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64 SCALI V., TINTI F., MANTOVANI B., MARESCALCHI O.
-BATT
-BGGR
-BGBE
-BGMA
-BREB
-BREP
1.6 1.4 0.8 0.6 0.4 0.2
Fig. 2 - Dendrogram obtained from Nei's genetic distances, showing
the genetic affinity relationships among Sicilian Bacillus. BATT, B. spermatic heterospecific nucleus and the female
atticus; BGGR, B. grandii grandit; BGBE, B. grandit benazzii,
BGMA, B. grandii maretimi; BREB, B. rossius redtenbacberi
bisexual samples; BREP, B. rossius redtenbacheri parthenogenetic
samples (from Mantovani & Scali, 1993b).
Downloaded by [31.185.101.124] at 08:47 23 March 2016 TABLE II - Nei's genetic distances (above diagonal) and identities
(below diagonal) among Sicilian Bacillus taxa: B. atticus (BATT), B. segregated grandii haploset does not degenerate to fuse
grandii grandii (BGGR), B. g. benazzii from Scopello and Levanzo
(BGBE), B. g. maretimi (BGMA),B. rossius redtenbacheri bisexual
samples (BREB)and parthenogenetic demes(BREP).
BATT BGGR BGBE BGMA BREB BREP
BATT 0.706 0.348 0.362 0.312 1.628 1.485
BGGR 0.696 - 0.217 1.498 1.358
BGBE 0.732 0.266 1.338 1.232
BGMA 0.196 0.805 - 1.431
BREB 0.226 0.109 1.315
BREP 0.766 0.897 - - 0.019
0.262
0.223 0.292 0.239 0.982
0.257 0.268
thenogens through the loss of an X chromosome, likely
in the very early embryogenetic stages (1 instance out of
about 1 000 field collected specimens).
The cytological feature, that appears to be shared by all
of the so far analyzed hybrid Bacillus, is an intrameiotic
extra-doubling of DNA content, leading to the produc-
tion of four-stranded chromosomes (Scali & Tinti, 1992).
This cytological trait seems to be present at a low
frequency even in B. rossius females (see Scali, 1969) and
it appears to be positively selected in hybrid
gametogenesis, during which the heterospecific strongly
differentiated haplosets encounter serious problems in
correct pairing and balanced segregation. Apparently, the
post-pachytene DNA doubling is an escape from a
meiotic blocking and allows, in various ways, the
production of balanced egg pronuclei, no matter
whether reduced or unreduced. This general picture
features the ephemeral condition of the males and the
foundation of an all-female taxon, which may become
fully self-sustaining in the parthenogenetic condition.
Hybridogenetic egg maturation is accomplished
through the segregation of the 2n tetrachromatidic
chromosomes, with 8C of DAPI-DNA content, into two
groups of n pseudobivalents (4C DNA content each),
given by the segregated unassorted parental sets. The
grandii chromosomes degenerate as pseudobivalents or
after a further division (1st polar body), while the rossius
complement originates a female pronucleus with n
pseudodiads (2C DNA content) following a division of
the pseudobivalents and the degeneration of one sister
nucleus (2nd polar body). Bacillus eggs are polyspermic
and several male pronuclei with 2C DNA content are form-
ed from sperm. Then a regular mixis occurs between one
pronucleus, thus restoring the hybrid structure of the
hybridogens (Scali et al, 1992; Tinti & Scali, 1992a).
Acareful analysis of B. whitei egg maturation mechanism
has revealed that its cytology is just the same as that of
hemiclonal eggs except for a single critical point: the
back to the rossius one, thus realizing the clonal trans-
mission of the maternal hybrid structure to the thelytokous
progeny (Fig. 3) (Marescalchi et al, 1991; Scali & Tinti,
1992; Tinti & Scali, 1992a). The peculiar but still very
similar features of the maturation processes observed in
hemiclonal and clonal hybrid complexes, strongly suggest
an evolution of the parthenogenetic clonáis (B. whitei)
from initially hybridogenetic strains. The issue will be more
specifically considered from the evolutionary standpoint in
the final section.
Cytological investigations on the meiotic mechanisms of
the remaining parthenogenons, B. atticus and B. lyn-
ceorum,have shown that each taxon follows a specific pat-
tern. The first division of B. atticus is normal and results in
two haploid nuclei. These, after a short interphase, fuse
together and the newly diploid nucleus per-
forms the 2nd division. One of the resultant sister nuclei
degenerates, whereas the other starts embryogenesis
(Marescalchi et al, 1993). This uncommon automictic type
of parthenogenesis explains both the clonal maintenance of
the various cytotypes and, on the genetic side, the trans-
mission of progressively lower levels of heterozygous con-
stitutions, depending on the chiasma position at prophase I
and chromosome segregation at metaphase I (Fig. 3).
Therefore, the multiclonal structure of B. atticus (Man-
tovani &Scali, 1993a) is, at least in part, certainly originated
by its specific parthenogenetic mechanism.
Maturation divisions of the triploid trihybrid B. lyn-
ceorum are «mechanically» normal, but, from the genetical
point of view, lead to the production of usually invariant
apomictic eggs (Fig. 3). This is due to the fact that
pachytenic oocytes, after a short synaptic phase, become
completely asynaptic and undergo an extra doubling of
DNA (tetrapachytene) allowing the formation of 3n four-
stranded chromosomes (autobivalents). Two divisions
follow, eventually leading to the production of a female
pronucleus with the unreduced number of unistranded
chromosomes and two degenerating polar bodies. This
mechanism, while keeping the cytological features of the
mother, clonally passes to the offspring the maternal
genetic structure (Marescalchi et al, 1991 ; Mantovani et al.,