Page 1 - Mantovani_alii_1999

Ital. J. Zool., 66. 265-272 (1999)
Downloaded by [31.185.101.115] at 11:07 10 April 2016Genomic evolution in parental and
hybrid taxa of the genus Bacillus
(Insecta Phasmatodea)
BARBARA MANTOVANI
MARCO PASSAMONTI
VALERIO SCALI
Dipartimento di Biologia Evoluzionistica Sperimentale,
Università di Bologna,
via Selmi 3, I-40126 Bologna (Italy)
ABSTRACT
Fifteen-year-long investigations have demonstrated that bisexual
Bacillus species (B. rossius and B. grandii), although sharply dif-
ferentiated for allozyme genes (Nei's D, 1.23-1.62), have hybridized
several times in Sicily, generating hybridogenetic strains (B. rossius-
grandii) and parthenogenetic taxa, the diploid B. whitei (B. ros-
sius/B. grandii) and, together with the telytokous taxon B. atticus,
the triploid trihybrid B. lynceorum (B. rossius/B. grandii/B. atticus). basin with eight races mainly defined on the basis of
B. atticus is much closer to B. grandii (D, 0.31-0.36) than to B. ros-
sius (D = 1.82). Furthermore, SEM and allozymatic investigations
allowed it to be established that body and egg characters are more
conserved than gene-enzyme systems. Recently, satellite DNA se-
quences (the Bag320 family) and a mitochondrial coding gene
(COII) were also analysed. It could be noticed that Bag sequences
provided differentiation values among taxa higher than coding nu-
clear genes. Furthermore, it was discovered that, while partheno-
genesis in B. atticus leaves random levels of individual variability
of repeats - likely due to the absence of chromosome shuffling in
the progeny -, the nucleotide sequences of B. grandii split into
subspecific clusters, indicating that sequence variant homogeniza-
tion/fixation are at work in Mendelian reproducing species. In B.
whitei and B, lynceorum, only a limited sequence variability of
grandii-like repeats was observed, that suggested a rather recent
origin for them, although sufficiently old to allow gene-conversion
to start between grandii-like and atticus-like sequences in B.
lynceorum. The invention of both grandii-like and atticus-like Bag
320 clones also established beyond doubt the B. atticus contibu-
tion to the structure of this triploid hybrid. MtDNA of the COII
gene gave indications of a more homogeneous degree of differen-
tiation among taxa than that suggested by nuclear compartments,
since B. rossius,B. grandii, and B. atticus appear to be differentiat-
ed from each other to a similar degree. Furthermore, COII unex-
pectedly demonstrated that, in addition to hybridogens and B.
whitei, also B. lynceorum has B. rossius as the maternal ancestor,
thus indicating a hybridization route different from that previously
thought for this hybrid. Finally, mtDNA served as a genetic marker
to demonstrate that androgenesis does occur in nature, since from
hybridogenetic females, B. grandii specimens were produced, car-
rying mtDNA of B. rossius.Androgens provide an unusual oppor-
tunity of genome evolution since gene-size fragments of DNA can
escape from mitochondria and migrate to the nucleus.
KEY WORDS: Stick insects - Genome evolution - mtDNA -
satDNA - Allozymes - Hybrid parentage - Parthenogenesis - Natur-
al androgenesis.
ACKNOWLEDGEMENTS
We appreciate the skilful suggestions made by Dr. Ombretta
Marescalchi for egg maturation process drawing.
Invited paper at the Symposium on "Structure and Evolution of
Genome", 59th National Congress of the Unione Zoologica Ital-
iana held in San Benedetto del Tronto in September 1998.
INTRODUCTION
An ample series of investigations earned out by means
of morphological, karyological, and allozymatic ap-
proaches (Mantovani et al, 1997, and references therein)
demonstrated that the holo-Mediterranean genus Bacillus
comprises the bisexuals B. rossius and B. grandii, the
unisexual B. atticus and, in Sicily, their related hybrids:
the diploids B. rossius-grandii and B. whitei, and the
triploid B. lynceorum (Figs 1, 2). These have been inves-
tigated also for their peculiar reproductive biology. Hy-
brid taxa can actually reproduce by parthenogenesis (B.
whitei and B. lynceorum) or hybridogenesis and andro-
genesis (B. rossius-grandii, Fig. 3). The genus Bacillus,
on the whole, constitutes a good example of reticulate
evolution (Scali et al, 1995, and references therein).
The bisexual B. rossius (2n = 36, XX female; 35, X0
male) ranges over most of the western Mediterranean
genetic distances obtained from gene-enzyme analyses.
Two such races are found in Italy: B. rossiusrossiusand
B. rossius redtenbacheri. The latter is widespread in
Sicily and represents the maternal ancestor of all Bacil-
lus hybrids (see below). The strictly bisexual B. grandii
(2n = 34, XX female; 33, X0 male) is endemic to the Si-
cilian area and is differentiated into three formally de-
scribed subspecies: B. grandii grandii located with a
few relict populations in the Iblean region; B. grandii
benazzii with demes scattered over a narrow coastal
belt in northwestern Sicily and on the Island of Levanzo
(Egadi Archipelago); B. grandii maretimi endemic to
the Island of Marettimo (Egadi Archipelago). Also the
all-female parthenogen B. atticus is differentiated into
three races on the basis of allozyme and karyological
characterization: the diploid B. atticus atticus (2n = 33-
34) widely distributed in the central Mediterranean
basin (Sardinia, Sicily, southern Italian peninsula, Croatia
and Greece); B. atticus carius including the Greek and
Turkish triploid demes (3n = 48-51) and a single diploid
Turkish population (2n = 34); B. atticus cyprius (2n =
32) from the island of Cyprus (Scali et al, 1995).
Morphological features, karyotype characters and ge-
netic distances on allozyme loci (Nei's D value = 0.31-
0.36) point to a close relationship between the bisexual
B. grandii and the unisexual B. atticus. For the sake of
argument, it should be borne in mind, though, that
parthenogenetic reproduction could have slowed down
the differentiation in B. atticus. On the contrary, when
these two species are compared to B. rossiusa high de-
gree of differentiation emerges. This is well supported
by several diagnostic morphological characters, high
Nei's distance values (Nei's D value = 1.23-1.62) and
clear differences in karyotypes (Scali et al, 1995).
Owing to the above-reported interspecific diver-
gences, the hybrid structure and the origin of the
diploid unisexuals B. rossius-grandii and B. whitei
have been clearly traced and defined. Both hybridoge-
netic and parthenogenetic females derive from B. ros-