Experimental elimination of parasites in nature leads to the evolution of increased resistance in hosts rspb.royalsocietypublishing.org Felipe Dargent1, Marilyn E. Scott2, Andrew P. Hendry3 and Gregor F. Fussmann1 1Department of Biology, McGill University, 1205 Doctor Penfield Avenue, Montreal, Que´bec, Canada H3A 1B1 2Institute of Parasitology, McGill University, 21,111 Lakeshore Road, Ste-Anne de Bellevue, Que´bec, Canada H9X 3V9 Research 3Redpath Museum, McGill University, 859 Sherbrooke Street West, Montreal, Que´bec, Canada H3A 2K6 Cite this article: Dargent F, Scott ME, Hendry A reduction in the strength of selection is expected to cause the evolution of AP, Fussmann GF. 2013 Experimental reduced trait expression. Elimination of a parasite should thus cause the elimination of parasites in nature leads evolution of reduced resistance to that parasite. To test this prediction in to the evolution of increased resistance in nature, we studied the fourth- and eighth-generation descendants of guppies hosts. Proc R Soc B 280: 20132371. (Poecilia reticulata) introduced into four natural streams following experimen- tal elimination of a common and deleterious parasite (Gyrodactylus spp.). http://dx.doi.org/10.1098/rspb.2013.2371 After two generations of laboratory rearing to control for plasticity and maternal effects, we infected individual fish to assess their resistance to the parasite. Contrary to theoretical expectations, the introduced guppy populations had rapidly and repeatably evolved increased resistance to the Received: 11 September 2013 now-absent parasite. This evolution was not owing to a resistance-tolerance Accepted: 14 October 2013 trade-off, nor to differences in productivity among the sites. Instead, a lead- ing candidate hypothesis is that the rapid life-history evolution typical in such introductions pleiotropically increases parasite resistance. Our study adds a new dimension to the growing evidence for contemporary evolution Subject Areas: in the wild, and also points to the need for a re-consideration of simple expectations from host–parasite theory. In particular, our results highlight evolution, ecology, health and disease the need for increased consideration of multiple sources of selection and and epidemiology pleiotropy when studying evolution in natural contexts. Keywords: relaxed selection, experimental evolution, 1. Introduction rapid evolution, resistance, tolerance Natural selection is the driving force behind adaptation in the wild [1]. As such, environmental changes that alter the direction or strength of selection should immediately initiate adaptive evolution—and a number of studies have con- Author for correspondence: firmed that such ‘contemporary evolution’ can indeed occur in very short Felipe Dargent time frames [2]. In a number of instances, environmental change can be so dra- e-mail: [email protected] matic as to cause the emergence of a new selective factor or the removal of an existing selective factor. The first situation (emergence) would be expected to cause an evolutionary increase in the ability of affected populations to cope with the new challenge. For instance, several studies have documented evol- utionary increases in the ability of formerly naive populations to cope with new contaminants [3], new prey [4], new competitors [5] or new parasites [6]. The second situation (removal) would be expected to cause an evolutionary decrease in the ability of populations to cope with the now-absent chal- lenge—at least when that ability trades off with another fitness component or is sensitive to mutation accumulation [7]. Although the loss of a selective factor is less often studied than the gain of a selective factor, cases have been documented of evolutionary decreases in the ability of populations to cope Electronic supplementary material is available with recently removed contaminants [8] or predators [9]. In this study, we pro- at http://dx.doi.org/10.1098/rspb.2013.2371 or vide a counter-example from a host–parasite system, where removal of a via http://rspb.royalsocietypublishing.org. selective pressure caused a rapid evolutionary increase in the ability to cope & 2013 The Author(s) Published by the Royal Society. All rights reserved. with a now-absent pressure. This finding invites a re- 2007 2 examination of the above tenets and points to the need for rspb.royalsocietypublishing.org Proc R Soc B 280: 20132371 laboratory source field new theory and experiments. Evolutionary models of resistance (the host’s ability to reduce its parasite load) predict that increases in parasite- 2008 induced mortality should drive the evolution of increased Lalaja Lower Upper Lalaja resistance in the host population—because individual hosts that are better able to avoid, control