Population Genetics of a Translocated Population of Mottled Ducks and Allies

Abstract

Translocating species is an important management tool to establish or expand the range of species. Success of translocations requires an understanding of potential consequences, including whether a sufficient number of individuals were used to minimize founder effects and if interspecific hybridization poses a threat. We provide an updated and comprehensive genetic assessment of a 1970s-1980s translocation and now established mottled duck (Anas fulvigula) population in South Carolina, USA. In addition to examining the population genetics of these mottled ducks, we simulated expected genetic assignments for generational hybrids (F1-F10), permitting formal purity assignment across samples to identify true hybrids and establish hybridization rates. In addition to wild mallards (A. platyrhynchos), we tested for presence of hybrids with migrant American black ducks (A. rubripes) and released domestic game-farm mallards (A. p. domesticus). We used wild reference populations of North American mallard-like ducks and sampled game-farm mallards from 2 sites in South Carolina that could potentially interbreed with mottled ducks. Despite 2 different subspecies of mottled duck (Florida [A. f. fulvigula] and the Western Gulf Coast [A. f. maculatlus]) used in original translocations, we determined the gene pool of the Western Gulf Coast mottled duck was overwhelmingly represented in South Carolina's current population. We found no evidence of founder effects or inbreeding and concluded the original translocation of 1,285 mottled ducks was sufficient to maintain current genetic diversity. We identified 7 hybrids, including an F1 and 3 late-staged (i.e., F2-F3 backcrosses) mottled duck × black duck hybrids, 1 F2-mottled duck backcrossed with a wild mallard, and 2 F3-mottled ducks introgressed with gamefarm mallard. We estimated a 15% hybridization rate in our mottled duck dataset; however, the general lack of F1 and intermediate hybrids were inconsistent with scenarios of high hybridization rates or presence of a hybrid swarm. Instead, our results suggested a scenario of infrequent interspecific hybridization between South Carolina's mottled ducks and congeners. We concluded that South Carolina's mottled duck population is sufficiently large now to absorb current hybridization rates because 85% of sampled mottled ducks were pure. These results demonstrate the importance in managing and maintaining large parental populations to counter hybridization. As such, future population management of mottled ducks in South Carolina will benefit from increased geographical and continued sampling to monitor hybridization rates with closely related congeners. We also suggest that any future translocations of mottled ducks to coastal South Carolina should originate from the Western Gulf Coast.

Mottled Duck introductions to South Carolina: The ugly, the bad, and the good?

“… Note I refer to this new population as the South Atlantic Coastal Zone (SACZ). Contemporary genetic structure of Mottled Ducks is well understood (e.g., Lavretsky et al, 2021 ), but potential ecological and future evolutionary consequences remain uncertain, as reflected here. …”

“…As such, this release of birds into a novel environment arguably has diverse short‐ and long‐term consequences and outcomes. Three specific negative concerns or hypotheses have been considered, including that currently established SACZ Mottled Ducks will: (1) Hybridize with feral Mallards; in North Carolina, for example, American Black Ducks ( Anas rubripes ) × Mallard introgression has resulted from gene flow through male feral Mallards (Lavretsky et al, 2019 ; Lawson et al, 2021 ); (2) hybridize with wild Mallards (Lavretsky et al, 2019 ), or (3) directly interbreed with the Florida Mottled Duck population, thereby disrupting this latter gene pool that has been generationally distinct (Bielefeld et al, 2010 ; Lavretsky et al, 2021 ; Peters et al, 2016 ). All three of these possibilities could occur and potentially deteriorate the integrity of the Florida Mottled Duck gene pool if subsequent hybrids or pure SACZ Mottled Ducks freely moved into Florida.…”

“…The current genetic structure of Mottled Ducks and other Mallard‐like species is well documented (Lavretsky et al, 2021 ; Peters et al, 2016 ; Weng, 2006 ). Although it is necessary to briefly overview those dynamics herein, my objective in this paper was to expand the view of the potential outcomes associated with this release (i.e., Table 1 ).…”

“…Some of these feral individuals along the Eastern Shore paired with wild Mallards and Black Ducks (USFWS, 2013 ). Feral Mallards can survive outside of shooting preserves, and Lavretsky et al ( 2019 ), Lavretsky et al ( 2021 ) recently determined that these birds are impacting genetics of North America's wild Mallard population, with one potential outcome being modified bill morphology as is evidenced in Europe (Söderquist et al, 2014 ).…”

“…f . fulvigula ) populations (Bielefeld et al, 2010 ; Lavretsky et al, 2021 ; McCracken et al, 2001 ) (Figure 1 ). Recent genetic analyses verify separation of the West‐Gulf Coast and Florida populations (Lavretsky et al, 2014 ; Weng, 2006 ).…”