Measurement of Genetic Variation and Subspecies Differentiation in Panthera tigris

Abstract

Isolated populations of Pan thera tigris have been assigned subspecific taxonomic designations according to geographic and morphological descriptions. Eight subspecies are recognized; however, three have gone extinct (P.t. virgata, P.t. sondaicus, and P.t. balica) during this century largely due to human encroachment and poaching. The five extant subspecies (P.t. tigris, P.t. corbetti, P.t. sumatrae, P.t. altaica, and P.t. amoyensis) survive in small, fragmented populations. This study was designed to measure genetic diversity and differentiation in these subspecies using a suite of molecular genetic markers. Twenty-eight tigers of confirmed geographic origin designated as "voucher specimens" were examined for genetic structure using four molecular genetic families of DNA markers. Sequence data from one mitochondrial gene (16S rRNA) and a nuclear major histocompatibility complex gene (DRB) revealed slight subspecific differences and markedly low genetic variability at the species level suggesting an historic loss of genetic variation for this species. To further resolve the question of subspecific differentiation, 20 microsatellite loci (STR) were analyzed for 28 voucher specimens and 17 captive tigers of uncertain background. A phylogenetic analysis revealed genotypic clustering of individuals in a pattern consistent with geographic origins. Highly significant differences in microsatellite allele occurrence and frequencies were found between subspecies. A likelihood ratio approach based upon the multi-locus genotypes was used to show that each individual can be assigned to a subspecies based upon a composite STR genotype. Subspecies designations of all captive tigers of unknown background were confidently implicated by this method. Evolutionary relationships among subspecies were also constructed with microsatellite data. P. t. amoyensis diverges first from a common ancestor in the phylogeny. This early divergence indicates that this subspecies is the oldest among all extant tiger subspecies.