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satDNA Structure in Unisexual Stick Insects 1289

FIG. 1.—Map of the Mediterranean Basin showing the distribution of Bacillus taxa. Inset: the Iblean region with B. lynceorum population Downloaded from http://mbe.oxfordjournals.org/ by guest on April 9, 2016
collecting sites (1, Cassibile; 2, Catania; 3, Mascalucia; 4, Melilli; 5, Noto; 6, Ponte Diddino; 7, Ragusa; 8, Siracusa; 9, Vendicari; 10, Villas-
mundo; 11, Vittoria).

hybridogenetic and parthenogenetic females derive from chromosome redoubling in pachytene, leading to the
B. rossius ϫ B. grandii crosses. However, the origin of production of autobivalents which correctly synapse and
the triploid B. lynceorum is uncertain. A first hypothesis segregate (Marescalchi, Pijnacker, and Scali 1993).
suggested that this taxon is the product of backcrosses
between B. whitei (B. rossius/grandii grandii) and B. Noncoding, highly repeated satellite DNA (sat-
grandii grandii (Bullini, Nascetti, and Bianchi Bullini DNA) sequences form a substantial fraction of the ge-
1984). Subsequent allozymic and karyological analyses nomes of most eukaryotes. They are typically organized
pointed to the contribution of the unisexual B. atticus to as large (up to 108 bp) clusters in the heterochromatic
the genetic structure of the triploid hybrid (Mantovani, regions of chromosomes. Several genetic mechanisms
Scali, and Tinti 1992; Manaresi, Marescalchi, and Scali may affect the dynamics of these tandem arrays and pro-
1993). However, the trihybrid constitution (B. rossius/ mote their low intraspecific variability, sometimes con-
grandii grandii/atticus) could be questioned owing to trasting with a high level of interspecific diversity (Do-
the affinities between B. grandii and B. atticus. Parthe- ver and Tautz 1986; Charlesworth, Sniegowski, and Ste-
nogenesis in B. lynceorum is apomictic and relies on a phan 1994). In particular, recent computer analysis sug-
gests a positive correlation between the rate of unequal

FIG. 2.—Schematic figure showing the origin of the unisexual taxa of the genus Bacillus from the parental species. It should be noted that
the Bag320 satellite DNA family is present in the bisexual B. grandii, in the automictic B. atticus, and in all of their hybrids. The same satDNA
family is lacking in the bisexual B. rossius (shaded arrows).