Karyological evolution of the angiosperm endemic flora of the Balearic Islands
The chromosome number and known karyological features (karyotype description, presence of secondary constrictions, presence of accessory chromosomes, karyotype asymmetry) are reviewed for the sexually-reproducing endemic flora of the Balearic Islands (100 taxa), a continental archipelago in the Mediterranean. Chromosome numbers range from 2n = 8 (Crepis triasii) to 2n = 94 (Helictotrichon crassifolium), with 2n = 30 as the modal value. Overall, the number of inferred diploid taxa (57) predominates over polyploid ones (42). Intraspecific karyological variation accounts for about 10% of the whole endemic flora and includes within-individual polymorphisms related to (1) euploid changes involving somatic segregation or endopolyploidy, (2) the presence of supernumerary (B) chromosomes, and (3) karyological variation in species showing holocentric chromosomes. True infraspecific cytological variation involving fixed euploid changes has been verified only in a single taxon, Ranunculus paludosus subsp. barceloi. Overall, the documented patterns of infraspecific karyological variation in the endemic Balearic flora are of limited evolutionary potential since (1) fixed number of accessory chromosomes within individuals has not been detected in any endemic species so far analyzed, and (2) the two ploidal cytotypes of Ranunculus paludosus subsp. barceloi are geographically structured (Eastern vs. Western Balearics), precluding the formation of hybrid zones. The available data (singularity of chromosome numbers and ploidy levels, as well as biosystematic and molecular evidence) strongly suggest that at least nine polyploid endemic species (Agrostis barceloi, Allium antonii-bolosii, Anthyllis hyxtrix, Brassica balearica, Bupleurum barceloi, Centaurium bianoris, Coronilla montserratii, Helictotrichon crassifolium, Polycarpon colomense) could have had autochthonous origins in the Balearic archipelago from diploid or low polyploid-level ancestors. This hypothesized polyploid element accounts for at least 26.2% of the endemic polyploid flora and contrasts with the low estimates of polyploid evolution reported from oceanic archipelagos.
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