Disentangling phylogenetic constraints from selective forces in the evolution of trematode transmission stages
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The transmission stages of parasites are key determinants of parasite fitness, but they also incur huge mortality. Yet the selective forces shaping the sizes of transmission stages remain poorly understood. We ran a comparative analysis of interspecific variation in the size of transmission stages among 404 species of parasitic trematodes. There are two transmission steps requiring infective stages in the life cycle of trematodes: transmission from the definitive to the first intermediate (snail) host is achieved by eggs and/or the miracidia hatched from those eggs, and transmission from the first to the second intermediate host is achieved by free-swimming cercariae. The sizes of these stages are under strong phylogenetic constraints. Our results show that taxonomy explains >50% of the unaccounted variance in linear mixed models, with most of the variance occurring at the superfamily level. The models also demonstrated that mollusc size is positively associated with egg volume, miracidial volume and cercarial body volume, but not with the relative size of the cercarial tail. In species where they encyst on substrates, cercariae have significantly larger bodies than in species penetrating chordates, although the relative size of the cercarial tail of species using chordates as second intermediate hosts was larger than in other trematode species. Habitat also matters, with larger cercarial tails seen in freshwater trematodes than in marine ones, and larger miracidial volumes in freshwater species than in marine ones. Finally, the latitude (proxy for local temperature) at which the trematode species were collected had no effect on the sizes of transmission stages. We propose that resource availability within the snail host, the probability of contacting a host, and the density and viscosity of the water medium combine to select for different transmission stage sizes.
KeywordsBody size Cercariae Latitude Habitat type Host type Tail size
We thank Isabel Blasco-Costa, Haseeb Randhawa and Shinichi Nakagawa for statistical advice, and Matthew Terry for help with references.
- Bray RA, Gibson DI, Jones A (eds) (2008) Keys to the Trematoda, vol 3. CAB International, WallingfordGoogle Scholar
- De Montaudouin X, Thieltges DW, Gam M, Krakau M, Pina S, Bazairi H, Dabouineau L, Russell-Pinto F, Jensen KT (2009) Digenean trematode species in the cockle Cerastoderma edule: identification key and distribution along the north-eastern Atlantic shoreline. J Mar Biol Assoc UK 89:543–556CrossRefGoogle Scholar
- Galaktionov KV, Dobrovolskij AA (2003) The biology and evolution of trematodes. Kluwer Academic Publishers, DordrechtGoogle Scholar
- Gibson DI, Jones A, Bray RA (eds) (2002) Keys to the Trematoda, vol 1. CAB International, WallingfordGoogle Scholar
- Jones A, Bray RA, Gibson DI (eds) (2005) Keys to the Trematoda, vol 2. CAB International, WallingfordGoogle Scholar
- Messina FJ, Fox CW (2001) Offspring size and number. In: Fox CW, Roff DA, Fairbairn DJ (eds) Evolutionary ecology: concepts and case studies. Oxford University Press, Oxford, pp 113–127Google Scholar
- R Development Core Team (2011) R: a language and environment for statistical computing. R Foundation for Statistical Computing, ViennaGoogle Scholar
- Roberts LS, Janovy J Jr (2010) Foundations of parasitology, 8th edn. McGraw-Hill, New YorkGoogle Scholar
- Roff DA (1992) The evolution of life histories: theory and analysis. Chapman & Hall, New YorkGoogle Scholar
- Stearns SC (1992) The evolution of life histories. Oxford University Press, OxfordGoogle Scholar
- Vogel S (1981) Life in moving fluids. Princeton University Press, Princeton 352 ppGoogle Scholar