Chromosomes and Karyotypes


Some organisms and cells have entire sets of chromosomes additional to the standard 2N diploid. Cells that have extra sets in the formula of 3N are called triploid. If they are 4N, they are called tetraploid. This is different than the case of Down’s syndrome, which has a chromosome complement of 2N+1. Any time there are abnormal numbers of chromosomes, cells are referred to as aneuploid. A special case of aneuploid occurs from having entire sets more of chromosomes – polyploid. Plants are especially robust in the regard of polyploidy and often have different species arise in such a way. Some plants become sterile in the case of polyploidy and will not produce seeds properly.

Inside a wild-type banana
Wild bananas or plantains (Musa acuminata or Musa balbisiana) are deemed inedible because of their large seeds.
Have you ever seen a banana with these large seeds? The answer is most likely “No!” since these are not regarded as being edible. However, due to selective breeding practices, most edible plantains and bananas are hybrids of the two species Musa acuminata or Musa balbisiana that are 3N or 4N. In this case, the fruit are sterile and the seeds don’t develop. Other seedless fruit are also developed this way and require propagation through clonal means.

Ideograms of triploid Musa accessions
Ideograms of triploid (2n = 3x = 33) Musa accessions: (A) Clones “Gros Michel” and “Poyo”; (B) East African Highland banana (EAHB) clone “Kagera”; (C) aneuploid EAHB clone “Imbogo” (2n = 3x − 1 = 32); (D) plantains “3 Hands Planty” and “Obino l’Ewai”; (E) aneuploid plantain clone “Amou” (2n = 3x − 1 = 32); (F) clones “Pelipita” and “Saba sa Hapon”. Chromosome paints were not used for the short arms of chromosomes 1, 2, and 10. Chromosomes are oriented with their short arms on top and the long arms on the bottom in all ideograms. For better orientation, translocated parts of the chromosomes contain extra labels, which include chromosome number and the chromosome arm that was involved in the rearrangement. (CC-BY 4.0)
Šimoníková, D.; Němečková, A.; Čížková, J.; Brown, A.; Swennen, R.; Doležel, J.; Hřibová, E. Chromosome Painting in Cultivated Bananas and Their Wild Relatives (Musa spp.) Reveals Differences in Chromosome Structure.  Int. J. Mol. Sci. 2020, 21 7915.

Tags: ,