(Photograph = Opuntia pinkavae, octoploid, Chris Ginkle)
Polyploidy in Cacti
Introduction
Polyploidy refers to the doubling (or more) of chromosome sets in plants, animals, and other organisms. In many cacti, the typical diploid number is 2n = 22, but genome duplications have produced tetraploids (44), hexaploids (66), and octoploids (88). Polyploidization is widely viewed as an important driver of evolution.
The Details
One immediate consequence of polyploidization is reproductive isolation. Because chromosome numbers differ, polyploids generally cannot interbreed successfully with their diploid progenitors, so a new lineage effectively splits from the old one.
Extra chromosome sets also change how plants look and function. With more DNA per cell, organs often enlarge (the classic “gigas” effect): flowers may be larger or more saturated in color, spines may be thicker, and overall growth may be more robust. These shifts can open ecological doors, allowing polyploids to occupy sites their parents did not.
Chromosome doubling usually happens in one of two ways. In autopolyploidy, a plant duplicates its own genome. In allopolyploidy, a hybrid between two species receives a second, matching set of chromosomes—stabilizing meiosis and producing offspring that differ markedly from either parent. Although such events are infrequent year to year, they accumulate over time, and many cactus lineages include polyploid species, especially within the:
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Opuntioideae,
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Browningieae,
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Pachycereeae,
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Notocacteae, and
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Cereeae.
Additional Reading: Evidence of Polyploidy in Haageocereus
Additional Reading: Polyploidy and Diversification in Pachycereus
