Muller Elements Description
The Muller element designation is a notation used to standardize the names of the chromosomal arms in different species of Drosophila. The names of chromosomes and their arms were arbitrarily assigned as Drosophila researchers began to work with other species of Drosophila. Beginning in the late 1930s, Sturtevant, Tan, and Novitski used comparative genetic mapping studies in a variety of species to show that the chromosomal arms of these species had similar gene content and hypothesized that the genes were syntenic (Sturtevant and Novitski 1941; Sturtevant and Tan 1937). Muller (1940) proposed a nomenclature system of six elements A to F as a standardized notation for the conserved chromosome arms among species that were later named in his honor as Muller elements (see the Table below). The 12 genomes study has largely confirmed that the genes of the six Muller elements are conserved among species (Schaeffer et al. 2008) with some notable exceptions noted below.
The table shows the syntenic relationships among the arms of the 12 sequenced genomes. We have included a view of the phylogenetic relationships of the species, their standard chromosomal numbering and the corresponding Muller element. It should be noted that there is not a simple one to one correspondence for all of the species chromosome arms and a single Muller element. This lack of correspondence is associated with three identified fusion and/or inversion events that reassociate all or portions of arms/elements in five of the species relative to D. melanogaster:
In D. erecta and D. yakuba there is a shared pericentric inversion at the base of the B.C element (2L.2R in D. melanogaster). Thus relative to D. melanogaster the B and C elements are now mixed from telomere to centromere. The new order is B/C and C/B.
In D. pseudoobscura, and D. persimilis the A and D elements are fused to form a metacentric X or first (1) chromosome from the X and autosomal 3L arms of D. melanogaster. In addition to this fusion there is a small pericentric inversion or transposition of material from the base of the A element to the base of the D element. It is unclear whether there is a reciprocal movement of D material into the base of A.
In D. willistoni there is a fusion of the F element into the proximal end of the E element. Thus there is no dot chromosome in this species and the new order is E/F. Note that D. willistoni has a similar if not identical A/D fusion as that seen in D. pseudoobscura and D. persimilis. However it does not show the A ⇒ D inversion/transposition.