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66 SCALI V., TINTI F., MANTOVANI B., MARESCALCHI O.

Downloaded by [31.185.101.124] at 08:47 23 March 2016 of two sperm nuclei among the several present in the nor- 7n titrnchromntidic
mally polyspermic hybrid eggs. Androgenesis of Bacillus chromosomes
represents the only instance so far reported among animals.
Androgenetics are homospecific with the host fathering Fig. 4 - Fertilization process of a clonal hybrid egg (RG) of Bacillus
male and this means that in the range of the northwestern whitei, leading to the formation of a triploid embryo (RGR') through
hybridogens, B. g. benazzii specimens are produced, a rossius genome (R') addition; the incorporation of the third
whereas in the area of the southeastern ones, B. g. grandit genome occurs during the automictic phase of the central nuclei.
androgenetics are obtained. We could also demonstrate The second triplet of sketched cromosomes of the embryo shows a
that more androgenetics are produced (about 1 3 ) when large submetacentric belonging to the grandii haploset, a medium-
non-syntopic host males, particularly B. grandii maretimi sized metacentric, deriving from the paieo-rossius karyotype frozen
or B. rossius, are utilized to fertilize hemiclonal females in B. whitei, and a corresponding acrocentric of similar size only
(Scali et al, 1991; Mantovani & Scali, 1992; Tinti, 1993a). found in nowaday B. rossius (from Tinti & Scali, 1994).
The production of natural androgens represents an un-
precedented instance of escape from hybridity as well as hybridogenetic eggs allow full development of rare all-
from sexual parasitism through the back-production of maternal descendants (Mantovani & Scali, 1992). Also, some
parental species specimens from their hybrids. We also inseminated egg batches of B. whitei showed a significantly
evidenced that it is possible to obtain bisexual an- different hatching than that of the corresponding unfer-
drogenetics from clonal parthenogenetic B. whitei females, tilized batches, suggesting either a facilitating or a
when inseminated by both natural or allopatric host males depressing sperm effect on embryonic development (Scali
(Tinti & Scali, 1994). et al, 1991; Tinti, 1993a). It is to be recalled that, in
Bacillus hybrids (as in Lumbricillus unisexuals), sperm is
The second main mechanism by which unisexual not needed for egg activation, because both par-
hybrids interact with their ancestors has been demon- thenogenetic and hybridogenetic eggs start dividing
strated in B. whitei: eggs of these clonal parthenogens can anyhow (Christensen & O'Connors, 1958; Mantovani &
incorporate an additional genome of their syntopic host Scali, 1992; Scali et al, 1992).
males to give rise to triploids. The genome addition to the
allodiploid zygoids (RG) has not been directly observed in The network of reproductive interactions between
the field, but only in experimental crosses. It must be hybrid unisexuals and bisexual species clearly demonstrates
stressed, however, that in Bacillus control crosses always that genetic invariance, reproductive isolation and very
gave the same results as the field-occurring ones. About 3
of triploid bisexual offspring were obtained with either
RGG' or RGR' structure, depending on the fathering male
contribution: G' or R' from B. grandii or B. rossius, respec-
tively (Fig. 4) (Tinti & Scali, 1992b, 1994).

Natural production of triploids by genome addition is
well known (Schultz & Kallman, 1968; Schultz, 1967, 1969;
Dawley & Bogart, 1989; Quattro et al., 1992b); now the
«synthetic» Bacillus represents a further instance. The
positive demonstration that an allodiploid stick-insect can
incorporate a third genome, makes the origin of the
trihybrid B. lynceorutn less conjectural and its suggested
pathway far more realistic. As a matter of fact we think that
a mictic parthenogenetic mechanism similar to the one
realized by B. whitei can allow incorporation of an ad-
ditional genome of spermatic derivation at the very
moment the segregated egg haplosets re-mix together to
keep the allodiploid clone. It is interesting to mention that
the «synthetic» RGG' and RGR' females we obtained are fer-
tile and can now be kept as an experimental new species
for investigating its cytological mechanism of egg
maturation. We would like to check directly whether the
addition of a third genome actually altered the existing
automictic process of B. whitei: this would support the
«hybrid» rather than the «spontaneous» theory of par-
thenogenesis.

Finally, a subtle and only statistically demonstrated kind
of reproductive interaction between unisexual hybrids and
bisexual species is the so called «gynogenetic effect» on
early embryogenesis. In fact, only inseminated