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|Title: ||Hybridisation between red deer (Cervus elaphus) and Japanese sika (C. nippon) on the Kintyre Peninsula, Scotland|
|Authors: ||Senn, Helen V.|
|Supervisor(s): ||Pemberton, Josephine|
|Issue Date: ||2009|
|Publisher: ||The University of Edinburgh|
|Abstract: ||Hybridisation between introduced and endemic species causes conservation concerns, but also
provides us with an opportunity to study the dynamics of gene flow between two species as they first
meet. Japanese sika deer (Cervus nippon) were introduced to the British Isles at a number of locations
at the beginning of the 20th century. In the intervening time, sika have spread and their range now
extends across approximately 40% of Scotland, where they overlap with that of native red deer (C.
elaphus), with which they hybridise. In this study we focus on the consequences of one particular
introduction that took place at Carradale, on the Kintyre Peninsula in 1893.
First, I assessed the current state of hybridisation using a sample of 735 red and sika deer samples
collected in 2006/7 from forestry blocks throughout the Kintyre Peninsula. Genetic analysis was
conducted with a panel of 22 highly differentiated microsatellite loci and one mtDNA marker.
Population admixture analysis of the microsatellite data was conducted with the Bayesian clustering
programme STRUCTURE. Over most of the study area, levels of introgression into red and sika deer
were low and were consistent with a scenario of very occasional F1 hybridisation followed by
backcrossing. There was, however, one forestry block where 43% of individuals could be defined as
Second, I developed a branching process model of introgression via backcrossing, to assess whether
variation in introgression across microsatellite loci could be interpreted as a signature of selection, or
could in fact be attributed to stochastic processes. If only a few hybridisation events have contributed
to the hybridising population, the pattern of introgression, even with a large number of genetic
markers, will be highly stochastic. This pattern of neutral variation in introgression can have high
enough variance that it could be mistaken for selection. Therefore, even if strong selection is acting, it
may not be possible to distinguish its effects from neutral variation.
Third, I analysed trends in hybridisation and introgression over 15 years on the peninsula, through
analysis of a dataset of 1513 red and sika deer samples at 20 microsatellite and a mtDNA marker.
There was little evidence of change in the extent of hybridisation and introgression over time. MtDNA
introgression was predominantly from red deer into sika. Recent introgression into sika on the
peninsula can be explained by a very small number of F1 hybridisation events (~10) via analysis of
the number of alleles that have introgressed from polymorphic red deer into the genetically
homogenous sika population (a similar analysis cannot be conducted for introgression into red deer).
Finally, I conducted a regression analysis of genetic hybrid scores against phenotypic traits to assess
the effect of hybridisation on phenotype. Hybridisation has caused changes in the weight of sika-like
deer and red-like females. Hybridisation has caused changes in incisor arcade breadth of both
populations and jaw length (a proxy for skeletal size) in sika-like females. However, there is no
evidence that hybridisation has caused changes in kidney fat (a measure of condition) or pregnancy
rates in either population.
In conclusion, even a small number of F1 hybridisation events can lead to extensive introgression and
the timing and spatial distribution of these events is likely to have a large impact on the structure of a
recently hybridising population - stochastic factors dominate both the distribution of hybrid
individuals and the distribution of the genes that introgress following a hybridisation event. In red
deer and sika deer, increasing phenotypic similarities of the two populations caused by hybridisation
are likely to facilitate further breakdown between the two species. It is possible that breakdown in
assortative mating between the two species could occur across their range.|
Japanese sika deer
|Appears in Collections:||Biological Sciences thesis and dissertation collection|
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