|MadSci Network: Botany|
No, chromosome numbers vary widely among plant species. Arabidopsis thaliana, a model plant for genetic research, has just five chromosomes (2n=10). It is the first plant species to have all its chromosomes, or genome, sequenced. One of the highest chromosome numbers is 2n=240 in adder's tongue fern (Ophioglossum vulgatum). The lowest number known for a plant species is 2n=4 in Shinners slender goldenweed (Machaeranthera gracilis or Haplopappus gracilis). Many plant species and especially cultivated plant species are polyploid, meaning they have more than two sets of chromosomes. Tetraploid (4n), hexaploid (6n) and even octaploid (8n) species exist. One way chromosome numbers increase is crossing of different species followed by spontaneous doubling. This is thought to be a mechanism in the origin of breadwheat, which has 42 chromosomes designated AABBCC. Einkorn wheat has 14 chromosomes designated AA. Emmer wheat has 28 chromosomes designated AABB. Botanists looked for wild plants with 14 chromosomes and found several goatgrasses that could have supplied the BB and CC chromosomes. Then they crossed the goatgrasses with Einkorn and Emmer and were able to obtain breadwheat after doubling the chromosome number. Chromosome numbers can decrease by accidental loss of a chromosome(s) or merging of two chromosomes into one. Crepis species started with 6 chromosomes but species evolved with 5, 4 or 3 chromosomes. References Arabidopsis thaliana Chromosome Numbers of Selected Organisms Karyotypes: Chromosome Numbers, Chromosome Shapes, and Phylogenetic Relations Botanical Society of America's Statement on Evolution Re: how do plants avoid fertilizing other plant species
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