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GENOMIC EVOLUTION IN BACILLUS STICK INSECTS 269
Downloaded by [31.185.101.115] at 11:07 10 April 2016 100 the detection of both atticus- and grandii grandii-Vike
sequences within the majority of populations, thus de-
Bg finitively demonstrating that this taxon embodies one
haploid set of both B. grandii grandii and B. atticus. B.
Ba lynceorum has therefore a double allotriploid hybrid
structure. In fact, karyological and allozymatic investiga-
0.1 tions clearly evidenced the presence of B. rossius and
B. grandii grandii genomes; the molecular approach
Fig. 5 - Dendrogram obtained from mt COH gene sequence com- established beyond doubt the additional contribution of
parisons in parental and hybrid taxa of Bacillus stick insects. B, atticus (Mantovani et al, 1992; Mantovani, 1998)
Br-gb, hemiclonal strains of B. rossius-grandii benazzii; Bw, B.
tvbitei; Bg, B. grandii; Br, B. rossius;Bl, B. lynceorum; Ba, B. atti- In dendrograms, the variability of B. lynceorum se-
cus. Figures on the branches indicate bootstrap values. Note that: quences of either atticus- or grandii grandii origin do
i) all hybrids have B. rossius as the maternal ancestor, thus rein- not follow any geographic trend (Fig. 4), thus revealing
forcing the asymmetry of hybridization crosses; ii) the Bg* refers a behaviour comparable to that observed for B. atticus
to B. grandii specimens derived through androgenesis from hy- monomers (Mantovani et al., 1997). The absence of in-
bridogenetic females: have a B. grandii nuclear genome together trapopulation or intrasubspecific higher sequence simi-
with B. rossius mt COIL larity appears, therefore, a common character of
parthenogenetic systems, possibly owing to the absence
grandii subspecies form specific cluster in phylogenetic of chromosomal reassortment in the progeny.
dendrograms, but, while the nucleotide sequences of
the three B. grandii subspecies split further into subspe- In the hybrid B. lynceorum, owing to the coexistence
cific clusters, B. atticus clones intermingle with no racial of two distinct kinds of sequences, the possibility was
or geographic trend (Fig. 4). The absence of intrasub- shown that gene conversion occurred between an; atti-
specific higher sequence similarity in B. atticus has cus- and a grandii grandii-like monomer, thus support-
been related to the influence of its automictic partheno- ing the occurrence of interchromosomal exchange even
genesis (Mantovani et al, 1997). This was also suggest- in an apomictic organism (Mantovani, 1998). The same
ed by a more detailed statistical investigation performed kind of evidence had been reached for ribosomal DNA
on B. grandii and B. atticus repeats sequence similarity: repeats in clonal parthenogens of Daphniapulex and in
B. grandii intersubspecific p-Distances are significantly hybrid lizards of the Heteronotia binoei complex
higher than intrasubspecific ones (i.e., sequence homo- (Crease & Lynch 1991; Hillis et al, 199D. On the other
geneity is higher within than between B. grandii sub- hand, the limited homogenization between atticus- and
species). On the contrary, B. atticus shows comparable grandii grandii-like monomers so far evidenced can be
levels of p-Distance values at both intra- and intersub- explained by taking into account: i) the apomictic ga-
specific ranks. It appears therefore that bisexuality acts metogenetic mechanism — which generally prevents the
strongly on sequence variant homogenization/fixation otherwise much more usual exchanges during meiosis;
in the same genomic pool, while parthenogenesis ii) the hybrid structure of B. lynceorum - probably re-
leaves random levels of individual variability owing to ducing homogenizing mechanism(s) during cell cycle;
the absence of chromosome shuffling among individu- and iii) a recent origin of the triploid (Mantovani, 1998).
als; chromosome clonality can hamper sequence variant
fixation (sensu Dover, 1986) in this unisexual taxon Mitochondria!DNA analysesand androgenesisin'nature
(Mantovani, 1998).
Among Bacillus stick insects, 25 sequences of the mi-
In the diploid hybrid B. whitei, sequences of B.
grandii kind were found to represent significantly dif- tochondrial COII gene have been sequenced (Manto-
ferent subsets of B. grandii grandii repeat variability.
On the whole, B. whitei variability is lower than that of vani et al, submitted). These have been derived from
its paternal ancestor and no intraspecific subclustering
of clones occurs (Scali & Tinti, 1999). These findings are both the parental species (B. rossius,B. grandii, B. atti-
mainly ascribed to the recent origin of B. whitei and to
the migration of hybrid females from the hybridization cus) and their hybrids (the diploid hybridogen B. ros-
focus.
sius-grandii, the diploid parthenogen B. whitei, and the
In the triploid B. lynceorum, random cloning led to
trihybrid parthenogen B. lynceorum). .;
Nucleotide and deduced amminoacidic sequences, as
well as statistical parameters, of the 16 Bacillus haplo-
types found, agree with published data on insects (Liu
& Beckenbach, 1992; Jermiin & Crozier, 1994; Frati et.
al, 1997), the only discordant feature being their very
high proportion of A+T (74.5%). In fact, the value falls
within the range of endopterygote insects (73-4-80%;
Jermiin & Crozier, 1994) rather than in that so far
recorded for neopterus Exopterigota (67-72.3%) to
which Bacillus belongs. Since comparable high levels
of A+T content are currently observed also in some ad-
ditional stick-insect species (data in progress), it can be
