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Satellite Sequence Turnover in Parthenogenetic Systems: The Apomictic
Triploid Hybrid Bacillus lynceorum (Insecta, Phasmatodea)

Barbara Mantovani

Dipartimento Biologia Evoluzionistica Sperimentale, Universita` di Bologna, Italy

In the genus Bacillus (Insecta, Phasmatodea) the Bag320 satellite DNA family is present in the bisexual B. grandii Downloaded from http://mbe.oxfordjournals.org/ by guest on April 9, 2016
and in the related automictic nonhybrid B. atticus; it is lacking in the other bisexual taxon of the genus, B. rossius.
This family of highly repeated sequences was analyzed for 11 populations of the apomictic triploid hybrid B.
lynceorum. In the neighbor-joining dendrogram, B. lynceorum nucleotide sequences distribute, regardless of geo-
graphical origin, among two clusters, one also including all clones of the three B. atticus races, and the other
including sequences of the B. grandii grandii subspecies. Thus, B. lynceorum is a trihybrid taxon: as the molecular
approach definitively demonstrates, it embodies one haploid complement each of both B. grandii grandii and B.
atticus, which must be added to that of B. rossius. The contribution of the latter species has already been assessed
on karyological and allozymic grounds. A statistical analysis performed on p-distances shows that for the parental
taxa, nucleotide substitution values are of comparable magnitudes at the population level but differ at the subspecific
level, being higher for the bisexual taxon. In the apomictic hybrid, atticus- and grandii grandii– like sequences
coexist with significantly different p-distance values. For three clones, the nucleotide compositions at the diagnostic
loci suggest that gene conversion can occur between atticus- and grandii grandii–like monomers. On the whole,
this supports bisexuality as a driving force in variant fixation and suggests that in Bacillus, different gametogenetic
processes and different origins of the unisexuals are mirrored in genomic turnover rates of satellite DNA.

Introduction row area in northwestern Sicily and on Levanzo Island
(Egadi Archipelago); and B. grandii maretimi, endemic
Speciation by hybridization in animals has gener- to Marettimo Island (Egadi Archipelago). The all-female
ally been thought to be rare and of low evolutionary parthenogen B. atticus is differentiated into three races
relevance. This opinion has become more and more un- on the basis of allozyme and karyological data: the dip-
tenable owing to the increasing number of hybrid spe- loid B. atticus atticus (2n ϭ 34), widely distributed in
cies detected in different groups (Bullini 1994). The the central Mediterranean Basin (Sardinia, Sicily, south-
stick insects of the holomediterranean genus Bacillus ern Italian Peninsula, Croatia and Greece); B. atticus
provide a wide array of suitable taxa for analyzing the carius, including Greek and Turkish triploid demes (3n
relationships between parental and derived hybrid taxa. ϭ 48–51) and a single diploid Turkish population (2n
The wide investigations carried out by means of mor- ϭ 34); and B. atticus cyprius, (2n ϭ 32) from the isle
phological, karyological, allozymic, and satellite DNA of Cyprus. In diploid B. atticus, parthenogenesis is re-
approaches (Mantovani et al. 1997 and references there- alized by an automictic mechanism which proceeds
in) demonstrate that the genus comprises the bisexuals through a normal first meiotic division with homolog
B. rossius and B. grandii, the unisexual B. atticus, and pairing and chiasma formation. The diploid condition is
their related Sicilian hybrids: the diploid B. rossius- restored by fusion of the first meiotic nuclei (Scali et al.
grandii and B. whitei, and the triploid B. lynceorum 1995 and references therein; Mantovani, Tinti, and Scali
(figs. 1 and 2). This complex of taxa represents a good 1995). A hybrid origin of B. atticus was initially sug-
example of reticulate evolution and is of particular in- gested (Goday et al. 1981). At present, the bulk of cy-
terest for the reproductive biology of hybrid taxa, which, tological and allozymic data does not support an inter-
in addition to parthenogenesis (B. whitei and B. lynceo- specific hybrid structure of this taxon. The diploid
rum), make use of hybridogenesis and androgenesis (B. strains possibly represent the parthenogenetic outcome
rossius-grandii; Mantovani, Scali, and Tinti 1992; Scali of a bisexual B. grandii–like ancestor, while the triploid
et al. 1995). ones may derive from initial interactions between the
diploid parthenogens and their bisexual ancestors (Man-
The bisexual B. rossius (2n ϭ 36, XX female; 35, tovani and Scali 1993; Marescalchi and Scali 1997).
X0 male) covers most of the western Mediterranean
area, with eight races defined mainly by allozyme dis- In interspecific comparisons, B. grandii and B. at-
tances. The strictly bisexual B. grandii (2n ϭ 34, XX ticus are highly related, sharing many morphological
female; 33, X0 male) is endemic to the Sicilian area and and karyological features and showing a mean Nei’s D
is differentiated into three subspecies: B. grandii gran- value of 0.35. On the other hand, a high degree of dif-
dii, located with a relict population in the Iblean region; ferentiation emerges when these two species are com-
B. grandii benazzii, with few demes ranging over a nar- pared with B. rossius, as demonstrated by several di-
agnostic morphological characters, high Nei’s distance
Abbreviation: satDNA, satellite DNA. values (averaging 1.2), and clear differences in karyo-
types (Scali et al. 1995).
Key words: Bacillus lynceorum, hybridization, parthenogenesis,
satellite DNA, stick insects. Owing to the above interspecific divergences, the
hybrid structure, and therefore the origin, of the diploid
Address for correspondence and reprints: Barbara Mantovani, Di- unisexuals B. rossius-grandii and B. whitei is clear. Both
partimento Biologia Evoluzionistica Sperimentale, Via Selmi 3, Univ-
ersita` di Bologna, I-40126, Italy. E-mail: barman@alma.unibo.it.

Mol. Biol. Evol. 15(10):1288–1297. 1998
᭧ 1998 by the Society for Molecular Biology and Evolution. ISSN: 0737-4038

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