Elsevier

Infection, Genetics and Evolution

Volume 28, December 2014, Pages 192-200
Infection, Genetics and Evolution

Unexpected absence of genetic separation of a highly diverse population of hookworms from geographically isolated hosts

https://doi.org/10.1016/j.meegid.2014.09.022Get rights and content

Highlights

  • Australian sea lion–hookworm model in remote South Australian localities.

  • Parasite diversity is retained after population decline across fragmented populations.

  • Low geographical differentiation of hookworm mtDNA from distinct populations.

  • Female host natal site fidelity has no effect on parasite gene flow between populations.

Abstract

The high natal site fidelity of endangered Australian sea lions (Neophoca cinerea) along the southern Australian coast suggests that their maternally transmitted parasitic species, such as hookworms, will have restricted potential for dispersal. If this is the case, we would expect to find a hookworm haplotype structure corresponding to that of the host mtDNA haplotype structure; that is, restricted among geographically separated colonies. In this study, we used a fragment of the cytochrome c oxidase I mitochondrial DNA (mtDNA) gene to investigate the diversity of hookworms (Uncinaria sanguinis) in N. cinerea to assess the importance of host distribution and ecology on the evolutionary history of the parasite. High haplotype (h = 0.986) and nucleotide diversity (π = 0.013) were seen, with 45 unique hookworm mtDNA haplotypes across N. cinerea colonies; with most of the variation (78%) arising from variability within hookworms from individual colonies. This is supported by the low genetic differentiation co-efficient (GST = 0.007) and a high gene flow (Nm = 35.25) indicating a high migration rate between the populations of hookworms. The haplotype network demonstrated no clear distribution and delineation of haplotypes according to geographical location. Our data rejects the vicariance hypothesis; that female host natal site fidelity and the transmammary route of infection restrict hookworm gene flow between N. cinerea populations and highlights the value of studies of parasite diversity and dispersal to challenge our understanding of parasite and host ecology.

Introduction

Parasite population structure is dictated by their ability to disperse (Badets et al., 2011, Poulin et al., 2011, Rohde, 2002). The parasite populations of canine hookworm (Ancylostoma caninum) and human hookworm (Ancylostoma duodenale) are not associated with the site of parasite collection because of the dispersal of the host, with no geographical barriers for both canine and human hosts (Hawdon et al., 2001, Hu et al., 2002, Miranda et al., 2008, Moser et al., 2007). In contrast, we expect that hookworm, Uncinaria sanguinis, from the endangered Australian sea lion, Neophoca cinerea, whose population of fewer than 15,000 individuals (Shaughnessy et al., 2011) is distributed across highly fragmented island colonies along the South and Western Australian coast, would exhibit geographical segregation through the process of vicariance; extreme natal site fidelity of their female host (Campbell et al., 2008, Lowther et al., 2012) and transmammary mode of transmission, imply restricted parasite dispersal (Fig. 1). Mitochondrial DNA analysis of Australian sea lion females indicates little or no interchange of females between most breeding colonies, with marked genetic restriction of mtDNA occurring between the three largest colonies, Dangerous Reef, Seal Bay, and The Pages Islands (Campbell et al., 2008, Lowther et al., 2012). We therefore hypothesise that, as with maternally derived genes, the extreme natal site fidelity of females will result in geographical barriers to maternally transmitted pathogens, such as hookworm.

Hookworms are blood-feeding intestinal worms responsible for high levels of morbidity and mortality in a wide range of mammals (Hotez et al., 2005, Hotez et al., 2004). Hookworm females lay eggs which are passed into the environment in host faeces (Fig. 1). The free-living infective larvae hatch and then infect new hosts percutaneously (Hotez et al., 2004, Loukas and Prociv, 2001, Olsen and Lyons, 1965). However, unlike in other hosts, percutaneous infection with hookworm larvae is not recognised to result in patent infection or cause clinical disease in otariids (fur seals and sea lions) (Olsen and Lyons, 1965). Larvae remain dormant in the ventral abdominal blubber until parturition, at which time they migrate to the mammary tissue and infect pups via the colostrum. Neonatal otariids exhibit high parasite burdens soon after birth but eggs are not detected in pup faeces after 2–8 months of age (Lyons et al., 2011, Olsen and Lyons, 1965). Based on this life cycle, adult males are considered to be dead end hosts (Castinel et al., 2007, Olsen and Lyons, 1965).

The aim of this study was to investigate the existence of geographical barriers (=vicariance) for hookworms (U. sanguinis) among the three largest, genetically restricted, island colonies of the Australian sea lion, N. cinerea. We used the mitochondrial cytochrome c oxidase subunit I gene (cox1) locus of mtDNA to evaluate the genetic diversity and population structure of hookworms infecting N. cinerea pups at these colonies. The use of the cox1 locus was validated against mtDNA marker genes across two newly determined complete U. sanguinis mtDNA genomes. The results are compared directly to the genetic diversity of canine and human hookworms reported previously. Apparent absence of geographical barriers is discussed, together with its implications to the ecology of N. cinerea populations.

