The Arctic is an excellent model system for the study of polyploidy. It is one the Earth's most polyploid-rich areas, in particular of high-level and recently evolved polyploids. Here we re-address previous hypotheses on arctic polyploidy based on a new analysis of the circumarctic flora, and review recent molecular, cytological and reproductive studies. The frequency and level of polyploidy strongly increase northwards within the Arctic. We found no clear-cut association between polyploidy and the degree of glaciation for the arctic flora as a whole, which contains many widespread species. However, for 'arctic specialist' taxa with restricted distributions, the frequency of diploids is much higher in the Beringian area, which remained largely unglaciated during the last ice age, than in the heavily glaciated Atlantic area. This result supports the hypothesis that polyploids are more successful than diploids in colonizing after deglaciation. There is abundant molecular evidence for recurrent formation of arctic polyploids at different scales in time and space. Examples are given of low-level polyploids formed after the last glaciation and of repeated and successively more high-level polyploidizations throughout the Quaternary. Recurrent polyploid origins, followed by interbreeding within and across ploidal levels, provide a major explanation for the taxonomic complexity of the arctic flora. In the well-studied, recently deglaciated archipelago of Svalbard, most species are mainly selffertilizing or clonal. All Svalbard polyploids examined so far are genetic allopolyploids with fixed heterozygosity at isozyme loci. The level of heterozygosity in 65 taxa increases dramatically from diploids to high-level polyploids. In the circumarctic area, there is evidence for numerous recently evolved sibling species within diploid taxonomic species. Rapid evolution of crossing barriers at the diploid level promotes further diversification after expansion from different refugia, and may provide new raw materials for allopolyploid formation. We conclude that the evolutionary success of polyploids in the Arctic may be based on their fixed-heterozygous genomes, which buffer against inbreeding and genetic drift through periods of dramatic climate change.

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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...

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Examples of recurrent homoploid hybrid speciation are few. One often-cited example is Argyranthemum sundingii. This example includes two described species, A. lemsii and A. sundingii, resulting from reciprocal hybridization between A. broussonetii and A. frutescens on Tenerife. The four species and artificial F1 and F2 hybrids have previously been investigated morphologically and cytologically. Here, we examine population differentiation based on amplified fragment length polymorphism to get a better understanding of the genetic relationships among the species and the extent of hybridization. We aim to investigate if there is molecular support for treating the hybrid species as one taxon. Seven parental and four hybrid species populations (149 individuals) were analysed and we scored 85 polymorphic markers. A few (2-5) were private to each species but variably present and mostly rare. Our principal coordinate, STRUCTURE and BAPS analyses and AMOVA resulted in a clear separation of the parental species. The hybrid species were genetically less divergent but not identical. Our data indicate that hybridization and introgression are common in all these species on Tenerife and support the hypothesis that homoploid hybrid speciation has occurred repeatedly. Intrinsic post-zygotic barriers are notoriously weak in Argyranthemum and reproductive isolation and speciation result primarily from strong ecological selection.