Abstract
BackgroundThe potential role hybridisation in adaptive radiation and the evolution of new lineages has received much recent attention. Hybridisation between roach (Rutilus rutilus L.) and bream (Abramis brama L.) is well documented throughout Europe, however hybrids in Ireland occur at an unprecedented frequency, often exceeding that of both parental species. Utilising an integrated approach, which incorporates geometric morphometrics, life history and molecular genetic analyses we identify the levels and processes of hybridisation present, while also determining the direction of hybridisation, through the analysis of mitochondrial DNA.ResultsThe presence of F2 hybrids was found to be unlikely from the studied populations, although significant levels of backcrossing, involving both parental taxa was observed in some lakes. Hybridisation represents a viable conduit for introgression of genes between roach and bream. The vast majority of hybrids in all populations studied exhibited bream mitochondrial DNA, indicating that bream are maternal in the majority of crosses.ConclusionsThe success of roach × bream hybrids in Ireland is not due to a successful self reproducing lineage. The potential causes of widespread hybridisation between both species, along with the considerations regarding the role of hybridisation in evolution and conservation, are also discussed.
Highlights
The potential role hybridisation in adaptive radiation and the evolution of new lineages has received much recent attention
The first relative warp (RW1) explained the majority of the variation recorded in both Lough Ramor (75.4%) and Ross Lake (77.6), the three taxa could be separated into clearly definable groups along this axis (Fig. 1.)
The output of Discriminant Function Analysis (DFA) was highly significant for the two lakes for the comparisons between hybrids and parental species (p < 0.01)
Summary
The potential role hybridisation in adaptive radiation and the evolution of new lineages has received much recent attention. Vertebrate hybrids are rarely as ecologically fit as parental taxa [11], there is an increasing body of evidence supporting the theory that hybridisation can lead to adaptation through the creation of novel genotypes and morphologies [1,5,9,12]. Bream had a patchy and limited distribution prior to an extensive stocking program in the 1950s which established populations around the country [22] Hybridisation between both taxa was first recorded following the establishment of an invasive roach population in waters containing resident bream stocks [23]. More recent surveys on the River Shannon, the largest river system in the country, estimate that roach × bream hybrids are the dominant fish in the system, outnumbering both parental species (Central Fisheries Board, internal reports). Hybrids present in both the UK and mainland Europe, are never reported as an abundant group [30,31,32,33,34,35,36,37]
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