Page 7 - Scani_et_al1995

EVOLUTION OF HYBRID STICK-INSECTS 65

Bacillus rossius: GAMETE DUPLICATION Bacillus lynceorum:APOMIXIS WITH DNA EXTRASYNTHESIS
very low recombination
with recombination without recombination

"y* 1 primary oocyte primary oocyte TETRA-PACHYTENE

1st division
secondary oocyte

/\ / \ 2nd division 1st division

/ \ Igamete duplication]

secondary oocyte

\ / cleavage nuclei \ / d genotype A1A2A3

ffjK zygoid /f^ Fig. 3 - Maturation processes of parthenogenetic Bacillus, a:
automictic facultative parthenogenesis of B. rossius;b, c: automictic
AA or aa genotype AAor obligate parthenogenesis of B. atticus and of B. whitei, respectively;
d: apomictic obligate parthenogenesis of B. lynceorum. Note that:
Downloaded by [31.185.101.124] at 08:47 23 March 2016 Bacillus atticus: AUTOMIXIS WITH FUSION OF THE PRODUCTS type a makes all loci homozygous at once; type b can produce
OF THE 1st MEIOTIC DIVISION homozygous genotypes from a heterozygous one following a
crossover; type c, although reductional and automictic, produces
without recombination with recombination clonal offspring; type d is apomictic, butallows rare recombinations
to occur in the early synaptic phase before tetra-pachitene.
primary oocyte

1st division 1992); rare recombination events appear possible,
however, very likely originating during the short, partially
secondary oocyte synaptic pachytenic phase (unpublished). As a consequen-
2nd division ce, B. lynceorum unisexuals do not remain completely in-
variant over long periods.

The comparative picture stemming from both facultative
and obligatory parthenogenesis of Bacillus supports the
idea that parthenogenetic mechanisms follow species-
specific patterns (Fig. 3).

Aa genotype INTERACTING HYBRIDS AND PARENTAL TAXA

Bacillus whitei: CENTRAL FUSION WITH UNASSORTED GENOME The data discussed above, besides demonstrating the oc-
SEGREGATION ANDEXTRASYNTHESIS OF DNA currence of interspecific interactions that ultimately
defined the composition of the genus, also showed thatall-
no recombination female taxa oî Bacillus didnot become completely isolated
from their bisexual ancestors. A first type of evidence is
primary oocyte TETRA-PACHYTENE represented by the temporary Sardinian complex of
atticus/rossius hybrids: since it kept crossing to B. rossius
grandii genome rossius genome and its meiotic system apparently allowed recombination,
it didnotpersist asa defined hybrid andonly produced the
2nd division already reported paleo-introgression of atticus genes into
secondary oocyte/ the rossius pool.

central fusion As far as stabilized hybrids are concerned, allozyme
zygoid markers and parallel karyological analyses consistently
evidenced two main distinct mechanisms of natural
genotype reproductive interactions occurring between allodiploid
Aa hybrids - either hemiclonal or clonal - andtheir ancestors:
«androgenesis» and «genome incorporation». Fertilized
hybridogenetic and parthenogenetic Bacillus allodiploids
are able to produce about 1 non-hybrid progeny, namely
all-paternal fertile bisexuals (androgenetics) from the mixis