or clear parasites gain approximately 4 higher lifetime reproductive success [10,11]. This expectation generations has been supported in laboratory studies on bacteria [12] 2009 and non-vertebrate organisms [13], as well as in comparative collection Caigual field studies [14]. On the flip-side, theoretical studies suggest Taylor that decreases in parasite-induced mortality and morbidity should drive the evolution of decreased resistance—because F2 experiments investing resources in resistance comes at the expense of 2010 investment in other fitness-related traits [10,11]. Supporting collection these ideas, resistance–fecundity trade-offs have been docu- mented in many organisms [15]. The handful of studies that F experiments have directly tested for the evolution of resistance under 2 relaxed selection, all laboratory-based, have found that Figure 1. Experimental design overview. Guppy introductions were made removal of parasites led to no evolutionary change or the evol- from a source population in the Guanapo River in 2008 to the guppy-free ution of decreased resistance ([16,17] and references within). Lower Lalaja and Upper Lalaja sites and in 2009 to the guppy-free Taylor However, planned experimental evolution studies that and Caigual sites. In 2009 and 2010, guppies collected from each site (red reduce or remove parasite loads have not been performed in dots) were bred for two generations in the laboratory and the F2 fish nature—and yet this is the context where inference is most were used in experiments. See Material and methods for details. critical because other environmental factors could modify evolutionary responses to changes in parasitism. garden in the laboratory, infected with Gyrodactylus turnbulli We used Trinidadian guppies to investigate the evolution and the exponential increase (and decline) of the parasite of resistance to an ectoparasite (Gyrodactylus spp.) after that population on the skin of each isolated fish was monitored parasite had been eliminated in nature. Gyrodactylus spp. for 24 days to assess their resistance to the parasite. We pre- are directly transmitted parasites that reproduce and dicted that removal of Gyrodactylus spp. in the field would browse on the skin of guppies [18]. Furthermore, gyrodacty- lead to the evolution of decreased resistance to that parasite. lids have important fitness consequences for their guppy Contrary to expectations from theory [10,11] and laboratory- hosts—they cause high levels of mortality in both laboratory based experimental evolution studies [16,17], the introduced [19] and field [20], are the most prevalent macroparasite in guppy populations were found to have rapidly and repeata- the wild [21], affect mate choice [22] and cause lesions that bly evolved increased resistance to the now-absent parasite. can serve as entry points for secondary fungal and bacterial infections [23]. Although the mechanisms of resistance to Gyrodactylus are still not fully characterized [24], involvement of the immune system is inferred from experiments on salmo- 2. Material and methods nids where macrophages produce Interleukin 1b which stimulates mucus production and Complement factor C3 (a) Field introductions which binds to and kills the parasite [24]. Guppies show heri- The guppy introductions were carried out as part of a United table [25] as well as a non-heritable [26] components of States National Science Foundation Frontiers in Integrative Bio- resistance, and those individuals that survive infection logical Research (FIBR) project led by D. Reznick. Guppies were captured as juveniles from the Guanapo ‘source’ population express acquired resistance upon challenge infection [27]. (1083802300 N, 6181405400 W and 1083901400 N, 6181501800 W) and As Gyrodactylus are easily visible using a dissecting scope, held in a laboratory in Trinidad. They were quarantined in aqua- repeated parasite counts can provide appropriate quantitative ria and treated with Fungus Eliminator (Jungle Laboratories, data throughout the course of an infection [24], and this is the Cibolo, TX, USA), Clout (Sentry AQ Mardel, Omaha, NE, most direct method to assess the host ability to control para- USA) and commercial forms of erythromycin and monocyclene site numbers. Gyrodactylus load on individual guppies is (Maracyn and Maracyn Two–Sentry AQ Mardel). These treat- known to vary both within and among populations, and ments remove parasites, including Gyrodactylus, and all fish this variation is attributed to variation in host resistance were monitored to ensure that they were in good health. [19], particularly when infections occur in isolated hosts Approximately 40 males and 40 females were then released raised in common garden. In this sense, fish with fewer para- into