The arctic flora is considered to be impoverished, but estimates of species diversity are based on morphological assessments, which may not provide accurate counts of biological species. Here we report on crossing relationships within three diploid circumpolar plant species in the genus Draba (Brassicaceae). Although 99% of parental individuals were fully fertile, the fertility of intraspecific crosses was surprisingly low. Hybrids from crosses within populations were mostly fertile (63%), but only 8% of the hybrids from crosses within and among geographic regions (Alaska, Greenland, Svalbard, and Norway) were fertile. The frequent occurrence of intraspecific crossing barriers is not accompanied by significant morphological or ecological differentiation, indicating that numerous cryptic biological species have arisen within each taxonomic species despite their recent (Pleistocene) origin.crossing relationships ͉ cryptic species ͉ Draba ͉ speciation ͉ postmating isolation T he Arctic f lora has long been viewed as depauperate.Indeed, the decrease in biological diversity with increasing latitude is one of the oldest recognized patterns in ecology (1). Diversity has, however, typically been quantified as the number of morphological or ''taxonomic'' species. Little is known about biological species diversity in the Arctic (2), because the recognition of biological species requires information on reproductive isolation.Here we present results from crossing experiments within three diploid circumpolar plant species: Draba fladnizensis, Draba nivalis, and Draba subcapitata (Table 1, which is published as supporting information on the PNAS web site). Low levels of genetic differentiation within and among the species suggest they originated recently, probably within the last one million years (3). The species are mainly self-pollinated (4), but they occasionally outcross as demonstrated by reports of natural hybrids between D. fladnizensis and D. nivalis (5). We show that, contrary to expectations based on observations of limited morphological, ecological, and genetic diversity, numerous cryptic biological species have arisen within each of the taxonomic species studied here. These results imply that biological species diversity may be considerably higher in arctic regions than previously believed. ResultsAll of the 79 parental plants of D. fladnizensis and D. nivalis and six of the seven parental plants of D. subcapitata were highly fertile ( Table 1). The within-population crosses mostly generated fertile hybrids (10 of 16 crosses attempted), but six crosses resulted in semisterile hybrids (Tables 2 and 3, which are published as supporting information on the PNAS web site). The within-region crosses (among populations in Alaska) generated hybrids that were semisterile in D. fladnizensis ( Fig. 1 and Tables 2 and 3) and mostly sterile in D. nivalis (Fig. 2 and Tables 2 and 3). Likewise, the majority of crosses among regions resulted in sterile Correlations between geographic and genetic amplified fragment length polym...

Random amplified polymorphic DNA (RAPD) markers are sensitive to changes in reaction conditions and may express polymorphisms of nongenetic origin. Taxa with variable chromosome numbers are particularly challenging cases, as differences in DNA content may also influence marker reproducibility. We addressed these problems by comparing RAPD and amplified fragment length polymorphism (AFLP) analyses of clonal identity and relationships in a chromosomally variable arctic plant, the polyploid Saxifraga cernua, which has been thought to be monoclonal over large geographical distances. Fifty-seven plants from four Greenland populations were analysed using a conservative scoring approach. In total, 26 AFLP and 32 RAPD multilocus phenotypes (putative clones) were identified, of which 21 were identical and each of the remaining five AFLP clones was split into two to three very similar RAPD clones. This minor difference can be explained by sampling error and stochastic variation. The pattern observed in Greenland corroborates our previous results from Svalbard, suggesting that rare sexual events in S. cernua are sufficient to maintain high levels of clonal diversity even at small spatial scales. We conclude that although AFLP analysis is superior in terms of efficiency, RAPDs may still be used as reliable markers in small low-tech laboratories.