Genetic differentiation among populations ofArabis serrata (Brassicaceae) along its geographic distribution

Levels and distribution of genetic variation were investigated in ten populations of the perennialArabis serrata distributed along a latitudinal gradient throughout Japan. Populations of this endemic species occupy predominantly three types of habitats: limestone and derived soils, volcanic and disturbed sites. Previous studies showed that plants ofA. serrata are differentiated in morphological and ecological traits under both natural conditions and common garden experiments suggesting genetic differentiation among populations. To assess the degree of genetic differentiation under different habitats ofA. serrata populations, we analyzed the isozyme genetic structure. Ten populations, located in mountains of central and northern Honshu and Hokkaido, were analyzed by starch gel enzyme electrophoresis. Fourteen loci of eight enzymes were resolved and six loci were monomorphic for all the populations. Populations sampled maintain low levels of genetic variation (P=0.16;H=0.05;A=1.16) compared to that maintained by other outcrossing seed plants. In some cases, few or no heterozygous individuals were detected, and consequently, mean observed heterozygosity was zero or near zero. Six (29%) of the fixation indices,F, estimated deviated significantly from Hardy Weinberg genotypic expectations indicating a deficiency of heterozygotes in most of the cases. The mean genetic identity (Nei'sI) between population pairs was 0.852 and indicates a moderate level of genetic differentiation among populations.Arabis serrata has most of its genetic variation distributed among rather than within populations. The among-population component of the total genetic diversity (G _ST mean value) was 0.416, indicating genetic differentiation between populations. There groups of populations were recognized in an unrooted tree generated by the Neighbor-Joining method. These results suggest groups of populations differentiated regionally. Estimates of interpopulational gene flow (Nm) were very variable (range 0.049–3.718) with a meanNm=1.203 for all populations.