with Coexisting Sexual and Asexual Lineages. Evolution. 65: 3202–3216.
Wallace, C. 1992. Parthenogenesis, Sex and Chromosomes in Potamopyrgus. Journal of Molluscan Studies. 58: 93–107.
Winterbourn, M. J. 1970. Population Studies on the New Zealand Freshwater Gastropod, Potamopyrgus antipodarum (Gray). Journal of Molluscan Studies. 39: 139–149.
Winterbourn, M. J. 1972. Morphological Variation of Potamopyrgus jenkinsi (Smith) from England and a comparison with the New Zealand species, Potamopyrgus antipodarum (Gray). Proceedings of the Malacological Society London. 40: 133.
Winterbourn, M. J. 1974. Larval Trematoda Parasitising the New Zealand Species of Potamopyrgus (Grstropodoa: Hydrobiidae). Mauri Ora. 2: 17–30.