Polyploidy in a phylogenetic context and correlation with the geography distribution of Cactaceae species in South America

Abstract

Cactaceae are an important floristic component of arid and semiarid areas of America and the hybridization and polyploidy are the major evolutionary forces in the family. Ploidy levels are often correlated with morphological and geographic distribution, and are crucial to the evolution and systematics of the family. The great diversification in Cactaceae has been associated with polyploidy and few chromosome rearrangements visible with conventional staining, i.e., large duplications, pericentric inversions, and reciprocal translocations of segments of unequal size. In this sense, cumulative small and cryptic structural changes are proposed to play an important karyoevolutionary role in Cactaceae. The objective of this work is to see the origin and how polyploidy events relate to the species distribution. The ancestor would have these characters: diploid (2n=22, x=11), with a small genome. Chromosome numbers in Cactaceae are nearly always a multiple of eleven (x=11). We also present a review of chromosome counts reported for Cactaceae species in South America. Ploidy in these taxa ranged from diploid, 2n=2x=22 to nonaicosaploid, 2n=29x=319. Of the 875 species of Cactaceae in South American, only chromosome counts have been carried out for 23 %, (wich 26.2% are diploid, 13.4% are both diploid and polyploid, and 60.4% are polyploidy) confirming that the frequency of genome duplication in the group is far more common than diploidy. From theecological point of view, the ability of polyploids to thrive under rigorous conditions is known. A correlation between polyploidy and aridity was determined in each of the subfamilies. Finally, a greater number of polyploid species wasobserved in extreme environments of aridity and cold.