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Ecology of Free-ranging Axis Deer (Axis axis) in the Edwards Plateau Ecoregion of Central Texas: Population Density, Genetics, and Impacts of an Invasive Deer Species

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Elk (Cervus canadensis) historically are among the most widely distributed members of the deer family, occupying much of the United States, Canada, and northern Mexico. The natural distribution of this species decreased substantially in the early 20 th century, presumably resulting in the extirpation of populations in Texas. In the past 40 years, several herds of free-ranging elk have reappeared in the Trans-Pecos region of Texas. For some herds, it is not known if the origin was: 1) the result of individuals that escaped from captive herds; 2) an expansion of previously transplanted individuals from South Dakota and Oregon into Texas; or 3) the result of natural emigrants from southeastern New Mexico into the Trans-Pecos region. The objective of this study was to use DNA sequences from the mitochondrial cytochrome-b gene and D-Loop region, in combination with nine microsatellite loci, to assess genetic divergence, relationships, and origin(s) of the contemporary elk herds in Texas. Findings of the mitochondrial sequence data depicted a high degree of relatedness among individuals throughout the sampling area; whereas, microsatellite data revealed differences in frequencies of alleles in the Glass Mountain populations of Texas compared to samples from South Dakota, New Mexico, and the Davis Mountains. Further, computer simulations of population genetic parameters based on the mi-crosatellite data supported a scenario depicting the origin of contemporary elk in Texas likely was the result of natural emigrants from New Mexico or descendants of previously introduced individuals from New Mexico. In addition, simulations did not detect evidence of a genetic bottleneck during the past 350 generations, indicating a long, shared history between Texas and New Mexico populations.
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The axis deer (Axis axis) is a species of ungulate native to the Indian subcontinent. In the 19th and 20th centuries, the axis deer was introduced to many regions of the world, where it established non-native free-ranging populations. The introduction of the axis deer to Croatia resulted in three populations that still live on the Adriatic islands. In this study, two new mitochondrial DNA control region (D-loop) haplotypes were identified in 39 axis deer samples from two Adriatic islands Rab and Dugi Otok in Croatia. Two distinct D-loop haplotypes found in Croatian axis deer populations indicate that axis deer in Croatia were introduced from at least two maternal lineages. Genetic differentiation between populations was quite low and not significant. Haplotype (0.497) and nucleotide (0.006) diversity of Croatian axis deer was similar to that of axis deer from Queensland, Australia (0.461 and 0.002, respectively). For a better understanding of the origin and genetic diversity of the introduced axis deer from Croatia, analysis of native populations and the addition of highly variable nuclear markers is required.
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PrP C variation at residue 96 (G/S) plays an important role in the epidemiology of chronic wasting disease (CWD) in exposed white-tailed deer populations. In vivo studies have demonstrated the protective effect of serine at codon 96, which hinders the propagation of common CWD strains when expressed in homozygosis and increases the survival period of S96/wt heterozygous deer after challenge with CWD. Previous in vitro studies of the transmission barrier suggested that following a single amplification step, wt and S96 PrP C were equally susceptible to misfolding when seeded with various CWD prions. When we performed serial prion amplification in vitro using S96-PrP C , we observed a reduction in the efficiency of propagation with the Wisc-1 or CWD2 strains, suggesting these strains cannot stably template their conformations on this PrP C once the primary sequence has changed after the first round of replication. Our data shows the S96-PrP C polymorphism is detrimental to prion conversion of some CWD strains. These data suggests that deer homozygous for S96-PrP C may not sustain prion transmission as compared to a deer expressing G96-PrP C .
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Axis deer (Axis axis) occur both in captivity and free-ranging populations in portions of North America, but to-date, no data exist pertaining to the species’ susceptibility to CWD. We sequenced the prion protein gene (PRNP) from axis deer. We then compared axis deer PrPC sequences and amino acid polymorphisms to those of CWD susceptible species. A single PRNP allele with no evidence of intraspecies variation was identified in axis deer that indicates axis deer PRNP is most similar to North American elk (Cervus canadensis) PRNP. Therefore, axis deer may be susceptible to CWD. We recommend proactively increasing CWD surveillance for axis deer, particularly where CWD has been detected and axis deer are sympatric with native North American CWD susceptible species.
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Chronic wasting disease (CWD) is a prion disease found in both free-ranging and farmed cervids. Susceptibility of these animals to CWD is governed by various exogenous and endogenous factors. Past studies have demonstrated that polymorphisms within the prion protein (PrP) sequence itself affect an animal’s susceptibility to CWD. PrP polymorphisms can modulate CWD pathogenesis in two ways: the ability of the endogenous prion protein (PrPC) to convert into infectious prions (PrPSc) or it can give rise to novel prion strains. In vivo studies in susceptible cervids, complemented by studies in transgenic mice expressing the corresponding cervid PrP sequence, show that each polymorphism has distinct effects on both PrPC and PrPSc. It is not entirely clear how these polymorphisms are responsible for these effects, but in vitro studies suggest they play a role in modifying PrP epitopes crucial for PrPC to PrPSc conversion and determining PrPC stability. PrP polymorphisms are unique to one or two cervid species and most confer a certain degree of reduced susceptibility to CWD. However, to date, there are no reports of polymorphic cervid PrP alleles providing absolute resistance to CWD. Studies on polymorphisms have focused on those found in CWD-endemic areas, with the hope that understanding the role of an animal’s genetics in CWD can help to predict, contain, or prevent transmission of CWD.
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Invasive species have the ability to colonize new habitats across distinct areas of the globe, rapidly adjusting to new biotic and abiotic conditions, and often experiencing little impact from the decrease in effective population size and genetic diversity. Still, as each invading population represents a subsample of the original native distribution, it is common to see variability in terms of the genetic makeup of invading populations and consequently differences in invasion success rates across their non‐native range (Blackburn et al. 2017). In a From the Cover article in this issue of Molecular Ecology, Stuart and Cardilini et al. (2020) used Genotyping‐by‐Sequencing to explore how landscape and environmental heterogeneity shaped the genetic population structure and adaptation of multiple invasions of the common starling in Australia, and compare it to the patterns observed in North America, examined in Hofmeister et al. (2019). Their results suggest that the common starling worldwide invasion has been driven by a handful of genes that allowed adaptation to extreme environmental conditions and might be the key for differences in invasion success.