The origin of multiple sex chromosomes in tiger beetles.

Abstract

In most holometabolous insects, sex determination involves differentiated sex chromosomes of the XY type (ZW, with heterogametic females in Lepidoptera). In Coleoptera (beetles) a general karyotype with meioformula 2n 5 9 1 XY is prevalent in the approximately 3,000 species studied to date (Smith and Virkki 1978; Petitpierre 1996), but the genetic system of Cicindelidae (tiger beetles) is unusually diverse, with many species exhibiting multiple sex chromosomes, XnY, where n varies between 2 and 4. During meiosis these heterosomes form a characteristic rosette-like multivalent linked by telomeric connections without forming chiasmata between the various X chromosomes (Giers 1977). The multiple sex chromosome system is widespread in cicindelids and is found in both the recognized subfamilies, including the Cicindelinae with a multiple system, which has been described for 55 species mostly in the species rich genus Cicindela (sensu lato), and the Collyrinae with three representatives studied to date. Other cicindelids, including two species of Megacephala (Serrano, Galian, and Ortiz 1986; Galian and Hudson 1999) and two species in the genus Cicindela, were found to have XY or X0 systems. Multiple sex chromosome systems in insects appear to be evolutionarily short-lived because they are generally confined to small taxonomic groups, where close relatives exhibit simple genetic systems. The unusually broad taxonomic distribution of multiple systems in Cicindelidae may suggest either a common ancestry of this trait and evolutionary stability (single-origin hypothesis) or a tendency to generate multiple systems repeatedly in this lineage (multiple origin). It is possible that lineagespecific properties canalize the evolution of structural features of the karyotype, and a repeated origin of a multiple system may represent the outcome of ‘‘karyotypic orthoselection’’ (White 1973). In either case the wide distribution of the multiple system in Cicindelidae and the comparison with related species exhibiting simple sex chromosome systems lend themselves to investigation of the evolution and the functional significance of this intriguing phenomenon. Here, we analyzed the evolutionary origin of multiple sex chromosomes in Cicindelidae by establishing the distribution of the multiple systems in major taxonomic groups in the context of a phylogenetic tree derived from 18S rRNA gene sequences. Chromosome