Who's your mama? Riverine hybridisation of threatened freshwater Trout Cod and Murray Cod.

Alan J Couch,Fiona J Dyer,Mark Lintermans,Peter J Unmack

doi:10.7717/peerj.2593

Abstract

Rates of hybridization and introgression are increasing dramatically worldwide because of translocations, restocking of organisms and habitat modifications; thus, determining whether hybridization is occuring after reintroducing extirpated congeneric species is commensurately important for conservation. Restocking programs are sometimes criticized because of the genetic consequences of hatchery-bred fish breeding with wild populations. These concerns are important to conservation restocking programs, including those from the Australian freshwater fish family, Percichthyidae. Two of the better known Australian Percichthyidae are the Murray Cod, Maccullochella peelii and Trout Cod, Maccullochella macquariensis which were formerly widespread over the Murray Darling Basin. In much of the Murrumbidgee River, Trout Cod and Murray Cod were sympatric until the late 1970s when Trout Cod were extirpated. Here we use genetic single nucleotide polymorphism (SNP) data together with mitochondrial sequences to examine hybridization and introgression between Murray Cod and Trout Cod in the upper Murrumbidgee River and consider implications for restocking programs. We have confirmed restocked riverine Trout Cod reproducing, but only as inter-specific matings, in the wild. We detected hybrid Trout Cod–Murray Cod in the Upper Murrumbidgee, recording the first hybrid larvae in the wild. Although hybrid larvae, juveniles and adults have been recorded in hatcheries and impoundments, and hybrid adults have been recorded in rivers previously, this is the first time fertile F1 have been recorded in a wild riverine population. The F1 backcrosses with Murray cod have also been found to be fertile. All backcrosses noted were with pure Murray Cod. Such introgression has not been recorded previously in these two species, and the imbalance in hybridization direction may have important implications for restocking programs.

Highlights

Hybridization and introgression in the wild occurs in more than 10% of animal species, and is most common in more recently diverged species (Mallet, 2005)
The genetic consequences of introgression are of increasing interest to conservation biologists as many species are on an irreversible path to extinction (Frankham, Ballou & Briscoe, 2002), which can be initiated without any obvious signs (Blomqvist et al, 2010; Fagan et al, 2005)
Juveniles and adults have been recorded in hatcheries and impoundments, and rare hybrid adults have been recorded in rivers previously (Douglas, Gooley & Ingram, 1994; Douglas et al, 1995), this is the first time fertile first generation (F1) hybrids have been recorded in the wild as evidenced by the finding of F1 × Murray Cod backcrosses (F1 × MC)

Summary

Introduction

Hybridization and introgression in the wild occurs in more than 10% of animal species, and is most common in more recently diverged species (Mallet, 2005). While introgression can initially increase genetic diversity (Grossen et al, 2014) and adaptive introgression may be important in response to change (Hamilton & Miller, 2016), introgression can eventually reduce genetic diversity of one or both of the parent species, if they are not naturally sympatric and one is introduced (Laikre et al, 2010). The genetic consequences of introgression are of increasing interest to conservation biologists as many species are on an irreversible path to extinction (the extinction vortex) (Frankham, Ballou & Briscoe, 2002), which can be initiated without any obvious signs (Blomqvist et al, 2010; Fagan et al, 2005). Introgression can be one of those drivers (Seehausen, 2006)

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