Section snippets

Specimens of hookworm (U. sanguinis) from Australian sea lion (N. cinerea) colonies in South Australia

Specimens of U. sanguinis were collected from Australian sea lion pups at three geographically isolated colonies in South Australia that show restriction of host mtDNA between colonies. All specimens were morphologically consistent with U. sanguinis (Marcus et al., 2014a). Samples were collected from Dangerous Reef, Spencer Gulf (34°48′54″S, 136°12′43″E), South Page Island, Backstairs Passage (35°46′37″S, 138°17′31″E) and Seal Bay, Kangaroo Island (35°59′40″S, 137°19′0″E) (Fig. 2). These

Results

Genetic diversity, based on cox1 from a single hookworm per host (n = 46, Dataset 1), revealed high haplotype diversity (h = 0.991) and high nucleotide diversity (π = 0.008) with 40 unique haplotypes identified (Table 1). Similarly, genetic diversity of hookworms within a single host was high, based on cox1 sequence of five worms from each of two individual pups, (Dataset 2 and 3); these demonstrated high haplotype and nucleotide diversity (Dataset 2: h = 1, π = 0.009, 5 haplotypes; Dataset 3: h = 0.900, π =

Discussion

The high diversity and low geographical differentiation of cox1 haplotypes of U. sanguinis from geographically and genetically distinct populations of N. cinerea rejects the vicariance hypothesis that female host natal site fidelity and the transmammary route of infection restricts hookworm gene flow between N. cinerea populations.

Lack of isolation by distance was previously documented for Nec. americanus and A. caninum with ΦST estimated as 0.12 and 0.28 respectively (Hawdon et al., 2001,

Acknowledgements

Funding for sample collection was provided by Australian Marine Mammal Centre, Australian Antarctic Division, Australian Federal Government, and the Winifred Violet Scott Foundation. Support was also provided by a Faculty of Veterinary Science Whitehead Bequest – Conservation. We thank Clarence Kennedy, Janet Simpson and the Staff of Seal Bay of the Department of Environment, Water and Natural Resources, South Australia and the many volunteers for their assistance in the field and sample

References (43)

  • Z. Zhang et al.

    KaKs_Calculator: calculating Ka and Ks through model selection and model averaging

    Genomics Proteomics Bioinformatics

    (2006)
  • M. Badets et al.

    Correlating early evolution of parasitic platyhelminths to Gondwana breakup

    Syst. Biol.

    (2011)
  • H.J. Bandelt et al.

    Median-joining networks for inferring intraspecific phylogenies

    Mol. Biol. Evol.

    (1999)
  • R.A. Campbell et al.

    Islands in the sea: extreme female natal site fidelity in the Australian sea lion, Neophoca cinerea

    Biol. Lett.

    (2008)
  • A. Castinel et al.

    Epidemiology of hookworm (Uncinaria spp.) infection in New Zealand (Hooker’s) sea lion (Phocarctos hookeri) pups on Enderby Island, Auckland Islands (New Zealand) during the breeding seasons from 1999/2000 to 2004/2005

    Parasitol. Res.

    (2007)
  • O. Folmer et al.

    DNA primers for amplification of mitochondrial cytochrome c oxidase subunit I from diverse metazoan invertebrates

    Mol. Mar. Biol. Biotechnol.

    (1994)
  • S.L. Fowler et al.

    Ontogeny of movements and foraging ranges in the Australian sea lion

    Mar. Mammal. Sci.

    (2007)
  • N.J. Gales et al.

    Breeding biology and movements of Australian sea lions, Neophoca cinerea, off the west coast of Western Australia

    Wildlife Res.

    (1992)
  • M. George-Nascimento et al.

    A case of parasite-mediated competition? Phenotypic differentiation among hookworms Uncinaria sp. (Nematoda: Ancylostomatidae) in sympatric and allopatric populations of South American sea lions Otaria byronia, and fur seals Arctocephalus australis (Carnivora: Otariidae)

    Mar. Biol.

    (1992)
  • J.M. Hawdon et al.

    Genetic structure of populations of the human hookworm, Necator americanus, in China

    Mol. Ecol.

    (2001)
  • L.V. Higgins et al.

    Birth to weaning: Parturition, duration of lactation, and attendance cycles of Australian sea lions (Neophoca cinerea)

    Can J Zool

    (1993)
  • Cited by (13)

    • Lack of genetic structure in pinworm populations from New World primates in forest fragments

      2017, International Journal for Parasitology
      Citation Excerpt :

      However, both pinworm species showed high haplotype diversity in all sampled populations, and nucleotide diversity similar to that found in free-living animals (Goodall-Copestake et al., 2012). Notably, the levels of nucleotide diversity found in these nematodes were considerably lower than those reported for cox1 in other parasites with direct life cycles (π = 0.012–0.021 (Miranda et al., 2008; Archie and Ezenwa, 2011; Haynes et al., 2014; Ács et al., 2016)), and instead resemble those of parasites with asexual reproduction (π = 0.006 (Keeney et al., 2009; Marigo et al., 2015)). The levels of nucleotide diversity found in the two species of Trypanoxyuris could be explained by their haplodiploid condition and transmission mode; however, other pinworm species and even Enterobius vermicularis, the sister genus of Trypanoxyuris and also a pinworm that parasitizes primates, show higher levels of nucleotide diversity (π = 0.014–0.049 (Falk and Perkins, 2013; Rodriguez-Ferrero et al., 2013)).

    • The diversity and impact of hookworm infections in wildlife

      2017, International Journal for Parasitology: Parasites and Wildlife
    View all citing articles on Scopus
    View full text