New chromosome number and cytomolecular characterization of the African Baobab (Adansonia digitata L.) – "The Tree of Life"

Authors: ISLAM-FARIDI, NURUL - Us Forest Service (FS); Sakhanokho, Hamidou; NELSON, C. DANA - Us Forest Service (FS)

Submitted to: Scientific Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/22/2020
Publication Date: 8/6/2020
Citation: Islam-Faridi, N., Sakhanokho, H.F., Nelson, C. 2020. New chromosome number and cytomolecular characterization of the African Baobab (Adansonia digitata L.) – "The Tree of Life". Scientific Reports. 10:13174. https://doi.org/10.1038/s41598-020-68697-6.
DOI: https://doi.org/10.1038/s41598-020-68697-6

Interpretive Summary: The African baobab (Adansonia digitata L.) is a majestic and multipurpose tree of sub-Saharan Africa. Internationally, a growing demand for baobab products in the food, pharmaceutical and cosmetics industries has been observed. This long-living tree (up to hundreds of years), also known as the “Tree of Life”, lives up to its name as it is reported to have over 300 purposes including providing nutritious food, livestock fodder, fiber, medicine, and income to local people. Despite all this, scientific information on the genetics and breeding of the baobab tree is scarce, including cytogenetics, gene conservation and reproductive biology. The objectives of our cytogenetics research were to determine the genome size, chromosome number, and ribosomal (45S and 5S) DNA chromosomal distributions of the African baobab. All these three characteristics are crucial in understanding the genetics and evolution of the baobab as well as developing sound breeding programs for the species. Conflicting reports on baobab genome size and chromosome numbers were published before, but no information on ribosomal DNA distribution was available. We determined the genome size and ribosomal DNA distribution, and using our improved chromosome preparation technique, we were able to unequivocally count the chromosomes resulting in 2n = 4x = 168, a new chromosome number for the “Tree of Life”.

Technical Abstract: The African baobab (Adansonia digitata L.), also referred to as the “Tree of Life”, is a long-lived, majestic, and multipurpose tree of sub-Saharan Africa. Internationally, a growing demand for baobab products in the food, pharmaceutical and cosmetics industries has been observed. Given this, there is a need for scientific information on the genetics and breeding of A. digitata, including cytogenetics, gene conservation and reproductive biology. The objectives of our cytogenetics research were to determine the genome size, chromosome number, and ribosomal (45S and 5S) DNA chromosomal distributions of the Africa baobab. Flow cytometry analysis revealed a 2C-DNA value of 3.8 + 0.6 pg (1C genome size 919.1 + 62.9 Mbp) for A. digitata. Using our improved chromosome preparation technique, we were able to unequivocally count the chromosomes resulting in 2n = 4x = 168, a new chromosome number for A. digitata. Fluorescent in situ hybridization (FISH) analysis revealed two massively large variants of 45S rDNA and their corresponding nucleolus organizer regions (NOR). The NOR was about three or four times larger than the main body of the respective chromosomes. To our knowledge, this is the first report of this phenomenon in plant species. Furthermore, we found that FISH analysis of the Arabidopsis telomere repeat sequence clarified and confirmed the new chromosome number and the 45S rDNA distribution.