Hybridization between two cestode species and its consequences for intermediate host range

Abstract

BackgroundMany parasites show an extraordinary degree of host specificity, even though a narrow range of host species reduces the likelihood of successful transmission. In this study, we evaluate the genetic basis of host specificity and transmission success of experimental F1 hybrids from two closely related tapeworm species (Schistocephalus solidus and S. pungitii), both highly specific to their respective vertebrate second intermediate hosts (three- and nine-spined sticklebacks, respectively).MethodsWe used an in vitro breeding system to hybridize Schistocephalus solidus and S. pungitii; hybridization rate was quantified using microsatellite markers. We measured several fitness relevant traits in pure lines of the parental parasite species as well as in their hybrids: hatching rates, infection rates in the copepod first host, and infection rates and growth in the two species of stickleback second hosts.ResultsWe show that the parasites can hybridize in the in vitro system, although the proportion of self-fertilized offspring was higher in the heterospecific breeding pairs than in the control pure parental species. Hybrids have a lower hatching rate, but do not show any disadvantages in infection of copepods. In fish, hybrids were able to infect both stickleback species with equal frequency, whereas the pure lines were only able to infect their normal host species.ConclusionsAlthough not yet documented in nature, our study shows that hybridization in Schistocephalus spp. is in principle possible and that, in respect to their expanded host range, the hybrids are fitter. Further studies are needed to find the reason for the maintenance of the species boundaries in wild populations.

Experimental parasite community ecology: intraspecific variation in a large tapeworm affects community assembly

“…The tapeworm S. solidus has a three‐host life cycle, infecting freshwater copepods, then three‐spined sticklebacks, before finally reproducing in fish‐eating birds (Clarke ). The relationship between S. solidus and sticklebacks is very specific; three‐spined sticklebacks are the only suitable fish host for S. solidus (Bråten ; Henrich, Benesh & Kalbe ). The size of S. solidus in sticklebacks is massive (i.e.…”

The role of prezygotic isolation mechanisms in the divergence of two parasite species

“…Negative fitness costs of self-fertilization mainly occur early in the parasite’s lifetime [60], and in sum lead to a lower probability of self-fertilized offspring reaching the age of sexual maturity. We could show in earlier experiments, that despite a lower hatching rate in hybrid crosses (which may be caused by a high proportion of self-fertilized offspring), hybrid cestodes that did hatch did not perform worse in their intermediate hosts, in both terms of infection rate and growth in the stickleback, than the parental species [36]. As hybrids are able to infect both stickleback species, they even extend their host range and therefore increase the probability of transmission to a suitable second intermediate host.…”

“…Both parasites are highly specific on this level of the life cycle: S. solidus can only infect three-spined sticklebacks ( Gasterosteus aculeatus ), while S. pungitii is only infective to nine-spined sticklebacks ( Pungitius pungitius ). Several experiments have already investigated this phenomenon of high host specificity [31, 32] while hybrids of the two parasites species show an expanded host range and are able to infect both stickleback species [36]. The life cycle is completed when piscivorous birds prey upon infected sticklebacks.…”

“…Eggs were washed and incubated at 20 °C in the dark for three weeks before coracidia were hatched [38]. 96 hatched coracidia were collected from the feces of each gull and typed using five different microsatellite markers [42], which are used for species discrimination as well as the identification of hybrids [36]. …”

“…The strong separation indicated by molecular data suggests low or no gene flow between these two parasite species, even though extensive studies on this have not yet been conducted. However, we were recently able to show that the two parasite species can still produce viable hybrids in the laboratory [36] which show an increased host range being able to infect both fish hosts. This study did not identify any obvious fitness disadvantages for several stages of the parasite life cycle (hatching rate, infection rate in first and second intermediate hosts as well as performance in the second intermediate host).…”