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

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.

Fresh is best: Accurate SNP genotyping from koala scats

“…Calling quality was assured by high average read depth per locus. This process is similar to that used in published literature using DArTseq ™ SNPs from animal genetic samples (e.g., Couch et al, 2016;Donnellan et al, 2015).…”

“…DArTseq ™ technology has been tested and used successfully for a wide range of genomic studies across a variety of vertebrate species (Melville et al, 2017). Examples of this include Cunningham's skinks (Egernia cunninghami) (Ofori, Beaumont, & Stow, 2017), North American green frog (Rana clamitans) (Lambert, Skelly, & Ezaz, 2016), trout cod (Maccullochella macquariensis) and Murray cod (Maccullochella peelii) (Couch, Unmack, Dyer, & Lintermans, 2016), yellowfin tuna (Thunnus albacares) (Grewe et al, 2015), eastern yellow robin (Eopsaltria australis) (Morales et al, 2017), and southern fiddler rays (Trygonrrhina dumerilii) (Donnellan et al, 2015).…”

Global reintroduction perspectives: 2018. Case studies from around the globe

“…Comprehensive monitoring of the lower Ovens River demonstrated the benefits of a long-term stocking program, with wide variation in the contribution of individual stocking events to the resultant population (Lyon et al, 2012). Genetic analysis of upper Murrumbidgee cod larvae from 2011 -2013 found hybridization between Trout cod and Murray cod, (Couch et al, 2016) demonstrating the problem of reestablishing a threatened species into the range of an established congeneric.…”

Barriers to hybridisation and their conservation implications for a highly threatened Australian fish species

“…However, recent broadscale genetic studies of Australian freshwater fishes have revealed high levels of genetic structuring between populations as well as many new cryptic species (Hammer et al, ; Hammer, Adams, Unmack, & Walker, ; Unmack, ; Unmack, Allen, & Johnson, ; Unmack & Dowling, ). As a result, introgressive hybridisation caused by introductions of “native” species outside their natural range or from one part of a species range to another, is a more recently recognised threat to the conservation for Australian freshwater fishes (Couch, Unmack, Dyer, & Lintermans, ; Harris, ; Lintermans et al, ). Keeping Australian fish in aquariums, ponds and stock troughs is becoming increasingly popular (e.g., Australia New Guinea Fishes Association).…”

“…As a result, introgressive hybridisation caused by introductions of "native" species outside their natural range or from one part of a species range to another, is a more recently recognised threat to the conservation for Australian freshwater fishes (Couch, Unmack, Dyer, & Lintermans, 2016;Harris, 2013;Lintermans et al, 2005).…”