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EVOLUTION OF HYBRID STICK-INSECTS 61
Downloaded by [31.185.101.124] at 08:47 23 March 2016(gamete multiplication, Suomalainen et al, 1987), which
make all loci homozygous at once (Pijnacker, 1969; Scali,
I969). A cline from high to low polymorphism is observ-
ed when B. g. tnaretimi, B. g. benazzii and B. g. grandit
are analyzed in that order (Table I). The first subspecies
appears to have kept a native genepool, as also its
ecology seems to support, whereas the last is the most
differentiated, both genetically (Fig. 2) and ecologically
(Mantovani & Scali, 1993b). The thelytokous B. atticus is
surprisingly polymorphic (Table I); the finding of very
many clones (Nascetti & Bullini, 1988; Mantovani & Scali,
1993a) can be explained in terms of both its origin from
multiple interracial crosses and of the peculiar cytology
of its parthenogenesis, allowing recombination with
production of homozygous genotypes from otherwise
fixed heterozygosities (Fig. 2) (Mantovani & Scali, 1993a;
Marescalchi et al., 1993; Marescalchi & Scali, 1995a).
The overall picture of the genetic similarities among
the three above-mentioned species is reported in Figure
2. The dendrogram, based on Nei's D values and ob-
tained from allozyme frequences of 20 gene-enzyme
systems, clearly shows that B. a. atticus is much closer to
B. grandii (D = 0.31 - 0.36) than to B. rossius (D = 1.48
- 1.63) (Table II). Accordingly, B. grandii and B. rossius
are highly differentiated from each other (D = 1.23 -
1.50) (Mantovani & Scali, 1993b).
Repeated hybridization events involving the same three
taxa led to quite different outcomes. In eastern Sardinia,
extensive crosses of unisexual B. a. atticus with bisexual
B. r. rossius originated diploid, apparently fertile, hybrids mating behaviour and later a general genetic differen-
since from their backcrosses to B. r. rossius an in-
trogression of atticus alíeles into the rossius gene-pool
took place (Mantovani & Scali, 1991). In northwestern
Sicily (Fig. IB), hybridization between B. r. redtenbacheri
and B. g. benazzii produced a hybridogenetic strain,
reported as B. rossius-g. benazzii (2n = 35,XX). In
southeastern Sicily (Iblean region) (Fig. 1C), at least four
distinct hybridization events occurred, originating two
clonal parthenogenetic species and an additional
hybridogenetic strain, namely B. whitei (= rossius/g.
grandii; 2n = 35,XX), B. lynceorum ( = atticus/rossius/g.
grandii; 3n = 52,XXX) and B. rossius-g. grandii (2n =
35,XX), respectively (Nascetti & Bullini, 1982, 1988;
Bullini et al, 1983; Scali & Mantovani, 1989; Mantovani et
al, 1991a, 1992; Scali et al, 1991, 1992).
Bacillus hybridogens are the first case recorded among
invertebrates; they achive a hemiclonal transmission of
the maternal rossius genome (R) to the progeny by
discarding the grandii haploset (G) and replacing it
through real fertilization with sperm provided by syn-
topic B. grandii host males (G'), thus renewing each
generation the F, hybrid structure (RG') of the
hybridogens (Mantovani etal, 1991a; Mantovani & Scali,
1992; Tinti & Scali, 1992a, 1993).
Both hybridogens show a «neo-» rossius chromosome
set, while both clonal parthenogenetic species embody a
«paleo-» rossius haploset; therefore the clonal taxa ap-
pear to be of more ancient origin than the related
hybridogenetic strains. Furthermore, these have an in-
variant karyotype, whereas several chromosome repat-
ternings have been detected in the parthenogens
(Manaresi et al, 1991, 1992a, b, 1993). B. whitei (= B.
rossius/g. grandii) is the clonal parthenogenetic com-
plex corresponding to the hybridogenetic southeastern
strains, from which it cannot be distinguished on the
basis of gene-enzyme systems (Mantovani et al., 1992).
The trihybrid B. lynceorum deserves a few more words
about its origin, which must have occurred in two
hybridization steps. On the basis of gene-markers and of
the unisexual condition of B. atticus, it has been
suggested that an atticus x rossius cross occurred first to
produce diploid hybrids, which then mated to B. gran-
dii and incorporated a third genome to achive the double
allotriploid structure of B. lynceorum (Mantovani et al.,
1992; Manaresi et al, 1993).
HETEROSPECIFIC MATE RECOGNITION
The ability to recognize mating partners is thought to
be either a by-product of natural selection directly
favouring isolation mechanisms or, alternatively, its
primary target. These opposite views appear to charac-
terize the biological species concept and the recognition
concept of species, respectively (Paterson, 1980, 1981,
1985; Paterson & Macnamara, 1984; Kimbel & Martin,
1993). Whatever the right theoretical background, it hap-
pens in practice that mating behaviour and genetic dif-
ferentiation generally go in parallel. Known exceptions
are rather in the direction of first developing specific
tiation, than vice versa. In nature, gene-flow between
related groups of organisms may be cut off (and
reproductive cohesion within each group developed)
even with a very low genetic differentiation: it com-
monly happens with the so called eco-ethological
speciation (see Bianchi et al, 1994).
Bacillus taxa appear to have opposite features: in spite
of their sharp or even very high genetic differentiation, as
judged from gene-enzyme systems (see preceding sec-
tion), their mate recognition system does not work very
specifically. Actually, in addition to the regularly oc-
curring B. rossius-grandii x B. grandii matings that sup-
port the hybridogenetic system, in the Canicattini Bagni
area (Siracusa, Sicily) B. grandii males have been found
copulating with B. whitei females. In the same way, B.
rossius males have been seen several times to copulate
with B. atticus females at Laghi Alimini (Apulia), where
natural B. atticus/rossius F, hybrids of both sexes could
be found. As a matter of fact, mate acceptance can be so
loose that, on occasions, B. rossius males do mate in the
field with females of the distantly related unisexual
bacilline Clonopsis gallica 0une 1990, La Giannella,
Grosseto, Tuscany). It seems therefore that, in Bacillidae,
genes controlling the mate-recognition system have been
highly conserved while structural genes coding for