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GENOMIC EVOLUTION IN BACILLUS STICK INSECTS 271
Downloaded by [31.185.101.115] at 11:07 10 April 2016 peats of the Bag$20 family, since no hybridization sig-
nal is detectable in FISH experiments confronting the
parthenogens' rossius haploset and Bag320 probes. Fur-
thermore, the Bag320 repeated sequences are found in
both B. grandit and B. atticus, although with different
copy number (15-20% of the genome in the former and
2-5% in the latter), but allow a clear specific distinction
(Mantovani et al, 1997)
MtDNA analysis of the COII gene gave indications of
a somewhat more homogeneous degree of differentia-
tion among compared Bacillus taxa than that suggested
by nuclear compartments. Actually, according to this cy-
toplasmic DNA domain, B. rossius, B. grandii, and B.
atticus appear differentiated from each other to a simi-
lar extent, although quite clearly being of specific rank
(Jukes-Cantor corrected distances ranging from 0.06 to
0.09; Mantovani et al, submitted). The genetic diversifi-
cation values derived from mtDNA are not influenced
by reproductive modes and therefore could give a more
adequate picture of relationships among taxa. In this
perspective B. atticus and B. grandii must have separat-
ed earlier than the allozymes suggest. Obviously, only
additional analyses on different mitochondrial genome
regions could tell us whether such a pattern and degree
of specific differentiation are of general occurrence in
the mitochondrial compartment of Bacillus, or whether
the COII values are biased by the constraints of the
coding function of the DNA sequence.
Finally, we would emphasize that the co-occurrence
of heterospecific mt and nuclear genomes provides an
unusual opportunity for the study of genome evolution.
It has been demonstrated in yeasts that an ongoing es-
cape of mtDNA fragments occurs; for gene-sized frag-
ments, the rate of escape from mitochondria and of
their subsequent migration to the nucleus is roughly
equivalent to the rate of spontaneous mutations of nu-
clear genes; smaller fragments may appear in the nucle-
us even more frequently (Thorsness & Weber, 1996).
The effects of such intracellular transfer of genetic infor-
mation between cell organelles and the nuclear genome
are still to be elucidated.
The cohexistence of mitochondrial and nuclear
genomes of different origin, besides in Bacillus, has al-
so been reported in other animals (Kraus & Miyamoto,
1990; Ruedi et al, 1997). We would like to point out
that such conditions may provide an experimental sys-
tem for the study of mitochondrion-nucleus relation-
ships in animals.
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