Genetics 1H
Mitochondrial DNA - mtDNA
mitochondrial genome apis melllifera
DNA is found in two forms in the cell.
The nucleus contains the chromosomal DNA which only become visible during cell division. The chromosomes define the cell’s genotype.
Mitochondrial DNA are small loops of double stranded DNA found within organelles in the cell - the mitochondria. The mitochondria supply most of the energy for nearly all the cells in the bee’s body. Most of the energy in nutrients is utilized inside the mitochondria by oxidation to form the ATP molecule that can be used everywhere in the cell as an all-purpose energy supplier.
The mitochondria are believed to be the descendants of ancient bacteria that participated symbiotically with ancient cells. In the course of evolution the bacteria became integrated into the cells as organelles. Thus mitochondrial DNA is found in the loop format, just like the DNA found in bacteria. (Chromosomal DNA is linear).
Whereas an organism receives its chromosomal DNA from both parents, mitochondrial DNA is only inherited from the mother. As such mtDNA does not give any information as to the purity of the bees in a colony. The bees all have the same mtDNA but may have different drone fathers.

Unlike nuclear DNA, which undergoes recombination from generation to generation, mitochondrial DNA only undergoes recombination with DNA from the same organelle, greatly limiting genetic change. Therefore, the only factor that introduces genetic change is mutation, rather than mutation plus recombination, as is the case with nuclear DNA. This makes mitochondrial DNA into a highly useful genetic marker that can be used to compare different queen lineages especially so since mDNA in a worker is only inherited from the queen. It follows that all bees in the colony possess the same mDNA inherited from the queen.
mtDNA - gene sequence

Whereas nuclear DNA consist of over 3 billion base-pairs and codes for over 20 00 proteins, mtDNA is much smaller and in the case of A. mellifera contains 16343 base-pairs consisting of only 37 genes: 2 ribosomal RNA genes, 22 transfer RNA genes, and 13 protein-encoding genes.
The mitochondrial genome for A.mellifera.L was first sequenced in 1992 by R.H. Crozier and Y.C. Crozier. The diagram above was adapted from this article but is colour coded to illustrate the various coding regions.
By way of interest the mitochondrial genome of A. cerana was sequenced in 2011by H-W Tan et al.
It was found that the order and orientation of the gene arrangement pattern for A. cerana was identical to that of A. mellifera, except for the position of the tRNA-Ser(AGN) gene shown as S1 in the diagram (top left).
A. mellifera: gene sequence is E S1 M Q A I
A. cerana: gene sequence is S1 E M Q A I

mtDNA is a valuable tool
in the study of bee genetics and the construction of phylogenetic trees either at the species or sub-species level. mtDNA is passed unchanged from generation to generation only along the maternal line i.e. without recombination as with nDNA. This makes it suited to the study of bee genetics for two reasons:
Firstly because all the bees in the colony are the progeny of the queen their mtDNA is identical. This holds when a new queen might be present in the colony after the old queen has swarmed or the old queen is superseded.
Secondly the spread of the bees by swarming to a new area ensures that the mtDNA is identical within the old and new colony.
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