Complete mitochondrial D loop sequences, 910-920 bp in length, were determined for two
animals from each of six European, four African, and three
Indian breeds of cattle (listed in Table 1). Comparisons of
these sequences revealed 24 mitochondrial types defined by
polymorphism at 63 sites: one sequence was shared by two
European animals and another by two African individuals. Of
the 63 variable positions, 2 represented insertion-deletion
events of a single base pair, 1 encompassed length variation
in a poly(C) tract, and the remaining 60 were nucleotide
substitutions. Only 1/60 of these substitutions was due to
transversion, indicating a strong transitional bias. This is a
characteristic of mammalian mitochondrial evolution and has
been demonstrated in a variety of other species (especially
between closely related sequences; ref. 14). An examination
of the distribution of mutations in the D loop revealed two
major hypervariable regions, one of =375 bp between the 5'
end and the central region, containing almost 58% of the
overall number of substitutions, and a second, less variable,
region toward the 3' end (Fig. 2). Studies of human mtDNA
D loops have revealed similar tracts of sequence. The first
400 bp of human control region DNA contain almost 64% of
the total polymorphism (15) and a second, somewhat less
polymorphic, region has been documented (16). The positions of these human and bovine hypervariable regions are
similar if a known 66-bp insertion at the 3' end of the bovine
D loop (17) is taken into consideration.