Introgression through rare hybridization: A genetic study of a hybrid zonebetween red and sika deer (genus Cervus) in Argyll, Scotland

In this article we describe the structure of a hybrid zone in Argyll, Scotl and, between native red deer ( Cervus elaphus) and introduced Japanese sika deer ( Cervus nippon), on the basis of a genetic analysis using II microsa tellite markers and mitochondrial DNA. In contrast to the findings of a pre vious study of the same population, we conclude that the deer fall into two distinct genetic classes, corresponding to either a sika-like or red-like phenotype. Introgression is rare at any one locus, but where the taxa overl ap up to 40% of deer carry apparently introgressed alleles. While most puta tive hybrids are heterozygous at only one locus, there are rare multiple he terozygotes, reflecting significant linkage disequilibrium within both sika - and red-like populations. The rate of backcrossing into the sika populati on is estimated as H = 0.002 per generation and into red, H = 0.001 per gen eration. On the basis of historical evidence that red deer entered Kintyre only recently, a diffusion model evaluated by maximum likelihood shows that sika have increased at similar to 9.2% yr(-1) from low frequency and dispe rse at a rate of similar to 3.7 km yr(-1). Introgression into the red-like population is greater in the south, while introgression into sika varies li ttle along the transect. For both sika- and red-like populations, the degre e of introgression is 30-40% of that predicted from the rates of current hy bridization inferred from linkage disequilibria; however, in neither case i s this statistically significant evidence for selection against introgressi on.